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Some observations on the coulometric determination of sulphur dioxide
Aunl~~ica Cltirnlca Acfu. ?6 ( 1975) 483486 ( *: Elsevicr Scientific Publishing Company.
SHORT
- Printed
in The Ncthcrlands
COMMUNICATION
Some observations
A. D. CAMPBELL,
on the coulometric
(the lotc) D. P. WUBBARD
Deprrv:rrwr~t oj’ Chendwy
(Rcccivcd
4x3 hmstcrdam
30th
Dcccmbcr
Uhwrsity
determination
of sulphur
dioxide
and N. l-l. TIOW
oj’ Otrrgu. Box
56. Dw~eclir~ (New
Zcc~lami)
1974)
During recent years the coulometric method has been extensively used for the determination of sulphur compounds, e.g. sulphur in hydrocarbons’-3, and sulphur dioxide both in the atmosphere4*5 and gas mixtures”. The advantages of the technique include selectivity, rapidity, wide range of analytical utility and, perhaps most important, the ease of automation. However, the above-mentioned methods all involve relatively expensive equipment; the work described here was concerned with the possibility of measuring sulphur dioxide in the atmosphere precisely and cheaply with a portable instrument. As a preliminary step, the coulometric titration of sulphur dioxide with electrogenerated iodinelW3 was assessed in terms of accuracy, precision and interferences. Experinwrttal Apparutus. A constant-current coulometer (Metrohm Model E211 A) was used in conjunction with a titration vessel (Metrohm 875-20) equipped with 5 standard ground glass joint inlets. Iodine was electrogenerated at a platinum foil anode separated from a platinum foil auxiliary electrode by a dialysis membrane (Metrohm EA 247). The end-pdint was detected by a pair of identical platinum end-point method described by electrodes in a variation of the “dead-stop” Hawkins’; this indicating unit was arranged so that the 0.3-V full-scale meter (Uchida Yoko Co. Ltd., Japan) acted as a high-resistance v&meter in parallel with the cell. The voltage across the cell decreased to a relatively low value at the end-point when free iodine was available to depolarize the cathode. The constant current was measured by a milliammeter (Uchida Yoko Co. Ltd., Japan) because the current settings on the coulometer were not accurate. Electrolyte solution was stored in a reservoir with an inlet into the titration vessel. Solutions containing sulphur dioxide were introduced through one of the other ground-glass joint inlets. Solutions were stirred magnetically. Electrolyte. Prepare a solution containing 0.2 M potassium iodide, 0.055% (w/v) sodium nzide and 0.68% (v/v) acetic acid with deoxygenated distilled water. Standard szrlpkur dioxide solution. Bubble sulphur dioxide through deoxygenated distilled water and standardize when required by adding acidified iodine solution and back-titrating with sodium thiosulphate solution. Standard sodium sttlphite solution. Dissolve a known amount of anhydrous sodium sulphite in 5% (v/v) glycerol solution.
SHORT
44-l
COMMUNICATION
Standard iodine and thiosulphatc solutions were prepared by suitable dilution of ‘“Volucon” standard cpncentrates (May and Baker, England). Analytical reagent-grade chemicals were used wherever possible. Z+oce&re. Transfer 30 cm3 of electrolyte solution to the titration vessel and generate iodine electrolytically at a constant current of 2.97 mA until the needle of the indicating meter registers half full-scale deflection. Introduce a known volume of sulphur dioxide solution into the cell and regenerate iodine to its previous level, recording the electrolysis time. Calculate the concentration of sulphur dioxidg in the added soIution from: I * t 103 - 64.064 p.p.m. PO23 = y-2*96487 l
where Z is the constant current (mA), t is the y is the volume of SO2 solution added (cm’).
