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Analysis of mixtures of sulphide, thiosulphate, dithionite and sulphite
Talanta, Vol. 30, No. 6, PP. 419-422, 1983 Printed in Great Britain
0039-9140/83/060419-04$03.00/O Pergamon Press Ltd
ANALYSIS OF MIXTURES OF SULPHIDE, THIOSULPHATE, DITHIONITE AND SULPHITE WILLIAM P. KILROY Naval
Surface
Weapons
Center,
White Oak Laboratory,
Silver Spring,
(Received 21 August 1982. Accepted 16 December Summary-This paper reports thiosulphate and an estimation
a procedure of sulphite
for the accurate
determination
MD 20910, U.S.A.
1982) of sulphide,
dithionite
and
in heterogeneous and/or inaccessible mixtures.
A procedure
for the analysis of certain mixtures of sulphur compounds, such as a mixture containing both sulphide and dithionite, has not hitherto been reported. There appear to be several reasons for this. (1) Sulphide and dithionite are both generally determined by employing their strong reducing power to reduce salts of mercury, lead or silver. (2) Conditions under which both co-exist may be rare, or if they do co-exist, this may occur under conditions which make them inaccessible to convenient methods of analysis; separation by dissolution complicates matters since both sulphide and dithionite are unstable in aqueous solution, readily hydrolysing or undergoing aerial oxidation. (3) In mixtures, dithionite has been determined in the presence of sulphite,’ thiosulphate2 and mixtures of both.3d Sulphide has also been determined in a mixture containing sulphite and thiosulphate by an iodometric method similar to that used for analysis of the corresponding dithionite mixture.’ However, in all the procedures reported, sulphide interferes with the dithionite determination and vice versa. This paper reports a method of analysing mixtures of soluble sulphides, thiosulphate, dithionite and sulphite. The selected method permits analysis of heterogeneous samples or samples not conveniently separated. The scheme combines several methods, including the iodometric procedure for analysis of containing dithionite,’ inaccessible mixtures modification of the dithionite determination with Methylene Blue,’ and the analysis of soluble sulphides by the iodate method of Bethge.”
and sodium carbonate just before use. The sodium acetate-acetic acid buffer was 3.5M in each constituent. The concentrations of the standard iodine solutions are expressed in moles of I, per litre. Solution preparation Argon (99.999% pure), after passing through acidic chromous chloride solution, was used to deaerate 500 ml of 0. I M sodium hydroxide in a 1-litre three-necked flask and maintain it oxygen-free. The flask was kept in an ice bath. Sulphide was introduced into the alkali and dissolved, and an aliquot was withdrawn for analysis to verify the sulphide concentration. Known amounts of thiosulphate, dithionite and sulphite were then introduced into the remaining solution. The apparatus is shown in Fig. 1.
T = TVOON
EXPERIMENTAL Fig. 1
Reagents A “purified” sample of J. T. Baker sodium dithionite, iodometrically analysed and found to be 89.9% dithionite, 6.97” sulphite and 2.5% thiosulphate, was used throughout. Anhydrous reagent-grade sodium thiosulphate and ultrapure lithium sulphide were analysed for purity under helium by the iodimetric and iodate methods”’ respectively. A standard aqueous 0.025M solution of Methylene Blue (99.9% purity, Matheson, Coleman and Bell) was prepared. The concentration was verified by titration of an oxygenfree aliquot of a solution of dithionite previously analysed by iodometric methods. Fresh zinc carbonate was prepared by mixing equal volumes of 1.OM solutions of zinc sulphate 419
Analysis The solution was forced under argon into an argon-filled burette. Measured 5-20-ml portions of this solution, accurately measured, were added to rapidly swirled solutions of iodine, efc. as described below. Titration A. To a 250-m] filter flask containing a magnetic stirring bar, add a mixture of approximately 100 ml of O.lM sodium hydroxide and 100 ml of methanol. Deaerate the solution by bubbling oxygen-free argon through it for 20 min. When the deaeration is complete, add the sample solution from the burette where it is kept under argon. Use
420
WILLIAM P. KILROY
