792 Analyst, December, 1968, Vol. 93, $9. 792-796 Oxidimetric Determination of Thiocarbonate Sulphur with Chloramine-T, Potassium Ferricyanide and Potassium Permanganate BY K. N. JOHRI AND N. K. KAUSHIK (Department of Chemistry, University of Delhi, Delhti 7, India) Oxidimetric methods for determining the concentration of aqueous solutions of potassium thiocarbonate by using chloramine-T, potassium ferricyanide and potassium permanganate are discussed. The chloramine-T method is based on the reaction of potassium thiocarbonate with a known excess of chloramine-T in alkaline medium at 6OoC and back-titration of the unreacted chloramine-T against a standardised solution of sodium thiosulphate, with starch as indicator. TwentyLfour equivalents of the oxidant per mole of potassium thiocarbonate are consumed, showing that the three sulphur atoms of thiocarbonate are oxidised to sulphate.When ferri- cyanide is used twenty-four equivalents of the oxidant per mole of potassium thiocarbonate are also consumed in an alkaline medium at 60" C. However, in acidic medium, potassium permanganate is found to oxidise the three sulphur atoms of thiocarbonate to elemental sulphur. The following molar relationships are established- 12 moles of chloramine-T = 1 mole of K&S, = 5 moles of K,CS, 24 moles of K,Fe(CN), = 1 mole of K,CS, 6 moles of KMnO, BECAUSE of the important analytical applications of potassium thiocarbonate reported lately in the literature,ls2Js4s6 an investigation of efficient and rapid chemical procedures for the determination of thiocarbonate sulphur present in aqueous samples of the reagent was considered necessary.A gravimetric methode in which thallium(1) nitrate is used for the quantitative precipitation of thiocarbonate sulphur and titrimetric procedures involving iodimetric, as well as iodatometric, oxidation of the thiocarbonate contents of samples have been rep~rted.~ Iodic acid alone was found to react with the three sulphur atoms of the thiocarbonate; iodine and potassium iodate each oxidised one of the sulphur atoms, the other two forming carbon disulphide. The iodimetric and gravimetric method~8s~~~O available relate only to the determination of sulphide sulphur and not to thiocarbonate sulphur. The results obtained by the present method are not only quantitative, but have confirmed that three moles of hydrogen sulphide per mole of potassium thiocarbonate are available in reacting solutions under optimum conditions.As chloramine-T is able to break the C-S, N-S and S-S bonds in a variety of sulphur compounds,ll to 17 oxidising all of the sulphur quantitatively to the sulphate form, it was of interest to investigate the reaction between chloramine-T and potassium thiocarbonate. The active constituent of chloramine-T is the hypochlorite ion, which is obtained by hydrolysis of chloramine-T. Chloramine-T is preferred to hypochlorite because of its relatively high stability. The oxidation of thiocarbonate sulphur was studied in both acidic and alkaline media, and a simple titrimetric procedure for the determination of potassium thiocarbonate has been developed by making use of the oxidation in alkaline medium.Potassium ferricyanide has also been studied as an oxidant for thiocarbonate sulphur in hot alkaline medium. An investigation of the use of this oxidant was necessary because, being a weak oxidant, it is more selective than others. Moreover, oxidation of various organic and inorganic sulphur compounds in alkaline medium has been reviewed by Sant,l* but no reference to thiocarbonate sulphur in this respect is found in the literature. However, recently Deshmukhlg has standardised potassium thiocarbonate amperometrically by using ferricyanide as oxidant, with osmium tetroxide as catalyst, and has reported that only one third of the thiocarbonate sulphur reacted at room temperature. 