M ercurimetric Esti mati o n of Thiols, Aryl TrityI Sulfides, and Disulfides DONALD C. GREGG, PAUL E. BOUFFARD, and ROGER BARTON' Deparfment o f Chemistry, The Universify o f Vermont, Burlingfon, Vt.
b Thiols and aryl trityl (triphenylmethyl) sulfides a r e estimated specifically by titration with mercury(l1) ion using s-diphenylcarbazone as t h e indicator. T h e amount of thiol and aryl trityl sulfide in a mixture can be estimated by determining the thiol indirectly. Many disulfides can b e esiimated by quantitative reduction and subsequent titration of the resultant thiols.
T
for the quantitative estimation of sulfides and disulfides reported by Siggia and Edsberg ( l a ) has been modified to estimate aryl trityl (triphenylmethyl) sulfides and benzhydryl (diphenylmethyl) sulfides (6, 8). The trityl and benzhydryl sulfides, however, consume 3 moles of bromine per mole of sulfide, whereas ordinary sulfides consume 1 mole of bromine per mole. Since thiols, disulfides, and benzhydryl sulfides interfere in the estimation of aryl trityl sulfides, a specific determination of the latter is desirable. Aryl trityl and benzhydryl sulfides are interesting academically because of their unique chemical behavior in contrast with most ordinary sulfides. So far as is known, they do not occur naturally. but some may be potential fungicides and pesticides. The methods described by Clarke ( I ) , Domask and Kobe (a), and Dubsky and Trtilek (3) for the determination of chloride ion are now used as the basis of a procedure for the quantitative estimation of thiols and aryl trityl sulfides. These compounds are titrated with mercury(I1) ion using s-diphenylcarbazoiie as the indicator. Disulfides, nontrityl sulfides, sulfox+des, and sulfones do not interfere. The method is sufficiently accurate t o be useful. If a thiol and an aryl trityl sulfide are together in a mixture, the thiol can be estimated indirectly. Primary and secondary disulfides can be estimated with reasonable accuracy, but disulfides that yield volatile thiols on reduction may lead to low results. As in the case of the HE PROCEDURE
1
Present address, Orleans High School,
Orleans, Vt.
methods reported by Karchmer and Walker (10) and by Hubbard, Haines, and Ball (Q),the method reported here is not applicable to tertiary disulfides. REAGENTS
Organic Sulfur Compounds. Each aryl trityl sulfide was prepared by using triphenylmethanol and the appropriate arylthiol according to methods described by Finzi and Bellavita (4)and Gregg, Iddles, and Stearns (7). The thiols were obtained from Distillation Products Industries and were redistilled before use. All other organic reagents were either obtained from Distillation Products Industries or synthesized by using established methods. The solutions of the sulfur compounds used in the titrations were usually 0.01M in 95% ethyl alcohol. s-Diphenylcarbazone. The indicator was obtained from Retort Pharmaceutical Co. and was used as received, The indicator solution used in the titrations was made by dissolving 1.0 gram of the indicator in 100 ml. of 95% ethyl alcohol. Because of possible deterioration, the indicator solution should be freshly prepared using a good quality solvent. Mercury(I1) Nitrate Standard Solution. T h e standard mercury(I1) nitrate solution mas prepared by dissolving mercury(I1) nitrate hexahydrate (Tested Purity Reagent, from Eimer and Amend) in distilled water. Concentrated nitric acid, 2 ml. per liter of solution, was added t o prevent hydrolysis of mercury(I1) ions. The solution was standardized by using either purified sodium chloride or a standard solution of hydrochloric acid. Mercury(I1) Acetate Standard Solution. The C.P. reagent grade mercury(I1) acetate was recrystallized from glacial acetic acid and dissolved in 5070 ethyl alcohol to yield a O.OO5M solution. Glacial acetic acid was added to adjust the solution to p H 3. The solution was standardized using purified sodium chloride. PROCEDURES