5 ml of sample solution for solutions -0.03M in dithionite, and IO ml for solutions -0.OlM in dithionite. Maintain the argon atmosphere in the filter flask, and replace the samplesolution burette with another containing standard Methylene Blue solution under argon. Titrate rapidly with the Methylene Blue, at room temperature. The end-point is characterized by a colour change from clear gold to a persistent purple. As the dithionite concentration decreases, the intensity of the gold colour decreases until the solution is nearly colourless, before the end-point change to a blue-violet. Let the number of mmoles of dithionite in 20 ml of the sample solution be A. Titration B. Add a IO or 20 ml portion of the sample solution (accurately measured) rapidly _ _ to a swirled flask containing approximately 12 ml of zinc carbonate suspension, 5 ml of water and four drops of 10M sodium hydroxide. Use a IO-ml sample for higher sulphide concentrations (-0.03M) and the 20-ml sample for lower ( - 0.01 M) concentrations. Immediately filter off the precipitate on a medium porosity sintered-glass filter, into a flask containing a mixture of approximately 6-10 ml of 0.5M iodine, 5 ml of water and 4 ml of acetic acid-acetate buffer. Wash with 0.1 M sodium hydroxide, then with several small portions of water. (The precipitate should be stirred in the washings and many washings applied; the precipitate must be thoroughly washed, otherwise the sulphide result will be high.) Transfer the filtrate quantitatively to a standard flask; the pH should be between 4 and 5, and sufficient excess of iodine should be left for the solution to be dark brown. Remove the excess of iodine with 10% sodium sulphite solution and add an additional 7 or 8 ml of sulphite solution, Neutralize the solution to phenolphthalein by dropwise addition of IOM sodium hydroxide. Let stand for 5 min, add 4 ml of 37% formaldehyde solution and dilute to the mark with 20”/, v/v acetic acid. Remove an ahquot, adjust the pH to 44.5 with 20% acetic acid, and titrate with 0.005M iodine to a starch end-point. Let the number of mmoles of iodine consumed for 20 ml of the original sample solution be B. Titration C. Transfer the zinc sulphide precipitate (from the titration B procedure) to a flask containing N 15 ml of 10M sodium hydroxide and a known excessive volume of standard 0.05M ootassium iodate. Boil eentlv for 10 min after the solution is clear, then cool in& ice-bath. Add
excess of potassium iodide solution and slowly acidify with 4M sulnhuric acid. addine about 3 ml more once iodine has appeared permanently. Titrate the iodine with O.lM thiosulphate. The number of mmoles of sulphide is equal to three-fourths of the number of mmoles of iodate consumed. Let the number of mmoles of sulphide from 20 ml of original sample solution be C. Titration D. Add 20 ml of the samnle solution to a
partially evacuated flask fitted with a one-hole stopper and containing a well stirred solution of 10-25 ml of water, 3 ml of acetic aciddacetate buffer and a known volume of standard iodine solution (in large excess). Let stand for 20 min in the stoppered flask. Titrate the remaining iodine with standard thiosulphate solution. Let the number of mmoles of iodine consumed by 20 ml of the sample solution be D. THEORY
Titration
A.
When Methylene Blue is added to an oxygen-free alkaline methanol solution containing dithionite, sulphide, thiosulphate, and sulphite, it reacts with only the dithionite (rapidly and quantitatively) at room temperature. The Methylene Blue is reduced from the intensely blue form to a solid leuco form that is soluble in the methanol. The colour intensity of the leuco form in the solution increases as more and more
dithionite is consumed. Consequently, the colour change at the end-point is dependent on the dithionite concentration in solution. The end-point is characterized by the persistence of the intense Methylene Blue colour. The number of mmoles of dithionite consumed in titration A is equal to the number of mmoles of Methylene Blue used. Titration
B
A sample containing sulphide, dithionite, thiosulphate and sulphite reacts with excess of iodine in acetic acid-acetate buffer as follows: s*- + I,+S S,O;-
+ 21-
(1)
+ 31, + 4H20+2HS0, 2s,o:-
HSO,
+ 6HI
(2)
+ I*+S@~
+ 21~
(3)
+ I, + H,O+HSOy
+ 2HI
(4)
If the sample is first added to a suspension of freshlv precipitated zinc carbonate, the sulphide is quantitatively precipitated, and the filtrate reacts with an acidic iodine solution according to reactions (2) (3) and (4). Excess of sulphite is then added (a) to remove unreacted iodine and (b) after alkalization of the solution, to convert the tetrathionate, S,Oi -, formed from reaction (3), quantitatively into half the original amount of thiosulphate: s,o;-
+ so:+s@-
+ S&.
(5)
The unreacted sulphite is complexed by formaldehyde in acetic acid, and the thiosulphate produced by reaction (5) is titrated with standard iodine (B mmoles of I*). Then 4B = the number of mmoles of S,O:- originally present. Titration
C
Hot alkaline iodate (in excess) oxidizes quantitatively according to the reaction 3S*- + 410,+3SO:~
sulphide
+ 4II.