0 SAC and the authors.JOHRI AND KAUSHIK 793 Furthermore, evaluation of thiocarbonate sulphur in acidic medium has been carried out with potassium permanganate, a strong oxidant that reacts quantitatively with the hydrogen sulphide liberated by the thiocarbonic acid produced in acidic solutions of potassium t hiocarbonate.CHLORAMINE-T METHOD REAGENTS- Chloramine-T, 0.1 N-This solution was kept in amber-coloured bottles and standardised iodimet rically.20 Sodium thiosulphate, 0.1 N-This was prepared from analytical-reagent grade material and standardised against potassium iodate. Starch solution, 1 $er cent., aqueous. Potassium thiocarbonate, 2 M-An aqueous solution was prepared by the direct method, and, after standardisation,, was used to prepare suitable dilutions.Other reagents used were of analytical-reagent grade. PROCEDURE- Transfer 10 ml of 0.02 M potassium thiocarbonate into a 250-ml conical flask containing 50 ml of standard chloramine-T solution, made alkaline with 5 ml of M sodium hydroxide. Heat to 60" C for half an hour by immersing the flask in a hot water bath. Cool to room temperature, acidify with 20 ml of 5 N sulphuric acid and add 25 ml of 10 per cent. potassium iodide solution. Titrate the liberated iodine with standard sodium thiosulphate solution. The amount of chloramine-T consumed by potassium thiocarbonate is thus obtained from the titre value. Calculate the thiocarbonate sulphur content of the sample from the relationship- Blank corrections were not necessary in these experiments. The titre values were found to be reproducible, and the results of a few representative experiments are given in Table I.1 ml of N chloramine-T = 7-75 x 10-3 g of K2CS3 = 4.008 x g of s. TABLE I RESULTS OF THE DETERMINATION OF SULPHUR IN POTASSIUM THIOCARBONATE WITH CHLORAMINE-T IN ALKALINE MEDIUM AT 60°C Volume of 0.02 M potassium thiocarbonate added, ml 10 10 5 5 3 3 Number of equivalents of oxidant per mole chloramine-T of potassium consumed, thiocarbonate ml used 47-98 23.99 47-84 23-92 24.02 24-02 24-00 24.00 14-38 23.96 14-40 24.00 Volume of 0.1 N Sulphur present, mg 19-20 19.20 9.60 9.60 5.76 5.76 Sulphur found, * g 19-23 19-18 9.63 9.62 5-76 6-77 Difference, *g + 0.03 - 0.02 + 0.03 + 0.02 0.0 + 0.01 DISCUSSION It is evident from the results shown in Table I that the reaction of chloramine-T with alkaline potassium thiocarbonate at 60°C is such that the three sulphur atoms undergo oxidation. For the complete oxidation of sulphur in potassium thiocarbonate to the sulphate ion twenty-four equivalents of oxidant per mole of potassium thiocarbonate would be needed according to the following equations- (;) CH3.C,H4.S02NC1 - Na+ + H20 -+ CH3.C,H4.S02NH2 + Na+ + Ocl-.(ii) CSS2- + 120Cl- + 40H- -+ 3S0,2- + 2HzO + 12C1- + C02. Thus, 12 moles of chloramine-T = 1 mole of K2CS3. Experiments carried out in acidic medium revealed incomplete oxidation, even at a higher temperature. This was caused by the partial oxidation of potassium thiocarbonate to elemental sulphur which, once formed, resists further oxidation by chloramine-T. The794 JOHRI AND KAUSHIK : OXIDIMETRIC DETERMINATION OF [Analyst, Vol.93 sulphur precipitated during these experiments in acidic media was observed to be suspended in solution. Oxidation in alkaline medium at room temperature (20" C) also did not proceed to completion, and separation of elemental sulphur was clearly observed as a white turbidity. However, when the temperature was raised to 60" C this turbidity vanished and the oxidation of the entire sulphur was found to be quantitative. The finely divided sulphur reacts with hot alkali,21 forming sulphide, sulphite and thiosulphate, all of which can be oxidised to sulphate by chloramine-T. FERRICYANIDE METHOD REAGENTS- Potassium ferricyanide, 0.1 this was