General Procedure. To 20 to 25 ml. of an ethanolic solution containing 0.0001 mole (0.1 mmole) of the organic compound(s) is added 3 drops of t h e diphenylcarbazone indicator solution. T h e solution is titrated with the standard mercury(I1) ion
solution to the initial appearance of a blue-violet coloration. If the titration occurs a t room temperature, the mercury arylmercaptides may precipitate near the end of the titrations of thiols and trityl sulfides. This precipitation does not seriously hamper detection of the end point. If the titration is run a t 40" to 50' C., the precipitation of the mercaptides can be prevented. During the standardization of the mercury(I1) ion solution, a 10-ml. aliquot is treated with 15 ml. of ethyl alcohol before addition of the indicator. If the solution to be titrated has a pH different from pH 3.0 to 3.3, the optimum p H for successful titrations, bromophenol blue (BPB) is added, and the solution adjusted until it is acidic to BPB. The light yellow color of the BPB enhances the detection of the diphenylcarbazone coloration a t the end point. Although titration with mercury(I1) acetate solution appears satisfactory, the use of mercury(I1) nitrate solution is recommended. Halide and sulfide ions should be absent from all solutions because they may interfere. For Thiols. If the sample being titrated contains onlv a thiol or a mixture of thiols, the"tota1 number of millimoles of thiol present is twice the number of millimoles of mercury (11) ions added. Ordinary disulfides, sulfoxides, sulfones, and all nontrityl sulfides do not interfere. Hence, thiols can be estimated directly in any mixture of these compounds. Since aryl trityl sulfides do interfere, a thiol cannot be estimated directly if a trityl sulfide is present. For Aryl Trityl Sulfides. If t h e sample contains only a n aryl trityl sulfide or is a mixture of sulfides and disulfides, the total number of millimoles of aryl trityl sulfide present is twice the number of millimoles of mercury(I1) ion added. Ordinary disulfides, sulfoxides, sulfones, and all nontrityl sulfides do not interfere. Since thiols do interfere, an aryl trityl sulfide cannot be estimated directly if a thiol is present. For a Mixture Containing a Thiol and an Aryl Trityl Sullide. If the sample contains a thiol and an aryl trityl sulfide, each can be estimated. A portion of the sample containing mole of compounds is about 1 X titrated with mercury(l1) nitrate solution. The total number of millimoles of thiol and tritgl sulfide present is VOL. 33, NO. 2, FEBRUARY 1961
269
Table
Substance(s) o-Toluenethiol Benzenethiol 1-Butanethiol o-Tolvl tritvl sulfide Phenyl tritGl sulfide Benzenethiol phenyl trityl sulfide Benxenethiol diphenyl disulfide Phenyl sulfide Dhenvl tritvl sulfide Bgnzenkthiol"+ n-butyl sulfide 1-Butanethiol n-butyl sulfide n-Butyl trityl sulfide Diphenyl disulfide Di-n-butyl disulfide Diphenyl sulfoxide m-Tolyl benzhydryl sulfone
+ + +
~
~~
+
I.
