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Amperometric determination of sodium hydrogen sulphite with chloramine-T
Tulrmrrr. Vol. 26. pp. 326-328 0 Pergamon Press Ltd 1979. hinted
0039-9140:79/0401-0326102.00!0 in Great Britain
AMPEROMETRIC
DETERMINATION
SULPHITE
WITH
OF SODIUM
HYDROGEN
CHLORAMINE-T
TOSHIO MATSUDA Department of Chemistry, Ritsumeikan University, Kyoto, Japan (Received
6 July
1978. Accepted
1 October 1978)
Summary-Amperometric titrations with chloramine-T at a rotating platinum electrode (RPE) and a dropping mercury electrode (DME) have been applied to the determination of sodium hydrogen sulphite over the concentration range 0.004O.lN. With the RPE, the indirect titration is best, at pH between 3 and 6, whereas the DME can be used for either direct or indirect titrations, at pH around 7. Relative standard deviations of 0.5% were obtained by both methods, with relative errors not exceeding &-1%.
In previous studies on the polarographic behaviour of chloramine-T (CAT) at a dropping mercury electrode (DME)’ and at a rotating platinum electrode (RPE),2.3 we found that CAT underwent a two-electron reduction in the pH range 2-13, though the stability of the diffusion current was dependent on the characteristics of the electrode reaction at both electrodes. This report describes amperometric titrations with CAT, using the DME and the RPE, for the determination of sodium hydrogen sulphite, and compares the results. The author and his co-workers4 have reported that sodium hydrogen sulphite can be determined over the concentration range O.Ol-O.lN by direct potentiometric titration with lead tetra-acetate in dilute acetic acid medium. However, this method could not be used for the determination of lower concentrations because of the volatility of sulphur dioxide. Although a considerable number of oxidizing reagents, including CAT, have been used for the titrimetric determination of hydrogen sulphite or sulphite,5-7 these cannot be used successfully for direct titration in acidic solution, again because of the volatility of sulphur dioxide, and back-titration has been recommended. In this study, therefore, the direct titration was performed in weakly acidic or neutral solutions in order to prevent the loss of sulphur dioxide. The possibility of extending the range of application of the method to relatively low.concentrations has been investigated.
Reagents Stock solution ofchloramine-T
Stock solution oisodium hydrogen sulphite, 0.05M (O.lN). Prepared fresh each day with air-free distilled water, and standardized by iodimetric titration.6 The solution, stored under nitrogen at 25”. was found to decrease in normality by about 0.3% in 9 hr. Lower concentrations were prepared by appropriate dilution with air-free water. The pH was adjusted to about 7 to prevent loss of sulphur dioxide. Other solutions such as potassium nitrate and BrittonRobinson buffer were prepared as reported earlier.‘*’ All solutions were prepared with doubly distilled water. Procedure Method A (reverse titration). Supporting electrolyte, (50 ml, O.lM in potassium nitrate and containing a suitable buffer) and standard CAT solution (5.00 ml) of appropriate concentration were ‘added to the cell and deoxygenated ‘completely by passage of nitrogen. This solution was titrated with sodium hydrogen sulphite solution. After each addition of titrant, the solution was mixed by passage of nitrogen for 6G90 set, and then the current was recorded. The measured currents were corrected for dilution. In order to prevent aerial oxidation of hydrogen sulphite, nitrogen was passed over the surface of the hydrogen sulphite solution in the burette during the titration. For titrations with the DME, 0.01% polyacrylamide (PAA) was added as a maximum suppressor. Method B (direct titration). After 50 ml of the supporting electrolyte had been deoxygenated in the cell, 5.00 ml of the sodium hydrogen sulphite solution were added. The solution was titrated with standard CAT solution. In order to prevent induced aerial oxidation of hydrogen sulphite, the CAT solution was deoxygenated and stored under nitrogen in the burette during the titration. Other details were as for method A.
RESULTS AND Amperometric
EXPERIMENTAL
Apparatus A Yanagimoto
Polarograph, Type PA-101, was used with the dropping mercury electrode (DME)’ and rotating platinum eltctrode (RPE)2V3described earlier. The potentials are referred to an SCE connected to the solution through a potassium nitrate-agar salt bridge.
