Coulometric Determination of Sulphur Compounds Using the Induced lodine-Azide Reaction

A C T A U N I V E R S I T A T I S L O D Z I E N S I S

FOLIA CHIMICA 1 3 ,2 0 0 4

COULOMETRIC DETERMINATION OF SULPHUR COMPOUNDS USING THE INDUCED IODINE-AZIDE REACTION

by W itold Ciesielski

D e p a rtm e n t o f In stru m en ta l A nalysis, U niversity o f Łódź, 163 P o m o rska Str., 90-236 Łódź, P oland

The review sum m arizes application o f coulom etry for determination o f the sulphur com pounds that induce the iodine-azide reaction. The article d iscusses advantages o f the coulom etric method. The influence o f potassium iodide on induction coefficien ts is also described. M oreover, determination ranges o f several inductors are presented and sensitivities o f determination by different methods are compared. K ey w ords: iodine-azide reaction, coulom etric determination, sulphur compounds.

1. Introduction

Iodine-azide reaction induced by sulphur com pounds was described by R aschig [1] and introduced to analytical chem istry by Feigl [2], All inorganic divalent sulphur com pounds as well as elem entary sulphur after cleavage o f the Sg ring have inducing activity. Am ong organic com pounds the follow ing are inductors o f iodine-azide reaction: com pounds containing thiol or thione groups, com pounds with P=S and PSH groups, disulphides (e.g. cystine), sulphur heterocycles (e.g. vitam in B |).

Inorganic inductors and thiol and thione com pounds induce the iodine- azide reaction quickly (in optim al conditions the reaction tim e ranges between less than one m inute to several minutes). The rate o f the reaction induced by organic disulphides, sulphur heterocycles and elem entary sulphur is considerably slower.

T he iodine-azide reaction proceeds as follows:

I

N

c I

N

I

(1)

RSH

-*RSI + 1T + 2I~

(

)

RSI + RSH

RSSR+H+ +

r

(3)

RSSR

IjNj-

RSI

RSN,

I

(4)

RSI

N3"

RSN

+ I'

(5)

RSNj

I

N

■

►RSI + 3N

+ r

(6)

RSI + 2I3' + 3H20

RSO

H+

+5H+

(7)

RSNj

2I3' +3H20

RSO,H

H+ (8)

In specified conditions, the consum ption o f iodine, the am ount o f nitrogen evolved during the reaction and the heat em itted are linearly dependent on the am ount o f the inductor and this is the reason why the reaction in question has found num erous applications for determ ination o f sulphur com pounds. The sensitivity o f determ inations heavily depends on the induction coefficient, which is defined by the formula:

n R S H

w here ni is m oles o f iodine consum ed in the induced reaction and nRsn is moles o f the inductor.

The higher the induction coefficient, the m ore sensitive the determ ination o f a given inductor.

2. Coulometric Titration

C oulom etric titration with the use o f iodine-azide reaction was first applied by Press and M urray [3] who determ ined sulphide ions in the range o f 0.01-0.08 ppm in a 70 ml sample.

F urther applications o f coulom etry for determ ination o f inductors o f the iodine-azide reaction were then presented by Jędrzejew ski and C iesielski [4-13].

C oulom etric titration with the anodically-generated iodine can be applied for determ ination of all sulphur com pounds that quickly induce llie iodine-azide reaction, inductors o f a long induction time can be determ ined by kinetic m ethods with anodically generated iodine, These are described in the subsequent section.

In the coulom etric m ethod the content o f inductors is calculated from the calibration graph show ing the dependency o f electric charge needed to generate iodine necessary for the induced reaction on the am ount o f the inductor. In this procedure it is not possible to calculate the content o f sulphur com pounds using Faraday laws. This method enables to determ ine very sm all am ounts o f inductors with a considerable accuracy. A nother advantage is that it requires no standard solutions. T he end-point is detected biam perom etrically.