(6)
Addition of iodide followed by acidification converts the unreacted iodate into iodine, which is titrated with standard thiosulphate solution. The number of mmoles of sulphide is given by
c = a [(mmoles
of iodate
- i (mmoles Titration
taken) of thiosulphate
used)]
D
The number of mmoles of I, consumed by the sample in titration D is the result of reactions (lt(4). Consequently, the number of mmoles of SOipresent is given by (D - 3A - 2B - C). DISCUSSION
The effects of temperature, concentrations of reagents, alkalinity, stability in solution, dilution, etc. were investigated to assess the procedure for the determination of each anion in the presence of the others. The procedure found most suitable is that
Sulphide, thiosulphate, dithionite and sulphite detailed in the experimental section. However, some of the variables cited above can influence the analysis and therefore some comments are appropriate. The anion stability and the possibility of anion interaction in the alkaline solution were investigated. A solution of each anion was prepared and analysed. In turn, each of the other anions was added and the solution re-analysed approximately 1 hr after each addition. Finally, a standard solution of all four anions (S*-, S,Oi-, S,O:-, SO:-) was prepared, and analysed immediately and again 6 hr later. No changes in concentration were observed. This was reconfirmed on several occasions during the course of the experiments. Interestingly, the yellow colour imparted to the solution by the sulphide slowly disappeared on addition of the dithionite. Analysis revealed that no change in either the sulphide or dithionite concentration had occurred. TO ensure stability of the solution, it should be kept oxygen-free and preferably at below 25”. The stability of dithionite in solution is a function of its concentration, temperature, and pH. I’ Analysis of higher concentrations of dithionite may warrant a different concentration of alkali solution. Sulphide and thiosulphate. For the determination of these anions, the complete recovery, effective separation and decomposition or oxidation of dithionite were the primary problems studied. Sulphide solutions, 0.01 and O.O3M, were prepared. Aliquots were analysed by iodate titration and then re-analysed by precipitation with freshly prepared zinc carbonate suspension. Recovery was 100% efficient. However, smaller sample volumes, 10 ml instead of 20, appeared to give slightly better reproducibility for the 0.03M solution when other anions were present. Other anions were added to the standard sulphide solution to determine their effect on the analysis. Thiosulphate had no effect on recovery of sulphide. Dithionite and sulphite increased the apparent sulphide value by approximately 4% and 2% relative respectively. The problem with dithionite was eliminated by adding several drops of 10M sodium hydroxide to the zinc carbonate suspension before adding the sulphideedithionite solution. Increasing the pH evidently stabilizes the dithionite sufficiently for it to be unaffected by the exposure to air for the 1-2 min required for the filtration. The apparent increase in sulphide in the presence of sulphite seems to be due to formation of some zinc sulphite. Thorough washing, first with O.OlM sodium hydroxide and then with many small volumes of water, minimizes the sulphite interference. No apparent interference by any of the other anions was observed in the thiosulphate determination. In mixtures, the results were generally - 1.5% high, especially for the more dilute (O.OlM) thiosulphate solutions. Dithionite. The best method for determination of dithionite in the presence of thiosulphate and/or sulphite and absence of sulphide has already been
421
reported.* Results from attempts to expand this method by adding an aliquot of sample solution to alkaline formaldehyde containing zinc carbonate or by adding the zinc carbonate later after permitting the formaldehyde mixture to stand for _ 15 min, followed by filtration of the sulphide, were generally -3% low and less reproducible. The use of Methylene Blue gives better results provided the analyst is familiar with the method. If the solution containing dithionite is used to titrate a standard solution of Methylene Blue in alkaline methanol, very erroneous and irreproducible results are obtained, especially with more concentrated dilthionite solutions. Instead, the sample solution must be added to a totally oxygen-free mixture of methanol and sodium hydroxide solution and this mixture titrated with Methylene Blue. The titration should be performed rapidly, under an inert gas and at room temperature. The same results are obtained if an additional 5 ml of O.lM sodium hydroxide are added to the methanolhydroxide mixture or if the methanol is replaced by acetone. Large volumes of the methanol-hydroxide solution are recommended for two reasons. First, to solubilize the relatively insoluble leuco form produced when Methylene Blue is reduced, because the insoluble leuco form appears to adsorb Methylene Blue and make the