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Titrimetric determination of certain mercaptans with mercuric chloride
0039-9l40/81/060397-02SO2.00/0 Pergamon Press Ltd
T‘llunru. Vol. 28. pp. 397 tcl 398. 1981 Printed in Great Britam
TITRIMETRIC DETERMINATION OF CERTAIN MERCAPTANS WITH MERCURIC CHLORIDE K.
and
M. VERMA
Department of Chemistry, University of Jabalpur, Jabalpur 482001, India (Received
30 September
1980. Accepted
26 December
1980)
Summary-A procedure is described for the semimicro determination of certain mercaptans, by means of their reaction with mercuric chloride. The acid liberated during the reaction is titrated with standard alkali after addition of sufficient potassium iodide to convert the surplus mercuric chloride into a stable soluble complex. The procedure has been applied to determine 0.12-l mmole of these compounds; the results are accurate to within 0.5%.
The high chemical reactivity of the mercapto group gives rise to a number of chemical reactions which can be used for its determination, these being mainly the oxidation to disulphide’-13 or the formation of a metal mercaptide. lkz4 The oxidative methods can suffer from over-oxidatio#*’ I-’ 3 or slow reactioqz5 and interference from some other sulphur-containing groups. The copper mercaptide methods1617 are regarded by Siggiaz6 as less accurate than the iodine method. Iodometric determination of excess. of copper” can suffer from partial oxidation of the mercaptide by the iodine liberated.14 Mercaptans react with mercuric chloride to liberate hydrochloric acid :
Orange or Methyl Red as indicator; they observe that the results are always low because with both the endpoint pH is too low, but has to be used to avoid interference by the formation of mercuric hydroxide. Ratkovics and Szepesvaryz7 used Bromocresol Green as the indicator and showed that the error was f2%. The difficulty caused by mercuric hydroxide is circumvented in the present work by adding an excess of potassium iodide before titrating the liberated acid. The iodide forms a highly stable soluble complex with the surplus mercury@), and phenolphthalein can then be used as the indicator. EXPERIMENTAL Reagents
RSH + HgClz = RSHgCl + HCI and Sampey and Reid” titrate the acid, using Methyl
Sodium hydroxide solution, O.lM. Standardized with oxalic acid and diluted to 0.05 and 0.02M as required.
Table 1. Determination of mercaptans with mercuric chloride
Compound 1-Thioglycerol 2-Mercaptoethanol t-Mercaptopropionic
acid?
Thioglycollic acidt 2-Mercaptosuccinic I-Propanethiol 2-Propanethiol 1-Dodecanethiol
acidt
Found, mg*
Comparison method. mg
Average deviation, “/:,
Relative standard deviation, %
108.3 13.56 78.2 9.80 106.1 13.30 92.1 11.55 75.1 15.06 76.1 9.54 76.2 9.54 202.2 25.37
108.0’ 13.50 78.0’ 9.75 106.0b 13.25 92.0’ 11.50 75.0b 15.00 76.0’ 9.50 76.00’ 9.50 202.0’ 25.25
0.3 0.5 0.3 0.5 0.1 0.4 0.1 0.4 0.1 0.4 0.1 0.5 0.3 0.5 0.1 0.5
0.3 0.5 0.3 0.4 0.1 0.4 0.1 0.4 0.2 0.3 0.3 0.6 0.3 0.4 0.2 0.6
* Average of 6 determinations. t Carboxyl group also taken into consideration in calculations. a Tetrathionate method. b Hexacyanoferrate method. ’ Iodine method. 397
398
SHORT COMMUNICATIONS
Mercuric chloride, saturated solution in water. Mercapran solutions. Standardized by different procedures, as shown in Table 1. Dodecyl mercaptan was dissolved in acetone. and both propyl mercaptans were dissolved in 3: 1 v/v water-acetone mixture.
Acknowledgement-The authors are thankful to University Grants Commission, New Delhi, for financial assistance to one of them (K.K.T.).