time
of electrolysis
(s),
and
Results and discussion
Sulphur dioxide reacts with the eoulomctrically generated iodine (which forms triiodide) in the normal manner, being oxidized to sulphur trioxide. In a comparison of end-points for the coulometric determination of arsenite with iodine, * Tackett’ reported that students generally found the amperometric (“dead-stop”) method to be more precise than either calorimetric or’ potentiometric methods. Moreover, the “dead-stop” method is widely accepted for Karl Fischer titrations, and was therefore chosen here. The electrolyte selected for study was recommended by Cedergren3 as a suitable absorption medium for sulphur dioxide after combustion of hydrocarbons. The high concentration of iodide is present to favour the formation of the less volatile triiodide. It has been shown’ that the presence of sodium azide eliminates the interference of nitrogen and chlorine compounds. Loss of SO2 from solution. It is well known that loss of SO2 from solution occurs on standing ‘. Table I shows the couIometric determination of the standard sulphur dioxide solution immediately aft& preparation and at 24-h intervals for 3 days; the results are compared with those of the iodine-thiosulphate back titration’O and the hydrogen peroxide methods Il. The peroxide method appears to TABLE TIiE DNJ
I
DETERMiNATION so2(*Io-4
OF SULPWUR Illd
DIOXIDE
IN A STANDARD
llr?i-J) 2 - b
CO~lO~Ctr_~*
ftlS203
0
6.54
c.57
6.36
: 3
4.85 3,23 2.42
3.26
6.26
w20
2c
o Mean of 6 or 7 readings, I’ Iodine nddcd and cxccss titrated with sadium thiosulphntc solution, p Oxidation to sulphur acid und titration with sodium hydroxide solution.
SOLUTXON
SWORT COMMUNICATION
-Is5
give
low results but the 30 min which elapsed between appl’ication of the first two methods and the peroxide method could account for the difference. Certainly there is excellent agreement between the coulometric and the back-titration methods which were carried out simultaneously. The results indicate that very little of the sulphur dioxide is lost by volatilization and that most is lost via oxidation to sulphur trioxide. Great care should be exercised whenever solutions containing sulphur dioxide cannot be analysed immedia:ely. Precision 0J the method. Syty I2 has suggested that the stability of sulphite solutions may be increased by the addition of glycerol. Consequently. the precision of the method was evaluated by determination of the SO2 content of a series of freshly prepared sodium sulphite solutions in 5% glycerol (Table II); it should be noted that the glycerol produced a blank value which had to be subtracted from the actual times recorded. In the case of solutions 4 and 5, it was necessary to add only 2 cm3 of standard solution to the coulometric cell instead of
TABLE
II
REPRODUCIBlLlTY S”/;;GLYCEROL
OF THE
COULOMETRK
ncrlral
Fo1r,7cP
1 :
3.783 19.93 II.83
3.783 112 0.0 16 I19.73rtO.18 1.76 f 0.04
4 ‘5
52.74 81.39
52.44 rt:0.2 1 81.12&0.19
METHOD
FOR
SULPHITE
” Mcnn of at least 4 values. TABLE
III
INTERFERENCE Iriterji’rerrt
STUDY
(rug)
SOJ” 2 45 NOz” i2.6 HCI* 7.29 H$’ 3.85 COzc 1.69 HCI-IO 18.9 Cl$ 1.09 COC 0.03 CHeC 0.02
AlfWC
Wit11 interfhwrt
0.171 0.171 0.157 0.157 0.131 0.112 0.112 0.125 0.125
0.172 0.171 0.157 0.157 0.131 0.112 0.088 0.125 0.124
u Added as 10 cmJ of standard sulphur dioxide solution to the clectrolytc ’ Added ns corresponding acid. c Added as saturated solution.