T able 1 show s determ ination ranges o f iodine-azide reaction inductors for which coulom etric determ ination m ethods were elaborated. In the case of certain com pounds - diethyldithiocarbam ate, dithiophosphates, organothiophosphorous com pounds, thioguanine and carbim azole - the delay time has to be set at 30 s in order to com plete the titration.

Table 2 com pares thioguanine induction coefficients in the following methods: coulom etric, spectrophotom etric and volum etric back-titration.

It was found that increasing iodide ions concentration causes the decrease ot the rate o f the thioguanine, induced iodine-azide reaction. If iodine is introduced slow ly and gradually, as it is the case in coulom etric titration, the iodine consum ption does not depend on the concentration o f iodide. When iodine is added quickly and is in excess in the reaction solution (in the spectrophotom etric and volum etric back-titration m ethods) the iodine consum ption and thus the induction coefficient increases with the increase o f iodide concentration. At high concentrations o f iodine in the volum etric back- titration m ethod, the induction coefficient is low er when com pared with the spectrophotom etric m ethod which is due to faster destruction o f the inductor caused by oxidation by iodine (reactions 7 and 8).

In the case o f many inductors the increase o f iodide ions concentration causes the increase o f the induction coefficient. In the case of diethyldithiocarbam ate at the potassium iodide concentration o f 3- 1 0 2 mol/1 the induction coefficient in the coulom etric method [8] reaches 1970, which is nine times higher than in the case o f volum etric back-titration [14].

Table 3 com pares induction coefficients o f diethyldithiocarbam ate for various iodide concentrations and two different currents. The induction coefficient depends on the rate o f iodine introduction. W hen the concentration o f iodide increases, the inductor is active for a longer period o f tim e and as a consequence the induction coefficients are higher. This is due to decreasing

iodine potential in the presence o f iodide ions. As a result, the level o f inductor deactivation becom es low er and the inductor can take part in a greater num ber of cyclical stages o f the induced reaction (reactions 5 and 6). At low iodide concentration, when the rate o f iodine introduction is increased (at greater current), the iodine consum ption decreases, which is due to faster inductor deactivation. This phenom enon, however, is not observed at higher iodide concentrations. Increasing the iodide concentration above 3- 10'2 mol/1 causes further increase o f the induction coefficient but the use o f such solutions appears to be unfavourable because the rate o f the induced reaction becom es too slow for coulom etric titration.

M ojski and M urawski [15] applied the iodine-azide reaction to coulom etric determ ination o f sulphur (0.2 - 20 pg) in organic solvents and liquid fuels. T he analysed sam ples were first transform ed to oxides and then to hydrogen sulphide. The titration end-point was detected potentiom etrically.

In the coulom etric determ ination o f inductors discussed here, constant current should be m aintained since the electric charge depends on the current. This phenom enon does not occur in com m only used coulom etric titration methods.

3. Kinetic Methods in Coulometry

C iesielski used kinetic m ethods for determ ination o f sulphur com pounds o f a long induction time. The am ounts o f inductors were determ ined based on the induced reaction rate. The m easurem ents were m ade in open system s in which during the reaction course a substrate (iodine) is introduced.

Kinetic m ethods o f analysis are com m only used in determ inations of com pounds that act as catalysts [16]. Since at the beginning the rate o f the iodine-azide reaction does not change in the presence o f vitam in B, and cystine - these com pounds act like catalysts - they may be determ ined by m eans of kinetic m ethods.

In earlier kinetic determ inations in open system s the substrate was added into the reaction solution in the form o f the standard solution using the autom atic burette [17,18],

C iesielski described anode iodine generation in steady-state determ ination o f cystine [19] and stat determ ination o f cystine and vitam in B, [20]. In the steady-state m ethod, iodine was generated at the constant current, in the solution containing azide and iodide ions and cystine. In this system a stationary condition is reached in which iodine is added at the sam e rate as it is being consum ed in the iodine azide reaction.

