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(1)AGH University of Science and Technology Doctoral Thesis. Impact of double layer modifiers on the signal registered on the renovated solid state electrodes. Supervisor:. Author: mgr in˙z. Filip Ciepiela. Dr hab. Malgorzata Jakubowska. July 13, 2012.

(2) 1. Acknowledgments At the very beginning, I must record my debt to my supervisor Dr hab. Malgorzata Jakubowska. I must acknowledge the accorded support in stimulating my ideas in different ways, incredible patience, understanding and all the knowledge with which she shared with me. Another Person with great contribution to this work and my life was Dr hab. in˙z. Boguslaw Ba´s, who was always enthusiastic and encouraging, even in the strangest ideas. The hours of meritocratic (and not only) conversations, directed this research and will not be forgotten. I would like to acknowledge Prof. Dr hab. Wladyslaw W. Kubiak for taking me under his wings at the begging of this work as well as his efforts to broaden my horizons during our weekly meetings. I also need to thank my coworkers - colleges - friends form the Department of Analytical Chemistry, for their warm hearts, willingness to any help or conversation and pleasant work atmosphere. Finally, I would like to thank my all my friends who supported me, entertained me and cheered for me. I hope that someday I can repay for all that efforts..

(3) List of Figures 2.1. Simplified experimental arrangement for voltammetric experiments: a) three electrode configuration, b) two electrode configuration. . . . . . . .. 2.2. −4. Typical LSV voltammograms. The calibration of l-Cysteine (5·10 −2. 1·10. 16. –. −1. molL ). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 17. 2.3. Helmholz double layer model. . . . . . . . . . . . . . . . . . . . . . . . .. 19. 2.4. Guy-Chapman-Stern double layer model. . . . . . . . . . . . . . . . . . .. 20. 2.5. Excitation signal for chronoamperometry . . . . . . . . . . . . . . . . . .. 24. 2.6. Excitation signal for linear sweep voltammetry . . . . . . . . . . . . . . .. 25. 2.7. Excitation signal for staircase voltammetry . . . . . . . . . . . . . . . . .. 26. 2.8. Excitation signal for normal-pulse voltammetry . . . . . . . . . . . . . .. 27. 2.9. Excitation signal for differential-pulse voltammetry . . . . . . . . . . . .. 28. 2.10 Excitation signal for square-wave voltammetry . . . . . . . . . . . . . . .. 28. 2.11 The excitation signal during full striping voltammetric measurement. The three phases are: conditioning, accumulation and measurement, where measurement can be any voltammetric technique. . . . . . . . . . . . . .. 29. 2.12 Triton X-100 molecule, n equal to 9–10. . . . . . . . . . . . . . . . . . . .. 37. 2.13 The sodium dodecyl sulfate molecule. . . . . . . . . . . . . . . . . . . . .. 38. 2.14 The cetyl trimethylammonium bromide molecule. . . . . . . . . . . . . .. 38. 2.15 PEG molecule - n can have value between 6 and 230,000. . . . . . . . . .. 38. 2.16 The example of the humic acid molecule. . . . . . . . . . . . . . . . . . .. 39. 3.1. The voltammetric measurement setup used: a) two electrode configuration b) three electrode configuration. . . . . . . . . . . . . . . . . . . . .. 3.2. 57. The change in the peak height resulting from different ways of background correction (equal fitting errors). a) registered voltammogram and fitted background b) the same voltammogram after background correction. The colors of peaks match colors of fits. . . . . . . . . . . . . . . . . . . . . . 2. 60.

(4) LIST OF FIGURES 3.3. 3. The background approximation problem: a) the registered voltammograms with standard additions of Pb2+ b) the results of determination, after different methods of background correction, considering only results where R2 is greater than 0.995. . . . . . . . . . . . . . . . . . . . . . . .. 3.4. 61. The idea of the novel background correction method a) problem with the regular methods: where is the real base line? b) the solution of the problem: two or more calibrations with the different sensitivities. . . . .. 62. 3.5. Self-Referencing background correction algorithm. . . . . . . . . . . . . .. 64. 3.6. Relation of DPASV Pb2+ peak potential (vs. AgQRE) and AgQRE potential (vs. AgREF) to the different Cl− activities. . . . . . . . . . . . .. 3.7. The schematic picture of the Renovated Metallic Annular Band Electrode a) the electrode construction b) the renovation/activation process . . . .. 3.8. 67. Stability of RAgABE as the response in pure electrolyte. Voltammograms measured every 2 minutes – colors represents the order of measurements.. 3.9. 65. 68. Calibration of Pb2+ (the default conditions) on RAgABE. a) registered voltammograms b) the same curves after background correction c) calibration plot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 69. 3.10 Activation of RAgABE electrode, presented as: a) CV O2 reduction wave in the presence of 500 µgL−1 Pb2+ b) DPASV of Pb2+ parallel to the CV. 70 3.11 Stability of RAuABE as the response in pure electrolyte (the default conditions). Voltammograms measured every 2 minutes – colors represents the order of measurements. . . . . . . . . . . . . . . . . . . . . . . . . . .. 71. 3.12 Calibration of Pb2+ (the default conditions) on RAuABE. a) registered voltammograms b) the same curves after background correction c) calibration plot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4+. 3.13 The calibration of Se. 72. (the default conditions) on RAuABE. a) registered. curves b) the same curves after background correction c) calibration plot. −1. 3.14 The registered DPASV (n=5) curves of 10 µgL. 2+. Pb. 73. on RAgABE in. 0 – 100 mmolL−1 KCl. a) registered voltammograms b) the same curves after background correction c) calculated Pb2+ peak current . . . . . . . −1. 3.15 The registered DPASV (n=5) curves of 10 µgL −1. – 100 mmolL. 2+. Pb. on RAgABE in 0. K2 SO4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . −1. 3.16 The registered DPASV (n=5) curves of 10 µgL. 2+. Pb. 74 75. on RAgABE in 0. – 100 mmolL−1 KNO3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 75.

(5) LIST OF FIGURES. 4. 3.17 The registered DPASV (n=5) curves of 10 µgL−1 Pb2+ on RAgABE in 0 – 100 mmolL−1 KBr. a) registered voltammograms b) the same curves after background correction c) calculated Pb2+ peak current . . . . . . . −1. 3.18 The registered DPASV (n=5) curves of 10 µgL. 2+. Pb. on RAgABE in 0. – 15 mmolL−1 K2 CO3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . −1. 3.19 The registered DPASV (n=5) curves of 10 µgL. 2+. Pb. 3.20 The registered DPASV (n=5) curves of 10 µgL. 2+. Pb. 77. on RAgABE in. 10 mmolL−1 KCl and 10 mmolL−1 HNO3 with different dE values. . . . . −1. 76. 77. on RAgABE in. 10mmolL−1 KCl and 10mmolL−1 HNO3 with different tp and tw (changed simultaneously) values. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 78. 3.21 The flowchart for the sequence of operations during the tests of the polishing powder particle size. The dashed line marks the ,,cycle” in the experiment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 80. 3.22 The comparison of the electrochemical activation effectiveness of RAgABE mechanically polished with the different alumina powders sizes: 1 µm, 0.3µm and 0.05µm. Pb2+ concentration: 10 µgL−1 . . . . . . . . . . . . .. 81. 3.23 The three methods of elechtrochemical activation: a) constant potential, b) cyclic potential, c) pulse potential . . . . . . . . . . . . . . . . . . . .. 82. 3.24 The flowchart for the sequence of operations during the tests of the activation potential. The dashed line marks the ,,cycle” in the experiment. .. 83. 3.25 The comparison of effectiveness of the electrochemical activation of the RAgABE with different potentials. Pb2+ concentration 10 µgL−1 . . . . .. 84. 3.26 The comparison of effectiveness of the electrochemical activation of the RAgABE with different activation techniques. Pb2+ concentration 10 µgL−1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 85. 3.27 The signal shape after the first and the last activation cycle Emax = 1.8 V, a) curves before background correction b) the same curves after background correction. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 86. 3.28 The activation process of RAgABE expressed by changes of: a) the redox potential difference from CV (sweep rate 500 mVs−1 ) experiment and peak potential from DPASV b) the redox potential difference from the same CV experiment and peak current from the same DPASV. All three values measured in parallel during the activation cycles (see Section 3.9.2). Pb2+ concentration 500 µgL−1 . . . . . . . . . . . . . . . . . . . . . . . .. 87.