solution,
SOLUTIONS
IN
-NC1
SHORT
COMMUNICATION
5 cm3 as for solutions l-3. otherwise inuccuracies resulted. i.e. the accuracy of the determination decreases with increasing sulphur dioxide content probably in these ciises an insufficient excess of because of oxidation losses. although iodine may have been present. It is suggested that the sulphur dioxide content of the solution should not cxcecd ~a.-+5 mg dm - 3 for optimal accuracy. Sodium thiosulphate solution also provides a suitable standard-the mean value of ten successive coulr~mctric determinations was 0.05OOk 0.0004 mol dme3. I/trer:/i~,~cr~ces.Some species likely to bc present in the atmosphere were added to the electrolyte: the results are summurized in Table III. The only interference was from chlorine but this is unlikely to be present in the atmosphere’3. It should be remembered that sulphides will interfere although the sulphide content of the atmosphere is usually very low compared with that of sulphur dioxide. These results show that the electrogeneration of iodine and coulometric determination of sulphur dioxide is an accurate and precise method *with few interferences. It should be possible to design a very simple coulometric portable meter for determining sulphur dioxide in the atmosphere. REFERENCES I F. C. A. Killer and K. E. Underhill. Arrcr/_vsr(Lorrclhrr). 95 (1970) 505. 2 .I. I? Dixon. ,htr/psf (Lor~th). 97 (1972) 412. 3 A. Ccdcrgrcn. Tdtwrcr. 20 ( 1973) 621. 4 W. L. Ehmesbcrgcr and D. F. Adams, Tuppi, 52 ( 1969) 1302. 5 D. F. Miller. W. E. Wilson Jr. and R. G. Kling. J. ,4/r Pollut. Cor~rr. Ass,. 21 (1971) 414. G S. I. Krichmar. V. E. Stcpuncnko and T. M. Galan. Z/I. nrrcrl. Klrlm.. 26 (1971) 1340. 7 A. E. Hawkins. Atrrr/_rst ( Lordon). 89 ( I9G4) 432. 8 S. L. Tackctt. J. Clren. Ehc*.. 49 ( 1972) 52. 9 P. L. Bailey and E. Bishop. Auu/.rsr (Lorrrlor~). 97 (1972) 31 I. IO A. I. Vogel. A Tc.whook oJ’ Qmnritc~tire /llCJr~J~lllic Arir~/,wls. Longmans. London. 3rd cdn.. 1961. I1 M. B. Jncobs. T/w C/wn~icw/ Amr/~~s/s of Air Po//~rfarrrs, Wiley Intcrscicncc. New York. l9GO. 12 A. Syty. nrtc~/. Clrerrr.. 45 (1973) 1746. I.1 P. K. Mucllcr in E. S. Starkman (Ed.). Corllblrsriorl-llcrter,crrcc/ Polhtrlor~. Plenum Press, New York. 1971.
SHORT
- Printed
in The Ncthcrlands
COMMUNICATION
Some observations
A. D. CAMPBELL,
on the coulometric
(the lotc) D. P. WUBBARD
Deprrv:rrwr~t oj’ Chendwy
(Rcccivcd
4x3 hmstcrdam
30th
Dcccmbcr
Uhwrsity
determination
of sulphur
dioxide
and N. l-l. TIOW
oj’ Otrrgu. Box
56. Dw~eclir~ (New
Zcc~lami)
1974)
During recent years the coulometric method has been extensively used for the determination of sulphur compounds, e.g. sulphur in hydrocarbons’-3, and sulphur dioxide both in the atmosphere4*5 and gas mixtures”. The advantages of the technique include selectivity, rapidity, wide range of analytical utility and, perhaps most important, the ease of automation. However, the above-mentioned methods all involve relatively expensive equipment; the work described here was concerned with the possibility of measuring sulphur dioxide in the atmosphere precisely and cheaply with a portable instrument. As a preliminary step, the coulometric titration of sulphur dioxide with electrogenerated iodinelW3 was assessed in terms of accuracy, precision and interferences. Experinwrttal Apparutus. A constant-current coulometer (Metrohm Model E211 A) was used in conjunction with a titration vessel (Metrohm 875-20) equipped with 5 standard ground glass joint inlets. Iodine was electrogenerated at a platinum foil anode separated from a platinum foil auxiliary electrode by a dialysis membrane (Metrohm EA 247). The end-pdint was detected by a pair of identical platinum end-point method described by electrodes in a variation of the “dead-stop” Hawkins’; this indicating unit was arranged so that the 0.3-V full-scale meter (Uchida Yoko Co. Ltd., Japan) acted as a high-resistance v&meter in parallel with the cell. The voltage across the cell decreased to a relatively low value at the end-point when free iodine was available to depolarize the cathode. The constant current was measured by a milliammeter (Uchida Yoko Co. Ltd., Japan) because the current settings on the coulometer were not accurate. Electrolyte solution was stored in a reservoir with an inlet into the titration vessel. Solutions containing sulphur dioxide were introduced through one of the other ground-glass joint inlets. Solutions were stirred magnetically. Electrolyte. Prepare a solution containing 0.2 M potassium iodide, 0.055% (w/v) sodium nzide and 0.68% (v/v) acetic acid with deoxygenated distilled water. Standard szrlpkur dioxide solution. Bubble sulphur dioxide through deoxygenated distilled water and standardize when required by adding acidified iodine solution and back-titrating with sodium thiosulphate solution. Standard sodium sttlphite solution. Dissolve a known amount of anhydrous sodium sulphite in 5% (v/v) glycerol solution.