(CAT), 0.05M (O.lN). Pre-
pared as described.
titration
DISCUSSION
at a rotating
platinum
electrode
WE)
The results of titrations by methods A and B for concentrations of around O.OlN in the pH range 3-7 are given in Table 1. All titrations except those at pH 7 were carried out at 0 V us. SCE, while a potential of -0.5 V was applied to the indicator electrode 326
321
SHORT COMMUNICATIONS
Table 1. Effect of pH on the amperometric titration at the RPE End-point
[CAT1
[NaHSO,] N
PH
Calc.. ml
N
Found.*
Average error, ?/;
ml
Method A 3.13 4.45 5.05 6.18 5.25t
1.037 1.075 1.090 1.091 1.060
x x x x x
lo-* lo-* 10-Z 1o-2 1o-2
0.984 0.984 0.984 0.984 0.984
x x x x x
10-l lo-’ 10-l lo-* lo-’
4.14 4.58 4.51 4.51 4.64
4.73 4.59 4.52 4.53 4.67
0.02 0.02 0.02 0.02 0.01
-0.2 +0.2 +0.2 +0.4 +0.6
Method B 3.55 5.14 6.18 7.30t
1.063 x 1.063 x 1.063 x 1.010 x
lo-’ lo-’ lo-’ 1o-2
0.973 0.973 0.973 0.944
x x x x
10-l 1o-2 lo-” lo-2
5.46 5.46 5.46 5.35
5.21 &-0.01 5.39 * 0.02 5.43 + 0.01 5.34 + 0.02
-4.7 -1.3 -0.6 -0.2
* * k + f
* Average of 3 titrations. t A potential of -0.5 V cs. SCE was applied.
for titrations at pH 7, because the chloramine-T reduction wave is shifted to more negative potentials as the pH rises. Titration curves of a normal L-shape and a reversed L-shape were obtained by methods A and B, respectively, because hydrogen sulphite showed neither a cathodic nor an anodic wave at the potentials applied over the pH range used. Over the pH range 36, method A gives good results, but method B gives poor results, being worse at lower pH and also when the Aow of nitrogen through the solution is increased, indicating that sulphur dioxide is being swept out of the solution under these conditions. At pH 7, both methods are satisfactory for concentrations of O.OlN, indicating that sulphur dioxide is not lost at this pH. However, for higher concentrations the reduction current for the CAT became increasingly unstable and the titration plot became curved after the end-point, causing difficulty in extrapolation. This instability of the current is considered to be due to a deactivation effect3 from some constituents adsorbed on the electrode surface. We can conclude that the titration at pH 7 cannot be successfully followed with the RPE.
From these results, it would seem that indirect titration (method A) with the RPE is best done at pH 5. Different concentrations within the range 0.001-0.1N were titrated under these conditions: the results are given in Table 2. Over the range 0.004-0.1N the relative errors did riot exceed +_l.Oo/, and the coefficient of variation for 6 titrations was 0.5%. It was also found that the results were not affected by the presence of chloride, sulphate or which are the reaction p-toluenesulphonamide, products, at concentrations up to at least twice those produced in the titration reaction. At concentrations below 0.002N the error became increasingly positive (Table 1); this is considered to arise from aerial oxidation of hydrogen sulphite during the preparation and storage of its solutions at concentrations around O.OOlN. Amperometric
titration
at a dropping
titration with
End-point
1.025 x 5.01 x 1.113 x 8.89 x 5.11 x 4.37 x 2.284 x
10-l 1o-2 1o-2 1O-3 1o-3 lo-’ lo-”
[CAT1 N 0.984 x 4.93 x 0.951 x 7.62 x 4.94 x 3.820 x 1.946 x
10-l 10-l 10-Z lo-” 10-a 1O-3 10-a
* s = 2.5 x lo-’ ml (6 titrations). f s = 2.0 x lo-’ ml (6 titrations).