Reproducibility of Titrations
Present, Mmole
8.70 17.25 8.60 8.50 6.35
8.65 17.10 8.55 8.60 6.50
8.80 17.35 8.75 8,70 6.45
8.65 17.30 8.65 8.65 6.46
15.25
14.90
15.10
15.11
0.10
8.55
8.65
8.70
8.65
0.10 0.10 0.10 0.10 0.10 0.15 0.15
8.80
8.73
8.68
8.65
8.60
8.75
8.80
8.65
8.65 0.0 0.1 0.0 0.0 0.0
8.70 0.05 0.05 0.0 0.0 0.0
8.60 0.0 0.07 0.0 0.0 0.0
8.65 0.0 0.0 0.0 0.0 0.0
0.10 0.20
0.10
0.10 0.075 0.10 0,075
0.10 0.10
0.10
0.10 0.10
twice the number of millimoles of mercury(I1) added. Another portion of the sample (in 20 to 25 ml. of ethyl alcohol) identical t o the one titrated directly is treated with 1 ml. of 3M sodium hydroxide and 2.0 ml. of 30% hydrogen peroxide. The mixture is heated t o 65" C. and then let stand on the bench top for 20 minutes. Two drops of 1% bromophenol blue indicator solution are added and the solution is acidified carefully with dilute nitric acid until a yellow coloration is attained. Three drops of diphenylcarbazone indicator solution are added and the solution is titrated with mercury(I1) nitrate or acetate solution. Since only the thiol has been oxidized, the number of millimoles of trityl sulfide is determined directly. By subtracting the number of millimoles of trityl sulfide from the total number of millimoles of thiol and trityl sulfide, the number of millimoles of t.hiol is determined. For a Disulfide. A s a m d e containing approximately 1 x 1 0 - 4 mole of disulfide is dissolved in 30 ml. of ethyl alcohol. To this is added 5 ml. of 1.OM sulfuric acid and about 2 grams of 20-mesh amalgamated zinc. (To prepare the zinc, stir 100 grams of 20mesh granulated zinc in 6 M hydrochloric acid for 1 minute, add 27 mg. of mercury(I1) chloride, stir 1 minute, and wash thoroughly with distilled water.) Heat the mixture to 55' C. and shake thoroughly for 30 minutes. Decant the solution and wash zinc with 5 to 10 ml. of ethyl alcohol. Add 2 drops of bromophenol blue indicator solution and carefully treat the solution until it is just acidic (yellow) to BPB. Since the disulfide has been reduced to thiol, titration with mercury(I1) nitrate solution indicates the number of millimoles of thiol produced. Each millimole of disulfide yields, on reduction, 2 mmoles of thiol. Therefore the number of millimoles of disulfide originally present in the sample is equal to the number of millimoles of mercury(I1) ion added. 270
e
0.0058M Hg(N032, MI. Actually Used Run 1 Run 2 Run 3 Theory
ANALYTICAL CHEMISTRY
All sulfides, sulfoxides, and sulfones do not interfere. If a disulfide is mixed with a thiol or a n aryl trityl sulfide or both of these, a sample of the mixture is titrated with mercury(I1) nitrate solution. This indicates the total number of millimoles of thiol and/or aryl trityl sulfide in the mixture. An identical sample of the mixture is treated to reduce the disulfide. When the resultant solution is titrated with mercury(I1) ions the increase in the number of millimoles of mercury(I1) ion added is equal to the number of millimoles of disulfide in the original sample. For a Mixture Containing Disulfide, Aryl Trityl Sulfide, and Aryl Benzhydry1 Sulfide. Titration of a sample of the mixture by using the method described by Gregg et al. (6, 8) indicates the total number of millimoles of bromine used in the oxidation of the compounds. The oxidation is accomplished by adding bromate and bromide ions t o an acidic solution of the compounds being titrated. Each millimole of sulfide requires 3 mmoles of bromine, whereas each millimole of disulfide requires 5 mmoles of bromine. An identical sample is titrated with mercury(I1) nitrate solution to determine the number of millimoles of trityl sulfide present. The number of millimoles of disulfide in the mixture is determined by treating another identical sample according to the procedure used in estimating disulfides. Since the total number of millimoles of trityl sulfide and disulfide is then known and the total number of millimoles of bromine needed to oxidize them and the benzhydryl sulfide is known also, the number of millimoles of benzhydryl sulfide is obtained by difference. If a mixture consists solely of aryl trityl and benzhydryl sulfides, the total number of millimoles of sulfide can be determined by titration of a sample of the mixture using bromate and bromide ions (6). Titration of another identical