(6) LIST OF FIGURES. 5. 3.29 The relative peak current of 10 µgL−1 Pb2+ plotted against a) Triton X-100 concentration (average of three repetitions) b) time, where vertical lines are Triton X-100 additions . . . . . . . . . . . . . . . . . . . . . . . −1. 3.30 The change in the signal of 30µgL. 4+. Se. registered on RAuABE after. addition of 1 mgL−1 Triton X-100. . . . . . . . . . . . . . . . . . . . . . . 2+. 3.31 Calibrations of Pb. 90 91. registered on RAgABE in the presence of four dif-. ferent concentrations of Triton X-100 a) 20, b) 40, c) 60 and d) 80 mgL−1 . 92 3.32 The flowchart for the sequence of operations during the tests of the influence of double layer modifiers (time series). . . . . . . . . . . . . . . . . . 3.33 The relative peak current of 10. µgL−1. Pb2+. 93. registered on RAgABE in fif-. teen successive (every 5, 30, 60, 120 and 180 s) DPASV measurements in the presence of 20 mgL−1 Triton X-100. . . . . . . . . . . . . . . . . . . . . . .. 93. 3.34 The relative peak current of 10 µgL−1 Pb2+ registered on RAgABE in fifteen successive (every 5, 30, 60, 120 and 180 s) DPASV measurements in the presence of 80 mgL−1 Triton X-100. . . . . . . . . . . . . . . . . . . . . . .. 3.35 The relative peak current of 10. µgL−1. Pb2+. 94. registered on RAgABE in fif-. teen successive (every 5, 30, 60, 120 and 180 s) DPASV measurements in the presence of 160 mgL−1 Triton X-100.. . . . . . . . . . . . . . . . . . . . . .. 95. 3.36 The chronoamperometric measurement (Eend = -300 mV) of O2 reduction on RAgABE. The black arrow indicates the time when Triton X-100 was added and blue arrow indicates the interval from which the data to analysis was taken from (plateau). The sampling rate is 12 points per second. Triton X-100 additions were 1st: 10, 2nd: 10 3rd: 100 mgL−1 , total 120 mgL−1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 96. 3.37 The relative peak current of 10 µgL−1 Pb2+ registered on RAgABE in the presence of different PEG 5M concentrations (average of three repetitions). 97 3.38 The relative peak current of 10 µgL−1 Pb2+ registered on RAgABE in the presence of different PEG 1k concentrations (average of three repetitions). 97 3.39 Calibration of Pb2+ registered on RAgABE in the presence of 50 mgL−1 PEG 5M: a) obtained voltammograms, b) the same curves after background correction, c) calibration plot. . . . . . . . . . . . . . . . . . . . . 2+. 3.40 Calibration of Pb. registered on RAgABE in the presence of 50 mgL. 98. −1. PEG 1k: a) obtained voltammograms, b) the same curves after background correction, c) calibration plot. . . . . . . . . . . . . . . . . . . . .. 99.

(7) LIST OF FIGURES. 6. 3.41 Voltammograms of 10 µgL−1 Pb2+ registered on RAgABE in the presence of different SDS concentrations. . . . . . . . . . . . . . . . . . . . . . . . 100 3.42 The change if signal of 30 µgL−1 Se4+ registered on RAuABE as a function of different SDS concentrations: a) obtained voltammograms, b) the same curves after background correction, c) calculated change peak current. . . 101 3.43 The relative peak current of 10 µgL−1 Pb2+ registered on RAgABE in fifteen successive (every 5, 30, 60, 120 and 180 s) DPASV measurements in the presence of 1 mgL−1 SDS. . . . . . . . . . . . . . . . . . . . . . . . . . . . 102. 3.44 The relative peak current of 10 µgL−1 Pb2+ registered on RAgABE in the presence of different CTAB concentrations (average of three repetitions). 102 3.45 The relative peak current of 10 µgL−1 Pb2+ registered on RAgABE in fifteen successive (every 5, 30, 60, 120 and 180 s) DPASV measurements in the presence of 1 mgL−1 CTAB. . . . . . . . . . . . . . . . . . . . . . . . . . . 103. 3.46 The relative peak current of 10 µgL−1 Pb2+ registered on RAgABE in fifteen successive (every 5, 30, 60, 120 and 180 s) DPASV measurements in the presence of 3 mgL−1 CTAB. . . . . . . . . . . . . . . . . . . . . . . . . . . 104. 3.47 The change in the signal of 20µgL−1 Se4+ registered on RAuABE before and after addition of 1 mgL−1 CTAB. . . . . . . . . . . . . . . . . . . . . 104 3.48 The change in signal of 2 µgL−1 Pb2+ registered on RAuABE as a function of different CTAB concentrations: a) obtained voltammograms, b) the same curves after background correction, c) calculated change peak current.105 3.49 The relative peak current of 10 µgL−1 Pb2+ registered on RAgABE during DPASV measurements in the presence of different HA concentrations.. 3.50 The relative peak current of 10. µgL−1. Pb2+. . . . . 106. registered on RAgABE in fif-. teen successive (every 5, 30, 60, 120 and 180 s) DPASV measurements in the presence of 10 mgL−1 HA. . . . . . . . . . . . . . . . . . . . . . . . . . . . 107. 3.51 The relative peak current of 10 µgL−1 Pb2+ registered on RAgABE in fifteen successive (every 5, 30, 60, 120 and 180 s) DPASV measurements in the presence of 50 mgL−1 HA. . . . . . . . . . . . . . . . . . . . . . . . . . . . 108. 3.52 The relative peak current of 10 µgL−1 Pb2+ registered on RAgABE in fifteen successive (every 5, 30, 60, 120 and 180 s) DPASV measurements in the presence of 100 mgL−1 HA.. . . . . . . . . . . . . . . . . . . . . . . . . . . 109. 3.53 The two methods tested of potential step during accumulation: a) advance I, b) advance II. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110.

(8) LIST OF FIGURES. 7. 3.54 The relative peak current of 10 µgL−1 Pb2+ obtained using advance I and regular accumulation methods. The delay between scans was 30 seconds. 110 3.55 Calibration of Pb2+ registered on RAgABE in the presence of 50 mgL−1 HA using the advance II accumulation method: a) obtained voltammograms, b) the same curves after background correction, c) calibration plot.111 3.56 Calibration plots of Pb2+ obtained when using regular accumulation method: a) with 0 mgL−1 HA, b) with 50 mgL−1 HA. . . . . . . . . . . . . . . . . 111 4.1. Methodology of the base correction. First position of the approximation intervals: solid line – experimental for 3.75 µgL−1 of Pb2+ , dotted line – approximated polynomial of 2nd degree, thick line – position of approximation intervals. . . . . . . . . . . . . . . . . . . . . . . . . . . . 118. 4.2. The flowchart of the algorithm which realizes proposed calibration strategy.119. 4.3. The interpretations of the obtained experimental calibration data: (a) one interpolative (iCal) with one extrapolative calibration (eCal) data, (b) 3 pairs of calibration data, (c) each interpolative with each extrapolative calibration data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121. 4.4. DPASV voltammograms registered on RAgABE: (a) blank (10 mM KCl + 10 mM HNO3 ) and added 0, 0.75, 1.5, 2.25, 3 and 3.75 µgL−1 Pb2+ ; (b) enlarged part of (a); (c) the same curve as in (b) after baseline subtraction; (d) CRM SPS-SW2 and added 0.75, 1.5, 2.25 and 3 µgL−1 standard addition of Pb2+ ; (e) enlarged part of (d); (f) the same curve as in (e) after baseline subtraction;. Conditions of the accumulation: Eacc = -0.65 V; tacc = 60 s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122. 4.5. Calibration lines for interpolative and extrapolative methods, thick lines on x-axis demonstrate dispersion of results for SPS-SW2 (10-fold dissolution) obtained using two calibration approaches. . . . . . . . . . . . . . 125. 4.6. Calibration of Pb2+ in the water from quarry - sample no. I a) registered voltammograms, b) the same curves after background correction, c) calibration plot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128. 4.7. Calibration of Pb2+ in the water from quarry - sample no. II a) registered voltammograms, b) the same curves after background correction, c) calibration plot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128.