electrode
The results of titrations by both methods in the pH range 5-7 are given in Table 3. A potential of -0.2 V was applied, because an anodic wave appeared at around 0 V for the hydrogen sulphite
Table 2. Results obtained by the indirect amperometric the RPE (method A) at pH 5.05
[NaHSO,] N
mercury
(DME)
Calc. ml 4.80 4.92 4.21 4.29 4.83 4.37 4.26
Found ml 4.81 4.93 4.27 4.30 4.83 4.40 4.36
f * + * f. + k
0.04* 0.02 O.Olt 0.01 0.02 0.02 0.04
Average error, 4, -0.2 +0.2 +o.o + 0.2 +0.2 +0.7 +2.3
328
SHORT
COMMUNICATM)NS
Table 3. Effect of pH on the amperometric titration at the DME End-point [NaHSO,] N
[CAT1
Calc. ml
Found*
N
ml
Average error. “/,
Method A 5.02 6.18 1.22
1.098 x lo-’ 1.098 x lo-’ 1.019 x 1o-2
0.991 x 1o-2 0.991 x 10-r 1.001 x 10-r
4.51 4.51 4.51
4.45 + 0.02 4.48 f 0.01 4.52 + 0.02
- 1.3 -0.9 +O.l
Method B 7.30
1.010 x 1o-2
0.944 x 1o-2
5.35
5.33 * 0.03
-0.4
PH
* Average of 3 titrations. solutions, as reported previous1y.s The titration curves obtained had the same shape as those obtained with the RPE. Indirect titrations (DME, method A) at pH < 6, gave low results with the error increasing as the pH decreased, in contrast to the good results obtained when the RPE was used. The errors also increased with increasing nitrogen flow-rate, suggesting that the negative errors could be attributed to reaction between CAT and the pool of mercury.’ The direct titration (method B) using the DME at pH < 6 gave low results similar to those obtained with the RPE. It is concluded that titrations using the DME should not be done at pH < 6. On the other hand, the titrations by both methods with the DME at pH 7 gave good results. For the determination of sodium hydrogen sulphite over the concentration range of 0.004-O.lN, the relative errors did not exceed + 1.0% and the coefficient of variation of 6 titrations was 0.5%. The optimum conditions as found experimentally are in agreement with the facts that the diffusion current for CAT at the DME is very stable in neutral solutions, but not in acidic solutions, and that loss of sulphur dioxide from neutral solutions cannot be detected. The presence of added amounts of chloride, sulphate or p-toluenesulphonamide, did not affect the performance of this method. CONCLUSIONS
Chloramine-T has been found to react quickly and quantitatively with hydrogen sulphite in neutral or weakly acidic solutions according to the equation RSO,NClH
+ HSO;
+ OH-
+ RS02NH2
+ HSO;
+ Cl-
where R = CH3C6H4. Based on this reaction, sodium hydrogen sulphite can be titrated directly with CAT over the concentration range 0.0040.1N with amperometric end-point indication by means of either an RPE or a DME. When the RPE is used, good results can be obtained when CAT is titrated in weakly acidic solution with sodium hydrogen sulphite solution at 0 V vs. SCE. When the DME is used, good results can be obtained when CAT is titrated in neutral solution with sodium hydrogen sulphite solution, or vice versa, at -0.2 V vs. SCE. It can be concluded that, for the standardization of sodium hydrogen sulphite solutions over the range 0.004-O.liV, the proposed amperometric methods with CAT are simpler and more accurate than previous methods, because the titrations are carried out under conditrons where sulphur dioxide is not lost as gas from the solution. Acknowledgements-The author wishes to express his sincere thanks to Professor Taitiro Fujinaga of Kyoto University and to Professor Toyoshi Nagai of Ritsumeikan ‘University. REFERENCES
1. T. Matsuda, J. Electroanal. Chem. Interfacial Electrothem., 1976, 69, 251. 2. Idem, Bull. Chem. Sot. Japan, 1977, 50, 1934. 3. Idem, ibid., 1978, 51, 639. 4. T. Nagai, T. Matsuda and N. Sugii, Bunseki Kagaku, 1972, 21, 337. 5. I. M. Kolthoff and R. Belcher, Volumetric Analysis, Vol. III, p. 293. Interscience, New York, 1957. 6. J. H. Karchmer, The Analytical Chemistry ofSu@r and its Compounds, Part I, p. 223. Interscience, New York, 1970. 7. E. Bishop and V. J. Jennings, Talanta, 1962, 9, 679. 8. I. M. Kolthoff and C. S. Miller, J. Am. Chem. Sot., 1941, 63, 1405.
