ANALYTICAL SCIENCES FEBRUARY 1990, VOL. 6 3

Reviews

Analytical Chemistry of Polythionates and Thiosulfate A Review

Tomozo KoH

Department of Chemistry, Faculty of Science, Tokai University, Hiratsuka, Kanagawa 259-12, Japan

The reaction of sulfide with dioxide in aqueous solution yields Wackenroder's solution, which contains polythionates (Sx062 x=3, 4 and 5) and thiosulfate. Various reactions of polythionates with such reagents as sulfite, cyanide, sulfide, permanganate and others are reviewed, in which the thiosulfate and/ or thiocyanate formed by these reactions of polythionates are measured titrimetrically, spectrophotometrically and coulometrically. As a result, simul- taneous equations can be obtained through corresponding different procedures and can be used for the determination of polythionates in their mixtures with other sulfur anions. Furthermore, modern liquid chromatography is described for the separation of polythionates and thiosulfate, in which quite different chromatographic techniques and conditions were employed.

Keywords Wackenroder's solution, polythionates reaction, polythionates mixture analysis, spectrophotometry, polythionates high performance liquid chromatography

1 Introduction 4 Pure Individual Polythionates 2 Titrimetric Methods 4.1 Spectrophotometry 2.1 Thiosulfate 4.2 Electroanalytical methods 2.2 Polythionates in mixtures 5 Polythionates and Other Sulfur Anions in 3 Thiosulfate Mixtures 3.1 Spectrophotometry 5.1 Spectrophotometry 3.2 Catalytic methods 5.2 Electroanalytical methods 3.3 Electroanalytical methods 5.3 Chromatographic separations

reagents such as cyanide and sulfite, in which the 1 Introduction thiosulfate formed is measured for the determination of polythionates. Compounds of composition H2Sx06 have been known Polythionates had been neglected for many years for a long time in the form of some salts that are rather because of their instability, together with the fact that unstable in aqueous solution. For many years the they are formed in complex mixtures. The determina- accepted values for x have been 3 -6, though x varies tion of individual polythionates in the same solution from 3 to 80;12 detailed knowledge of the chemistry of has long been a tedious, and in many cases impossible the compounds designated as polythionic acids is still task. However, they play important roles in technical restricted to the first four members. They were found processes, environmental chemistry, the sulfur meta- in an aqueous solution of and hydrogen bolism of certain sulfur bacteria and, generally, in many sulfide, known as Wackenroder's solution3 or Samans basic reactions of inorganic sulfur chemistry. Hence, solution.4 Polythionic acids can also be designated as the analytical chemistry of the polythionates is of disulfonic acids of the sulfanes. This nomenclature practical significance to both basic chemistry and would show the fact that the dithionic acid, H2S206, industry. In the present review, a comprehensive survey formed by an oxidative treatment of sulfites, has concerning the determination of each polythionate up chemically nothing to do with the real polythionic to hexathionate (tetrasulfanedisulfonate) in both the acids, which are reduction products of sulfites. absence and presence of other sulfur anions is de- Therefore, is excluded here. Thiosulfate is scribed, including chromatographic separations of poly- reviewed because polythionates are formed as mixtures thionate mixtures. with thiosulfate. Additionally, polythionates are often converted to thiosulfate by their reactions with some 4 ANALYTICAL SCIENCES FEBRUARY 1990, VOL. 6