end-point more drawn-out and difficult to locate. Secondly, the larger volume decreases the overall concentration of dithionite present and thereby enhances detection of the end-point. Best results are obtained by using a small sample volume, preferably 5 ml, large amounts of methanol-hydroxide mixture, and a preliminary titration to become familiar with the end-point. The end-point is remarkably sharp for dilute solutions of dithionite (-O.OOSM) but at concentrations of -O.O3M, the methanol-hydroxidedithionite solution progressively becomes more deeply golden in colour as the Methylene Blue is added. Even under these latter conditions, however, the end-point, characterized by the persistence of a purple colour in the solution, due to unreacted Methylene Blue, is remarkably reproducible. Sulphite. Sulphite can be determined by difference provided all four anions can be quantitatively oxidized. Two methods were examined; oxidation by hot alkaline iodate and oxidation by acidic iodine. Alkaline iodate was found unsuitable owing to slow oxidation of the dithionite (accompanied by some decomposition). If the solution containing the sulphur anions is added to a rapidly stirred solution containing excess of iodine and a few ml of the acetic acid-acetate buffer, good agreement is consistently found between the amount of iodine consumed and the theoretical amount needed. However, if a large excess of iodine is not used or if too much water (_ 100 ml) is added along with the iodine, the analysis is generally low by -2%. Analysis for the individual anions shows that the dithionite is re-
422
WILLIAMP. KILROY Table I. Determination S,O:-, mmole
of thiosulphate, sulphide, dithionite, and sulphite*
S2-, mmole
S,O:- ,mmole
SO:-, mmole
Runt
Present
Found
Present
Found
Present
Found
Present
Founds
1 2 3 4 5 6 7 8 9 10
0.111 0.205 0.202 0.210 0.596 0.210 0.224 0.208 0.612 0.605
0.113 0.207 0.205 0.212 0.602 0.215 0.228 0.212 0.615 0.600
0.205 0.214 0.628 0.198 0.625 0.21 I 0.614 0.195 0.209 0.202
0.206 0.215 0.630 0.196 0.631 0.211 0.605 0.199 0.206 0.202
0.591 0.200 0.202 0.627 0.604 0.205 0.624 0.204 0.614 0.207
0.586 0.198 0.201 0.637 0.599 0.202 0.628 0.203 0.624 0.207
~ -
-
0.206 0.251 0.598 0.251 0.203
0.204 (0.197) 0.233 (0.237) 0.575 (0.578) 0.239 (0.259) 0.200 (0.198)
*Expressed as mmoles per 20ml from the original 5OOmlof solution. tRun S-the solution contained -0.5 g of carbon black; run l&the solution contained LiBr (0.02M). $The value in parentheses is the number of mmoles of sulphite found by using the original initial concentrations
of S*-, S,@-,
S,O:- (as opposed to those experimentally found).
sponsible for this error, apparently undergoing some acidic decomposition. The procedure described for titration D consistently gives a consumption of iodine that agrees with the theoretical consumption within f 1%. Consequently, in theory, the determination of sulphite should be correct, but in practical analyses, of course, the sulphite result is affected by the errors in all four titrations, especially A and B [because of the stoichiometry of reactions (l)-(4)]. Careful analysis permits sulphite to be determined with a relative error below 5%. High surface-area (60 m’/g) carbon black and the presence of 0.02M lithium bromide has no effect on either catalysing the solution interactions or influencing the analysis scheme. RESULTS
Table 1 summarizes the analyses of solutions containing a variety of anion concentrations, corrected for the thiosulphate and sulphite present in the dithionite sample. The findings are reported as mmoles per 20 ml of the original 500 ml of test solution, corresponding to the sample volumes most frequently taken for analysis, Two analyses were performed for each constituent of each sample solution. Excellent agreement was found and the average
values are reported. The experimental conditions under which titration D gives successful analyses for sulphite were discovered during the later part of the analytical investigation. Consequently, the first part of Table I does not include sulphite in the test mixtures. Additional investigation of titration D was done with mixtures similar to those of runs 6 and 8. The experimental iodine titration volume used was found to be within 1% of the theoretical value predicted from the composition of a standard solution of the four anions. Table 1 reports the sulphite values found by use of both the original concentrations of the anions and those experimentally measured. REFERENCES 1. R. L. Kaushik and R. Prosad, J. ~~~~~~Chem. Sot., 1969, 46, 405. 2. F. Solymosi and A. Varga, Mugy. Chem. Folyoirut, 1959, 65, 52. 3. R. Wollak, Z. Anal. Chem., 1930, 80, 1. 4. V. R. Nair and C. R. Nair, Res. Ind., 1971, 16, 47. 5. W. P. Kilroy, Talunru, 1978, 25, 359. 6. Idem, ibid., 1979, 26, 1Il. 7. A. Kurtenacker and R. Woliak, Z. Anorg. A&em. Chem., 1927, 161, 201. 8. W. P. Kilroy, Talanru, 1980, 27, 343. 9. E. Knecht and E. Hibbert, Ber., 1907, 40, 3819. IO. P. Beth8e, Anal. Chim. Acta, 1954, 10, 310. 11. W. P. Kilroy, J. Znorg. Nucl. Chem., 1980, 42, 1071.
