Determination of Tetrathionate, Thiosulfate, Sulfite and Trithionate in Their Mixtures by Spectrophotometry

ANALYTICAL SCIENCES FEBRUARY 1989, VOL. 5 79 Determination of Tetrathionate, Thiosulfate, Sulfite and Trithionate in Their Mixtures by Spectrophotometry Tomozo KOH, Yasuyuki MIURA, Masahiro IsHIMORI and Norihito YAMAMURO Department of Chemistry, Faculty of Science, Tokai University, Hiratsuka, Kanagawa 259-12, Japan The proposed method consists of four procedures; excess iodine for reactions with thiosulfate and/or sulfite under Procedures I, II and III, and the thiocyanate formed under Procedure IV are measured spectrophotometrically after proper chemical treatments. The absorbance obtained by Procedure I corresponds to the sum of the amount of tetrathionate and that of thiosulfate in the mixture. The absorbance obtained by Procedure II corresponds only to the amount of thiosulfate in the mixture. The absorbance obtained by Procedure III corresponds to the sum of the amount of thiosulfate and twice that of sulfite in the mixture. The absorbance obtained by Procedure IV corresponds to the sum of the amount of both thiosulfate and trithionate and twice that of tetrathionate in the mixture. The proposed method was applied to the determination of tetrathionate, thiosulfate, sulfite and trithionate mixed in various ratios in amounts of more than 0.05 µmol with an error below ±0.02 µmol. Keywords Polythionates determination, tetrathionate sulfitolysis, trithionate cyanolysis, tetrathionate-trithionate- thiosulfate-sulfite mixture, spectrophotometry The determination of various sulfur anion species in of four sulfur species of tetrathionate, thiosulfate, their mixtures is desirable for interpreting their redox sulfite and trithionate in a mixture. chemistry in aqueous solution systems. However, this is difficult owing to similarities in their chemical and physical properties. Spectrophotometric methods"2 for Experimental the determination of trithionate, thiosulfate and tetrathionate in a mixture, based on the formation of Reagents and apparatus thiocyanate by their cyanolysis, have been proposed. In All chemicals used, except tetrathionate and trithio- an experiment the cyanolysis of trithionate was carried nate, were of analytical grade and used without further out at pH 9.6 for 30 min at a temperature of boiling purification. The water used in these experiments was water. Under these conditions, only 87% of trithionate redistilled. Potassium tetrathionate was prepared was converted into thiocyanate, the rest into thio- according to a procedure described by Stamm et al.16 sulfate. Accordingly, these methods can not be con- The obtained tetrathionate was further recrystallized sidered accurate. Recently, a number of publications with water at temperatures below 60° C and then dried dealing with high performance liquid chromatography at room temperature before storage at -10±2°C. A (HPLC) for the separation of mixtures of polythionates standard tetrathionate solution (1X103-M) was pre- have appeared.3-10 However, no consideration has been pared by dissolving 151.3 mg of the potassium tetra- given to the determination of tetrathionate, thiosulfate, thionate (water content: 0.07%) in water and diluting to sulfite and trithionate in a mixture. 500 ml. The concentration of the tetrathionate solution The present authors have developed methods for the was checked by the sulfitolysis method" and the determination of tetrathionate 11 pentathionate12 and solution stored at 5±2° C. A stock solution of hexathionate13 by means of their sulfitolysis. On the thiosulfate was prepared by dissolving a known amount other hand, lanthanum(III) has been found to have a of sodium thiosulfate pentahydrate in oxygen-free catalytic effect on the conversion of trithionate14 and water containing a small amount of sodium carbonate thiosulfate15 into thiocyanate. In this work, the as a stabilizer. It was standardized by iodometryl' one conditions under which both thiosulfate and trithionate week after preparation. Working standard thiosulfate are simultaneously and completely converted into thio- solutions were prepared by suitable dilution with cyanate were established. This paper is concerned with oxygen-free water. These standards were used to the sulfitolysis of tetrathionate and the cyanolysis of ascertain the stoichiometry and completion of the both thiosulfate and trithionate for the determination sulfitolysis of tetrathionate. A sulfite solution was 80 ANALYTICAL SCIENCES FEBRUARY 1989, VOL. 5 prepared by dissolving sodium hydrogen sulfite in 2 ml of the 0.5 M formaldehyde solution and 1.5 ml of oxygen-free water and standardized by iodometry.17 the acetate buffer solution (pH 3.5) in a 25-ml volu- Working standards were obtained by suitable dilutions metric flask. Then allow the mixture to stand for 5 min