(9) LIST OF FIGURES 4.8. 8. Calibration of Pb2+ in the water from quarry - sample no. III a) registered voltammograms, b) the same curves after background correction, c) calibration plot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129. 4.9. Calibration of Pb2+ in the water from quarry - sample no. IV (accumulation time = 60 s). a) registered voltammograms, b) the same curves after background correction, c) calibration plot. . . . . . . . . . . . . . . 129. 4.10 Calibration of Pb2+ in the water from quarry - sample no. V a) registered voltammograms, b) the same curves after background correction, c) calibration plot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130.

(10) List of Tables 2.1. The recent studies on voltammetric detection of Pb2+ . . . . . . . . . . .. 44. 2.2. The recent studies on voltammetric detection of Se4+ . . . . . . . . . . . .. 46. 3.1. The default measurement conditions of Pb2+ determination on RAgABE (the two electrode setup). . . . . . . . . . . . . . . . . . . . . . . . . . .. 3.2. The default measurement conditions of Pb2+ determination on RAuABE (the two electrode setup). . . . . . . . . . . . . . . . . . . . . . . . . . .. 3.3. 55 55. The default measurement conditions of Se4+ determination on RAuABE (the three electrode setup). . . . . . . . . . . . . . . . . . . . . . . . . . .. 55. 3.4. The tp and tw optimization, dE = 30 mV (n=5). . . . . . . . . . . . . . .. 78. 3.5. The dE optimization, tp and tw = 10 ms (n=5). . . . . . . . . . . . . . .. 78. 3.6. The activation parameters, base potential Ebeg = -200mV. . . . . . . . .. 83. 3.7. 2+. Parameters of calibration plots of Pb. in the presence of different Triton. X-100 concentrations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8. Chronoamperometry of oxygen in the presence of different Triton X-100 concentrations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3.9. 91 92. Chronoamperometry of oxygen in the presence of different SDS concentrations.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 99. 3.10 Chronoamperometry of oxygen in the presence of different CTAB concentrations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 3.11 Chronoamperometry of oxygen in the presence of different HA concentrations.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106. 3.12 The influence of the RAgABE renovation procedure (1 – 4th cycle) on the Pb2+ peak current. The procedure was carried out after 15th cycle of measurements in the presence of the double layer modificator (see sections 3.10.1 –3.10.5). The percent value is calculated in the relation to the first measurement (100%) – least affected by double layer modifier. . 113 9.

(11) LIST OF TABLES 4.1. 10. Calibration line parameters and results for determination of lead by application of interpolative and extrapolative methods (1 repetition of the whole calibration procedure in each case, final results with 0.95 confidence interval). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123. 4.2. Results for determination of Pb2+ by the application of interpolative and extrapolative methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124. 4.3. The comparison between different techniques in determination of lead (Pb/Pb2+ ) in the samples from the quarry. All results in µgL−1 . . . . . 130.

(12) Chapter 1. Contents 1 Contents. 11. 2 Introduction. 14. 2.1. 2.2. Voltammetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 14. 2.1.1. Fundamental concepts . . . . . . . . . . . . . . . . . . . . . . . .. 15. 2.1.2. Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 23. 2.1.3. Working Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . .. 30. 2.1.4. Surface preparation and activation . . . . . . . . . . . . . . . . .. 34. Double layer modifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 36. 2.2.1. Surface active species . . . . . . . . . . . . . . . . . . . . . . . . .. 37. 2.2.2. Humic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 38. 2.2.3. Effects of double layer modifiers presence on electrochemical experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 39. 2.3. Adsorption models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 40. 2.4. Lead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 43. 2.4.1. Lead in the environment and its influence on living organisms . .. 43. 2.4.2. Voltammetric determination of lead . . . . . . . . . . . . . . . . .. 44. Selenium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 45. 2.5.1. Selenium in the environment and its influence on living organisms. 45. 2.5.2. Voltammetric determination of selenium . . . . . . . . . . . . . .. 45. Signals processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 46. 2.6.1. Noise in voltammetry . . . . . . . . . . . . . . . . . . . . . . . . .. 47. 2.6.2. Digital filtering - smoothing . . . . . . . . . . . . . . . . . . . . .. 47. 2.6.3. Background correction . . . . . . . . . . . . . . . . . . . . . . . .. 51. 2.5. 2.6. 3 Study of laboratory samples 3.1. 54. Default experimental settings . . . . . . . . . . . . . . . . . . . . . . . .. 11. 54.

(13) Chapter 1. Contents. 12. 3.2. Reagents and supporting materials . . . . . . . . . . . . . . . . . . . . .. 56. 3.3. Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 56. 3.3.1. The electrode system . . . . . . . . . . . . . . . . . . . . . . . . .. 57. 3.3.2. Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 57. 3.4. EALab software and its modifications . . . . . . . . . . . . . . . . . . . .. 58. 3.5. Signals processing procedures and results interpretation . . . . . . . . . .. 59. 3.6. The study on application of the silver quasi-reference electrode . . . . . .. 63. 3.7. Renovated Metallic Electrodes - construction and preliminary evaluation. 66. 3.7.1. Renovated Silver Annular Band Electrode - RAgABE . . . . . . .. 67. 3.7.2. Renovated Gold Annular Band Electrode - RAuABE . . . . . . .. 69. 3.8. Optimization of experiments conditions . . . . . . . . . . . . . . . . . . .. 70. 3.9. Electrodes surface preparation . . . . . . . . . . . . . . . . . . . . . . . .. 79. 3.9.1. Mechanical pretreatment . . . . . . . . . . . . . . . . . . . . . . .. 79. 3.9.2. Electrochemical activation . . . . . . . . . . . . . . . . . . . . . .. 81. 3.9.3. The activation performance measurements . . . . . . . . . . . . .. 86. 3.10 Influence of the double layer modifiers . . . . . . . . . . . . . . . . . . .. 88. 3.10.1 Triton X-100 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 89. 3.10.2 Polyethylene glycols . . . . . . . . . . . . . . . . . . . . . . . . .. 96. 3.10.3 Sodium dodecyl sulfate . . . . . . . . . . . . . . . . . . . . . . . .. 98. 3.10.4 Cetyl trimethylammonium bromide . . . . . . . . . . . . . . . . . 101 3.10.5 Humic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 3.10.6 Alteration of the influence of the double layer modifiers . . . . . . 108 3.10.7 Surface renovation . . . . . . . . . . . . . . . . . . . . . . . . . . 112 3.10.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 4 Environmental application of RMeABE 4.1. 115. Integrated Calibration Method for determination of Pb and Cd in natural waters on RAgABE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115. 4.2. 4.1.1. Experimental and Mathematical Procedure . . . . . . . . . . . . . 116. 4.1.2. Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . 118. 4.1.3. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . 120. 4.1.4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124. Comparison between methods (ICP-MS, Potentiometry, Voltammetry) for an environmental sample . . . . . . . . . . . . . . . . . . . . . . . . . 126.

(14) Chapter 1. Contents. 13. 4.2.1. Sample acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . 126. 4.2.2. Sample preparation for DPASV . . . . . . . . . . . . . . . . . . . 127. 4.2.3. The measurement procedure . . . . . . . . . . . . . . . . . . . . . 127. 4.2.4. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127. 4.2.5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131. 5 Final considerations. 133. A Appendix. 159. A.1 Implementation in MATLAB of the ,,self-referenced” calibration method 159 A.2 EALab file containing the experiment description . . . . . . . . . . . . . 163.