0039-9140:79/0401-0326102.00!0 in Great Britain
AMPEROMETRIC
DETERMINATION
SULPHITE
WITH
OF SODIUM
HYDROGEN
CHLORAMINE-T
TOSHIO MATSUDA Department of Chemistry, Ritsumeikan University, Kyoto, Japan (Received
6 July
1978. Accepted
1 October 1978)
Summary-Amperometric titrations with chloramine-T at a rotating platinum electrode (RPE) and a dropping mercury electrode (DME) have been applied to the determination of sodium hydrogen sulphite over the concentration range 0.004O.lN. With the RPE, the indirect titration is best, at pH between 3 and 6, whereas the DME can be used for either direct or indirect titrations, at pH around 7. Relative standard deviations of 0.5% were obtained by both methods, with relative errors not exceeding &-1%.
In previous studies on the polarographic behaviour of chloramine-T (CAT) at a dropping mercury electrode (DME)’ and at a rotating platinum electrode (RPE),2.3 we found that CAT underwent a two-electron reduction in the pH range 2-13, though the stability of the diffusion current was dependent on the characteristics of the electrode reaction at both electrodes. This report describes amperometric titrations with CAT, using the DME and the RPE, for the determination of sodium hydrogen sulphite, and compares the results. The author and his co-workers4 have reported that sodium hydrogen sulphite can be determined over the concentration range O.Ol-O.lN by direct potentiometric titration with lead tetra-acetate in dilute acetic acid medium. However, this method could not be used for the determination of lower concentrations because of the volatility of sulphur dioxide. Although a considerable number of oxidizing reagents, including CAT, have been used for the titrimetric determination of hydrogen sulphite or sulphite,5-7 these cannot be used successfully for direct titration in acidic solution, again because of the volatility of sulphur dioxide, and back-titration has been recommended. In this study, therefore, the direct titration was performed in weakly acidic or neutral solutions in order to prevent the loss of sulphur dioxide. The possibility of extending the range of application of the method to relatively low.concentrations has been investigated.
Reagents Stock solution ofchloramine-T
Stock solution oisodium hydrogen sulphite, 0.05M (O.lN). Prepared fresh each day with air-free distilled water, and standardized by iodimetric titration.6 The solution, stored under nitrogen at 25”. was found to decrease in normality by about 0.3% in 9 hr. Lower concentrations were prepared by appropriate dilution with air-free water. The pH was adjusted to about 7 to prevent loss of sulphur dioxide. Other solutions such as potassium nitrate and BrittonRobinson buffer were prepared as reported earlier.‘*’ All solutions were prepared with doubly distilled water. Procedure Method A (reverse titration). Supporting electrolyte, (50 ml, O.lM in potassium nitrate and containing a suitable buffer) and standard CAT solution (5.00 ml) of appropriate concentration were ‘added to the cell and deoxygenated ‘completely by passage of nitrogen. This solution was titrated with sodium hydrogen sulphite solution. After each addition of titrant, the solution was mixed by passage of nitrogen for 6G90 set, and then the current was recorded. The measured currents were corrected for dilution. In order to prevent aerial oxidation of hydrogen sulphite, nitrogen was passed over the surface of the hydrogen sulphite solution in the burette during the titration. For titrations with the DME, 0.01% polyacrylamide (PAA) was added as a maximum suppressor. Method B (direct titration). After 50 ml of the supporting electrolyte had been deoxygenated in the cell, 5.00 ml of the sodium hydrogen sulphite solution were added. The solution was titrated with standard CAT solution. In order to prevent induced aerial oxidation of hydrogen sulphite, the CAT solution was deoxygenated and stored under nitrogen in the burette during the titration. Other details were as for method A.
RESULTS AND Amperometric
EXPERIMENTAL
Apparatus A Yanagimoto
Polarograph, Type PA-101, was used with the dropping mercury electrode (DME)’ and rotating platinum eltctrode (RPE)2V3described earlier. The potentials are referred to an SCE connected to the solution through a potassium nitrate-agar salt bridge.