sample with mercury(I1) nitrate solution yields the number of millimoles of trityl sulfide only. The benzhydryl sulfide is determined by difference. DISCUSSION
The results presented in Table I indicate that the maximum deviations from the theoretical volumes of mercury(I1) nitrate solution required in the sample titrations do not exceed 2%. Considering the maximum deviations between the volumes of titrating solution used in three different titrations of a given sample mixture, the methods seem reproducible within =k2.5%. Deviations from the average of three different titrations of a given sample mixture are less than &2.5%. The detection of the titration end point is probably the largest source of error. Roberts (11) reported that only the alkaline form of the indicator sdiphenylcarbazone forms the deep blueviolet complex with mercury(I1) ion. Hence, it is imperative that the acidity of the titration mixtures be controlled at p H 3.0 0.3 as directed in the general procedure. The behavior of thiols under the titration conditions indicates that mercury(I1) ion has a greater affinity for the thiol and its conjugate base than for the s-diphenylcarbazone molecule. For example, the titration of benzenethiol (phenylmercaptan) with either mercury(I1) nitrate or acetate solutions yields mercury phenylmercaptide according t o the reaction
*
2 CGHbSH
+ Hg+*
-+
(C,H,S)&g
+ 2 H+
The mercury phenylmercaptide isolated from a mercurimetric titration of benzenethiol was identical to a n authentic sample. A titration of o-toluenethiol (0-tolylmercaptan) yielded mercury otolylmercaptide. The behavior of the aryl trityl sulfides indicates that mercury(I1) ions cleave the sulfide molecules and displace trityl cations, (CBHJIC+, from the sulfides to yield the mercury mercaptides of the thiol fragments of the sulfide molecules. In the case of phenyl trityl sulfide, the reaction is
+
2 Ce,HsSC(CoHj)3
Hg+* (Cd&S),Hg
+ 2 C(CsHd3'
+
The trityl cations react with water in the titration mixtures to yield triphenylmethanol. Mercury phenylmercaptide !%-as isolated after titrating phenyl trityl sulfide with mercury(I1) nitrate solution. Earlier work ( 7 ) has shown that arylthiols and aryl trityl sulfides compete favorably with chloride ion for mercury(I1) ion and that aryl trityl sulfides cleave in the presence of mer-
Table II. Estimation of Aryl Trifyl Sulfide in Mixtures Containing Benzenethiol
Composition of Original Mixture, Sfmoles Phenyl Phenyl bensBenzene- trityl hydryl thiol= sulfide sulfide n 10 0 1.5 0 0
a 10 0 10 0 05 0 05
0 0 0 0
075 0375 075 15
0 1 0 1 0 0 0.1
Trityl Sulfide Titrated, Rlmoles 0 0 0 0 0
146 074 0373 073 148
Destroyed by oxidation before the aryl trityl sulfide Tas titrated with mercury( 11) acetate solution. 5
cury(I1) chloride more readily than do the aryl benzhydryl sulfides (8). The results shown in Table I1 indicate that it is possible to estimate
indirectly within =t2% the amount of benzenethiol in a mixture that also contains phenyl trityl sulfide. Earlier work (6) has shown that aryl trityl sulfides are not easily oxidized by hydrogen peroxide in an alkaline medium. Hence, the thiol in a mixture can be removed by oxidation leaving the trityl sulfide unchanged. Results using mixtures of either o-toluenethiol or 1-butanethiol and a t least two different aryl trityl sulfides were comparable t o those reported in Table 11. Determinations of diphenyl disulfide, di-p-tolyl disulfide, and di-n-butyl disulfide alone and in mixtures appeared to be as accurate as those of the related thiols. If samples were sufficiently pure, the mercurimetric method might be useful in estimating molecular weights. ACKNOWLEDGMENl
Roger Barton and Donald Gregg are indebted to the National Science Foundation for assistance during the 1960
Summer Research Participation Program. LITERATURE CITED
(1) . , Clarke. F. E.. AXAL.CHEM.22, 553-5, 1458 (1950). ’ (2) Domask, W. G., Kobe, K. A., Ibid., 24,989 (1952). (3) Dubsky, J. V., Trtilek, J., Mikrochcmae 12,315 (1933). (4) Finzi, C., Bellavita, V., Gazz. chim. ital. 62, 699 (1932). (5) Gregg, D. C., The University of
Vermont, Burlington, Vt., unpublished data, (1953). (6) Gregg, D. C., Blood, C. A., J . Org.