(15) Chapter 2. Introduction In today’s world, analytical chemistry plays a major role in many parts of life. In the immediate environment, toxic or harmful substances can be exposed in large quantities by simple human mistake, minor equipment malfunction or even as a natural process [1]. In order to protect the environment and human health the detection and determination of chemical species needs to be conducted on-line or on-site, and return the results instantly [2]. However, interference for the environment (no matter anthropogenic or natural) can disturb, falsify or prevent the analysis from obtain the true information. The interference like double layer modifiers (such as surface active species) are considered harmful in many analytical techniques [3, 4, 5]. Moreover, the concentration of many species, which modifies double layer, increase in the environment as a result of human activity [6, 7]. As a consequence it is crucial to know, understand and be able to suppress the effects of their interference in the registered signal. Therefore, the aim of this work was to develop the methodology of testing the influence of the double layer modifiers on signal obtained on Renovated Annular Band Electrodes. Additionally, the work had to be directed towards conducting the future work outside of the laboratory (e.g. on-site), what required fast and simple preparation of the working electrode. This approach resulted in the necessity to minimize the complexity of used equipment, reagents and procedures.. 2.1. Voltammetry. Voltammetry is one of the electrochemical methods of analytical chemistry. The main interest of the electrochemistry is in the relationship between electrical quantities (current, potential, charge) and chemical parameters of the measured system [8]. As opposed to many analytical techniques, those quantities - so the parameters of the whole system, are measured on the interface between the solid phase (electrode) and the liquid phase 14.

(16) Chapter 2. Introduction. 15. (solution) [9]. Electrochemical methods can be divided into following groups: • Voltammetry • Potentiometry • Coulometry. • Amperometry. • Polarography [8]. Voltammetry is a technique which measures change in current resulting from enforced change of potential e.g., potential linearly changing with time (Linear Sweep Voltammetry). The graph where current is plotted as a function of potential is called a voltammogram. As the potential is changed, more species are able to exchange electrons with the electrode. As a result of that exchange, the increase of the current is registered. As the potential requires a reference, minimum two electrodes need to be used. One working (or indicator) electrode, where process of interest is taking place, and one reference electrode used as as the other half of the cell. Often, also third electrode is used. The counter (or auxiliary) electrode to increase the stability of the reference potential. In the three electrode setup, the current flows between counter and working electrode, leaving the third electrode to act as reference in measuring and controlling the working electrode potential. For precise control of the potential change, a potentiostat is used. Two types of the voltammetric experimental arrangements are presented in Figure 2.1. When the species, which can exchange the electrons, are depleted in the proximity of the electrode, the current becomes proportional to the diffusion rate of the new species to the electrode. The diffusion rate is proportional to the concentration of the species in the bulk of the solution and the diffusion coefficient (Section 2.1.1). Another noteworthy propriety is that each specie exchange electrons at a certain, quite narrow potential range (the potential depends on the type of electrode, electrolyte, temperature, technique, etc.), making it possible to distinguish between species by measuring the potential at which the exchange has started [9]. A typical voltammogram is in Figure 2.2 [10]. The method is highly flexible, low-cost, provides information about speciation and has low limits of detection thanks to the development of new experimental techniques for last 70 years.. 2.1.1. Fundamental concepts. The idea of voltammetry is to measure the electron exchange rate of the redox reactions on the working electrode. In the ideal thermodynamic system the potential of the redox.

(17) Chapter 2. Introduction. 16. a) Microcontroler. i(t) E controlled. D/A Potentiostat A/D. reference electrode. working electrode. Cell counter electrode i(t) measured. b) Microcontroler. i(t). D/A Potentiostat E controlled. working electrode Cell. A/D quasi-reference electrode i(t) measured. Figure 2.1: Simplified experimental arrangement for voltammetric experiments: a) three electrode configuration, b) two electrode configuration. reaction: O + ne− R. (2.1). should be given by the Nernst equation: E = E0 +. 2.3RT CO (0, t) log nF CR (0, t). (2.2). where E0 is the standard potential for the redox reaction, R is universal gas constant (8.314 JK−1 mol−1 ), T is the Kelvin temperature, n is the number of electrons exchanged in the single reaction, and F is the Faraday constant (96,487 C) [9]. If in this system the potential more negative than E 0 is applied, the oxidized form is reducing and the balance is more favorable for the species on the right-hand side of the Equation 2.1. The current resulting from the reaction is called Faradaic because it obeys the Faraday’s law (reduction of 1 mol of substance involves the flow of n · 96, 487 C) [9]. Nevertheless,. reactions taking place at the electrodes are rarely so straightforward to explain [e.g..

(18) Chapter 2. Introduction. 17. Current / μA. -16. -8. 0. 0.2. -0.3 Potential / mV. -0.8. Figure 2.2: Typical LSV voltammograms. The calibration of l-Cysteine (5·10−4 – 1·10−2 molL−1 ). they often have a few intermediate states (i.e. active complexes)]. The current, which is flowing in the voltammetric experiment, is usually mass-transfer controlled - what means that the reaction rate is much faster than diffusion of new species, from the bulk of the solution. This current was described by Frederic Cottrell in the form of Equation 2.3. r Dj (2.3) i = nF ACjO πt Where i is current resulting from the redox reaction, j is the index of ion which is reducing, D is the diffusion coefficient, t is time in seconds and A is the area of the working electrode (planar) [11]. The Cottrell equation is particularly useful from the educational point of view, although, more complex models are developed, which provide closer approximation to the real life experiment results. Similarly, the Nernst equation does not provide accurate results. The consideration of kinetics, in addition to thermodynamics, is required to provide the reaction potential due to the temporal component of voltammetry [12]. Solid-liquid interface Interface between the electrode and the solution is the area of the highest interest in the voltammetric experiment. On the interface the charge transfer takes place, gradients in.

(19) Chapter 2. Introduction. 18. electrical and chemical potentials constitute the ,,driving forces” of the electrochemical reactions [13]. The early models presented the interface as a capacitor with one plate being the electrode and the other plate being the solution, whence the name of this interface: ,,the double-layer” [9]. This simple model, called Helmholz model (the detailed description in Section 2.1.1), works surprisingly well in many applications but even it recognizes the problem of the state of the electrode surface. Surface of the solid dipped into the solution is not a perfect crystalline lattice. The most obvious is a fact that lattice should be infinite and since there is an interface, there is an end to the lattice. Another source of deformation are adsorbed species, which will interfere with the surface structure. A lot of other problems with the electrode surface exists, which were discussed in details in many publications [13, 14, 15]. Because of this imperfection the surface of the solid does not have a homogeneous distribution of the electrons energy, what leads to the fact that the energy of the bulk as well as the surface depends on the local electrostatic potential. Furthermore, the strong potential gradient at the interface is created, hence, the species near the surface of the electrode will be affected by different portion of the externally applied potential than the bulk does. This may lead to a phenomenon, where on the different parts of the electrode different processes are taking place – the more imperfect the surface of the electrode is, the more processes with higher reaction rates can be involved [13]. When the reaction rate of the interfering processes is comparable to the rate of process of interest, no or little information can be obtained. Double layer models The double layer creates spontaneously on the interface contact between two different phases (phases with different surface potential). This phenomenon is very complicated and its description requires to include many different interactions. Nevertheless, the problem of double layer was noticed as early as 1853, when the first model of double layer by Hermann von Helmholtz appeared. The first model was very simple and soon new models, which added complexity to it, followed. Below, two selected models are presented, Helmholz model – as the first double layer model, and Guy-Chapman-Stern model – which resembles the real word double layer..

(20) Chapter 2. Introduction. 19 d. potential. Esolution electrode. solution. Eelectrode distance. Figure 2.3: Helmholz double layer model. Helmholz model Helmholz proposed that, since in the case of metal the excess charge is located on the surface, in the liquid the same is happening – the counter-charge layer of solution particles is created at the interface liquid-metal. The result of that assumption is appearance of two charged planes, with opposite polarity, separated by a distance of molecular order (Figure 2.3). The structure resembles a parallel plate capacitor and should be governed by the same equations:. 0 V (2.4) d where, σ is the charge stored by capacitor, is the dielectric constant of the medium, 0 σ=. is the permittivity of vacuum, d is the distance between the plates and V is the voltage drop. From the equation above the differential capacitance can be derived as: ∂σ 0 = Cd = ∂V d. (2.5). This model introduces a lot of assumptions and simplifications – e.g., infinite diffusion speed, all unpaired ions in the solution are at the same distance d from the electrode etc. The number of simplifications used, make this models predictions close to registered values only for very high concentrations of electrolyte in the solution [8]..