T he stationary concentration o f iodine is m easured biam perom etrically. Linear dependence between the reciprocal o f the indicator current in biam perom etric current and cystine concentration is used as a calibration curve. In the stat m ethod, the current in the generating circuit was proportional to the concentration o f the catalyst - vitamin B | or cystine, which enables to plot a calibration curve. The concentration o f the indicator substance - iodine - was controlled biam perom etrically and m aintained at constant level. In the described methods inductors o f a short induction time do not cause interference as they are oxidised during the initial stage o f determ ination.

T he advantage o f the discussed kinetic determ ination in the open system is short analysis tim e and the fact that standard solutions are not required.

The m ethods described above were used to determ ine vitamin B| in drugs and cystine in wool and hair.

4. Conclusions

The coulom etric m ethods discussed here can be said to be ones o f the most sensitive and accurate m ethods o f analysis o f these sulphur com pounds w hich induce the iodine-azide reaction. In Table 4 the sensitivity o f the determ ination o f thiourea by means o f different m ethods - with application of iodine-azide reaction - is presented.

Iodine consum ption in the induced iodine-azide reaction for a given sulphur com pound depends on reaction conditions i.e. concentrations o f azide, iodide, iodine, pH o f the solution as well as on the rate o f iodine introduction. The increase o f iodide ions concentration decreases the rate o f the induced reaction since the concentration of I2N3 produced in the reversible reaction decreases:

I3 + N j * I2N3 + I

At the sam e time, the rate o f inductor oxidation becom es low er and thus the inductor takes part in the reaction for a longer period o f time. As a result, in the case o f m ajority o f sulphur com pounds the iodine consum ption increases. In most cases, at low iodine concentration, which is observed during coulom etric determ ination o f inductors, the rate o f inductor destruction decreases. Therefore, the induction coefficients are higher when com pared to those in other m ethods, in which the iodine concentration is higher.

In the proposed procedures coulom etric titration is not an absolute m ethod and thus the standard curve has to be used, but it shares all other advantages with other coulom etric methods.

Table 1. Determination ranges o f sulphur compounds for which coulom etric determination methods were elaborated.

Compound Determination range References

Thiocyanate 1 - 7 jig 4 Sulphide 2 0 - 1000 ng 5 , 9 Thiosulphate 50 - 7000 ng 5 , 9 Thiourea 1 0 - 3 0 0 0 ng 4 , 9 C ysteine 50 - 4 0 0 ng 6 Glutathione 50 - 6000 ng 6 , 9 Dithiooxam ide 1 - 6 n g 7 Sodium diethyldithiocarbamate 20 - 200 ng 8 Thioguanine 1 0 - 2 0 0 ng 10 Dithiophosphates 1 - 20 nmol 11

Organothiophosphorus com pounds 1 - 20 nmol 12

Carbimazole 2 - 2 0 nmol 13

Table 2. Induction coefficien ts for thioguanine.

C (KI) mol/l Coulometric Spectrophotometric Volum etric

back-method method titration method

3- 10'3 2630 2080 1840

3- 10'2 2630 2430 2260

T able 3. Induction coefficien ts for diethyldithiocarbamate.

C (KI) mol/l I = 1mA I = 5mA

3- I O'3 1520 1080

I- 10‘2 1850 1460

Table 4. Sensitivity ot the determination o f thiourea by different methods.

M ethod Sensitivity according

given procedure Accuracy References Volum etric back-titration 45 0 0 ng/50 ml ± 90 ng 21

Com petitive reactions 150 ng/15 ml 22

Stat method _{180 ng/5 ml ± 8 ng 18}

S low titration 25 ng/15 ml ± 5 ng 23

Flow injection analysis 2 n g /10/j.l ± 0.4 ng 24

Flow continuous analysis 2 n g/l ml ± 1 ng 25

Spectrophotom etric method 20 ng/7 ml 26

C oulom etric titration _{l0 n g /2 0 ml ± 1 ng 9}

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