from the sulfite is determined after distillation in the 2 Titrimetric Methods presence of mercury(II) chloride from hydrochloric acid medium. In a third sample the thiosulfate is determined 24 Thiosulfate by iodometric titration in the presence of cadmium Thiosulfate is oxidized to by iodine acetate and formaldehyde. according to the following equation: On the other hand, Ikeda and Satake12 have developed a method for a successive determination of thiosulfate, sulfide and sulfite by potentiometric titra- 2S2032+12 --~ S4062+21. (1) tion using a silver-silver sulfide electrode for sulfide and a silver-silver iodide electrode for thiosulfate and The iodometric method based on this reaction is the sulfite as the indicator electrodes. Both the sulfide and simplest and the best method for the determination of thiosulfate are titrated directly with a standard solution thiosulfate, because the above reaction proceeds to of silver nitrate, whereas sulfite is determined by stoichiometric completion in an acidic medium. The titrating the iodide generated from the reaction of the end-point of the reaction is usually detected visually sulfite with the added iodine. These three sulfur species using a starch solution. Other oxidizing agents such as in their mixtures can be determined directly by the bromine5, iodate6 and manganate' have been recom- three successive argentometric titrations without any mended for the oxidation of thiosulfate. For practical separation procedure. purposes only iodine is of prime importance for the Thiosulfate was also determined in a mixture containing determination of thiosulfate. sulfide, and sulfite by iodometric titration.13 2+1 Thiosulfate-sulfite or sulfide mixture The sample solution was first added to a suspension of The method8'9 using formaldehyde gives excellent freshly precipitated zinc carbonate. The zinc sulfide results for the analysis of thiosulfate-sulfite mixtures. precipitate formed was filtered off and the filtrate was Formaldehyde forms an additional compound with allowed to react with iodine in an acetic acid-acetate sulfite, which is stable toward iodine. An aliquot of the buffer solution; both dithionite and sulfite are oxidized sample solution is directly titrated with a standard to , whereas thiosulfate to tetrathionate. Then iodine solution and a further aliquot is titrated in the sulfite is added (a) to remove the unreacted iodine and same way after the addition of formaldehyde. The first (b) to sulfitolyze the tetrathionate formed. After titration gives the sum of thiosulfate and sulfite; the masking the excess sulfite by formaldehyde, the thio- second gives only the amount of thiosulfate. When sulfate formed by sulfitolysis of the tetrathionate is thiosulfate and sulfide are present together in a sample titrated with standard iodine. The number of mmol of solution, sulfide is expelled as H2S by bubbling the thiosulfate originally present is given by four times the solution with nitrogen gas after the addition of mmol of iodine used. hydrochloric acid; thiosulfate can then be determined iodometrically. In a second sample both anions are 2.2 Polythionates in mixtures titrated iodometrically. Individual polythionates are very similar in their 2.F2 Thiosulfate-sulfide-sulfite mixture chemical and physical properties. Therefore, a direct Kurtenacker and Wollak10 have proposed an iodo- determination of a polythionate in mixtures with other metric titration method for the determination of polythionates is not practicable. A large number of thiosulfate, sulfide and sulfite in a mixture, which has procedure equations which can be obtained through been extensively employed in the past. According to corresponding different procedures are necessary for the this method, the total amounts of these three are determination of each polythionate in mixtures. titrated iodometrically in one aliquot. After precipita- 221 Total polythionates in mixtures tion and filtration of zinc or cadmium sulfide, the sum Polythionates (SX062-: x=3, 4 and 5) react with of thiosulfate and sulfite can be titrated in one aliquot mercury(II) chloride to form 4 mol of hydronium ions of the filtrate. To another aliquot of the filtrate, which can be determined alkalimetrically.14 The formaldehyde is added in order to mask the sulfite, and stoichiometric equation is as follows: the thiosulfate is determined iodometrically. A poten- tial difficulty here is the coprecipitation of thiosulfate 2SxO62-+3HgC12+4H2O --~ and sulfite on the bulky zinc or cadmium sulfide HgC12.2HgS+8H++4CL+4SO42-+(2x-6)S. (2) precipitate. A precipitate of smaller surface area, and therefore causing less coprecipitation, is produced by A reaction of hexathionate with mercury(II) chloride adding a fresh suspension of zinc or cadmium carbonate has not yet been investigated. The proposed method is hydroxide. Wiele1' also proposed an iodometric readily applicable to various samples, and is not method for the analysis of a thiosulfate-sulfide-sulfite affected by the presence of weak acids. mixture in magnesium or calcium carbonate. In one A titrimetric method15 has been described for the aliquot sample the total amounts of three sulfur species determination of various forms of sulfur, in order to are determined by iodometric titration in an acid distinguish between non-oxidizable sulfate and the medium. In a new sample the sulfur dioxide (SO2) oxidizable thio salts including thiosulfate, dithionate, ANALYTICAL SCIENCES FEBRUARY 1990, VOL. 6 5 tri- and tetrathionate. Metal ions are removed, after an Thus, a standard iodine solution can be readily oxidizing step, by a cation-exchange resin, and the prepared whenever needed. A molar absorptivity of sulfate is precipitated with a known excess of barium triiodide at 350 nm is much higher than that of iodine chloride. The excess of barium is back-titrated with a in carbon tetrachloride at 515 nm.18 Hence, thiosulfate standard EDTA solution. The method consists of can be determined sensitively by measuring the ab- determining: (a) the initial sulfate plus the sulfate sorbance of excess iodine (as triiodide) for the reaction obtained by oxidizing sulfite with iodine, (b) the total of Eq. (1) at 350 nm. sulfate obtained by oxidizing the polythionates (including Thiosulfate reacts with cyanide in the presence of the tetrathionate produced by oxidation of thiosulfate copper(II) to form thiocyanate according to by iodine) with acidic hydrogen peroxide, and (c) the total sulfate obtained by oxidation of all thio salts S2032-+CN- ----, S032-+SCN-, (9) including dithionate with a mixture of hydrogen peroxide, potassium chlorate and concentrated nitric in which the thiocyanate can be determined photo- acid. This method is suitable for monitoring mining metrically as the iron(III)-thiocyanate complex at effluents for the total content of thio salts rather than 460 nm.19,2° A further investigation by Nor and the individual species. Tabatabai21 indicated that thiosulfate and tetrathionate 222 Individual polythionates in mixtures can be determined in the presence of very large Goehring et a1.16have reported an iodimetric method amounts of sulfate. Optimal conditions established for for the determination of individual polythionates in stoichiometric conversion of thiosulfate into thiocyanate mixtures of trithionate to hexathionate, in which the have been used for the determination of two species of mixtures were treated in separate aliquots with sulfite, polythionates in their mixtures.22 The sensitivity for the cyanide, sulfide and alkali. The thiosulfate produced is determination of thiosulfate could be increased by then titrated iodimetrically. The reactions of poly- about 60-fold23 by extracting the -pair formed thionates with sulfite, cyanide, sulfide and alkali are as between the cation of Methylene Blue (MB)24 and the follows: anion of thiocyanate formed from the thiosulfate. Lanthanum(III)25 and gold(III)26 (as tetrachloroaurate) SxO62+(x-3)SO32 -`' S3062 +(x-3)S2032- have also been found to catalyze the reaction of (x=4, 5 and 6) (3) thiosulfate with cyanide, and the optimum conditions for quantitative lanthanum(III)-catalyzed cyanolysis of SxO62-+(x-1)CN-+H2O --~ thiosulfate have been established. Thiosulfate was S,042-+(x-3)SCN-+2HCN+S2032- found to react with mercury(II) thiocyanate to yield (x=4, 5 and 6) (4) thiocyanate; this reaction was clarified with special reference to the determination of thiosulfate27, in which Sx062 +s2 ---~ (x-3)S+2S2032 the thiocyanate formed is photometrically measured. (x=3, 4, 5 and 6) (5) The reaction 2S6062-+60H- ---' 2S+3H2O+5S2032- (6) 4S2032-+6Hg(SCN)2+HP042-+ 16H20 Hg6S4(OH)2HP04+4SO42-+10H30++12SCN- (10) 2SxO62-+6OH- (10-2x)S032-+3H2O+(2x-5)S2032- proved to proceed to stoichiometric completion under (x=3, 4 and 5) (7) certain conditions. 311 Thiosulfate-sulfide-sulfite mixture These reactions were not all investigated in detail. Particularly when thiosulfate, sulfide and sulfite are Therefore, the method described by Goehring et al. mixed with one another in solution, the identification gave serious errors with a recovery of 56 -140%. and determination of these sulfur species are difficult, because they are labile and participate in complex equilibria with the other sulfur compounds formed by 3 Thiosulfate their auto- reactions. A method28 has been proposed for the analysis of this mixture, which is 34 Spectrophotometry based on their oxidation with iodine after proper Ozawa17 has proposed an excellent method for the chemical treatments, followed by a measurement of any determination of thiosulfate, which has been extensively excess iodine at 350 nm. In this study, the conditions of employed for the determination of polythionates. When stabilization of sulfide and sulfite mixed with each an acid is added to a mixture solution of iodate and other have been investigated in order to avoid any time- excess iodide, the iodine equivalent to the iodate is dependent change in the chemical composition of the produced according to the following equation: sample solution containing soluble sulfide, sulfite and thiosulfate. These three sulfur species in their mixture IO3-+8I-+6H+ 3I3-+3H2O. (8) give the following equivalents in three procedures (I- III) used. 6 ANALYTICAL SCIENCES FEBRUARY 1990, VOL. 6