SHORT
44-l
COMMUNICATION
Standard iodine and thiosulphatc solutions were prepared by suitable dilution of ‘“Volucon” standard cpncentrates (May and Baker, England). Analytical reagent-grade chemicals were used wherever possible. Z+oce&re. Transfer 30 cm3 of electrolyte solution to the titration vessel and generate iodine electrolytically at a constant current of 2.97 mA until the needle of the indicating meter registers half full-scale deflection. Introduce a known volume of sulphur dioxide solution into the cell and regenerate iodine to its previous level, recording the electrolysis time. Calculate the concentration of sulphur dioxidg in the added soIution from: I * t 103 - 64.064 p.p.m. PO23 = y-2*96487 l
where Z is the constant current (mA), t is the y is the volume of SO2 solution added (cm’).
time
of electrolysis
(s),
and
Results and discussion
Sulphur dioxide reacts with the eoulomctrically generated iodine (which forms triiodide) in the normal manner, being oxidized to sulphur trioxide. In a comparison of end-points for the coulometric determination of arsenite with iodine, * Tackett’ reported that students generally found the amperometric (“dead-stop”) method to be more precise than either calorimetric or’ potentiometric methods. Moreover, the “dead-stop” method is widely accepted for Karl Fischer titrations, and was therefore chosen here. The electrolyte selected for study was recommended by Cedergren3 as a suitable absorption medium for sulphur dioxide after combustion of hydrocarbons. The high concentration of iodide is present to favour the formation of the less volatile triiodide. It has been shown’ that the presence of sodium azide eliminates the interference of nitrogen and chlorine compounds. Loss of SO2 from solution. It is well known that loss of SO2 from solution occurs on standing ‘. Table I shows the couIometric determination of the standard sulphur dioxide solution immediately aft& preparation and at 24-h intervals for 3 days; the results are compared with those of the iodine-thiosulphate back titration’O and the hydrogen peroxide methods Il. The peroxide method appears to TABLE TIiE DNJ
I
DETERMiNATION so2(*Io-4
OF SULPWUR Illd
DIOXIDE
IN A STANDARD
llr?i-J) 2 - b
CO~lO~Ctr_~*
ftlS203
0
6.54
c.57
6.36
: 3
4.85 3,23 2.42
3.26
6.26
w20
2c
o Mean of 6 or 7 readings, I’ Iodine nddcd and cxccss titrated with sadium thiosulphntc solution, p Oxidation to sulphur acid und titration with sodium hydroxide solution.
