Anal. Chem. 1982, 54, 2633-2635
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Titrimetric Determination of Mercaptans Krishna K. Verma* and Anil K. Gulati Department of Chemistry, University of Jabalpur, Jabalpur 482001, India
aromatic mercaptans alone with sodium methoxide in acetone using thymol blue as indicator and then determining a sum of two mercaptans by mercurimetry. Xanthates and dithiocarbamates react with mercuric cyanide with the formation of an equivalent amount of cyanide which can be titrated with hydrochloric acid.
Sampey and Reid (1) determined mercaptans by shaking the sample with aqueous mercuric chloride and titrating the liberated acid with sodium hydroxide.
RSH + HgCla
—
RSHgCl + HC1
The results obtained were always low since the end point was at a pH that was too low, when methyl red or methyl orange was used as indicators, to avoid precipitation of mercuric hydroxide. Complexation of surplus mercuric ion with large excesses of alkali halides permitted a direct titration of acid with phenolphthalein. as indicator (2-7). The determination of mercaptans as acids has long been known (8-10), but this approach, despite its simplicity, could not find wider use because of the general interference of acids. The titration of acids with alkali is also vitiated by the presence of mercaptans. The fact that previous methods of mercaptan determination do not differentiate aliphatic and aromatic mercaptans and are not applicable to analyze mixtures of mercaptans with acids makes the other procedures desirable. Mixtures of mercaptans and xanthates are used
—
(ROCSS)2Hg + 2KCN
2R2NCSSK + Hg(CN)2
—
(R2NCSS)2Hg + 2KCN
Mercaptans do not interfere. Thus, mixtures of mercaptans and xanthates or dithiocarbamates can be analyzed by two separate reactions with mercuric chloride and mercuric cyanide followed by titration with alkali or acid, respectively.
EXPERIMENTAL
SECTION
Reagent. Sodium methoxide, 0.05 M, was prepared by dissolving sodium in methanol, diluting with the same solvent, and standardizing against potassium hydrogen phthalate. Sodium hydroxide and hydrochloric acid, 0.05 M each, were made in water and standardized. Thymol blue, methyl red, and phenol red, 0.05% each, were made in methanol. Most of the mercaptan samples were obtained from Evans Chemetics Inc., New York, and were used as such. As a check, their purity was determined by previously established methods (Table I). All other chemicals were high purity reagents. Procedures. Determination of Mercaptans. An accurately weighed amount of sample, containing 0.1-1.0 mmol of mercaptan, is dissolved in 20 mL of methanol and treated with 25 mL of 5% aqueous solution of mercuric chloride and 25 mL of water. The contents are shaken vigorously and then 10% aqueous solution of potassium iodide is introduced in portions till the initially formed brick red precipitate of mercuric iodide dissolves. The liberated acid is determined by adding 2 to 3 drops of phenol red indicator and titrating with 0.05 M sodium hydroxide to a pink color. If the mercaptan has a carboxyl group (e.g., in 2-mercapto-
flotation agents. All mercaptans are acidic to phenolphthalein; therefore, the concomitant acids cannot be neutralized before treating the mixture with mercuric chloride for mercaptan determination in the method of Sampey and Reid (1). It has been found that aliphatic mercaptans are neutral to phenol red and acid impurities can be neutralized without any interference. The mercaptan is then determined by reaction with excess mercuric chloride followed by titration of liberated hydrochloric acid with alkali using phenol red as indicator after the residual mercuric ion is complexed by an excess of potassium iodide. There is no precipitation of mercuric hydroxide from dias
potassium mercuric tetraiodide. Aromatic mercaptans do act acidic to phenol red. Mixtures of aromatic mercaptans and acids are analyzed first by determining a sum by mercurimetry and then acid alone after masking mercaptan with allyl isothiocyanate; mercapttin is obtained by difference. Mixtures of aliphatic and aromatic mercaptans are analyzed by titrating Table I.