Procedure To aqueous sample solution containing the 0.12-l mmole of mercaptan. add about 5 ml of saturated mercuric chloride solution. Swirl the solution for 1 min. Add 1-2g of potassium iodide and shake until the red precipitate first formed disappears. Some insoluble mercaptides will also dissolve at this stage. Add 4-S drops of 1% phenolphthalein solution and titrate with standard sodium hydroxide solution. In the case of dodecyl mercaptan, add about 20ml of water after the mercaptide formation, because in pure acetone medium the colour change of phenolphthalein is not sharp.
1. J. W. Kimball, R. L. Kramer and E. E. Reid, J. Am. Chem. Sot., 1921, 43, 1199. 2. J. A. R. Cooper and G. S. Maingot, Anal. Chem., 1955, 27, 1479. 3. J. MacLeod, .I. Gen. Physiol., 1951, 34, 705. 4. D. S. Mahadevappa and N. M. H. Gowda, Talanta, 1975, 22, 771. 5. K. K. Verma, J. Ahmed and S. Bose, Z. Anal. Chem., 1976, 128, 278. 6. L. Hellerman, F. P. Chinard and P. A. Ramsdell, J. Am. Chem. Sot.. 1941.63. 2551. I. K. K. Verma and S. Bose, Anal. Chim. Acto, 1973, 65, 236. 8. A. E. Mirsky, J. Gen. Physiol., 1941, 24, 725. 9. A. Shrivastava and S. Bose, J. Indian Chem. Sot., 1974. 51, 736. 10. A. Shrivastava, Talanta, 1979, 26, 917. 11. J. P. Danhey, fnt. J. Sulphur Chem., 1971, C6, 159. 312. J. P. Danhev and M. Y. Oester. J. Ora. ” Chem.. 1967. ,.32. 1491. 13. 1.M. Kolthoff and W. E. Harris, Anal. Chem., 1949. 21. 963. 14. E. W. Ellis, ibid., 1951, 23, 1777. 15. G. R. Bond, Ind. Eng. Chem., Anal. Ed., 1933, 5, 257. 16 S. B. Sant and B. R. Sant, Anal. Chem., 1959, 31, 1879. 17. E. Turk and E. E. Reid, Ind. Eng. Chem., Anal. Ed.. 1945, 17, 713. 18 H. Roth, Mikrochim. Acta, 1958, 769. J. l?. Buckley and G. Gorin, Anal. Chem.. 19. R. K. Ku&l, 1959, 31, 1098. and E. E. Reid, Ind. Eng. Chem., Anal. 20. P. Borgstrom Ed., 1929, 1, 186. 21. J. S. Fritz and T. A. Palmer, Anal. Chem., 1961. 33,98. 22. M. Wroliski. Analvst. 1964. 89. 800. 23. C. A. Brown, Anal. Chem., 1967.39, 1882. P. E. Boufford and R. Borton. ibid., 1961. 24. D. C. Greg. 33, 269. 25. J. R. Sampey and E. E. Reid, J. ‘Am. Chem. Sot., 1932, 54,3404. 26. S. Siggia and I. G. Hanna, Quantitariue Organic Analysis Via Functional Groups, p. 723. Wiley-Interscience, New York, 1979. 27. F. Ratkovics and P. Szepesvary, Magy. Kern. Folyoirar, 1958, 64, 472.
RESULTS
AND DISCUSSION
The method has been applied to various simple mercaptans and to some containing hydroxy and carboxy1 groups as well, and the results presented in Table 1 show that quantities as little as 0.125 meq of these mercaptans can be determined with an average deviation of O.l&OSo,b Unlike the oxidimetric procedures, the method does not require strict reaction conditions. However, mercaptans containing the amino-group, such as cysteine or 2-mercaptoethylamine hydrochloride, cannot be determined by this method because of the basic nature of the amino-group. Carbon tetrachloride, carbohydrates, sulphur and disulphides do not interfere, but acidic salts and compounds with an aminogroup or of basic nature interfere. Alcohols do not interfere if present in small amount but larger quantities cause overconsumption of alkali. The red mercuric iodide precipitate does not appear, and the solution becomes yellow (a yellow turbidity appears in the case of insoluble mercaptides) and the colour interferes with detection of the end-point.