prepared by dissolving analytical-reagent grade material in redistilled water and standardised by titrating against standard sodium thio- sulphate solution.Sodium thiosulphate, 0.1 N. Starch solution, 1 per cent., aqueous. Sulphuuric acid, 5 N. Zinc sulphate, 0.5 M-This was prepared by dissolving an analytical-reagent grade sample in redistilled water. Potassium thiocarbonate, 2 M. PROCEDURE- In a series of experiments to determine the optimum conditions for the complete oxida- tion of thiocarbonate sulphur with ferricyanide, carried out at room temperature, the time of reaction was varied and the amount of potassium thiocarbonate used kept constant. The excess of ferricyanide was determined by back-titrating against standardised thio- sulphate solution. The reaction was slow at room temperature, the number of equivalents of oxidant per mole of potassium thiocarbonate increasing from 2.60 to 8-06 in 45 minutes at &minute intervals.However, in the second set of experiments the reactants were heated to 60" C and the reaction was found to be completed within 15 to 20 minutes. RECOMMENDED PROCEDURE- Introduce a measured aliquot of the test solution containing not more than 17 mg of sulphur into a measured excess volume of standard 0.1 N ferricyanide, previously made alkaline with 5 N sodium hydroxide so that its alkalinity is about 3 N. Heat to 60" C for 15 to 20 minutes by immersing the flask in a hot water bath. Cool to room temperature, and titrate the excess of ferricyanide against standardised thiosulphate after acidifying with 5 N sulphuric acid, adding 25 ml of 10 per cent.potassium iodide solution and an excess of zinc sulphate solution so that all of the resulting ferrocyanide can be precipitated as zinc ferrocyanide. Calculate the thiocarbonate sulphur content of the sample from the relationship- Use starch as the indicator. 1 ml of N ferricyanide = 7.75 x g of K, CS, = 4.008 x g of s. The results of a few representative experiments are given in Table 11. TABLE I1 RESULTS OF THE DETERMINATION OF SULPHUR IN POTASSIUM THIOCARBONATE WITH FERRICYANIDE IN ALKALINE MEDIUM AT 60°C Volume of 0.06 M potassium Volume of 0.1 N added, consumed, 1 14.38 1 14.40 2 28.76 2 28-78 3 43.20 3 43-22 thiocarbonate f emcyanide ml ml Number of equivalents of oxidant per mole of potassium t hiocarbona te used 23-96 24-00 23-96 23.98 24.00 24-01 Sulphur present, mg 5-76 5-76 11-52 11.52 17.28 17-28 Sulphur found, mg 5-76 5-77 11-52 11.53 17.32 17-33 Difference, mg 0.0 +o*or 0.0 + 0.01 + 0.04 + 0-06December, 19681 THIOCARBONATE SULPHUR WITH CHLORAMINE-T 795 DISCUSSION It was seen from the results that the reaction between ferricyanide and potassium thiocarbonate is time consuming.It takes about 45 minutes to oxidise only one of the three sulphur atoms to sulphate at room temperature, after which the titre value remains constant. However, the reaction at 60°C is rapid, and the three sulphur atoms of the thiocarbonate are oxidised to sulphate, as seen in Table 11. The reaction of ferricyanide with potassium thiocarbonate in alkaline medium at 60°C can be expressed- CS,2- + 24Fe(CN),3- + 260H- -+ 24Fe(CN),4- + 3so,2- + 13H20 + CO Thus, 24 moles of ferricyanide = 1 mole of K2CS,.Before titrating the excess of ferricyanide, sufficient zinc sulphate must be added to precipitate all the ferrocyanide as zinc ferrocyanide. Otherwise, the end-point would not be sharp because of the formation of Prussian blue, the presence of which has been verified by making the solution alkaline (colour fades). POTASSIUM PERMANGANATE METHOD PROCEDURE- Transfer 10ml of 0.1 N potassium permanganate solution to a 250-ml conical fiask and add 10 ml of 0.1 N sulphuric acid. Introduce, gradually, freshly prepared potassium thio- carbonate solution from a microburette with a bent nozzle, keeping the tip of the