2ROCSSK + Hg(CN)2
Determination of Mercaptans
purity, mercaptan
present method0
CV
%
comparison method
reagent
98.7 iodine (1 7) 0.25 98.9 2-mercaptoethanol 1-dodecanethiol 98.5 0.20 iodine (1 7) 98.7 1 -mercapto-2-propene 99.5 0.28 99.4 copper(II) (18) 99.6 0.21 100.1 2-methyl-2-propanethiol copper(II) (18) 98.2 0.30 tetrathionate (14) 98.5 mercaptosuccinic acid 6 6 99.9 0.26 99.7 o-iodosobenzoate (19) cysteine hydrochloride 6 N- acetylcysteine 99.2 0.31 99.4 tetrathionate (14) 99.4 0.28 lead tetraacetate (20) 99.6 2-mercaptobenzoic acid 6 99.6 0.33 99.3 o-iodosobenzoate (19) glutathione 6 97.8 lead tetraacetate (20) 0.30 98.0 2-mercaptobenzoxazole 98.7 0.19 lead tetraacetate (20) 98.5 2-naphthalenethiol b 98.2 iodine (1 7) 0.24 98.4 mercaptoacetic acid toluene-a-thiol 98.5 0.17 98.7 lead tetraacetate (20) 98.4 0.34 98.6 tetrathionate (14) 3-mercaptopropionic acid6 99.2 iodine (1 7) 0.22 98.9 2-propanethiol 89.8 0.24 iodine monochloride (21) 89.5 3-mercaptopropane-l,2-diol 0 6 of six determinations; CV, coefficient of variation. Test material neutralized to phenol red before adding Average mercuric chloride. 0003-2700/82/0354-2633$01.25/0
©
1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
Table II.
Determination of Mixtures of Mercaptans, Xanthates, Dithiocarbamates, and Acids I
amt added,® mg II
4-toluene-
1-dodecane-
amt found,6 mg (CV)
III
I
II
thiol
thiol
11.71 29.29 41.02 58.46
67.22 53.78 20.89 13.45
11.94 (0.3) 29.29 41.29 (0.2) 58.43 (0.2)
67.23 (0.5) 53.80
benzene-
2-methyl-2propanethiol 60.49
10.05(0.1) 49.50 (0.2)
60.03 (0.4) 48.35 29.80 (0.4) 12.14 (0.4)
32.13 (0.2) 22.48 12.80
7.45 (0.3) 14.90 26.01
6.45(0.2)
37.15(0.4)
thiol 9.99 24.98 34.20 49.41 acetic acid 32.08 22.46 12.83 6.42
oxalic acid 30.76 21.05 12.30 6.15 mandelic acid 59.36 44.54 29.68 14.84 2-mercaptoethanol 6.38 9.25 12.46 16.25 2-propane-
thiol
5.69 7.24 15.22 20.16 1-dodecane-
thiol 30.36 25.70 19.11 10.32 3-nitrobenzoic acid 15.92 31.84 39.79 47.75
48.28 30.25 12.10
24.96
34.42(0.2)
2-mercaptoethanol 7.42 14.83 25.95 37.08
4-toluenethiol 13.18 26.37 46.15 65.86
30.93
21.28(0.3)
12.32 6.16 (0.5)
III
20.92(0.2)
13.50 (0.4)
13.06 26.24 (0.2) 46.24 65.75 (0.4)
benzene-
thiol 11.75 23.51 47.00 58.75
59.64 (0.2) 44.58 29.76 (0.3) 14.96
ethyl xanthate 20.28 15.46
6.42 (0.3) 9.20 (0.2) 12.52
8.99 6.15
16.35(0.3)
11.68(0.3)
23.30 46.89 (0.3) 58.85
20.39 (0.3)
15.28(0.2) 9.06 6.06 (0.3)
2-diethyldithiocarbamate 20.29 16.80 10.90 8.16
5.73 (0.2) 7.19 15.16 20.26 (0.3)
20.34 (0.3) 16.75 10.86
30.43 (0.3) 25.69 19.00 10.40 (0.3)
12.72(0.2)
15.90(0.3)
9.04 (0.4) 17.86 22.36 27.10 (0.5)
8.25(0.3)
1-butylxanthate 12.69 18.23 25.66 29.71
18.14 25.58 29.83 (0.3)
4-chloro2-propane-
thiol 8.98 17.96 22.45 26.95
benzene-
thiol 13.42 26.83 33.55 40.25
31.99 40.30 48.18 (0.3)
13.49 (0.4) 26.68 33.32 40.43 (0.4)
0 Amounts found by standardizing acids by alkali, mercaptans by methods of Table I, ethyl xanthate by mercuric chloride titration (22), diethyldithiocarbamate by argentimetry (23), and butyl xanthate by iodine monochloride titration 6 Average of six determinations. (24).