(21) Chapter 2. Introduction. 20 IHP OHP. diffusive layer. potential. Esolution. electrode. solution. Eelectrode distance. Figure 2.4: Guy-Chapman-Stern double layer model. Guy-Chapman-Stern model This is an extended by Stern of Guy-Chapman model, called Guy-Chapman-Stern(GSC) (Figure 2.4). According to the model, the charge carrier density is low compared to the density of non charged particles. Additionally, the diffusion speed is limited. Therefore, it may take some significant thickness of solution to accumulate the excess charge needed to neutralize the electrode charge. Since, the charge carriers are a subject of thermal diffusion, the additional layer is created called ,,diffusive layer”. The highest concentration of charge carriers would be in the vicinity of the interface, where electrostatic forces are strongest, while progressively lesser concentrations would be found at greater distances as those forces become weaker. The GSC model, also predicts that when an electrode is immersed the solvatation process occurs. Thus, any interaction has to take into account a layer of solvent molecules adsorbed at the interface (Inner Helmholz Plane) and solvatation of charge carriers themselves (Outer Helmholz Plane). The model allows for a free diffusion of all ions except for those adsorbed within the outer Helmholz plane (OHP). The model has a complex mathematical derivation and the result of it is presented in Equation 2.6. xi 1 1 = +p 2 2 −1 Cd 0 0 z e c0 (kT ) cosh[zeEi (2kT )−1 ]. (2.6).

(22) Chapter 2. Introduction. 21. where, Ei is a potential at xi , e is electron charge, k is Bolzman constant, T is absolute temperature, z is oxidation state of the charge carrier and c0 is the bulk concentration of the charge carrier [8, 9]. The GSC model provides much more reliable results than Helmholz model, even with low electrolyte concentrations. However, still many interactions are not taken into account; e.g., the structure of the dielectric in the compact layer, ion pairing effects in the double layer, strong nonspecific interactions of the ions with the surface charge on the electrode. Nevertheless, nowadays, only some very complex models resulting from mathematical modeling give more accurate information about the double layer structure [16]. Mass transfer There are three major types of mass transfer which can be involved in voltammetric experiment: • diffusion,. • migration,. • convection.. Diffusion is the spontaneous movement, caused by the difference in local concentrations. Species diffuse from regions of high concentration to regions of low concentration. The aim of diffusion is to lower the concentration gradient. In voltammetric experiment the diffusion always plays a major role, since an analyte needs to diffuse through the double layer in order to reach the electrode. Convection is the transport of species by gross physical movement, it is used during accumulation process in stripping voltammetry and is caused by stirring of the solution. Migration is move of charged particles along an electrical fields. In voltammetry this type of movement is undesirable. Its influence is nullified by adding a supporting electrolyte and by minimizing the distance between the electrodes [9]. All three types of mass transport are responsible for flux. The flux is a common measure of the rate of mass transport. It is defined as the number of molecules moving through a unit area of an imaginary plane in a unit of time and is expressed in units of mol(cm2 s)−1 . The flux is described by the Nernst–Planck equation: J(x, t) = −D. ∂C(x, t) zF DC ∂E(x, t) − + C(x, t)V (x, t) ∂x RT ∂x. (2.7). where D is the diffusion coefficient (cm2 s−1 ); ∂C(x,t)∂x−1 is the concentration gradient (at distance x and time t) – together it describe the diffusion component; ∂E(x,t)∂x−1.

(23) Chapter 2. Introduction. 22. is the potential gradient, z and C are the charge and concentration, respectively, of the electroactive species – together it describes the migration component; and V (x, t) is the hydrodynamic velocity (in the x direction) – it is the convection component. Underpotential deposition Underpotential deposition (UPD) is a phenomenon only visible in the case of solid state electrodes. It is the formation of uniform submonolayers of reduced species at potentials a few hundred mV more positive than the Nernst equilibrium potential [17]. This formation occurs only at ,,active-sites” on the surface of the electrode - the adatom coverage is in the range of 0.01–1% [18]. The formation is followed by the deposition of the bulk-phase metal. UPD occurs at solid electrodes due to stronger interactions of the deposited atoms with the substrate than with each other (i.e. less energy is required to reduce the first monolayer of metal). This means that, at potentials lower than predicted by the Nernst equation (Equation 2.2) up to a monolayer of the metal may deposit on the metal electrode substrate. This can be regarded as an adsorption process (Section 2.3), in which the Gibbs energy of the adsorbate–substrate interaction is stronger than for the adsorbate–adsorbate interaction. Therefore, at a solid electrode, two (or more) reduction and stripping processes (for stripping process please see Section 2.1.2) can occur, increasing complexity of the interpretation of the analytical signal. Interesting is the fact, that in concentrated electrolyte solutions UPD active sites can be ,,poisoned” by adsorption of alkali metals, decreasing the electrode sensitivity. Even at the low potential ranges, where alkali ions are apparently electrochemicaly non-active, ions such as Na+ or K+ can adsorb on the active sites of the electrode [19]. Despite its complicated nature, UDP has proven to be be analytically-useful. The voltammetric methods based on these phenomena, such as stripping voltammetry (Section 2.1.2), offer outstanding sensitivity and selectivity for a number of trace metal species in different matrices [15]. Bias of the measured current The two main difficulties arise from conducting the current measurements in solution: double layer charging effect and Ohmic drop. The double layer charging current or just charging current is a result of rearrangement in the double layer structure. Such a rearrangement happens at every potential change;.

(24) Chapter 2. Introduction. 23. e.g., in LSV experiment (Section 2.1.2), where the potential change is continuous, the charging current is always present at high level. It is the most important interfering current in the majority of experiments. This current on planar electrode can be described by equation:. Es −t exp (2.8) R RCdl is the double layer capacitance and R is a electrical resistance between the idlc (t) =. where Cdl. electrodes, Es is a potential step in experiment (Section 2.1.2) and t is time [20]. Therefore, the charging current decays exponentially with time. When compared with the Cottrell equation, where Faradaic current is proportional to t−1/2 , it is clear that given enough time, with no potential change, the charging current should become lower than Faradaic current - what allowed new voltammetric techniques to develop. Ohmic drop is a consequence of the limited conductivity of a solution. Resistance of the solution is responsible for a potential drop between the electrodes in the cell, called the Ohmic drop. The magnitude of the Ohmic drop, EiR , is given by Ohm’s Law: EiR = icell · R. (2.9). where icell is the current flowing in the cell, and R is the solution resistance. Ohmic drop causes that the potential observed by solution species at the working electrode is lower than the one indicated by the potential measuring device. There is several methods of decreasing the magnitude of Ohmic drop: • when working in the three electrode configuration, placing the reference electrode very close to the working electrode will decrease the effect,. • using the microelectrodes will decrease flowing current, and hence Ohmic drop,. • nowadays, apparatus used in measurements can partially prevent the effects of the Ohmic drop e.g., by application of higher potential based on the resistance measurements [21], • using high concentration of supporting electrolyte increases the solution conductivity, decreasing the Ohmic drop [21].. 2.1.2. Techniques. The development of voltammetry for almost a century, provided the researchers with many techniques which can be applied in a different experiments. In this subsection following voltammetric techniques will be described in more detail:.

(25) Chapter 2. Introduction. 24. • chronoamperometry. • linear sweep voltammpetry • staircase voltammetry. • normal pulse voltammetry. • differential pulse voltammetry • square wave voltammetry. As well as modifications to these techniques - cycle voltammetry and stripping voltammetry which are often applied. The description used in this section is consistent with the settings of 8KCA analyzer and EALab software (see section 3.4 and article [22]) and may differ from traditionally used systems. Chronoamperometry In chronoamperometric experiment the current is measured, which results from change of the potential, from value where no Faradaic current occur to the value where the concentration of the electroactive species is effectively zero (i.e. where applied E is higher than E from equation 2.2). The potential vs. time plot is given in Figure 2.5. When no stirring of the system is applied, the current equal should be proportional to the diffusion rate of the electroactive species. The result of chronoamperometric experiment is presented as a plot of time vs. current and not potential vs. current.. potential. E end. E beg. t 0. time. Figure 2.5: Excitation signal for chronoamperometry.