REFERENCES
T‘llunru. Vol. 28. pp. 397 tcl 398. 1981 Printed in Great Britam
TITRIMETRIC DETERMINATION OF CERTAIN MERCAPTANS WITH MERCURIC CHLORIDE K.
and
M. VERMA
Department of Chemistry, University of Jabalpur, Jabalpur 482001, India (Received
30 September
1980. Accepted
26 December
1980)
Summary-A procedure is described for the semimicro determination of certain mercaptans, by means of their reaction with mercuric chloride. The acid liberated during the reaction is titrated with standard alkali after addition of sufficient potassium iodide to convert the surplus mercuric chloride into a stable soluble complex. The procedure has been applied to determine 0.12-l mmole of these compounds; the results are accurate to within 0.5%.
The high chemical reactivity of the mercapto group gives rise to a number of chemical reactions which can be used for its determination, these being mainly the oxidation to disulphide’-13 or the formation of a metal mercaptide. lkz4 The oxidative methods can suffer from over-oxidatio#*’ I-’ 3 or slow reactioqz5 and interference from some other sulphur-containing groups. The copper mercaptide methods1617 are regarded by Siggiaz6 as less accurate than the iodine method. Iodometric determination of excess. of copper” can suffer from partial oxidation of the mercaptide by the iodine liberated.14 Mercaptans react with mercuric chloride to liberate hydrochloric acid :
Orange or Methyl Red as indicator; they observe that the results are always low because with both the endpoint pH is too low, but has to be used to avoid interference by the formation of mercuric hydroxide. Ratkovics and Szepesvaryz7 used Bromocresol Green as the indicator and showed that the error was f2%. The difficulty caused by mercuric hydroxide is circumvented in the present work by adding an excess of potassium iodide before titrating the liberated acid. The iodide forms a highly stable soluble complex with the surplus mercury@), and phenolphthalein can then be used as the indicator. EXPERIMENTAL Reagents
RSH + HgClz = RSHgCl + HCI and Sampey and Reid” titrate the acid, using Methyl
Sodium hydroxide solution, O.lM. Standardized with oxalic acid and diluted to 0.05 and 0.02M as required.
Table 1. Determination of mercaptans with mercuric chloride
Compound 1-Thioglycerol 2-Mercaptoethanol t-Mercaptopropionic
acid?
Thioglycollic acidt 2-Mercaptosuccinic I-Propanethiol 2-Propanethiol 1-Dodecanethiol
acidt
Found, mg*
Comparison method. mg
Average deviation, “/:,
Relative standard deviation, %
108.3 13.56 78.2 9.80 106.1 13.30 92.1 11.55 75.1 15.06 76.1 9.54 76.2 9.54 202.2 25.37
108.0’ 13.50 78.0’ 9.75 106.0b 13.25 92.0’ 11.50 75.0b 15.00 76.0’ 9.50 76.00’ 9.50 202.0’ 25.25
0.3 0.5 0.3 0.5 0.1 0.4 0.1 0.4 0.1 0.4 0.1 0.5 0.3 0.5 0.1 0.5
0.3 0.5 0.3 0.4 0.1 0.4 0.1 0.4 0.2 0.3 0.3 0.6 0.3 0.4 0.2 0.6
* Average of 6 determinations. t Carboxyl group also taken into consideration in calculations. a Tetrathionate method. b Hexacyanoferrate method. ’ Iodine method. 397
398
SHORT COMMUNICATIONS
Mercuric chloride, saturated solution in water. Mercapran solutions. Standardized by different procedures, as shown in Table 1. Dodecyl mercaptan was dissolved in acetone. and both propyl mercaptans were dissolved in 3: 1 v/v water-acetone mixture.
Acknowledgement-The authors are thankful to University Grants Commission, New Delhi, for financial assistance to one of them (K.K.T.).