(CAT), 0.05M (O.lN). Pre-
pared as described.
titration
DISCUSSION
at a rotating
platinum
electrode
WE)
The results of titrations by methods A and B for concentrations of around O.OlN in the pH range 3-7 are given in Table 1. All titrations except those at pH 7 were carried out at 0 V us. SCE, while a potential of -0.5 V was applied to the indicator electrode 326
321
SHORT COMMUNICATIONS
Table 1. Effect of pH on the amperometric titration at the RPE End-point
[CAT1
[NaHSO,] N
PH
Calc.. ml
N
Found.*
Average error, ?/;
ml
Method A 3.13 4.45 5.05 6.18 5.25t
1.037 1.075 1.090 1.091 1.060
x x x x x
lo-* lo-* 10-Z 1o-2 1o-2
0.984 0.984 0.984 0.984 0.984
x x x x x
10-l lo-’ 10-l lo-* lo-’
4.14 4.58 4.51 4.51 4.64
4.73 4.59 4.52 4.53 4.67
0.02 0.02 0.02 0.02 0.01
-0.2 +0.2 +0.2 +0.4 +0.6
Method B 3.55 5.14 6.18 7.30t
1.063 x 1.063 x 1.063 x 1.010 x
lo-’ lo-’ lo-’ 1o-2
0.973 0.973 0.973 0.944
x x x x
10-l 1o-2 lo-” lo-2
5.46 5.46 5.46 5.35
5.21 &-0.01 5.39 * 0.02 5.43 + 0.01 5.34 + 0.02
-4.7 -1.3 -0.6 -0.2
* * k + f
* Average of 3 titrations. t A potential of -0.5 V cs. SCE was applied.
for titrations at pH 7, because the chloramine-T reduction wave is shifted to more negative potentials as the pH rises. Titration curves of a normal L-shape and a reversed L-shape were obtained by methods A and B, respectively, because hydrogen sulphite showed neither a cathodic nor an anodic wave at the potentials applied over the pH range used. Over the pH range 36, method A gives good results, but method B gives poor results, being worse at lower pH and also when the Aow of nitrogen through the solution is increased, indicating that sulphur dioxide is being swept out of the solution under these conditions. At pH 7, both methods are satisfactory for concentrations of O.OlN, indicating that sulphur dioxide is not lost at this pH. However, for higher concentrations the reduction current for the CAT became increasingly unstable and the titration plot became curved after the end-point, causing difficulty in extrapolation. This instability of the current is considered to be due to a deactivation effect3 from some constituents adsorbed on the electrode surface. We can conclude that the titration at pH 7 cannot be successfully followed with the RPE.
From these results, it would seem that indirect titration (method A) with the RPE is best done at pH 5. Different concentrations within the range 0.001-0.1N were titrated under these conditions: the results are given in Table 2. Over the range 0.004-0.1N the relative errors did riot exceed +_l.Oo/, and the coefficient of variation for 6 titrations was 0.5%. It was also found that the results were not affected by the presence of chloride, sulphate or which are the reaction p-toluenesulphonamide, products, at concentrations up to at least twice those produced in the titration reaction. At concentrations below 0.002N the error became increasingly positive (Table 1); this is considered to arise from aerial oxidation of hydrogen sulphite during the preparation and storage of its solutions at concentrations around O.OOlN. Amperometric
titration
at a dropping
titration with
End-point
1.025 x 5.01 x 1.113 x 8.89 x 5.11 x 4.37 x 2.284 x
10-l 1o-2 1o-2 1O-3 1o-3 lo-’ lo-”
[CAT1 N 0.984 x 4.93 x 0.951 x 7.62 x 4.94 x 3.820 x 1.946 x
10-l 10-l 10-Z lo-” 10-a 1O-3 10-a
* s = 2.5 x lo-’ ml (6 titrations). f s = 2.0 x lo-’ ml (6 titrations).
