through a Bakelite panel led into the box to operate the filament rheostat, a battery switch, the panel-type potentiometer, and the control grid switch. The vacuum tube was placed inside the box in a stoppered glass cylinder, in the bottom of which was placed a layer of Desicchlora to maintain a dry atmosphere.
AXLS
of
Filament Mercury leads
Rubber
lead Plate lead
Glass Rod screen
lead
ANALYTICAL EDITION 477
Mi l l i l i t e r s BaC l2
Fi g u r e 3 . Ti t r a t i o n o r Su l f a t e w i t h Ba r i u m
ment of Ri the microammeter needle was set at any convenient value. The grid was then set a t free grid potential by closing switch S 3 and adjusting Rs until the microammeter needle was again at its original setting. By adjusting the grid to free grid potential before titration was begun and by connecting the elec
trodes in the manner indicated, the grid was prevented from be
coming positive during the titration.
M any vacuum tube voltmeters described in the literature are more elaborate, costly, and sensitive than necessary for use with bimetallic electrodes in titrations such as those de
scribed in this paper. The apparatus as described here is characterized not only by its simplicity but also by its sta
bility. Its free grid potential with respect to the negative end of the filament is 0.23 volt and a t the beginning of a titration adjustm ent to this voltage is easily made in the manner described. The filament of the vacuum tube operates on only 0.13 ampere. Although the storage cells furnish a current of this magnitude with little fluctuation, the same storage cells are used to balance out the initial steady portion of the plate circuit. Thus any fluctuation in the current from the filament battery is minimized in its effect on the microammeter in the plate circuit. Under the conditions of operation no fluctuation of the microammeter needle could be detected over a 30-minute period. During the course of a titration the change of potential between the electrodes of the titration cell would produce corresponding changes in the microammeter reading. Except near the end point only a few seconds were required for the potential between the elec
trodes to reach a steady state. If a well-defined end point was not apparent from the titration data, it was obtained graphically by plotting the change in microamperes per unit volume of titra n t against the corresponding volume of titrant.
P r e c ip it a t io n R e a c tio n s
Bimetallic electrodes have not been extensively used in the potentiometric titration of ions by precipitation reactions.
One reason for this has been the small and somewhat variable
potential change a t the end point in most titrations. In the titration of the halides w ith the silver ion various electrode pairs have been used. I t has been suggested (6) th a t in this titration the indicating electrode becomes plated w ith silver and then acts as a silver electrode. On the other hand, in acid-base titrations French and Kahlenberg (1) believed the potential between the electrodes to be due largely to absorbed gases, th a t caused the electrodes to function as gas electrodes.
In the titration of magnesium (4) or copper (£) ions w ith sodium hydroxide it is probable th a t the indicating electrode acts as a gas electrode.
Using the apparatus described above, it was easy to detect the end point of certain precipitation reactions. N ot only was chloride titrated w ith silver ion and magnesium ion with hydroxyl but sulfate was titrated with barium ion. The titration time varied between 5 and 15 minutes, depending upon the titration reaction, the electrodes, and the rate of stirring.
D e t e r m i n a t i o n o f S u l f a t e . For the titration of sulfate 10 ml. of a standard potassium sulfate solution were placed in a 150-ml. titration beaker and 50 to 60 ml. of water plus 25 ml.
of acetone were added. Natural graphite was used as the reference electrode and approximately 7.5 cm. (3 inches) of No.
20 tungsten or platinum wire as the indicating electrode. The solution was vigorously stirred as the barium chloride solution was added. After each addition of barium chloride sufficient time was allowed for the electrodes to reach a steady state as indicated by the microammeter. Table I shows typical data ob
tained during a titration. The end point can be determined by inspection of the data, but they have been plotted in Figure 3 to show graphically the sharpness of the end point.
The barium chloride solution was prepared 0.5000 M by weighing pure barium chloride dihydrate as a primary standard.
Analysis of this solution for its barium content by precipitating and weighing barium sulfate verified this concentration at least to within 1 part in 2500. For titration of 10-ml. aliquot portions of the potassium sulfate solution 9.58, 9.57, 9.54, 9.56, 9.56, and 9.60 ml. of the barium chloride solution were required. This indicated 0.383 gram of sulfur trioxide to be present per aliquot, whereas 0.399 gram of sulfur trioxide had been the value ob
tained by weighing the potassium sulfate as a primary standard as well as by precipitating and weighing barium sulfate.
The precision of the potentiometric titration was satis
factory, b u t the results were about 4 per cent low. Though its accuracy was such th a t this method of titration cannot be recommended for quantitative work, the results have con
siderable theoretical interest because the end point of such a titration can be detected with bimetallic electrodes. How
ever, no theory is presented to account for the manner in which the indicating electrode functioned. The change in
Ta b l e I. Ti t r a t i o n o f Su l f a t e w i t h Ba r i u m
BaClj
Microampcrea 0.01 Ml.
M l. M icroamperes
9 .0 0 28
9 .2 0 28
0 . 0
9 .4 2 30
0 .1
9 .4 7 32
0 .4
9 .5 0 36 1 .3
9 .5 4 45
2 .2
9 .5 8 57 3 .0
9 .6 1 66 3 .0
9 .6 3 70
2 .0
9 .7 1 74 1 .3
9 .8 2 76
0 .5
1 0 .1 0 78 0 .1
E n d poin t, 9.58 ml.