Procedure To aqueous sample solution containing the 0.12-l mmole of mercaptan. add about 5 ml of saturated mercuric chloride solution. Swirl the solution for 1 min. Add 1-2g of potassium iodide and shake until the red precipitate first formed disappears. Some insoluble mercaptides will also dissolve at this stage. Add 4-S drops of 1% phenolphthalein solution and titrate with standard sodium hydroxide solution. In the case of dodecyl mercaptan, add about 20ml of water after the mercaptide formation, because in pure acetone medium the colour change of phenolphthalein is not sharp.
1. J. W. Kimball, R. L. Kramer and E. E. Reid, J. Am. Chem. Sot., 1921, 43, 1199. 2. J. A. R. Cooper and G. S. Maingot, Anal. Chem., 1955, 27, 1479. 3. J. MacLeod, .I. Gen. Physiol., 1951, 34, 705. 4. D. S. Mahadevappa and N. M. H. Gowda, Talanta, 1975, 22, 771. 5. K. K. Verma, J. Ahmed and S. Bose, Z. Anal. Chem., 1976, 128, 278. 6. L. Hellerman, F. P. Chinard and P. A. Ramsdell, J. Am. Chem. Sot.. 1941.63. 2551. I. K. K. Verma and S. Bose, Anal. Chim. Acto, 1973, 65, 236. 8. A. E. Mirsky, J. Gen. Physiol., 1941, 24, 725. 9. A. Shrivastava and S. Bose, J. Indian Chem. Sot., 1974. 51, 736. 10. A. Shrivastava, Talanta, 1979, 26, 917. 11. J. P. Danhey, fnt. J. Sulphur Chem., 1971, C6, 159. 312. J. P. Danhev and M. Y. Oester. J. Ora. ” Chem.. 1967. ,.32. 1491. 13. 1.M. Kolthoff and W. E. Harris, Anal. Chem., 1949. 21. 963. 14. E. W. Ellis, ibid., 1951, 23, 1777. 15. G. R. Bond, Ind. Eng. Chem., Anal. Ed., 1933, 5, 257. 16 S. B. Sant and B. R. Sant, Anal. Chem., 1959, 31, 1879. 17. E. Turk and E. E. Reid, Ind. Eng. Chem., Anal. Ed.. 1945, 17, 713. 18 H. Roth, Mikrochim. Acta, 1958, 769. J. l?. Buckley and G. Gorin, Anal. Chem.. 19. R. K. Ku&l, 1959, 31, 1098. and E. E. Reid, Ind. Eng. Chem., Anal. 20. P. Borgstrom Ed., 1929, 1, 186. 21. J. S. Fritz and T. A. Palmer, Anal. Chem., 1961. 33,98. 22. M. Wroliski. Analvst. 1964. 89. 800. 23. C. A. Brown, Anal. Chem., 1967.39, 1882. P. E. Boufford and R. Borton. ibid., 1961. 24. D. C. Greg. 33, 269. 25. J. R. Sampey and E. E. Reid, J. ‘Am. Chem. Sot., 1932, 54,3404. 26. S. Siggia and I. G. Hanna, Quantitariue Organic Analysis Via Functional Groups, p. 723. Wiley-Interscience, New York, 1979. 27. F. Ratkovics and P. Szepesvary, Magy. Kern. Folyoirar, 1958, 64, 472.
RESULTS
AND DISCUSSION
The method has been applied to various simple mercaptans and to some containing hydroxy and carboxy1 groups as well, and the results presented in Table 1 show that quantities as little as 0.125 meq of these mercaptans can be determined with an average deviation of O.l&OSo,b Unlike the oxidimetric procedures, the method does not require strict reaction conditions. However, mercaptans containing the amino-group, such as cysteine or 2-mercaptoethylamine hydrochloride, cannot be determined by this method because of the basic nature of the amino-group. Carbon tetrachloride, carbohydrates, sulphur and disulphides do not interfere, but acidic salts and compounds with an aminogroup or of basic nature interfere. Alcohols do not interfere if present in small amount but larger quantities cause overconsumption of alkali. The red mercuric iodide precipitate does not appear, and the solution becomes yellow (a yellow turbidity appears in the case of insoluble mercaptides) and the colour interferes with detection of the end-point.
REFERENCES