0039-9140/83/060419-04$03.00/O Pergamon Press Ltd
ANALYSIS OF MIXTURES OF SULPHIDE, THIOSULPHATE, DITHIONITE AND SULPHITE WILLIAM P. KILROY Naval
Surface
Weapons
Center,
White Oak Laboratory,
Silver Spring,
(Received 21 August 1982. Accepted 16 December Summary-This paper reports thiosulphate and an estimation
a procedure of sulphite
for the accurate
determination
MD 20910, U.S.A.
1982) of sulphide,
dithionite
and
in heterogeneous and/or inaccessible mixtures.
A procedure
for the analysis of certain mixtures of sulphur compounds, such as a mixture containing both sulphide and dithionite, has not hitherto been reported. There appear to be several reasons for this. (1) Sulphide and dithionite are both generally determined by employing their strong reducing power to reduce salts of mercury, lead or silver. (2) Conditions under which both co-exist may be rare, or if they do co-exist, this may occur under conditions which make them inaccessible to convenient methods of analysis; separation by dissolution complicates matters since both sulphide and dithionite are unstable in aqueous solution, readily hydrolysing or undergoing aerial oxidation. (3) In mixtures, dithionite has been determined in the presence of sulphite,’ thiosulphate2 and mixtures of both.3d Sulphide has also been determined in a mixture containing sulphite and thiosulphate by an iodometric method similar to that used for analysis of the corresponding dithionite mixture.’ However, in all the procedures reported, sulphide interferes with the dithionite determination and vice versa. This paper reports a method of analysing mixtures of soluble sulphides, thiosulphate, dithionite and sulphite. The selected method permits analysis of heterogeneous samples or samples not conveniently separated. The scheme combines several methods, including the iodometric procedure for analysis of containing dithionite,’ inaccessible mixtures modification of the dithionite determination with Methylene Blue,’ and the analysis of soluble sulphides by the iodate method of Bethge.”
and sodium carbonate just before use. The sodium acetate-acetic acid buffer was 3.5M in each constituent. The concentrations of the standard iodine solutions are expressed in moles of I, per litre. Solution preparation Argon (99.999% pure), after passing through acidic chromous chloride solution, was used to deaerate 500 ml of 0. I M sodium hydroxide in a 1-litre three-necked flask and maintain it oxygen-free. The flask was kept in an ice bath. Sulphide was introduced into the alkali and dissolved, and an aliquot was withdrawn for analysis to verify the sulphide concentration. Known amounts of thiosulphate, dithionite and sulphite were then introduced into the remaining solution. The apparatus is shown in Fig. 1.
T = TVOON
EXPERIMENTAL Fig. 1
Reagents A “purified” sample of J. T. Baker sodium dithionite, iodometrically analysed and found to be 89.9% dithionite, 6.97” sulphite and 2.5% thiosulphate, was used throughout. Anhydrous reagent-grade sodium thiosulphate and ultrapure lithium sulphide were analysed for purity under helium by the iodimetric and iodate methods”’ respectively. A standard aqueous 0.025M solution of Methylene Blue (99.9% purity, Matheson, Coleman and Bell) was prepared. The concentration was verified by titration of an oxygenfree aliquot of a solution of dithionite previously analysed by iodometric methods. Fresh zinc carbonate was prepared by mixing equal volumes of 1.OM solutions of zinc sulphate 419
Analysis The solution was forced under argon into an argon-filled burette. Measured 5-20-ml portions of this solution, accurately measured, were added to rapidly swirled solutions of iodine, efc. as described below. Titration A. To a 250-m] filter flask containing a magnetic stirring bar, add a mixture of approximately 100 ml of O.lM sodium hydroxide and 100 ml of methanol. Deaerate the solution by bubbling oxygen-free argon through it for 20 min. When the deaeration is complete, add the sample solution from the burette where it is kept under argon. Use
420
WILLIAM P. KILROY