Chem. 16, 1255 (1951). (7) Gregg, D. C., Iddles, H.’ A., Stearns, P. W., Jr., Ibid., p. 246. (8) Gregg, D. C., Vartuli, F., Risner, J. W., Jr., J . Am. Chem. SOC.77, 6660 (1955). (9) Hubbard, R. L., Haines, W. E., Ball, J. S.,ANAL.CHBIM. 30, 91 (1958). (10) Karchmer, J. H., Walker, M. T., Ibid., 30,85 (1958). (11) Roberts, I., IND. ENG.CHEM.,ANAL. ED. 8, 365 (1936). (12) Siggia, S., Edsberg, R., ANAL. CHERI.20,938 (1948).
RECEIVEDfor review August 22, 1960. Accepted September 29, 1960.
Gradient Elution in the Separation of Chlorophenols by ion Exchange NORMAN E. SKELLY Special Services laboratory, The Dow Chemical Co., Midland, Mich.
b A method has been developed for the separation of the isomers of tetrachlorophenol from each other and from pentachlorophenol. Milligram quantities of the chlorophenols are adsorbed on a strongly basic anion exchange resin in the acetate form. By means of a gradient elution with acetic acid in methanol the compounds are selectively and quantitatively desorbed. Concentrations of the chlorophenols in the eluate are determined b y ultraviolet spectrophotometry. This procedure has been applied to the analysis of commercial grade pentachlorophenol. A method for the determination of 2,4-dichlorophenol is also described.
T
of ion exchange chromatography has found wide application in the last few years. Until recently most separations were made using aqueous solvent systems. It has been demonstrated (2, 3, 5 ) that the separation of chlorophenols and other weak acids may be accomplished on strongly basic ion exchange resins using hydrochloric or acetic acid in alcohols as the eluent. Seki (4) showed t h a t substituted phenols HE USE
could be separated on cation exchange resins. With the introduction of a gradient elution (1) technique the method developed by Logie (2) for the analytical separation of the isomers of di- and trichlorophenols has been improved and extended to cover the analysis of mixtures containing pentachlorophenol and the isomers of tetrachlorophenol. The application of the gradient elution has the following advantages over the fixed-concentration elution. I t reduces the experimentation required for the separation of acids having extremely close pK values. Mixtures of
7
11
4
Figure 1
Gradient elution equipment
unknown composition are readily separated. The problem of tailing, whichis a difficulty in the separation of tetrachlorophenol and pentachlorophenol, is eliminated. The complete analysis of a chlorophenol mixture can be made on one ion exchange column. Separations not possible by fixed-concentration elution can be made using a gradient elution. The nonaqueous ion exchange method by Logie (2) coupled with the gradient elution has found a-ide application in our laboratory. EXPERIMENTAL
Apparatus. Chromatography tubes 13 X 200 mm., with a 2-mm. stopcock a t the bottom and a 28/15 socket joint a t the top. Chromatography reservoir, a 28/15 ball joint was attached to a 250-ml. mercury leveling bulb. Gradient elution equipment, Figure 1. A 250-ml. round-bottomed flask was used for the mixing flask and a 1or 2-liter flask was used for the reservoir. Recording ultraviolet absorption meter, fraction collector, and recorder for measuring eluate of chromatography columns, Gilson Medical Electronics, Middleton, Wis. VOL. 33, NO. 2, FEBRUARY 1961
271