electrode
The results of titrations by both methods in the pH range 5-7 are given in Table 3. A potential of -0.2 V was applied, because an anodic wave appeared at around 0 V for the hydrogen sulphite
Table 2. Results obtained by the indirect amperometric the RPE (method A) at pH 5.05
[NaHSO,] N
mercury
(DME)
Calc. ml 4.80 4.92 4.21 4.29 4.83 4.37 4.26
Found ml 4.81 4.93 4.27 4.30 4.83 4.40 4.36
f * + * f. + k
0.04* 0.02 O.Olt 0.01 0.02 0.02 0.04
Average error, 4, -0.2 +0.2 +o.o + 0.2 +0.2 +0.7 +2.3
328
SHORT
COMMUNICATM)NS
Table 3. Effect of pH on the amperometric titration at the DME End-point [NaHSO,] N
[CAT1
Calc. ml
Found*
N
ml
Average error. “/,
Method A 5.02 6.18 1.22
1.098 x lo-’ 1.098 x lo-’ 1.019 x 1o-2
0.991 x 1o-2 0.991 x 10-r 1.001 x 10-r
4.51 4.51 4.51
4.45 + 0.02 4.48 f 0.01 4.52 + 0.02
- 1.3 -0.9 +O.l
Method B 7.30
1.010 x 1o-2
0.944 x 1o-2
5.35
5.33 * 0.03
-0.4
PH
* Average of 3 titrations. solutions, as reported previous1y.s The titration curves obtained had the same shape as those obtained with the RPE. Indirect titrations (DME, method A) at pH < 6, gave low results with the error increasing as the pH decreased, in contrast to the good results obtained when the RPE was used. The errors also increased with increasing nitrogen flow-rate, suggesting that the negative errors could be attributed to reaction between CAT and the pool of mercury.’ The direct titration (method B) using the DME at pH < 6 gave low results similar to those obtained with the RPE. It is concluded that titrations using the DME should not be done at pH < 6. On the other hand, the titrations by both methods with the DME at pH 7 gave good results. For the determination of sodium hydrogen sulphite over the concentration range of 0.004-O.lN, the relative errors did not exceed + 1.0% and the coefficient of variation of 6 titrations was 0.5%. The optimum conditions as found experimentally are in agreement with the facts that the diffusion current for CAT at the DME is very stable in neutral solutions, but not in acidic solutions, and that loss of sulphur dioxide from neutral solutions cannot be detected. The presence of added amounts of chloride, sulphate or p-toluenesulphonamide, did not affect the performance of this method. CONCLUSIONS
Chloramine-T has been found to react quickly and quantitatively with hydrogen sulphite in neutral or weakly acidic solutions according to the equation RSO,NClH
+ HSO;
+ OH-
+ RS02NH2
+ HSO;
+ Cl-
where R = CH3C6H4. Based on this reaction, sodium hydrogen sulphite can be titrated directly with CAT over the concentration range 0.0040.1N with amperometric end-point indication by means of either an RPE or a DME. When the RPE is used, good results can be obtained when CAT is titrated in weakly acidic solution with sodium hydrogen sulphite solution at 0 V vs. SCE. When the DME is used, good results can be obtained when CAT is titrated in neutral solution with sodium hydrogen sulphite solution, or vice versa, at -0.2 V vs. SCE. It can be concluded that, for the standardization of sodium hydrogen sulphite solutions over the range 0.004-O.liV, the proposed amperometric methods with CAT are simpler and more accurate than previous methods, because the titrations are carried out under conditrons where sulphur dioxide is not lost as gas from the solution. Acknowledgements-The author wishes to express his sincere thanks to Professor Taitiro Fujinaga of Kyoto University and to Professor Toyoshi Nagai of Ritsumeikan ‘University. REFERENCES
1. T. Matsuda, J. Electroanal. Chem. Interfacial Electrothem., 1976, 69, 251. 2. Idem, Bull. Chem. Sot. Japan, 1977, 50, 1934. 3. Idem, ibid., 1978, 51, 639. 4. T. Nagai, T. Matsuda and N. Sugii, Bunseki Kagaku, 1972, 21, 337. 5. I. M. Kolthoff and R. Belcher, Volumetric Analysis, Vol. III, p. 293. Interscience, New York, 1957. 6. J. H. Karchmer, The Analytical Chemistry ofSu@r and its Compounds, Part I, p. 223. Interscience, New York, 1970. 7. E. Bishop and V. J. Jennings, Talanta, 1962, 9, 679. 8. I. M. Kolthoff and C. S. Miller, J. Am. Chem. Sot., 1941, 63, 1405.