5 ml of sample solution for solutions -0.03M in dithionite, and IO ml for solutions -0.OlM in dithionite. Maintain the argon atmosphere in the filter flask, and replace the samplesolution burette with another containing standard Methylene Blue solution under argon. Titrate rapidly with the Methylene Blue, at room temperature. The end-point is characterized by a colour change from clear gold to a persistent purple. As the dithionite concentration decreases, the intensity of the gold colour decreases until the solution is nearly colourless, before the end-point change to a blue-violet. Let the number of mmoles of dithionite in 20 ml of the sample solution be A. Titration B. Add a IO or 20 ml portion of the sample solution (accurately measured) rapidly _ _ to a swirled flask containing approximately 12 ml of zinc carbonate suspension, 5 ml of water and four drops of 10M sodium hydroxide. Use a IO-ml sample for higher sulphide concentrations (-0.03M) and the 20-ml sample for lower ( - 0.01 M) concentrations. Immediately filter off the precipitate on a medium porosity sintered-glass filter, into a flask containing a mixture of approximately 6-10 ml of 0.5M iodine, 5 ml of water and 4 ml of acetic acid-acetate buffer. Wash with 0.1 M sodium hydroxide, then with several small portions of water. (The precipitate should be stirred in the washings and many washings applied; the precipitate must be thoroughly washed, otherwise the sulphide result will be high.) Transfer the filtrate quantitatively to a standard flask; the pH should be between 4 and 5, and sufficient excess of iodine should be left for the solution to be dark brown. Remove the excess of iodine with 10% sodium sulphite solution and add an additional 7 or 8 ml of sulphite solution, Neutralize the solution to phenolphthalein by dropwise addition of IOM sodium hydroxide. Let stand for 5 min, add 4 ml of 37% formaldehyde solution and dilute to the mark with 20”/, v/v acetic acid. Remove an ahquot, adjust the pH to 44.5 with 20% acetic acid, and titrate with 0.005M iodine to a starch end-point. Let the number of mmoles of iodine consumed for 20 ml of the original sample solution be B. Titration C. Transfer the zinc sulphide precipitate (from the titration B procedure) to a flask containing N 15 ml of 10M sodium hydroxide and a known excessive volume of standard 0.05M ootassium iodate. Boil eentlv for 10 min after the solution is clear, then cool in& ice-bath. Add
excess of potassium iodide solution and slowly acidify with 4M sulnhuric acid. addine about 3 ml more once iodine has appeared permanently. Titrate the iodine with O.lM thiosulphate. The number of mmoles of sulphide is equal to three-fourths of the number of mmoles of iodate consumed. Let the number of mmoles of sulphide from 20 ml of original sample solution be C. Titration D. Add 20 ml of the samnle solution to a
partially evacuated flask fitted with a one-hole stopper and containing a well stirred solution of 10-25 ml of water, 3 ml of acetic aciddacetate buffer and a known volume of standard iodine solution (in large excess). Let stand for 20 min in the stoppered flask. Titrate the remaining iodine with standard thiosulphate solution. Let the number of mmoles of iodine consumed by 20 ml of the sample solution be D. THEORY
Titration
A.
When Methylene Blue is added to an oxygen-free alkaline methanol solution containing dithionite, sulphide, thiosulphate, and sulphite, it reacts with only the dithionite (rapidly and quantitatively) at room temperature. The Methylene Blue is reduced from the intensely blue form to a solid leuco form that is soluble in the methanol. The colour intensity of the leuco form in the solution increases as more and more
dithionite is consumed. Consequently, the colour change at the end-point is dependent on the dithionite concentration in solution. The end-point is characterized by the persistence of the intense Methylene Blue colour. The number of mmoles of dithionite consumed in titration A is equal to the number of mmoles of Methylene Blue used. Titration
B
A sample containing sulphide, dithionite, thiosulphate and sulphite reacts with excess of iodine in acetic acid-acetate buffer as follows: s*- + I,+S S,O;-
+ 21-
(1)
+ 31, + 4H20+2HS0, 2s,o:-
HSO,
+ 6HI
(2)
+ I*+S@~
+ 21~
(3)
+ I, + H,O+HSOy
+ 2HI
(4)
If the sample is first added to a suspension of freshlv precipitated zinc carbonate, the sulphide is quantitatively precipitated, and the filtrate reacts with an acidic iodine solution according to reactions (2) (3) and (4). Excess of sulphite is then added (a) to remove unreacted iodine and (b) after alkalization of the solution, to convert the tetrathionate, S,Oi -, formed from reaction (3), quantitatively into half the original amount of thiosulphate: s,o;-
+ so:+s@-
+ S&.
(5)
The unreacted sulphite is complexed by formaldehyde in acetic acid, and the thiosulphate produced by reaction (5) is titrated with standard iodine (B mmoles of I*). Then 4B = the number of mmoles of S,O:- originally present. Titration
C
Hot alkaline iodate (in excess) oxidizes quantitatively according to the reaction 3S*- + 410,+3SO:~
sulphide
+ 4II.