[S2032] I=[S2032]+2[S2]+2[S032], thiosulfate was shaken with a solution of iodine in [S2032] II=[S2032]+2[S2], carbon tetrachloride. The organic phase was separated, the excess of iodine extracted into an aqueous sulfite and solution and the iodide liberated measured with an [S2032]III=[S2032 ]. iodide-selective electrode. The measured potential was converted directly to the concentration of iodide using The absorbance obtained by procedure I corresponds to two iodide standards approximating the concentration the sum of the amount of thiosulfate, twice that of of the unknown. It was possible to determine thio- sulfide and twice that of sulfite in the mixture. The sulfate in various concentration ranges by changing only absorbance obtained by procedure II, where sulfite was the concentration of the standard iodine in a carbon masked by formaldehyde, corresponds to the sum of tetrachloride solution. For example, this method is the amount of thiosulfate and twice that of sulfide. The applicable to the determination of thiosulfate in the absorbance obtained by procedure III corresponds only concentration range 4X107 - 8X106 M when a 10-ml to the amount of thiosulfate, because sulfite was aliquot of 1X105-M iodine in carbon tetrachloride is masked by formaldehyde and sulfide was eliminated used under the conditions specified in procedure, and in with the suspension of zinc carbonate hydroxide. The the concentration range 4X105 - 8X104 M when a 10- following equations can thus be obtained: [S2032-]=III, ml aliquot of 1X103.M iodine in carbon tetrachloride is [S2-]=(II_III)/2 and [S032-]=(I-II)/2. Here, I, II and used. A linear calibration graph was obtained over any III denote the molar concentrations of thiosulfate, concentration ranges, and thiosulfate was selectively determined using the absorbance obtained by procedures determined, even in the presence of sulfite and sulfide in I, II and III, respectively. The proposed method is 100-fold amounts. The proposed method can be applicable to the determination of the three sulfur expected to be used for the determination of poly- species mixed in amounts of more than 0.05 µmol and thionates at the 10-6 M level. gave a relative standard deviation of 3% at 0.4 µmol level. This method was applied to the determination of thiosulfate, sulfide and sulfite in various amounts in 4 Pure Individual Polythionates fumarolic condensate samples. The identification and determination of polythionates 3.2 Catalytic methods in their mixtures are difficult owing to similarities in The catalytic effect of thiosulfate, sulfide and thiocya- their chemical and physical properties. Hence, various nate on the azide-iodine reaction, reactions of individual polythionates should be investi- gated in detail in order to determine a specific 2N3-+I2 3N2+2I~, (11) polythionate in the presence of other polythionates. In addition, it is desirable to establish, if possible, the has been well known.29 Michalski and Wtorkowska3o conditions under which all the polythionates concerned have reported a catalytic method for the determination are simultaneously and stoichiometrically converted to of ultramicro amounts of thiosulfate, in which amper- thiosulfate and/ or thiocyanate. There are many ometry using a rotating platinum electrode was used to difficulties in the analysis of polythionate mixtures, but follow the reaction rate. Utsumi and Okutani31 the determination of individual polythionates in pure employed the same reaction for the determination of form is relatively easy. thiosulfate in the range 0.001- 0.15 ppm; in this case the excess iodine (as triiodide) was spectrophotometrically 4.1 Spectrophotometry measured at 350 nm. Iron(III), copper(II), sulfite and Many investigations have been carried out on the cyanide interfere. sulfitolysis [Eq. (3)], cyanolysis [Eq. (4)], sulfidolysis [Eq. (5)] and alkalinolysis [Eq. (7)] of polythionates for 3.3 Electroanalytical methods the determination of polythionates, in which the There is a need for secondary coulometric titration of thiocyanate and/ or thiosulfate produced from the thiosulfate since it reacts completely, rapidly and polythionates were spectrophotometrically measured. stoichiometrically with iodine. This requirement was 411 Cyanolysis met by the addition of excess iodide with a subsequent Nietzel and DeSesa34 employed the cyanolysis reac- titration of the electrolytically generated iodine by tion for the determination of tetrathionate, in which the thiosulfate; the end point was determined by amperom- thiocyanate formed was photometrically measured as etry.32 This coulometric procedure has been extended the iron(III)-thiocyanate complex at 460 nm. They to the determination of polythionates, in which the pointed out that their method might be suitable for thiosulfate produced from the polythionates was cou- pentathionate and hexathionate, because the higher lometrically titrated. polythionates react more easily with cyanide to form Recently, a method for the determination of thio- thiocyanate. However, both of penta- and hexathio- sulfate in the range 0.045-0.9 ppm using an iodide ion nate were found not to be stoichiometrically converted selective electrode has been proposed.33 Aqueous to thiocyanate under the conditions of the Nietzel- ANALYTICAL SCIENCES FEBRUARY 1990, VOL. 6 7