with oxygen-free water. A 0.15 M sulfite solution, to completely mask any sulfite in the sample. which was not standardized, proved to be useful for the Procedure III. For total amounts ofthiosulfate and sulfite sulfitolysis of tetrathionate for 2 weeks after its in the mixture. Pipette a l0-ml aliquot of the sample preparation. A thiocyanate solution was standardized solution into a 25-m1 volumetric flask. by the Volhard method18, and working standards were To these three mixture solutions from Procedures I, prepared by suitable dilution. These standards were II and III, add 2 ml of 5 M acetic acid and 2.4 ml of the used to ascertain the stoichiometry of the cyanolysis of standard 1.OOX10-3 N iodate in a 0.87 M iodide solu- thiosulfate and trithionate. Potassium trithionate was tion; then dilute to the mark with water. Mix the prepared according to a procedure described by Stamm contents of the flask well and measure the absorbance et al.16 The obtained trithionate was recrystallized with of the solution against water at 350 nm of maximum water at a temperature below 35° C and then dried at absorption for triiodide ion. room temperature before storage at --10±2°C. A Procedure IV. For total amounts of tetrathionate, thiosul- trithionate solution was standardized against a standard fate and trithionate in the mixture. Pipette 1.8 ml of thiocyanate solution by the cyanolysis method;14 the the carbonate buffer solution (pH 9.9), 2 ml of the solution was stored at 5±2° C. A standard iodate- 0.15 M sulfite solution and a 10-m1 aliquot of the iodide solution was prepared by adding 50.0 ml of sample solution into a 50-m1 volumetric flask. Allow 1.OOX10-2 N (=1.67X 10-3M) standard iodate to a the mixture to stand for 20 min in order to sulfitolyze solution containing 0.2 g of sodium carbonate and the tetrathionate completely. Then add 0.6 ml of 0.5 M 72.6 g of potassium iodide, and diluting it to 500 ml to acetic acid and 2 ml of 0.5 M formaldehyde to the give a 1.OOX10-3 N iodate in a 0.87 M iodide solution. reaction mixture, and allow the mixture to stand for Then, 1.5 M iron(III) in a 4 M perchloric acid 5 min to mask the sulfite. To this mixture add 2 ml of solution was prepared by dissolving 306.05 g of iron- 1 M acetic acid, 1.5 ml of 1.5 M lanthanum(III) nitrate (III) nitrate nonahydrate (99%) in a small volume of and 4.5 ml of 4 M cyanide solution; the resulting water containing 217.4 ml of 60% perchloric acid and solution has a pH of 9.5. Allow the mixture to stand at diluting it to 500 ml. The buffer solutions of pH 9.9 15° C for 1.5 h in order to completely convert both the used in Procedures I, II and IV and of pH 3.5 used in thiosulfate and the trithionate into thiocyanate. Then Procedures I and II were prepared by mixing 100 ml of add 1 ml of 0.01 M iodine in methanol, 4 ml of 60% 0.3 M sodium hydrogencarbonate with 100 ml of 0.3 M perchloric acid and 3 ml of 1.5 M iron(III) nitrate in a sodium carbonate, and 160 ml of 0.5 M acetic acid with 4 M perchloric acid solution. Dilute to the mark with 10 ml of 0.5 M sodium acetate, respectively. water, mix the contents well and measure the absor- All spectrophotometric measurements were made bance of the solution of the iron(III)-thiocyanate with a Shimadzu Model UV-100-02 spectrophotometer complex, thus formed, against water at 460 nm. with 10-mm quartz cells. The pH was measured with a Hitachi-Horiba Model M-1 pH meter. Results and Discussion Procedure The proposed method for the determination of Calibration graphs tetrathionate, thiosulfate, sulfite and trithionate in It is well known that polythionates do not consume mixtures consists of the following four procedures. iodine, but that thiosulfate and sulfite react with iodine Procedure 1. For total amounts of tetrathionate and in an acidic medium as follows: thiosulfate in the mixture. Pipette 1.8 ml of the carbonate buffer solution (pH 9.9), 2 ml of the 0.15 M 252032- + 12 S4062- + 21- (1) sulfite solution, and then a 10-ml aliquot of a sample SO32- + 12 + H2O --i SO42- + 21- + 2W. (2) solution containing tetrathionate, thiosulfate, sulfite and trithionate, into a 25-ml volumetric flask. The pH In a previous paper", the conditions under which of the solution is thereby brought to 8.0. Allow the tetrathionate is converted into thiosulfate according to mixture to stand at room temperature for 20 min, to completely convert tetrathionate into thiosulfate. Add S4062 + S032 --i S2032 + S3062 (3) 2 ml of 0.5 M formaldehyde solution and 1.5 ml of the acetate buffer solution (pH 3.5) to the reaction mixture. have been established, in which 1 mol of thiosulfate is In this case, the solution was buffered to a pH of 5.0. formed by the sulfitolysis of 1 mol of tetrathionate. It Allow the mixture to stand for 5 min to completely can be seen in Fig.

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