SOLUTXON
SWORT COMMUNICATION
-Is5
give
low results but the 30 min which elapsed between appl’ication of the first two methods and the peroxide method could account for the difference. Certainly there is excellent agreement between the coulometric and the back-titration methods which were carried out simultaneously. The results indicate that very little of the sulphur dioxide is lost by volatilization and that most is lost via oxidation to sulphur trioxide. Great care should be exercised whenever solutions containing sulphur dioxide cannot be analysed immedia:ely. Precision 0J the method. Syty I2 has suggested that the stability of sulphite solutions may be increased by the addition of glycerol. Consequently. the precision of the method was evaluated by determination of the SO2 content of a series of freshly prepared sodium sulphite solutions in 5% glycerol (Table II); it should be noted that the glycerol produced a blank value which had to be subtracted from the actual times recorded. In the case of solutions 4 and 5, it was necessary to add only 2 cm3 of standard solution to the coulometric cell instead of
TABLE
II
REPRODUCIBlLlTY S”/;;GLYCEROL
OF THE
COULOMETRK
ncrlral
Fo1r,7cP
1 :
3.783 19.93 II.83
3.783 112 0.0 16 I19.73rtO.18 1.76 f 0.04
4 ‘5
52.74 81.39
52.44 rt:0.2 1 81.12&0.19
METHOD
FOR
SULPHITE
” Mcnn of at least 4 values. TABLE
III
INTERFERENCE Iriterji’rerrt
STUDY
(rug)
SOJ” 2 45 NOz” i2.6 HCI* 7.29 H$’ 3.85 COzc 1.69 HCI-IO 18.9 Cl$ 1.09 COC 0.03 CHeC 0.02
AlfWC
Wit11 interfhwrt
0.171 0.171 0.157 0.157 0.131 0.112 0.112 0.125 0.125
0.172 0.171 0.157 0.157 0.131 0.112 0.088 0.125 0.124
u Added as 10 cmJ of standard sulphur dioxide solution to the clectrolytc ’ Added ns corresponding acid. c Added as saturated solution.
solution,
SOLUTIONS
IN
-NC1
SHORT
COMMUNICATION
5 cm3 as for solutions l-3. otherwise inuccuracies resulted. i.e. the accuracy of the determination decreases with increasing sulphur dioxide content probably in these ciises an insufficient excess of because of oxidation losses. although iodine may have been present. It is suggested that the sulphur dioxide content of the solution should not cxcecd ~a.-+5 mg dm - 3 for optimal accuracy. Sodium thiosulphate solution also provides a suitable standard-the mean value of ten successive coulr~mctric determinations was 0.05OOk 0.0004 mol dme3. I/trer:/i~,~cr~ces.Some species likely to bc present in the atmosphere were added to the electrolyte: the results are summurized in Table III. The only interference was from chlorine but this is unlikely to be present in the atmosphere’3. It should be remembered that sulphides will interfere although the sulphide content of the atmosphere is usually very low compared with that of sulphur dioxide. These results show that the electrogeneration of iodine and coulometric determination of sulphur dioxide is an accurate and precise method *with few interferences. It should be possible to design a very simple coulometric portable meter for determining sulphur dioxide in the atmosphere. REFERENCES I F. C. A. Killer and K. E. Underhill. Arrcr/_vsr(Lorrclhrr). 95 (1970) 505. 2 .I. I? Dixon. ,htr/psf (Lor~th). 97 (1972) 412. 3 A. Ccdcrgrcn. Tdtwrcr. 20 ( 1973) 621. 4 W. L. Ehmesbcrgcr and D. F. Adams, Tuppi, 52 ( 1969) 1302. 5 D. F. Miller. W. E. Wilson Jr. and R. G. Kling. J. ,4/r Pollut. Cor~rr. Ass,. 21 (1971) 414. G S. I. Krichmar. V. E. Stcpuncnko and T. M. Galan. Z/I. nrrcrl. Klrlm.. 26 (1971) 1340. 7 A. E. Hawkins. Atrrr/_rst ( Lordon). 89 ( I9G4) 432. 8 S. L. Tackctt. J. Clren. Ehc*.. 49 ( 1972) 52. 9 P. L. Bailey and E. Bishop. Auu/.rsr (Lorrrlor~). 97 (1972) 31 I. IO A. I. Vogel. A Tc.whook oJ’ Qmnritc~tire /llCJr~J~lllic Arir~/,wls. Longmans. London. 3rd cdn.. 1961. I1 M. B. Jncobs. T/w C/wn~icw/ Amr/~~s/s of Air Po//~rfarrrs, Wiley Intcrscicncc. New York. l9GO. 12 A. Syty. nrtc~/. Clrerrr.. 45 (1973) 1746. I.1 P. K. Mucllcr in E. S. Starkman (Ed.). Corllblrsriorl-llcrter,crrcc/ Polhtrlor~. Plenum Press, New York. 1971.