Fig. 2 Calibration graphs for thiocyanate, tetra-, penta- and hexathionate obtained by Methylene Blue method (reagent blank subtracted). •, SCN-; Q, S4062-; ®, S5062-; J, S6062-. Fig. 1 Calibration graphs for tetra-, penta- and hexathionate obtained by their cyanolysis. 0, reagent blank; Q, S4062-; ®, S5062; J, S6062. for thiocyanate when plotted as molar concentrations (Fig. 2). This proves that all three polythionates can be quantitatively converted to thiocyanate, even at the 10-6 DeSesa's method, owing to the partial alkaline decom- M level, according to Eq. (4). position. The reactions of pentathionate35 and hexathio- Urban39 and Kelly40 investigated the cyanolysis of nate36 with cyanide have been investigated in detail and trithionate and utilized this reaction for the determina- the conditions for the cyanolysis of penta- and hexathio- tion of trithionate. Urban's method has the disadvan- nate to go well to stoichiometric completion have been tage of requiring 16 h for the cyanolysis at room established. The optimum pH ranges for the cyanolysis temperature; even then, the cyanolysis was not quanti- of penta- and hexathionate are, respectively, 8.0 - 8.8 tative. Kelly attempted to complete the cyanolysis of and 7.8 - 8.9 when cyanolyzed for 30 min at 40° C. On trithionate as fast as possible; the cyanolysis was carried the other hand, tetrathionate37 proved to be completely out at pH 9.6 for 45 min at a temperature of boiling converted to thiocyanate over the pH range 8.3 -12.1 . Under these conditions, only 87% of trithionate under the same conditions as those for both penta- and was converted into thiocyanate and the rest into hexathionate. Hence, when these three polythionates thiosulfate. Accordingly, these methods can not be to were cyanolyzed in the solution buffered to the pH be considered accurate. Lanthanum(III)41 has been range 8.3 - 8.8 for 30 min at 40° C, the calibration found to have a catalytic effect on the conversion of graphs obtained for tetra-, penta- and hexathionate trithionate into thiocyanate, and a rapid and accurate were, as can be seen in Fig. 1, in good agreement with method for the determination of trithionate has been one another, respectively, in which the molar concentra- developed. This method is based on the formation of tion scales for penta- and hexathionate were drawn to thiocyanate equivalent to trithionate in the presence of twice and three times the concentration scale for lanthanum(III) according to tetrathionate. This indicates that all three of these polythionates could be completely converted to thio- S3062-+3CN-+H2O cyanate according to Eq. (4) under the identical con- S 032-+S 042-+2 H CN+S CN-, (12) ditions. In addition, it was shown that the sensitivity for the determination of polythionates (SxO62-:x=4, 5 and on the photometric measurement of the thio- and 6) can be increased approximately 60-fold by cyanate thus formed as the iron(III)-thiocyanate com- extracting the thiocyanate formed as an ion pair with plex at 460 nm. Complete cyanolysis was obtained in Methylene Blue into an organic solvent.38 The optimal 20 min at 10° C over the pH range 9.3 -9.6. pH ranges for the cyanolysis of polythionates at the 10-6 44.2 Sulfitolysis and sulfidolysis M level were, respectively, 7.2 -10.8 for tetrathionate Iwasaki and Suzuki42 proposed a method for the and 7.2 - 7.6 for both of penta- and hexathionate when determination of tetrathionate which is based on the cyanolyzed at 40°C for 4.5 h. The calibration graphs sulfitolysis of tetrathionate [Eq. (3)], followed by a obtained for tetra-, penta- and hexathionate in the photometric measurement of any excess iodine (as solutions buffered to pH range 7.2 - 7.6 are, respec- triiodide) for the thiosulfate formed at 350 nm. tively, once, twice and three times as sensitive as that Pentathionate43 and hexathionate44 were stoichiomet- 8 ANALYTICAL SCIENCES FEBRUARY 1990, VOL. 6

quantitatively converted into thiosulfate over the pH range 5.1- 7.0 when sulfidolyzed at 20° C for 10 min. There was no need for pentathionate to be sulfidolyzed at lower temperatures, such as 10°C, because the optimum pH for the stoichiometric sulfidolysis is lower than that for tetrathionate. 44.3 Other reactions It has been stated that tetrathionate reacts with hydroxide according to Eq. (13) in a weak alkaline medium and according to Eq. (14) in a strong alkaline solution:48,49 4S4062-+6OH- ----~ 5S2032-+253062-+3H2O (13) 2SxO62-+6OW ----~ (2x-5)S2032~+(10-2x)S032-+3 H2O (x=3, 4 and 5) . (14) Miura and Koh proposed methods for the determination of tetrathionateSO and pentathionate51, in which the excess iodine for the thiosulfate formed by the reaction Fig. 3 Calibration graphs for tetra-, penta- and hexathionate of the polythionates with hydroxide was photometrically obtained by their sulfitolysis. 0, reagent blank; Q, S4062-; measured as triiodide at 350 nm. It was impossible to ®, S5062; ~, S6062. find the conditions under which the reaction of the polythionates with hydroxide proceeds according to Eq. (13) or (14); both tetra- and pentathionate were rically converted to thiosulfate in the pH ranges found to be converted not only to thiosulfate and 6.5 - 8.2 and 6.9 - 9.5, respectively, when sulfitolyzed for sulfite, but also to sulfide. Under the optimum con- 20 min at room temperature. The sulfitolysis of ditions for pentathionate in which 2 mol of thiosulfate tetrathionate45 was further investigated ii detail and are produced from 1 mol of pentathionate, the reaction tetrathionate proved to be completely converted into of tetrathionate with hydroxide proceeded according to thiosulfate over the pH range 7.7 -12.5 under the same the following equation: conditions as those for both penta- and hexathionate. Therefore, the optimal pH range for all three of these 1054062-+340H- ---~ polythionates was found to be 7.7 - 8.2 when sulfi- 13S2032-+38/3SO32-+4/3S2~+17H2O. (15) tolyzed for 20 min at room temperature. The calibra- tion graphs obtained for tetra-, penta- and hexathionate Recently, sensitive methods for penta-52 and hexathio- were, as can be seen in Fig. 3, respectively, in good nate53 have been published. The procedure involves agreement with one another when plotted in terms of oxidation of the polythionates with a given amount of equivalent concentrations. permanganate in sulfuric acid medium, followed by Koh et al. investigated the sulfidolysis of polythio- spectrophotometric determination of the triiodide nates in detail and proposed methods for the determina- formed by the oxidation of iodide with an excess of tion of tetra-46 and pentathionate47 which are based on permanganate. The analytical conditions were esta- the formation of thiosulfate equivalent to the poly- blished by varying the temperature, reaction time and thionates, followed by a measurement of the excess amounts of sulfuric acid and permanganate. It has iodine (triiodide) for the thiosulfate formed. The been confirmed that under the identical conditions, conditions under which tetra- and pentathionate are each 2 mol of pentathionate and hexathionate react stoichiometrically converted into thiosulfate according completely with 7 and 9 mol of permanganate, respec- to Eq. (5) were established by varying the reaction time, tively. The proposed method is applicable to the pH and amount of sulfide. The excess of sulfide used determination of pentathionate in the range 2X107 - for the sulfidolysis of the polythionates was removed 1.4X 10-5M and hexathionate in the range 1.8X 10-7- with a suspension of freshly precipitated zinc carbonate 1.1X10-5M. hydroxide. The polysulfide formed by the reaction of Trithionate54 was found to react with mercury(II) sulfide with the free sulfur produced by the sulfidolysis thiocyanate to yield thiocyanate according to the of polythionates might readily undergo air-oxidation to following equation: form thiosulfate at both higher temperatures and higher pH levels. Tetrathionate was completely converted to 453062-+6Hg(SCN)2+HPO42-+28H2O ---~ thiosulfate according to Eq. (5) over the pH range of Hg6S4(OH)2HP04+8SO42-+18H30++12SCN-. (16) 7.0 - 9.5 when allowed to react with sulfide at 10° C for 20 min. On the other hand, pentathionate was A detailed examination of this reaction led to the ANALYTICAL SCIENCES FEBRUARY 1990, VOL. 6 9