benzoic acid, mercaptosuccinic acid, etc.) or any other acidity (e.g., in cysteine hydrochloride), the sample is dissolved in 20 mL of
methanol or water, diluted to 40 mL with water, mixed with 2 to 3 drops of phenol red, and neutralized by titration with 0.05 M sodium hydroxide to a pink color. Then the mercapto group is determined by the method described before. Determination of Mixtures of Aromatic and Aliphatic Mercaptans. A known aliquot of mixture, containing not more than 1.0 mmol of total mercaptan, is diluted to 40 mL with acetone,
mixed with 2 to 3 drops of thymol blue, and titrated with 0.05 M sodium methoxide to a blue color. This titer gives aromatic mercaptan only. To determine a sum of two mercaptans, 25 mL of 5% mercuric chloride and 25 mL of water are added to an equal but separate aliquot of mixture which is shaken vigorously. Then, the liberated hydrochloric acid is titrated with 0.05 M sodium methoxide using phenol red as indicator by the method described before. Aliphatic mercaptan is obtained by difference.
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
Determination of Mixtures of Aromatic Mercaptans and Acids. To a known aliquot of mixture in acetone or methanol, containing not more than 1.0 mmol of total determinants, are added 25 mL of 5% mercuric chloride and 25 mL of water. The contents are shaken vigorously and the acid is titrated, after complexing the residual mercury(II) with potassium iodide, using 0.05 M sodium hydroxide and phenol red as indicator. For determination of acid alone, an equal but separate aliquot of mixture is mixed with 20 mL of methanol and 1 mL of allyl isothiocyanate and allowed to stand for 15 min. Two to three drops of phenol red and 40 mL of water are added and titrated with 0.05 M sodium hydroxide to a pink color. Mercaptan is yielded by difference. Determination of Mixtures of Aliphatic, Aromatic Mercaptans and Acids. A known aliquot of mixture solution in acetone or methanol is diluted to 40 mL with acetone, mixed with 2 to 3 drops of thymol blue, and titrated with 0.05 M sodium methoxide to a blue color. This titer (Vj) gives a total of aromatic mercaptan and acid. A second but equal aliquot of mixture is combined with 20 mL of methanol and 1 mL of allyl isothiocyanate and kept for 15 min. Thereafter, 40 mL of water and 2 to 3 drops of phenol red are added and the acid content is titrated with 0.05 M sodium methoxide (titer V2). The difference of V'1 and V2 produces aromatic mercaptan. A third equal portion of mixture is mixed with 25 mL of 5% mercuric chloride and 25 mL of water and shaken vigorously. The total acid is titrated with 0.05 M sodium methoxide using phenol red as indicator after complexing mercury(II) with potassium iodide (titer V3). Aliphatic mercaptan is calculated from the difference of Vs and Vj. Determination of Mixtures of Mercaptans and Xanthates or Dithiocarbamates. To an aliquot of sample solution in methanol or water, containing not more than 1.0 mmol of total analyte, 25 mL of 5% aqueous solution of mercuric cyanide and 25 mL of water are added. The contents are shaken vigorously, mixed with 2 to 3 drops of methyl red indicator, and titrated with 0.05 M hydrochloric acid to a pink color. This titer corresponds to xanthate or dithiocarhamate only. An equal but separate aliquot of mixture is analyzed for mercaptan content by the mercuric chloride method.