(26) Chapter 2. Introduction. 25. Linear Sweep Voltammetry In the linear sweep voltammetry (LSV) the potential is changed linearly from potential where no Faradaic current is present to potential above potential from equation 2.2 Figure 2.6. The rate of change is described by equation: rate =. dE dt. (2.10). This rate varies between 0.1 mVs−1 and 1000 Vs−1 . The resulting current for redox reaction is a wave shaped signal where half-wave potential is given by equation: r DR RT 0 log (2.11) E1/2 = E + nF DO where DR and DO are diffusion coefficients of the redox couple. The maximal current is given by the Cottrell equation. The method is designed for an analog devices – the. potential. E end. dE dt. Ebeg 0. time. Figure 2.6: Excitation signal for linear sweep voltammetry current results should be continuous. In the digital devices the current is sampled usually at maximal rate allowed by analog-to-digital converter, but more often the staircase voltammetry is used to approximate the LSV [22]. Staircase Voltammetry In staircase voltammetry (SCV), the potential of the working electrode is stepped from the potential where no Faradaic current is flowing to the potential above Nernst potential in increasing steps of constant amplitude (Es ). The length of each step depends of two parameters tw and tp . The parameter tw (waiting time) describes the delay of probing.

(27) Chapter 2. Introduction. 26. the current after each potential step is applied (this time should allow double layer to recharge). Time of probing - tp describes for how long the current should be probed to obtain a single data point. This time should be long enough, so that more than one value of current is registered (the final result should be an average) - the recommended value is at least three samples per step [8, 22]. Additionally, SCV can take full advantage of the resolution of the potentiostat’s digital-to-analog converter, what is a problem in LSV experiment [23]. The method is more sensitive than LSV because of, at least partial,. potential. E end tw tp Es. Ebeg 0. time. Figure 2.7: Excitation signal for staircase voltammetry elimination of the charging current by appropriate selection of tw and tp . Normal Pulse Voltammetry Normal pulse voltammetry consists of a series of pulses of linearly increasing amplitude (Es ) applied at constant time intervals (Figure 2.8). Between the pulses the electrode is kept at potential E0 where no Faradaic current should flow. The current is sampled twice just before and right at the pulse (Figure 2.8). The first current is subtracted from the second and the difference is plotted against the pulse potential. The technique allowed to increase the Faradaic current (given by Equation 2.3) about 5–10 times compared to LSV [9]. Differential Pulse Voltammetry In differential pulse voltammetry fixed amplitude pulses (dE) are imposed over linear potential ramp (or its staircase counterpart Es [22]). The current in DPV, similarly as in NPV, is sampled twice just before and right at the end of the pulse (Figure 2.9)..

(28) Chapter 2. Introduction. 27. potential. Eend Es twtp. Ebeg. E0. twtp 0. time. Figure 2.8: Excitation signal for normal-pulse voltammetry The first current is subtracted from the second and the difference is plotted against the potential applied before pulse. In the resulting voltammogram the signal from analytes is in a form of peaks which height and area are directly proportional to the concentration. The maximum useful pulse magnitude is around 50 mV - depending of the reaction rate and type [9]. The current is given by: r ij = nF ACj. Dj πtm. . 1−σ 1+σ. (2.12). Where tm is the time after application of the pulse at which the current is sampled, σ is equal to: . nF σ = exp RT The potential of peak maximum is equal to:. . E = E1/2 −. dE 2. . dE 2. (2.13). (2.14). Square Wave Voltammetry Square wave voltammetry (SQW) is technique, where a symmetric square waves are superimposed over staircase potential base (Figure 2.10). Since the wave amplitude is usually very large (dE) the reverse pulse causes the reverse reaction of the product (the method is designed for reversible reactions) [8, 9]. The technique is also designed for very fast scan rates - frequencies of 1 – 100 cycles per second, what allows to increase the sample throughput in batch and flow analytical operations [9]..

(29) Chapter 2. Introduction. 28. potential. E end. tw tp. dE. Es. Ebeg. tw tp 0. time. Figure 2.9: Excitation signal for differential-pulse voltammetry. potential. E end tw tp dE. Es. Ebeg. tw tp 0. time. Figure 2.10: Excitation signal for square-wave voltammetry Cyclic Voltammetry Cyclic voltammetry (CV) is a technique in which the solution is scanned two-ways. The first scan is from more negative to more positive potentials and the second scan is other way around (or vice-versa). The CV can provide much more information about the analyzed sample – e.g. is the analyte reaction reversible. Moreover, if CV scan is made in special conditions the width of a double layer can be derived from the results. It is widely used and powerful technique. Stripping Voltammetry Stripping analysis is voltammetric technique of extreme sensitivity for measuring trace metals [9]. The limits of detection can go as low as 10−11 , 10−12 molL−1 [24, 25]. The.

(30) Chapter 2. Introduction. 29. reason for such a high sensitivities is preconcentration. The stripping analysis is a two step technique: the first step (accumulation step) is electrolytic deposition of small portion of analyte onto working electrode, the second step is dissolution (stripping) of the deposit. The stripping part can be any of the previously presented voltammetric techniques. The general scheme of stripping measurement is presented in Figure 2.11. The stripping voltammetry can be divided in three groups - depending on the potential conditioning. accumulation. I. Econd. I II t cond t cond. t acc. measurement LSV, SCV, NPV, DPV or SQW Ebeg. potential. Eacc. II. Econd. Eend time. Figure 2.11: The excitation signal during full striping voltammetric measurement. The three phases are: conditioning, accumulation and measurement, where measurement can be any voltammetric technique. applied during the accumulation [9]. Anodic stripping voltammetry (ASV) is the most widely techniques of electrochemical analysis. The preconcetration is done by cathodic deposition (thus, it usually used in the determination of metal ions) at a controlled time and potential. The deposition potential Eacc should be around 100 – 400 mV more negative that resulting from Equation 2.2. The time is usually between 10 and 600 seconds [26] but there are even examples of longer times used. The metal ions are transported onto the electrode by convection and diffusion. The convectional movement comes from stirrer which is mixing the solution during accumulation period - the stirrer is turn off short before the measurement to calm the solution. The concentration of the deposited metal on the surface of the electrode is given by the equation: Cj =. ij tacc nF A. (2.15).

(31) Chapter 2. Introduction. 30. and is directly proportional to the concentration in the bulk solution. Cathodic striping voltammetry (CSV) is a mirror technique to ASV. During CSV the process of anodic deposition is conducted followed by negative-going potential scan. CSV allows also to determinate some non-metallic species like Cl− or Br− or even organic molecules. Adsorptive stripping voltammetry (AdSV) greatly improved the number of species possible to measure with stripping voltammetry. The AdSV involves the formation, adsorptive accumulation and reduction of the surface active complexes of metals and other electroactive species. The response signal is directly related to the surface concentration of analyte (or its ligand), while Langmuir adsorption isotherm (see section 2.3) provides the relationship between surface and bulk concentration of the adsorbate [9]. Other techniques include Abrasive stripping voltammetry and Potentiometric stripping voltammetry.. 2.1.3. Working Electrodes. Working electrode is the most important part of voltammetric system. The working electrode material and its surface characteristic and condition determines the reactions which can take place as well as the available potential range and currents [9, 27, 28]. A perfect working electrode should be characterized by: • high electrical conductivity • chemical stability. • rapid heterogeneous electron transfer rate with analyte of choice • reproducibility. • simple construction. • high hydrogen evolution reaction (HER) overpotential • easy fabrication. • cost effectiveness [29, 30] • non-toxicity [9, 31] • portability [32].. The advances in voltammetry resulted in the development of many types of electrodes, which were possessing several of those features. Below, a few selected and categorized types of electrodes, is presented..