development of an accurate method for the determina- 511 Two-polythionates (SxO62-: x=4, 5 and 6) mix- tion of trithionate, in which the thiocyanate liberated is tures photometrically measured as the iron(III)-thiocyanate As can be seen in Fig, 1, when polythionates (SxO62-: complex at 460 nm. The optimum conditions under x=4, 5 and 6) are cyanolyzed in the solutions buffered which the reaction of trithionate proceeds to stoichio- at the pH range 8.3 - 8.8 for 30 min at 40° C, all of their metric completion according to Eq. (16) have been reactions proceed to stoichiometric completion accor- established with special reference to the determination ding to Eq. (4).37 The optimal copper(II)-catalyzed of trithionate. cyanolysis conditions of the thiosulfate formed from the polythionates have been established by adding copper- 4.2 Electroanalytical methods (II) to the reaction mixture in which the reaction of Tetrathionate is reducible at the dropping-mercury Eq. (4) was completed. Accordingly the overall reac- electrode. In a supporting electrolyte solution of tion is as follows:22 potassium nitrate55, the polarogram showed the peak of a pronounced polarographic maximum. This peak SxO62-+xCN~+H2O ---' could be suppressed by the addition of gelatin or S 042-+(x-2) S CN-+2 H CN+5 032-. certain dyes, but quinoline and acridine are especially (x=4, 5 and 6) (19) suitable as maximum suppressors. For the determina- tion of tetrathionate, polarograms were best recorded in The proposed method for two-species of polythionates 1 M solutions of phosphoric acid or of the ammonium in their mixtures consists in a determination of different phosphate at pH range 0.93-8.15 and with 0.001% amounts of the thiocyanate formed according to quinoline. The half-wave potential is independent of Eqs. (4) and (19) on two 10-m1 aliquots of sample the pH value of 1 M phosphate supporting electrolyte, solution containing two polythionates; one is treated by but it becomes more negative as the concentration of procedure I and the other by procedure II. For the tetrathionate increases. The proposed method is tetrathionate-pentathionate mixture, the two species of applicable to the determination of tetrathionate in the the polythionates give the following equivalents in the presence of dithionate, trithionate, sulfite or thiosulfate, two procedures: but pentathionate interferes. Murayama56 reported a polarography for tri- and tetrathionate in a supporting [SCN ]1=[S4062]+2[S5062] electrolyte solution of potassium chloride or calcium and chloride. The reduction of tri- and tetrathionate at the dropping-mercury electrode is irreversible and consi- [SCN ]11=2[S4062]+3[S5062]. dered to be as follows: Hence, the following equations can be obtained: [S4062-] S3062 +2e --' S2032 +S032 (17) =2II-3I and [S5062-]=21-II . Here, I and II denote the S4062-+2H2O+2e 4HS02~. (18) molar concentrations of thiocyanate determined by procedures I and II, respectively. Similarly, for the An alternating current polarography57 also showed the tetrathionate-hexathionate mixture [54062-]=(3II-4I)/ 2 determination of pure polythionates, but not of their and [S6062i=(2I-II)/2, and for the pentathionate- mixtures because of the small differences in their hexathionate mixture [S5062-]=3II-4I and [56062-] potentials. =3I-2II . By using these equations, tetrathionate- A coulometric method for the determination of pentathionate, tetrathionate-hexathionate and pentathio- polythionates has been described by Blasius and nate-hexathionate mixtures can be analyzed. The Munch.S8 This method involves sulfitolysis of the sensitivity for the determination of two species of polythionates followed by coulometric titration of the polythionates in their mixtures could be increased thiosulfate formed with electrolytically generated io- about 60-fold by extracting the thiocyanate formed into dine. Procedures are also described for the indirect an organic solvent as an ion pair with Methylene determination of tetrathionate by its cyanolysis. Blue.59 The proposed method is suitable for the determination of polythionates at the 10-7-10.6 M level and is more sensitive than any photometric method. 5 Polythionates and Other Sulfur Anions in The values of x in the formula, S062, for aqueous Mixtures polythionates have been evaluated by the determination of different amounts of the thiocyanate formed accord- 5.1 Spectrophotometry ing to Eqs. (4) and (19).22,59 Suppose that I and II Since various decomposition reactions of polythio- denote the molar concentrations of thiocyanate deter- nates such as cyanolysis, sulfitolysis and sulfidolysis mined by the respective procedures I and II. Then, have been investigated in detail, it is possible to I/ II=(x-3)/ (x-2), where x is the number of sulfur determine individual polythionates in their mixtures by atoms in the formula for Sx062-. When arranged with solving the simultaneous equations which can be respect to x, the equation x=I/(II-I)+3 can be obtained through corresponding different procedures. obtained. By using this equation, values of x were 10 ANALYTICAL SCIENCES FEBRUARY 1990, VOL. 6