nozzle beneath the liquid surface. Continue adding until the colour of the potassium permanganate is discharged.Repeat the observations with different amounts of potassium permanganate. Calculate the potassium thiocarbonate sulphur content of the sample solution from the relationship- 1 ml of N KMnO, = 31.006 x lO-,g of K,CS, = 16.03 x 10-3g of s. DISCUSSION The results in Table I11 show that the reaction of potassium thiocarbonate with acidified potassium permanganate is such that the three sulphur atoms of thiocarbonate undergo oxidation to elemental sulphur according to the following equations- MnO,- + 8Hf + 5e- -+ Mn2+ + 4H20 CS,2- + 3(0) -+ C032- + 3s + 6e- k, 6KMn0, + 9H2S04 + 5K2CS, --3 3K2S0, + GMnSO, + 9H20 + 5K,C03 + 15 S Thus, 6 moles of KMnO, = 5 moles of K,CS3. The results of titrimetric evaluation with permanganate are accurate for 0.006 to 0.06 M concentrations of potassium thiocarbonate.The volume of permanganate taken should be such that not more than 8 mg of sulphur are precipitated out after the complete reaction. TABLE I11 RESULTS OF THE DETERMINATION OF THIOCARBONATE SULPHUR WITH POTASSIUM PERMANGANATE IN THE PRESENCE OF 0.05 TO 0.1 N SULPHURIC ACID Volume of 0-05 N potassium permangana te taken, ml 5 5 10 10 15 15 Titre of 0-03 M potassium thiocarbonate. ml 1-38 1.40 2.80 2-82 4.46 4.44 Number of equivalents of oxidant per mole of potassium thiocarbonate used 6.03 5.95 5.95 5.91 5.60 5.63 Sulphur present, mg 4.00 4.00 8-12 8.12 12.18 12-18 Sulphur found, mg 3.97 4.03 8-06 8.12 12-84 12-78 Difference, mg - 0.03 + 0-03 - 0.06 0.0 + 0-66 + 0.60 The fact that the three sulphur atoms of thiocarbonate are oxidised by permanganate and no trace of carbon disulphide separated was confirmed by the negative result of a colori- metric test.22 In this test a drop of the solution should produce a stable, pink colour if acetone796 JOHRI AND KAUSHIK and elemental sulphur are present together with free carbon disulphide. The reaction of permanganate with potassium thiocarbonate in alkaline medium at room temperature was also studied, and it was found that only one sulphur atom of thiocarbonate had undergone oxidation to sulphate, while the other two formed carbon disulphide.Furthermore, results at 60” C are not reproducible. To ensure that the liberated hydrogen sulphide undergoes oxidation, potassium thio- carbonate is added to the permanganate, dropwise, from a burette.CONCLUSIONS The results are quantitative, and suitable dilution of more concentrated solutions of potassium thiocarbonate is necessary to obtain accurate results. Under optimum conditions the three sulphur atoms of the thiocarbonate are oxidised, thus giving a rapid method for determining and identifying thiocarbonate from other sulphur compounds. The authors thank Professor R. P. Mitra, F.N.I., Head of the Department of Chemistry, University of Delhi, for the facilities provided, and one of us (N.K.K.) is also grateful to the Council of Scientific and Industrial Research, New Delhi, India, for the award of a Research Fellowship. REFERENCES 1. 2. 3. 4. 6. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. Johri, K. N., J .Scient. Ind. Res., 1969, 18B, 430. -, J . Indian Chem. Soc., 1961, 38, 935. -, Analyst, 1961, 86, 487. -, Indaan J . Appl. Chem., 1963, 26, 114. -, “Chemical Analysis without Hydrogen Sulphide using Potassium Trithiocarbonate,” Asia Johri, K. N., and Singh, K., Indian J . Chem., 1965, 3, 158. -- , Analyst, 1965, 90, 746. Jacobs, M. B., “The Analytical Chemistry of Industrial Poisons, Hazards and Solvents,” Second Edition, Interscience Publishers Inc., New York and London, 1949, p. 323. Scott, W. W., “Standard Methods of Chemical Analysis,” Fifth Edition, D. Van Nostrand, New York, 1939, Volume 1, pp. 911 and 923. 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