RESULTS AND DISCUSSION Methods based on mercaptide formation with silver (I) or mercury(II) are accurate and widely used for the determination of mercaptans (11). Oxidations with iodine constitute a simple route; however, the presence of oxidizable materials and unsaturated compounds vitiates the analysis (12). This method is also affected by the structure of mercaptan. Thus, 0-mercapto acids tend to oxidize beyond the disulfide stage (13, 14), reaction with secondary and long chain alkyl mercaptans is slow and that with tertiary mercaptans goes only up to sulfenyl iodide (15). Copper(II) reacts in stoichiometric ratio but the results are less accurate than iodimetry (16). The determination of mercaptans using reaction with mercuric chloride and titration with alkali is rapid and accurate and unaffected by the structure of mercaptan (Table I). A variety of mercaptans including primary, secondary, and tertiary compounds have been analyzed. The presence of iodide, bromide, cyanide, thiocyanate, cyanate, diethyldithiocarbamate, ethyl dithiocarbonate, formate, diphenyl sulfide, methionine, acrylonitrile, carbon disulfide, diethyl
·
2635
disulfide, dimethyl sulfoxide, and unsaturated compounds (the double bond in l-mercapto-2-propene does not add mercuric ion) does not interfere with this method. As such, sulfide does not interfere but the black precipitate formed obscures the visual end point detection. A potentiometric titration of acid is preferable. The present method also yields any acidic material that happens to be present in the test sample. However, a correction for acid impurities can be made by their separate titration after masking the mercaptan with allyl isothiocyanate. The latter compounds react as below giving S-sub-
stituted allyl dithiocarbamate. RSH
+
C3H5NCS
—
RSCNHC3H5
The efficacy of mercaptan binding was noted by titrating known amounts of acids in the presence of mercaptans (Table II). Allyl isothiocyanate is lachrymatory and therefore the titrated solutions should be treated with ammonia or morpholine (for 15 min) before disposal. Mercaptans which gave erratic results when tested by the present method include 2-mercaptoethylammonium chloride, 2-diethylaminoethanethiol hydrochloride, 2-mercaptobenzothiazole, and 2-mercaptobenzimidazole.
ACKNOWLEDGMENT Thanks are due to Evans Chemetics Inc., New York, for generous gifts of samples of mercaptans.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) ¡9)
(10) (11) (12) (13) (14) (15) (16) (17)
(18) (19) (20) (21) (22) (23) (24)
Sampey, J. R.; Reid, E. E. J. Am. Chem. Soc. 1932, 54, 3404. Martin, J. W. Anal. Chem. 1959, 31, 921. Johnson, J. B.; Fletcher, J. P. Anal. Chem. 1949, 21, 1563. Marquardt, R. P.; Luce, E. N. Anal. Chem. 1949, 21, 1194. Marquardt, R. P.; Luce, E. N. Anal. Chem. 1950, 22, 363. Jay, R. R. Anal. Chem. 1953, 25, 288. Tlwari, K. K.; Verma, R. M. Talanta 1981, 28, 397. Fritz, J. S.; Llsickl, N. M. Anal. Chem. 1951, 23, 589. Malmstadt, A. V.; Vassallo, D. A. Anal. Chem. 1959, 31, 862. Verma, K. K. Talanta 1975, 22, 920. Ashworth, M. R. F. “The Determination of Sulphur-containing Groups”; Academic Press: London, 1976; Vol. II, p 56. Ryland, L. B.; Tamele, M. W. "The Analytical Chemistry of Sulfur and Its Compounds"; Karchmer, J. H., Ed.; Wlley-Intersclence: New York, 1970; Part I, p 481. Danehy, J. P.; Oester, . Y. J. Org. Chem. 1967, 32, 1491. Verma, K. K. Talanta 1979, 26, 277. Kolthoff, I. M.; Harris, W. E. Anal. Chem. 1949, 21, 963. Slggla, S.; Hanna, J. G. "Quantitative Organic Analysis via Functional Groups"; Wlley-Intersclence: New York, 1979; p 725. Kimball, J. W.; Kramer, R. L; Reid, E. E. J. Am. Chem. Soc. 1921, 43, 1199. Bose, S.; Sahasrabuddhey, . P.; Verma, K. K. Talanta 1976, 23, 725. Verma, K. K.; Bose, S. Analyst (London) 1975, 100, 366. Verma, K. K.; Bose, S. Anal. Chlm. Acta 1973, 70, 227. Srlvastava, A.; Bose, S. J. Indian Chem. Soc. 1974, 51, 736. Verma, K. K.; Bose, S. J. Indian Chem. Soc. 1974, 51, 927. Verma, K. K. Chem. Anal. (Warsaw) 1979, 24, 671. Verma, K. K.; Ahmed, J.; Sahasrabuddhey, . P.; Bose, S. J. Indian Chem. Soc. 1977, 54, 699.
Received for review July 2,1982. Accepted August 16,1982. A.K.G. is grateful to the Council of Scientific and Industrial Research (New Delhi) for a Senior Research Fellowship.