Table 1 Evaluation of x in SO 62 for aqueous polythionates lyzed, corresponds to the sum of the amount of thiosulfate and twice that of sulfite in the mixture. The following equations can therefore be obtained: [SxO62-] =I-II , [S2032]=II and [S032-]=(III-II)/2. Here, I, II and III denote the molar concentrations of thiosulfate determined by the procedures I, II and III, respectively. This technique can be successfully applied to the determination of total polythionates, thiosulfate and sulfite mixed in various ratios. 54.3 Polythionate(SXO62-: x=4, 5 or 6)-thiosulfate- sulfite mixture Sulfitolysis of tetra-45, penta-43 and hexathionate44 was used for the determination of polythionate (SxO62-: x=4, 5 or 6), thiosulfate and sulfite mixed in various ratios. For the tetrathionate-thiosulfate-sulfite mix- ture, they give the following equivalents in three procedures: determined for aqueous solutions of pure polythionates. [S2032 ]I=[S4062]+[S2032], The results are shown in Table 1. Urban60 deduced that [S2032 ]11-[S2032 ], aqueous polythionates, with the exception of tetrathio- nate, are present as mixtures containing other polythio- and nates from tetrathionate upwards. However, his [S2032 ]III-[S2O32 ]+2[SO32~]. conclusion is not acceptable; the lower values of x obtained under Urban's conditions were caused by a The absorbance obtained by procedure I corresponds to partial alkaline decomposition of the polythionates the sum of the amount of tetrathionate and that of accompanied with hydrolysis of the cyanide used. thiosulfate in the mixture, because the sulfite in the Table 1 shows that tetra-, penta- and hexathionate, mixture was masked by the formaldehyde added for respectively, are present as pure polythionates even at removing the excess sulfite. The absorbance obtained 10-6M levels. by procedure II, in which sulfitolysis of tetrathionate 5.F2 Total polythionates (SxO62-: x=4, 5 and 6)-thio- was not carried out, corresponds to the amount of sulfate-sulfite mixture thiosulfate in the mixture. The absorbance obtained by When polythionates undergo cyanolysis, the reaction procedure III corresponds to the sum of the amount of of Eq. (4) proceeds to stoichiometric completion; each thiosulfate and twice that of sulfite in the mixture. 1 mol of the polythionates produces 1 mol of thio- Hence, the following equations can be derived: [S4062-] sulfate.61 Total polythionates (tetra-, penta- and =I-II , [S2032-]=II and [S032-]=(III-II)/ 2, in which I, hexathionate) were determined by photometrically II and III denote the molar concentrations of thio- measuring the excess iodine for the thiosulfate formed sulfate determined by the procedures I, II and III, from the polythionates after masking the excess of respectively. Similarly, for the pentathionate-thiosulfate- cyanide with formaldehyde. Total polythionates, sulfite mixture [55062-]=(I-II)/2, [S2032]=II and thiosulfate and sulfite in a mixture give the following [5032-]=(III-II)/2, and for the hexathionate-thio- equivalents in three procedures: sulfate-sulfite mixture [56062-]=(I-II)/ 3, [52032-]=I1 and [5032]=(III-II)/2. By using these equations, [S2032]1 - [SxO62]+[S2O32 ]~ tetrathionate-thiosulfate-sulfite, pentathionate-thio- [S2032 ]II=[S2032], sulfate-sulfite and hexathionate-thiosulfate-sulfite mix- tures can be analyzed. and As can be seen in Fig. 3, the calibration graphs for [S2032 ]III-'[S2032 ]+2[S032 ]. tetra-, penta- and hexathionate, obtained under the identical procedure, were in good agreement with one The absorbance obtained by procedure I corresponds to another, respectively. Hence, the sulfitolysis of poly- the sum of the amount of total polythionates and that thionates has played, and will play further, a critical of thiosulfate in the mixture, sulfite being masked by role in the development of methods for the analysis of the formaldehyde added for removing the excess of mixtures of the polythionates. cyanide. The absorbance obtained by procedure II, 5.1.4 Polythionate (SXO62-:x=4 or 5)-thiosulfate-sulfide where cyanolysis of polythionates was not carried out mixture and the sulfite in the mixture was masked with The mixture of tetrathionate (or pentathionate), formaldehyde, corresponds only to the amount of thiosulfate and sulfide was analyzed by using sulf- thiosulfate in the mixture. The absorbance obtained by idolysis of the polythionates. For the tetrathionate- procedure III, where polythionates were not cyano- thiosulfate-sulfide mixture46, these three sulfur species ANALYTICAL SCIENCES FEBRUARY 1990, VOL. 6 11 in the mixture give the following equivalents in three procedure II, is depressed to 0.7% by the addition of procedures: organic solvents such as acetone and methanol. When "[SCN-] 1=[52032]" is used, the amounts of the three [S2032]1- 2[S4062]+[S2032], sulfur species in their mixture are given by the [S2032 ]II-'[S2032], following