(32) Chapter 2. Introduction. 31. Mercury electrodes Th search for an electrode which will provide presented above proprieties started with the beginning of voltammetry. It was not until Heyrovsky in 1922 introduced the electrode, which characterized with excellent surface conditions and good reproduciblity - the Dropping Mercury Electrode (DME) [33]. Liquid mercury electrodes have many advantages: • the surface of the electrode is always smooth even at the microscopic level, • the electrode can be easily renewed by forming a new mercury drop,. • the formation of liquid metal amalgams during the reduction process prevents the formation of intermetallic compounds. • mercury has high HER overpotential. The main problem at the time with DME was that it consumed very large amounts of mercury during measurements and mercury had to be additionally purified before measurements. The first problem was addressed by new designs of the electrode. In 1959 Kemula constructed first Hanging Mercury Drop Electrode (HMDE) [34] and in 1989 Kowalski developed Controlled-Growth Mercury Drop Electrode (CGMDE), which reduced the use of mercury to the minimum [35]. The second problem was solved due to increased purity of commercially available mercury, which nowadays is up to 99.99995% [36]. However, mercury is a toxic material and its use has been prohibited it is allowed to use only in very specialized applications and under extreme safety conditions [37, 38]. Another drawback of mercury electrode is very high rate of adsorption of different surface active species (SAS) and organic matter on the surface [39, 40]. Also, easy oxidation of mercury occurring from about 0 V makes it unusable for determination of metals in the anodic potential area [41]. Therefore, new designs of electrodes and electrodes materials are being developed. New designs cover a broad range of used materials (carbon, nobel metals, bismuth, antimony [42] etc.) and electrode types. Some of more widely used electrodes are presented in following sections. All described applications include only unmodified (i.e., non doped and not modified by organic films), mercury-free and used without any complexion agents electrodes. This strict distinction is made to keep the introduction within the aim of this work - i.e. simplicity of used electrodes and reagents..

(33) Chapter 2. Introduction. 32. Mercury-free metal electrodes While a wide choice of metals is available, platinum, gold, silver and bismuth are the most widely used materials for metal electrodes. Such electrodes offer very high electrical conductivity, good fast electron transfer kinetics and a large anodic potential range. Also on their surface the UPD process can take place, which may additionally increase their sensitivity towards other metals [15]. The metallic electrodes are used as bulk electrodes in the form of wires, nets, discs or cylinders or as a films deposited on the carbon or metal substrate. Platinum electrodes Platinum is a noble metal with one of the highest densities of all elements. Application of platinum electrodes in voltammetry was constantly increasing, since the beginning of XX century when the platinum black electrode was introduced [43]. One of the most famous use of the platinum electrode is standard hydrogen electrode (SHE), where platinum black electrode is used in the hydrogen oxidation process. The anodic potential range of the Pt electrode is limited by reaction between Pt and water or oxygen molecules, which is taking place between 850 and 1100 mV vs. SHE [44, 45]. From the cathodic side by HER around -100 – -200 mV (vs. SHE) [9]. The platinum electrodes have proven to be useful in the detection of many inorganic species, including: N2 O [46], Ag+ [47], Cr3+ [48] and As3+ [49]. Moreover, unmodified platinum electrodes are used very often in determination of different organic species and constituents of drugs like: glucose [50], naproxen [51], morphine [52], riboflavin [53], rutin [54], trazodone [55] etc. Platinum electrodes are widely used as substrates for different types of films, what allows to expand their usage on a very broad range of organic and nonorganic analytes [56, 57]. Gold electrodes Gold is a noble metal with very high density and high electrical conductivity. Gold electrodes are one of the most inert bulk electrodes, and hence are less prone to the formation of stable oxide films or surface contamination [9]. Gold electrodes have low overpotential of HER, the reaction starts at -200 – -300 mV vs. SHE [58]. On the other hand, they have very high anodic potential range, up to 1100 – 1500 mV (vs. SHE) in conventionally used electrolytes [59, 45]. Because of this high anodic potential windows the gold electrodes are used for determination of metals with high reduction potentials.

(34) Chapter 2. Introduction. 33. such as Hg2+ , Bi3+ [60] and Te4+ [61]. Despite low cathodic potential range, bulk gold electrodes are successfully used in determination of Pb2+ and Cd2+ with high limits of detection [17, 62]. It is possible because of UPD of these metals on the surface of gold electrode. Other more important inorganic species measurable on the gold electrode include CO [63], CN− [64], As3+ [65], Cu2+ [60] and Se4+ [66]. Gold electrodes, similarly as platinum ones, are widely used as substrates for different types of films [67, 68]. Silver electrodes Silver (as all previous solid electrode material) is a nobel metal. It has the highest electrical conductivity of all elements. Silver electrodes are used mostly because of their relatively high HER overpotential, which is between -300 and -450 mV (vs. SHE) [69, 70]. However, Ag dissolves in water at potentials above 800 mV [45]. Silver reference electrode (Ag|AgCl) is one of the most often used reference electrodes, because of its stable potential, simplicity and non-toxicity [29, 9]. The determination of Pb2+ by UPD on the silver electrode is one of the most widely studied and described. The method provide very high limits of detection and have solid theoretical basis [14, 15, 18, 70, 71]. The high HER overpotential on silver electrodes allowed direct measurements of metals, in nanomolar concentrations, with lower reaction potentials such as Cd2+ [18], Tl+ [72] or Te4+ [73]. Other important inorganic applications include Se4+ [74], As3+ [75] and S2+ [76]. Bismuth electrodes Bismuth has been considered as the highest-atomic-mass element that is stable. In 2003 it was discovered that its main isotope bismuth-209 decays via alpha-particle emission to thallium-205, with half-life time of 1.9·1019 years [77]. Bismuth electrodes are used in forms of film and bulk electrodes [78]. Although, bismuth films are more often used and are considered to be an alternative to mercury electrodes for stripping voltammetry of many trace metals [29]. Bismuth electrodes are characterized by a very high HER, the top limit is between -900 and -1500 mV (vs. SHE), what makes it comparable with mercury electrodes [79, 78]. Unfortunately, the cathodic limit is much shorter – the oxidation of the metallic bismuth occurs at potentials less negative than -200 mV (vs. SHE) [80]. Exceptionally high HER potential allowed the bismuth electrodes to be used in direct determination of In3+ [81], Zn2+ [82], Ga3+ [83] and Cd2+ [82]. Other.

(35) Chapter 2. Introduction. 34. important inorganic applications include Pb2+ [84, 78], Sn4+ [85] and Tl+ [86]. Carbon electrodes Carbon is a nonmetal, with one the highest (graphite) or one of the lowest (diamond) conductivity among nonmetals. Carbon based electrodes became commonly used, primarily because of their broad potential window, rich surface chemistry, low cost and chemical inertness. The usable potential window of carbon electrodes (glassy carbon, graphite and fiber) is between -900 mV and 1300 mV (vs. SHE) [87]. Additionally, metal electrodes have higher Cdl than most carbon electrodes, what contributes to the larger background current. However, in contrast, electron transfer rates observed at carbon surfaces are often much slower than those observed at metal electrodes [29]. Four general types of carbon electrode exist: (a) glassy carbon, (b) carbon paste, (c) carbon nano-tubes and fibers – existing only in the form of films or modifiers, (d) diamond – which needs to be modified, because of very high electrical resistance. Below a brief description of the first two types is presented. • Glassy carbon electrodes are one of the most commonly used solid electrodes [9]. Their mechanical and chemical proprieties make them great substrate for deposition of thin-films. However, its main issue is the requirement of very rigorous pretretment methods [88]. The unmodified glassy carbon electrodes were used in determination of very large number of organic species [89, 90, 91, 92, 93, 94] (including DNA [95]). They have been also applied in the detection of such a inorganic species as Fe2+ [96], Hg2+ [97] and NO− 3 [98] to name a few. • Carbon paste electrodes are made by mixing graphite powder with various waterimmiscible nonconducting organic binders (pasting liquids)[9]. They offers wide possibilities of modification as both surface and bulk can be modified. The unmodified carbon paste electrodes are used in detection of in example Ag+ , Cu2+ [99], Fe2+ [100], and also many enzymes [29, 101].. 2.1.4. Surface preparation and activation. A challenge related to use of solid electrodes is to understand the conditions on the surface. In contrast to the mercury electrodes it is difficult to produce a new clean and reproducible surface on solid electrodes [9]. The pretreatment is an essential part when working with any type of solid electrode and it is vital in terms of the sensing parameters.