(6)
Addition of iodide followed by acidification converts the unreacted iodate into iodine, which is titrated with standard thiosulphate solution. The number of mmoles of sulphide is given by
c = a [(mmoles
of iodate
- i (mmoles Titration
taken) of thiosulphate
used)]
D
The number of mmoles of I, consumed by the sample in titration D is the result of reactions (lt(4). Consequently, the number of mmoles of SOipresent is given by (D - 3A - 2B - C). DISCUSSION
The effects of temperature, concentrations of reagents, alkalinity, stability in solution, dilution, etc. were investigated to assess the procedure for the determination of each anion in the presence of the others. The procedure found most suitable is that
Sulphide, thiosulphate, dithionite and sulphite detailed in the experimental section. However, some of the variables cited above can influence the analysis and therefore some comments are appropriate. The anion stability and the possibility of anion interaction in the alkaline solution were investigated. A solution of each anion was prepared and analysed. In turn, each of the other anions was added and the solution re-analysed approximately 1 hr after each addition. Finally, a standard solution of all four anions (S*-, S,Oi-, S,O:-, SO:-) was prepared, and analysed immediately and again 6 hr later. No changes in concentration were observed. This was reconfirmed on several occasions during the course of the experiments. Interestingly, the yellow colour imparted to the solution by the sulphide slowly disappeared on addition of the dithionite. Analysis revealed that no change in either the sulphide or dithionite concentration had occurred. TO ensure stability of the solution, it should be kept oxygen-free and preferably at below 25”. The stability of dithionite in solution is a function of its concentration, temperature, and pH. I’ Analysis of higher concentrations of dithionite may warrant a different concentration of alkali solution. Sulphide and thiosulphate. For the determination of these anions, the complete recovery, effective separation and decomposition or oxidation of dithionite were the primary problems studied. Sulphide solutions, 0.01 and O.O3M, were prepared. Aliquots were analysed by iodate titration and then re-analysed by precipitation with freshly prepared zinc carbonate suspension. Recovery was 100% efficient. However, smaller sample volumes, 10 ml instead of 20, appeared to give slightly better reproducibility for the 0.03M solution when other anions were present. Other anions were added to the standard sulphide solution to determine their effect on the analysis. Thiosulphate had no effect on recovery of sulphide. Dithionite and sulphite increased the apparent sulphide value by approximately 4% and 2% relative respectively. The problem with dithionite was eliminated by adding several drops of 10M sodium hydroxide to the zinc carbonate suspension before adding the sulphideedithionite solution. Increasing the pH evidently stabilizes the dithionite sufficiently for it to be unaffected by the exposure to air for the 1-2 min required for the filtration. The apparent increase in sulphide in the presence of sulphite seems to be due to formation of some zinc sulphite. Thorough washing, first with O.OlM sodium hydroxide and then with many small volumes of water, minimizes the sulphite interference. No apparent interference by any of the other anions was observed in the thiosulphate determination. In mixtures, the results were generally - 1.5% high, especially for the more dilute (O.OlM) thiosulphate solutions. Dithionite. The best method for determination of dithionite in the presence of thiosulphate and/or sulphite and absence of sulphide has already been
421
reported.* Results from attempts to expand this method by adding an aliquot of sample solution to alkaline formaldehyde containing zinc carbonate or by adding the zinc carbonate later after permitting the formaldehyde mixture to stand for _ 15 min, followed by filtration of the sulphide, were generally -3% low and less reproducible. The use of Methylene Blue gives better results provided the analyst is familiar with the method. If the solution containing dithionite is used to titrate a standard solution of Methylene Blue in alkaline methanol, very erroneous and irreproducible results are obtained, especially with more concentrated dilthionite solutions. Instead, the sample solution must be added to a totally oxygen-free mixture of methanol and sodium hydroxide solution and this mixture titrated with Methylene Blue. The titration should be performed rapidly, under an inert gas and at room temperature. The same results are obtained if an additional 5 ml of O.lM sodium hydroxide are added to the methanolhydroxide mixture or if the methanol is replaced by acetone. Large volumes of the methanol-hydroxide solution are recommended for two reasons. First, to solubilize the relatively insoluble leuco form produced when Methylene Blue is reduced, because the insoluble leuco form appears to adsorb Methylene Blue and make the