478 INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 12, NO. 8 potential at the end point was not great but was definite.
The potential across the titration cell attained a steady state in the presence of a water-acetone mixture more readily than in a water medium. Thus the presence of the acetone en
abled greater speed in titration and it was added for this reason.
determined by gravimetric analysis in which MgNH(POJ. 6H20 was precipitated and Mg2P207 was weighed. The magnesium chloride solution had a pH of 6.4 as prepared. The data for the titration of magnesium ions with sodium hydroxide are listed in Table II and a typical titration curve is shown in Figure 5.
In passing from an excess of magnesium to an excess of hy
droxyl at the end point, the potential change was so small that it was advisable to substitute a galvanometer for the micro
ammeter. The change was only about 10 to 15 microamperes, whereas in the titration of a strong acid with a strong base an abrupt change of about 70 microamperes was noted. Not
withstanding the small change in potential at the end point, it was sufficiently abrupt to be detected with considerable precision and accuracy by the use of the galvanometer.
M IL L IL IT E R S 0 - 2 8 N A g N 0 3
Fi g u r e 4 . Ti t r a t i o n o f Ch l o r i d e w i t h Si l v e r
E.P 2 3 .0 8 m l
23 M I L L I L I T E R S N a O H
Fi g u r e 5 . Ti t r a t i o n o p Ma g n e s i u m w i t h Hy d r o x y l
De t e r m i n a t i o n o f Ch l o r i d e. Solutions containing chloride were titrated with 0.28 N silver nitrate. The same electrodes and titration technique described in the previous section were used except that the addition of acetone was unnecessary. Using 25-ml. aliquots of a sodium chloride solution 49.83 ml. of 0.28 N silver nitrate were required for each of three potentiometric titrations. A Mohr titration checked this value to within 0.02 ml. Figure 4 shows the curve for one of the potentiometric titrations. The end point is so sensitive th at it is possible to use more dilute silver nitrate solution in the titration of smaller chloride samples. No attem pt was made to establish the mini
mum feasible concentration of silver nitrate, but a very sharp end point resulted with 0.028 N solution.
De t e r m i n a t i o n o f Ma g n e s i u m. For the potentiometric titration 25 ml. of magnesium chloride solution were diluted to 70 ml. to prevent splattering during titration. In some cases (Table II) 20 ml. of acetone or ethyl aclohol were included in this volume. These solvents allowed the end point to be de
tected more easily than when using only an aqueous medium.
Platinum and tungsten were found to be the best indicating electrodes and a buret electrode (7) was used as a reference elec
trode because of the protection it offered against absorption of carbon dioxide from the air by the sodium hydroxide solution during titration. The sodium hydroxide solution was prepared carbonate-free by precipitating the carbonate with an excess of barium ions. This solution was determined to be 0.2283 N by standardizing against potassium acid phthalate using phenol- phthalein as the indicator. The magnesium chloride solution contained 0.0642 gram of magnesium per 25 ml. of sample, as
Ta b l e II. Ti t r a t i o n o f Ma g n e s i u m w i t h Hy d r o x y l (0.0642 gram of M g present per sam ple)
Sam ple Indicatin g
N o. Electrode M g Found Error
Oram %
1 P t 0 .0 6 4 0 - 0 . 3
2a P t 0 .0 6 4 1 - 0 . 1 6
3° P t 0 .0 6 4 0 - 0 . 3
4 a P t 0.0 6 4 1 - 0 . 1 6
5& P t 0 .0 6 4 0 - 0 . 3
P t 0 .0 6 4 0 - 0 . 3
7b W 0 .0 6 4 0 - 0 . 3
8& W 0 .0 6 4 0 - 0 . 3
a 20 ml. of acetone present in 70 ml. total volum e.
b 20 m l. of eth yl alcohol present in 70 m l. to ta l volum e.
S u m m a r y
A one-tube vacuum tube voltmeter which is inexpensive and easy to operate allows rapid titrations to be performed with simple electrodes. The instrument has been applied to the titration of sulfates, chlorides, and magnesium as well as to the more common oxidation-reduction titrations.
L ite r a t u r e C ite d
(1) French, S. J., and Kahlenberg, L., Trans. A m . Eleclrochem. Soe., 54, 163 (1928).
ANALYTICAL EDITION 479 (2) Fuoss, R. M., I n d . E n g . C h e m ., Anal. E d ., X, 125 (1929).
(3) Kinney, G. P., and Gannan, It. L., J . Chem. Education, 13, 190 (1936).
(4) Malvea, B. B., and Withrow, J. R., J . Am. Chem. Soc., 54, 2243 (1932).
(5) RCA Cunningham Radiotron Manual, Technical Series RC-12, p. 67, RCA Manufacturing Co., Inc., Harrison, N. J.
(6) Van Name, R. G., and Fenwick, Florence, J . Am . Chem. Soc., 4 7 ,9 (1925).
(7) Willard, H. H., and Boldyreff, A. W„ Ibid., 51, 471 (1929).
Ab s t r a c t e d from a th esis by L loyd E . W est subm itted to the G raduate F aculty of th e U niversity of W ashington, 1939, in partial fulfillm ent of th e requirem ents for th e degree of doctor of philosop hy. Contribution N o. 96 Oceanographic Series.