equations: [52032-]=I, [53062-]=III-II and and [54062-]=(II-I)/2. Here, I, II and III denote the molar concentrations of thiocyanate determined using the [S2032]III[S2032 ]+2[S2 ]. absorbance obtained by procedures I, II and III, respectively. This technique was applied to the deter- The absorbance obtained by procedure I corresponds to mination of thiosulfate, trithionate and tetrathionate in the sum of the amount of thiosulfate and twice that of flotation mill solutions.63 However, it is doubtful, as tetrathionate in the mixture, the sulfide in the mixture stated before41, whether the cyanolysis of both tri- and being removed by the zinc carbonate hydroxide suspen- tetrathionate proceeds to stoichiometric completion sion. The absorbance obtained by procedure II, in under the conditions in the procedure III4°,62,because which the sulfidolysis of tetrathionate was not carried they were cyanolyzed at a high temperature of boiling out, corresponds only to the amount of thiosulfate in water for 30 - 45 min. the mixture. The absorbance obtained by procedure III 5.F6 Tetrathionate-thiosulfate-sulfite-trithionate mix- corresponds to the sum of the amount of thiosulfate ture and twice that of sulfide in the mixture. Hence, the The proposed method64 for the determination of following equations can be obtained: [S4062-]=(I-II)/2, tetrathionate, thiosulfate, sulfite and trithionate in a [S2032]=II and [S2-]=(III-II)/ 2, where I, II and III mixture consists of four procedures; the excess iodine denote the molar concentrations of thiosulfate, deter- for thiosulfate and/ or sulfite under procedures I, II and mined using the absorbance obtained by procedures I, III, and the thiocyanate formed under procedure IV are II and III, respectively. Similarly, for the pentathio- measured spectrophotometrically after proper chemical nate-thiosulfate-sulfide mixture47, [55062-]=(I-II)/2, treatments. These four sulfur compounds in their [52032]=II and [52-](III-II)/ 2 were obtained. mixture give the following equivalents in the four The optimum conditions for the sulfidolysis of procedures: tetrathionate is different from those for pentathionate. In addition sulfidolysis of hexathionate has not yet [52032_]I=[S4O62]'[52032 ], been investigated. It is desirable for the conditions, [52032 ]II-[S2032], under which all three polythionates of tetra-, penta- and [520321111 - [S20321+2[520321, hexathionate are simultaneously and stoichiometrically converted into thiosulfate according to Eq. (5), to be and established, for the analysis of mixtures of various [SCN ]Iv-2[S4062_]+[52032_]+[53062_]. polythionates. 5.1.5 Thoosulfate-trithionate-tetrathionate mixture Under procedure I, the thiosulfate formed by sulfi- Kelly et a1.40 have proposed a method for the tolysis of tetrathionate reacts with iodine. Therefore, determination of thiosulfate, trithionate and tetrathio- the absorbance obtained by procedure I corresponds to nate in a mixture using their cyanolysis. This method the sum of the amount of tetrathionate and that of consists in the determination of different amounts of thiosulf ate in the mixture [see Eqs. (1) and (3), and the thiocyanate formed by three procedures on three Fig. (3)]. The absorbance obtained by procedure II, in 10-ml aliquots containing thiosulfate, trithionate and which the sulfitolysis of tetrathionate was not carried tetrathionate. These three sulfur compounds in their out and the sulfite in the mixture was masked by mixture give the following equivalents in the three formaldehyde, corresponds only to the amount of procedures: thiosulfate in the mixture. The absorbance obtained by procedure III, in which neither the sulfitolysis of [SCN ]1=[S4062 ], tetrathionate nor the masking of sulfite was carried out, [SCN ]II 2[54062]+[52032], corresponds to the sum of the amount of thiosulfate and twice that of sulfite in the mixture. The and absorbance obtained by procedure IV, in which tetra- [SCN ]III-2[54062]+[52032]+[53062]. thionate was converted into both thiosulfate and trithionate according to Eq. (3), followed by their The results obtained by the proposed method give conversion into thiocyanate by the cyanolysis, corre- serious errors with a recovery ranging from 80 to 90%. sponds to the sum of the amount of thiosulfate [Eq. (9)], Mizoguchi and Okabe62 investigated the cyanolysis that of trithionate [Eq. (12)] and twice that of tetra- conditions and reported an improved method in which thionate [Eq. (20) and Fig. (4)] in the mixture. Hence, the amount of thiosulfate in the mixture was directly the following equations can be obtained for the four determined by procedure I. The cyanolysis of trithio- sulfur compounds in their mixture: [S4062]=1-II, nate, which can not be ignored under the conditions of [S2032-]=II, [S032-]=(III-II)/ 2 and [53062]=IV-2I+ 12 ANALYTICAL SCIENCES FEBRUARY 1990, VOL. 6