end-point more drawn-out and difficult to locate. Secondly, the larger volume decreases the overall concentration of dithionite present and thereby enhances detection of the end-point. Best results are obtained by using a small sample volume, preferably 5 ml, large amounts of methanol-hydroxide mixture, and a preliminary titration to become familiar with the end-point. The end-point is remarkably sharp for dilute solutions of dithionite (-O.OOSM) but at concentrations of -O.O3M, the methanol-hydroxidedithionite solution progressively becomes more deeply golden in colour as the Methylene Blue is added. Even under these latter conditions, however, the end-point, characterized by the persistence of a purple colour in the solution, due to unreacted Methylene Blue, is remarkably reproducible. Sulphite. Sulphite can be determined by difference provided all four anions can be quantitatively oxidized. Two methods were examined; oxidation by hot alkaline iodate and oxidation by acidic iodine. Alkaline iodate was found unsuitable owing to slow oxidation of the dithionite (accompanied by some decomposition). If the solution containing the sulphur anions is added to a rapidly stirred solution containing excess of iodine and a few ml of the acetic acid-acetate buffer, good agreement is consistently found between the amount of iodine consumed and the theoretical amount needed. However, if a large excess of iodine is not used or if too much water (_ 100 ml) is added along with the iodine, the analysis is generally low by -2%. Analysis for the individual anions shows that the dithionite is re-
422
WILLIAMP. KILROY Table I. Determination S,O:-, mmole
of thiosulphate, sulphide, dithionite, and sulphite*
S2-, mmole
S,O:- ,mmole
SO:-, mmole
Runt
Present
Found
Present
Found
Present
Found
Present
Founds
1 2 3 4 5 6 7 8 9 10
0.111 0.205 0.202 0.210 0.596 0.210 0.224 0.208 0.612 0.605
0.113 0.207 0.205 0.212 0.602 0.215 0.228 0.212 0.615 0.600
0.205 0.214 0.628 0.198 0.625 0.21 I 0.614 0.195 0.209 0.202
0.206 0.215 0.630 0.196 0.631 0.211 0.605 0.199 0.206 0.202
0.591 0.200 0.202 0.627 0.604 0.205 0.624 0.204 0.614 0.207
0.586 0.198 0.201 0.637 0.599 0.202 0.628 0.203 0.624 0.207
~ -
-
0.206 0.251 0.598 0.251 0.203
0.204 (0.197) 0.233 (0.237) 0.575 (0.578) 0.239 (0.259) 0.200 (0.198)
*Expressed as mmoles per 20ml from the original 5OOmlof solution. tRun S-the solution contained -0.5 g of carbon black; run l&the solution contained LiBr (0.02M). $The value in parentheses is the number of mmoles of sulphite found by using the original initial concentrations
of S*-, S,@-,
S,O:- (as opposed to those experimentally found).
sponsible for this error, apparently undergoing some acidic decomposition. The procedure described for titration D consistently gives a consumption of iodine that agrees with the theoretical consumption within f 1%. Consequently, in theory, the determination of sulphite should be correct, but in practical analyses, of course, the sulphite result is affected by the errors in all four titrations, especially A and B [because of the stoichiometry of reactions (l)-(4)]. Careful analysis permits sulphite to be determined with a relative error below 5%. High surface-area (60 m’/g) carbon black and the presence of 0.02M lithium bromide has no effect on either catalysing the solution interactions or influencing the analysis scheme. RESULTS
Table 1 summarizes the analyses of solutions containing a variety of anion concentrations, corrected for the thiosulphate and sulphite present in the dithionite sample. The findings are reported as mmoles per 20 ml of the original 500 ml of test solution, corresponding to the sample volumes most frequently taken for analysis, Two analyses were performed for each constituent of each sample solution. Excellent agreement was found and the average
values are reported. The experimental conditions under which titration D gives successful analyses for sulphite were discovered during the later part of the analytical investigation. Consequently, the first part of Table I does not include sulphite in the test mixtures. Additional investigation of titration D was done with mixtures similar to those of runs 6 and 8. The experimental iodine titration volume used was found to be within 1% of the theoretical value predicted from the composition of a standard solution of the four anions. Table 1 reports the sulphite values found by use of both the original concentrations of the anions and those experimentally measured. REFERENCES 1. R. L. Kaushik and R. Prosad, J. ~~~~~~Chem. Sot., 1969, 46, 405. 2. F. Solymosi and A. Varga, Mugy. Chem. Folyoirut, 1959, 65, 52. 3. R. Wollak, Z. Anal. Chem., 1930, 80, 1. 4. V. R. Nair and C. R. Nair, Res. Ind., 1971, 16, 47. 5. W. P. Kilroy, Talunru, 1978, 25, 359. 6. Idem, ibid., 1979, 26, 1Il. 7. A. Kurtenacker and R. Woliak, Z. Anorg. A&em. Chem., 1927, 161, 201. 8. W. P. Kilroy, Talanru, 1980, 27, 343. 9. E. Knecht and E. Hibbert, Ber., 1907, 40, 3819. IO. P. Beth8e, Anal. Chim. Acta, 1954, 10, 310. 11. W. P. Kilroy, J. Znorg. Nucl. Chem., 1980, 42, 1071.