(36) Chapter 2. Introduction. 35. and long-term stability. The first phase is usually the mechanical polishing [88] or micrometric layer cutting [28], during which ,,the history” of electrode is wiped. The polishing should be proceeded with caution, from emery paper - in case of heavily contaminated or rough electrodes, to micrometric alumina polishing powder. The next step of the pretreatment process can be very different [29]. Below, there is a list of the most often appearing in the literature techniques used as a second or even third step in the electrode pretreatment (activation) process. • Electrochemical activation is the most popular electrode activation method. Usually understood as an either linear sweep between potential of hydrogen evolution. and potential of the electrode oxidation (e.g., formation of AgCl on the surface of Ag electrode) [13] or application of constant potential within that range [14]. It can utilize a pulsing potential within that range [102]. The duration of the process varies between 1 and 20 min. Electrochemical activation leads to the change of the surface structure and may lead to desorption (oxidation/reduction) of impurities [103]. • Solvent cleaning is dissolving and/or desorbing adsorbed contaminants from the surface by a special solvent. It usually conducted by immersing the electrode for. a few minutes (usually 10 – 30 min) – different solvents can be used including acetonitrile, dichloromethane, toluene [104] or isopropanol [105]. • Exposure to radio frequency plasma can be used to activate electrode. The procedure is conducted by treating electrodes in plasma chambers at powers up to about. 100 mW and pressures in the 100 mTorr range. The procedure usually lasts 5 – 20 min and involves a sputtering type of mechanism in which energetic particles collide with the surface. The alterations in the electrode surface chemistry and microstructure can range from mild to severe depending on the plasma conditions (e.g., gas phase, power, pressure, and duration) [29, 106] • Laser activation is method for electrode pretreatment that uses a short laser pulse (wavelength usually 400 – 1100 nm, power up to 100 MWcm−2 ) to irradiate the electrode. It can be used to renew solid electrodes passivated by adsorbed or polymeric materials and to improve analytical voltammetry by enhancing the electron transfer rate [107, 108]. • Ultrasound activation uses the power of ultrasound in the processes surface activation by means of electrode erosion/roughening. The surface activation and/or. cleaning is controlled by continuous application of ultrasound which modifies the.

(37) Chapter 2. Introduction. 36. mechanism of electrode reactions. When applied correctly, ultrasounds, can provide continuous activation of the electrode during measurement [109]. • During heat treatment in temperature between 400 and 800o C most of contaminants from the surface of the electrode is vaporized. The high temperature also leads to the relaxation of the electrode surface. To increase the performance of activation the process usually takes place in vacuum or under reduced pressure (below 1 Torr) for 2 – 24 hours [110]. • UV/ozone treatment is an method for cleaning and activating electrodes (usually carbon electrodes). UV/ozone treatment works by oxidizing the impurities the on surface of the electrode. It is able to remove micrometer-thick layers of polymers in a cleaning process which takes 10-20 min [111].. 2.2. Double layer modifiers. The double layer can be modified not only by direct alteration of the solution composition or the electrode surface but also due to specific adsorption. The specific adsorption is taking place usually within IHP and it is driven by short-range interactions (e.g. chemisorption) [112]. The alteration by double layer modifiers is usually reversible with the respect to the potential and the bulk concentration of modifier [113]. Specific adsorption alters the double layer capacitance, its width and kinetics, therefore, some general methods of detecting specific adsorption involve measurements of ∂Cdl /∂E. The type of interactions depends on the electrode material, its texture and the adsorbed specie. The adsorption of neutral molecule requires the displacement of water molecules form the surface. Therefore, it is unlikely that when the electrode is strongly polarized, less dipolar substance will displace water molecules due to negative energetic balance [8]. In general almost every ionic or non-ionic molecule can be double layer modifier, although, some require more specific conditions or concentrations than other [114, 115, 116, 117, 118]. The one type of molecules which alter the interfacial structure are surfactants (surface active species - SAS). SAS are usually organic molecules with amphiphilic proprieties. The hydrophobic part is a long carbon chain while hydrophilic part is ionic or strongly polar [119]. Four examples of SAS are presented in following subsection. The other type are humic and fluvic acids. Humic acids (HA) are large organic molecules created by biodegradation of organic matter, their structure is complicated but usually consist of both hydrophobic and hydrophilic elements [120]..

(38) Chapter 2. Introduction. 2.2.1. 37. Surface active species. The surface active species became very important substance in human life. Their main use is as detergents - soaps, dish washing liquids and washing powders, while other usage, to named a few, includes: fabric softeners [121], emulsions [122], paints [123], pesticides [124], cosmetics [125], fire-fighting foams [126], liquid drag reducing agent [127] and leak detectors [128]. Thus, their anthropogenic concentration in the environment is increasing and it is established that increased concentration of surface active species has negative effects on environment and higher concentrations are toxic [129]. Therefore, it became important to do both, monitor their concentration and develop methods to monitor other species in their presence. Below, the brief characteristics of surface active species used in this work is presented. Triton X-100 Triton X-100 is commercial name of 4-(1,1,3,3-Tetramethylbutyl)phenyl-polyethylene glycol (other names are t-Octylphenoxypolyethoxyethanol and Polyethylene glycol tertoctylphenyl ether; Figure 2.12). Its average molecular weight varies between 623 and 627. Triton X-100 is widely used non-ionic surfactant for recovery of membrane components under mild non-denaturing conditions [130]. In analytical science it is commonly used. Figure 2.12: Triton X-100 molecule, n equal to 9–10. as a model of non-ionic surface active agent [17, 131, 132, 133, 134, 135]. Sodium dodecyl sulfate Sodium dodecyl sulfate (SDS - Figure 2.13) is an anionic surfactant for protein solubilization. Its molecular weight is 288.38 [136]. It is found in high concentrations in industrial products including engine degreasers, floor cleaners, car wash soaps, also in toothpastes, shampoos, shaving foams and bubble bath formulations in part for its thickening effect and its ability to create a lather [137, 138]..

(39) Chapter 2. Introduction. 38. Figure 2.13: The sodium dodecyl sulfate molecule. Cetyl trimethylammonium bromide Cetyl trimethylammonium bromide (Cetrimonium bromide - CTAB) is a cationic surfactant, presented in Figure 2.14, used in a buffer solution for the extraction of DNA as well as for hair conditioners [137, 139]. Its molecular weight is 364.45.. Figure 2.14: The cetyl trimethylammonium bromide molecule.. Polyethylene glycol Polyethylene glycol (PEG), otherwise known as poly(oxyethylene) or poly(ethylene oxide) (PEO), is a synthetic polyether that is available in a range of molecular weights - from 300 – 10,000,000. The molecule is presented in Figure 2.15, where n can have value between 6 and 230,000 (possibly even more). The polymers are amphiphilic and soluble in water [140]. PEG has been found to be nontoxic and is approved by the FDA for use as excipients or as a carrier in different pharmaceutical formulations, foods, and cosmetics [141].. Figure 2.15: PEG molecule - n can have value between 6 and 230,000.. 2.2.2. Humic acid. Humic acid is a principal component of humic substances, which are the major organic constituents of soil (humus), peat, coal, many streams, and ocean water. The molecular structures vary, one of possible arrangements is presented in Figure 2.16. The functional.

(40) Chapter 2. Introduction. 39. groups that contribute most to surface charge and reactivity of humic substances are phenolic and carboxylic groups [142]. It has molecular weight range of 2,000–500,000 [143] (no average provided).. Figure 2.16: The example of the humic acid molecule.. 2.2.3. Effects of double layer modifiers presence on electrochemical experiments. The specific adsorption is a part of every voltammetric experiment when ions (e.g., species of supporting electrolyte or surface active species) are adsorbed on the surface of the electrode. When that happens, the potential at OHP (EOHP ) is changed. The results of that change is the change of the diffusive layer thickness and structure, and hence, the diffusion speed of analyte to the electrode surface. Specific adsorption of an anion causes EOHP to be more negative, while specific adsorption of a cation causes EOHP to be more positive [8]. The specific adsorption can block active-sites involved in the electrochemical reaction, what can alter both the reaction kinetics and the mechanism. This can lead to variability in the electroanalytical measurement [144]. The specific adsorption can also increase the reaction rate, as is for example in the case of Cu2+ reaction in the presence of SCN− ions adsorbed on the surface of Cu electrode [145]..