II. Here, I, II and III indicate the molar concentrations such as trithionate-tetrathionate-pentathionate-hexa- of thiosulfate determined using the absorbance obtained thionate-thiosulfate-sulfite-sulfide mixture, can be ex- by procedures I, II and III, respectively, and IV denotes pected to be available provided that, in addition to the molar concentrations of thiocyanate determined by sulfitolysis and cyanolysis of polythionates, sulfidolysis procedure IV. and other chemical reactions of the polythionates are The thiosulfate and trithionate formed by the used as additional procedures. sulfitolysis of tetrathionate were quantitatively con- verted into thiocyanate over the pH ranges 9.3 -9.6 and 5.2 Electroanalyticalmethods 8.9 - 9.6, respectively, when cyanolyzed for 1.5 h25 at The polarographic behavior of tri-, tetra- and 15°C.41 Consequently, both of the thiosulfate and pentathionate has been investigated using potassium trithionate formed from tetrathionate and those in the chloride, potassium iodide and hydrochloric acid in the mixture were stoichiometrically converted into thio- base solutions.65 The half-wave potentials vs. SCE cyanate over the pH range 9.3 -9.6. The following obtained are, respectively, -1.34 V for trithionate in a overall reaction for tetrathionate therefore can be medium of 1 M potassium chloride in 50% alcohol, -0 obtained under the conditions in procedure IV: .285 V for tetrathionate in 1 M hydrochloric acid and -0.3 V in 1 M potassium chloride-0.01% gelatin, S4062-+4CN-+H20 and -0.211 V for pentathionate in 2 M hydrochloric 5032-+SO42-+2HCN+2SCN-. (20) acid. The tetrathionate in a mixture of tri- and tetrathionates can be determined by using 1 M hydro- The calibration graph obtained for tetrathionate by chloric acid as the supporting electrolyte. The total measuring the thiocyanate formed from tetrathionate, amounts of tri- and tetrathionate are then determined shown in Fig. 4, coincided with that for thiocyanate by using a polarographic medium of 1 M potassium when the molar concentration scale for tetrathionate is chloride-0.0l% gelatin-50% alcohol; in this base solu- drawn to twice the scale for the thiocyanate concentra- tion the reduction waves of the two polythionates tion. Figure 4 confirms that tetrathionate was com- overlap. Pentathionate can be determined in the pletely converted into thiocyanate according to Eq. (20) presence of tetrathionate only when polarographed in and further that the thiosulfate and trithionate in the 2 M hydrochloric acid in 50% alcohol; the tetrathionate mixture were converted into thiocyanate according to does not give any reduction process in this medium. Eqs. (9) and (12), respectively. Trithionate could not be determined in the presence of Of course, the proposed method is applicable not pentathionate since the polarogram of the latter shows only to a mixture of trithionate and tetrathionate, but a depression at about -1.15 V vs. SCE in alcoholic also to a mixture of thiosulfate, trithionate and potassium chloride solutions. tetrathionate. [S3062-]=IV-2I and [S4062-]=I can be On the other hand, the mixture containing tetra-, obtained for the trithionate-tetrathionate mixture, and penta- and hexathionate was analyzed by coupling thin- [S2032]=II, [S3O62]=IV-2I+II and [S4062]1-II for layer high-voltage electrophoresis with coulometric the thiosulfate-trithionate-tetrathionate mixture. Ana- titration or ultraviolet spectrophotometry.66 lytical methods for mixtures of six to seven sulfur species, 5.3 Chromatographic separations 5.3.1 Classicalanion-exchange chromatography Di-, tri-, tetra- and pentathionate in their mixture, as well as the constituents of Wackenroder's solution, were separated by using Dowex 1-X2 in the chloride form of 50 to 100mesh.67 Dithionate is first eluted with 1 M hydrochloric acid; at this stage any oxyanions of sulfur other than polythionates are eluted out before the dithionate. Then trithionate is eluted with 3 M HCI, tetrathionate with 6 M HCI, and finally pentathionate with 9 M H:CI. Pollard et al.68began the elution first with potassium hydrogen phthalate and then with hydrochloric acid, and achieved the separation of the polythionates up to pentathionate, thiosulfate and sulfite. In addition, mixtures of S2032--S032--S3062- and S3062-- S4062-- 55062-could be separated without difficulty using sodium chloride or hydrochloric acid as eluents.69 However, the separation of hexathionate was not possible due to its decomposition on the column. Fig. 4 Calibration graphs for tetrathionate and thiocyanate 5.3.2 Modern liquid chromatography obtained by sulfitolysis-cyanolysis procedure. 0, reagent A number of publications dealing with the liquid blank; Q, S4062 ®, SCN-. chromatography of polythionates have appeared and ANALYTICAL SCIENCES FEBRUARY 1990, VOL. 6 13 quite different chromatographic conditions have been Table 2 Retention times, capacity factors and chromato- used for the separation of the polythionates. Chapman graphic data for thiosulfate and polythionates and Beard70 used an activated carbon column with ultraviolet absorption detection at 254 nm for the separation and determination of polythionates and thiosulfate in Wackenroder's solution. However, difficulty in preparing reproducible columns of ac- tivated carbon has been encountered and the sensitivity for trithionate is quite low, leading at times to problem with detection. Wolkoff and Larose71 suggested a Eluent, 5.OX10-3 M TPABr in 8% CH3CN/ H2O; flow rate, method using a commercially available anion-exchange 2.42 ml/min. column and applied it to the determination of the polythionates and thiosulfate in mining wastewaters and environmental samples. The detection system employed herein involves reaction of the polythionates in the column effluent with base and then oxidation of both the thiosulfate and sulfite formed with cerium(IV) with subsequent fluorometric detection of cerium(III). The relative molar responses obtained for polythionates were not close to the theoretical values probably because the alkaline decomposition of the polythionates did not go to stoichiometric completion. Story72 also employed anion-exchange liquid chromatography to separate polythionates. His technique requires a postcolumn reaction for the polythionates to be detectable; the polythionate anions in the column effluent are oxidized with bromine and the resulting sulfate is measured as FeSO4+ at 335 nm after addition Fig. 5 Separation of thiosulfate, tri-, tetra-, penta- and 2 4 of iron(III) to the effluent stream. Takano et al.73 used hexathionate. 1, 52032 , 2, 53062 , 3, 5406 , 55062 a differential pulse polarographic technique to deter- 5, 56062 . mine the polythionates and thiosulfate in the anion- exchange column effluent. However, they failed to separate tri- and tetrathionate chromatographically. A sample aliquot was therefore sulfitolyzed45 prior to a absorbing quaternary ammonium ion-pair reagents. high performance liquid chromatographic (HPLC) Steudel and Holdt76 separated polythionates in their separation, followed by a subsequent polarographic mixture by ion-pair chromatography on a Dionex measurement of the resulting thiosulfate and trithio- MPIC-NS 1 column with 254-nm UV detection using an nate. On the other hand, retention times in anion- eluent containing 27% acetonitrile, 4% methanol, 69% exchange liquid chromatography for polythionates water, 0.001 M sodium carbonate and 0.002 M tetra- (SXO62-: x=3, 4, 5 and 6) could be fairly readily butylammonium hydroxide. The chromatographic predicted using a plot of log retention time vs. x, which separation of thiosulfate and the polythionates with up gives a straight line.74 to six sulfur atoms is shown in Fig. 5. In addition, they Though reversed-phase ion-pair chromatography has prepared a mixture containing other higher polythio- proven to be a valuable technique for the separation of nate anions (x>6) by treating a mixture of dichloro- organic solute, this technique had not been widely sulfanes SXCl2(x=1-7) with thiosulfate according to utilized for inorganic ions. This can be attributed to the following equation: the poor detectability of most inorganic ions by the 254-nm ultraviolet detector prevalent in HPLC systems. 2Na2S2O3+SXCl2+2HC1 Ion-pair chromatography75 on a Hamilton Resin PRP- H2S4+X06+4NaCl. (21) 1 reversed phase column with conductivity detection made possible the separation of polythionates in The peak identification in the chromatograms obtained mixtures with thiosulfate. Optimization conditions for the above polythionate mixtures, however, causes include the use of tetrapropylammonium bromide problems since pure or almost pure polythionates (TPABr) as an ion-pair reagent. Retention was higher than hexathionate are not available. dependent on the nature of both the cation and the anion of the ion-pair reagent. Results of a typical References chromatogram of a mixture of thiosulfate, tri-, tetra- and pentathionate are presented in Table 2. A 1. E. Weitz, F. Becker, K. Gieles and B. Alt, Chem. Ber., 89, convenient alternative has been offered by using UV- 2353 (1956). 14 ANALYTICAL SCIENCES FEBRUARY 1990, VOL. 6

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Acta, 71, (Received August 4, 1989) 367 (1974). (Accepted November 4, 1989)