Detection of Mercaptans by the Weisz Ring Oven Method M. H. Saxe 64 Williams Road, Lexington, Mass.. 02173
The Weisz ring oven (1) has been found useful in a technique for detecting small quantities of mercaptans in organic solvents. The technique developed proved suitable for detecting very small quantities of octadecylmercaptan, nz-thiotoluol, mercaptobenzothiazole, 4-mercaptopyridine, and 2mercaptoquinoline and permitted their semiquantitative determination. Mercaptans have been determined quantitatively in hydrocarbons by titrations with cupric oleate (Z), by titrations with silver nitrate using sodium nitroprusside (3) as an internal indicator, and by titrations with silver nitrate using electrometric techniques ( 4 ) . Feigl (5) has described a sensitive spot test for mercaptobenzothiazole. As little as 5 pg of this material gave a yellow precipitate when deposited on filter paper and then treated with a freshly prepared aqueous solution of cuprous ion. It appeared reasonable that this technique might be adapted to a semiquantitative method for determining mercapto compounds in organic solvents. A sample of solvent could be applied to a filter paper disk positioned on the ring oven, dried, and developed with a mercaptan solvent such as ethanol. Spraying with cuprous ion solution might be expected to produce a yellow ring. Applications of a series of samples from mercaptan solutions of progressively increasing concentrations would provide a standard series, useful for semiquantitative determinations of unknown samples. The first mercaptan tried was mercaptobenzothiazole. It was found, however, that while spray application of the cuprous ion solution did indeed produce a yellow ring, the sheet changed its appearance a few moments after its preparation: a yellow background developed which soon obscured the sample ring. An aqueous tenth-normal silver nitrate solution was found a usefuI reagent for the determination of octadecylmercaptan, m-mercaptotoluol, mercaptobenzothiazole, 4-mercaptopyridine, and 2-mercaptoquinoline. The silver solution was spray-applied, The results are described below. EXPERIMENTAL Apparatus. The ring oven was made by the National Appliance Co., Portland, Ore. Samples were separated and treated on Whatman No. 44 filter paper. Sample application and development were carried out using Weisz ring oven pipets from Fisher Scientific Co. The development pipet was joined to a small Becton-Dickinson hypodermic syringe with the minimal length of Tygon tubing required. The mercaptobenzothiazole used was MBT-XXX (American Cyanamid), the octadecylmercaptan, Humphrey Chemical (1) H. Weisz, “Micro-analysis by the Ring Oven Technique,” Pergamon Press, Ltd., Oxford, 1961 pp. 17, 69. (2) G. R. Bond, Ind. Eng. Chem., 5,257 (1933). ( 3 ) C . E. Mapstone, J . Pvoc. Australian Chem. Inst., 1948, p. 236.
Siggia, “Quantitative Analysis via Functiodal Groups,” 2nd ed., Wiley, New York, 1954, p. 135. ( 5 ) F. Feigl, “Spot Tests, Volume 11, Organic Applications,” 4th ed., Elsevier, Amsterdam, 1954, p. 167 ff. (4) S.
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ANALYTICAL CHEMISTRY
Company; the 4mercaptopyridine and the 2-mercaptoquinoline, K & K Laboratories; and the rn-toluenethiol, Eastman. Tenth normal silver nitrate solution was sprayed from a DeVilbiss atomizer. Ultraviolet irradiation was provided by a Glo-Craft Model 106 lamp (Switzer Brothers, Cleveland, Ohio.) Procedure. Ring oven operation, sample application, and development were carried out by the method described by Weisz. After the sample had dried, development was carried out by 10 applications of alcohol. When the sample appeared free of alcohol, it was sprayed with 0.1N aqueous silver nitrate until it appeared uniformly moistened. The moist sheet was suspended about 3 inches from the face of the ultraviolet lamp and irradiated for approximately 7 minutes. Standards. A 74-mg sample of MBT-XXX was placed in a 100-ml volumetric flask and diluted to the mark with 23-A denatured ethanol. A series of 5 dilutions decreasing in turn in concentration by a factor of 10 was prepared. A sample of n-octadecylmercaptan, 0.425 gram was placed in a 500-ml volumetric flask and diluted to the mark with odorless mineral spirits (OMS). The first dilution was prepared by taking 10.0 ml of this solution to the mark in a 100-ml volumetric flask with OMS. The second dilution was prepared by taking 1.0 ml of the first dilution to the mark in a 100-ml volumetric flask with OMS. Solutions of 4mercaptopyridine (107 mg) and m-mercaptotoluene (202 mg) and 4-mercaptoquinoline (147 mg) were prepared by taking the weighed sample to the mark in a 100-ml volumetric flask with 23-A ethanol. Dilutions were prepared from each solution by diluting 1.0 ml of stock solution to the mark in a 100-ml volumetric flask with 23-A ethanol. RESULTS
Irradiation of the silver treated sample sheets produced a darkened sheet bearing a yellow ring for each of the compounds tested. The inner diameter of the yellow rings appeared independent of mercaptan concentration within the ranges tested. The width of the light ring was related to concentration, affording a means of semiquantitative analysis. A 1.5-plsample of 7.4 X 10-5 gram of MBT per 100 ml of ethanol yielded a clearly detectable ring. This corresponds to this method’s being capable of detecting 10-3 pg of MBT. This method has been found capable of detecting about 10- 3 pg of n-octadecylmercaptan, m-toluenethiol, 4-mercaptopyridine, and 2-niercaptoquinoline, Each of the mercaptans used in these studies has a relatively low volatility. The lower mercaptans are much more volatile. Increasing volatility of the sample would be expected to lessen the sensitivity of the test. The bonding of mercaptan to the test disk could tend to lessen sample loss during the determination. Studies with these more volatile mercaptans have not yet been carried out.
RECEIVED for review July 12, 1967. Accepted August 14, 1967.
