30 G. J. MISRA AND J. P. TANDON

Potentiometrie Determination of some Thio-compounds using Copper(I) Chloride as the Titrimetric Reagent

G. J. M isr a and J. P. T a n d o n Department of Chemistry, University of Rajasthan, Jaipur (India)

(Z. Naturforsch. 25 b, 30—31 [1970] ; eingegangen am 3. September 1969) Copper (I) chloride has been used for the potentiometric determination of sulphur compounds, namely, n-dodecane-, toluene-a-thiol, , monothioglycerol, 2-mercaptoethanol and ß-mer- captopropionic, thioglycollic, thiobenzoic and thiosalicylic acids. The determinations are possible even in the presence of their corresponding oxygen derivatives and results within an accuracy of ± 0.4% have been obtained.

The thiol group (-SH ) is mainly characterised by Experimental its ease of salt formation (mercaptides) and its ready oxidation by mild oxidising agents to the Reagents: 0.5 N copper (I) chloride solution was prepared by dissolving the required amount of pure corresponding disulphides. Methods, based upon the copper (I) chloride in deoxygenated solution of 1.0 N above mentioned properties, have been developed for potassium chloride. It was stored and standardised as the quantitative determination of the sulfur func­ described in an earlier communication 12. tions. In a number of titrimetric procedures such as Aqueous solutions of 2-mercaptoethanol, monothio­ already described in the literature, iodine1, bro­ glycerol and thioglycollic acid in deoxygenated water mine 2, bromate 3, iodate 4, lead tetraacetate 5 and and ethanolic solutions of n-dodecanethiol, toluene-a- thiol, thiophenol, /?-mercapto-propionic, thiobenzoic and mercury (II) 6 and silver (I)7, 8 compounds have been thiosalicylic acids in absolute alcohol were prepared employed as the titrimetric reagents. B o n d s 9 w as and standardised iodometrically. The required volume the first to report the use of organic derivatives of was then diluted to 50 ml to make 0.025 N solution of copper for the visual determination of mercaptans. the thio-compounds. As these compounds undergo T u r k and R e id 10 suggested the use of copper alkyl- aerial oxidation, all the solutions were prepared, stored and used under an inert atmosphere of nitrogen. phthalate as a reagent for such titrations. S a n t and SANT 11 described a procedure for the determination A pparatus: Potentiometric titrations were carried out in a tall beaker fitted with a five-bored cork (two of thioglycollic acid with copper(II) solution at a for the electrodes, one for the tip of the burette and pH of 4 — 4.4. the remaining two as inlet and outlet for purified nitro­ In the present investigations, copper (I) chloride gen) , using platinum and saturated calomel electrodes. solution has been employed for the first time as a The e.m.f. was measured with the help of Cambridge bench type pH-meter. titrimetric reagent for the potentiometric determina­ tion of sulphur compounds such as n-dodecane-thiol, Procedure: 8 or 10 ml of the solution of thio-com- pound was taken in a titration vessel and platinum and toluene-a-thiol, thiophenol, monothioglycerol, 2- saturated calomel electrodes were put in. Purified nitro­ mercapto- and /?-mercaptopropionic, thiogly­ gen was bubbled for about 10 minutes to deaerate the collic, thiobenzoic and thiosalicylic acids. The deter­ solution and then copper (I) chloride solution wras minations are also possible in presence of their cor­ added in small increments. The e.m.f. was recorded responding oxygen derivatives. The equivalence after each addition and an atmosphere of nitrogen was maintained throughout the titration. However, in the point is marked by a sharp inflection in all the cases case of monothioglycerol and thiosalicylic acid, 10 ml and results within an error of have been i 0.4% of 0.2 N sodium acetate was also added in the begin­ obtained. ning of the titration.

1 A. K e k u l e and E. L in n e m a n n , L ie b ig s Ann. C h em . 123, 7 E. M . C a r r and M . M i t -A l k h o s , A n a ly t. Biochem. [New 277 [1862]. York] 1, 158 [I960]. 2 J. W. S e a s e , C. N ie m a n n , and E. M. S w if t , Analytic. 8 S. Rosenberg, J. A. P e r r o n e , and P . L. K ir k , Analyt. Chem. 19, 197 [1947]. Chem. 22. 1186 [1950]. 3 R. W il l e m a r t and P. F a b r e . Ann. Pharm. France 16, 676 9 G. R . B o n d , Ind. Engng. Chem., anal. Edit. 25, 257 [1953]. [1958]. 10 E. T u r k and E. E. R e i d . Ind. Engng. Chem., anal. Edit. 17, 4 K . J. S t e e l , J. Pharmacy Pharmacol. 10, 574 [1958]. 713 [1945]. 5 L . S u c h o m e l o v a and J. Z y k a , J. electroanalyt. Chem. 11 S . B. S a n t and B. R . S a n t , Analtic. Chem. 31, 1 8 9 9 [Amsterdam] 5,57 [1963]. [1959], 6 D. C. G r e g g y . P. E. B o n n f f a r d , and R. B a r t o n , Ana­ 12 G. J. M is r a and J. P . T a n d o n , Z. a n a lv tic . Chem. 214, 9 4 lytic. Chem. 33, 269 [1961]. [1965],

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Results and Discussion

Some typical results obtained have been shown in Table 1. The results agree fairly well with those obtained iodometrically. Approximately 14 —45 mg of the thiocompounds have been determined within

Thio-compound Volume of Volume of Weight of thio- thio- copper (I) compound [mg] compound chloride ------solution required Found Present taken (obtained [ml] from graph) [ml] n-dodecanethiol 8 3.99 40.4 40.5 n-dodecanethiol 10 4.99 50.5 50.6 Toluene-a-thiol 8 4.02 24.9 24.8 Toluene-a-thiol 10 5.01 31.1 31.1 Thiophenol 8 4.01 22.1 22.0 Thiophenol 10 5.00 27.5 27.5 2-mercaptoethanol 8 3.99 15.6 15.6 2-mercaptoethanol 10 4.99 19.5 19.5 * Mono-thiogly­ Fig. 1. Potentiometric titration of dodecane-thiol (8 ml of cerol 8 4.00 21.6 21.6 0.025 n ; Curve 1), thiophenol (8 ml of 0.025 n ; Curve 2), * Mono-thiogly­ toluene-a-thiol (8 ml of 0.025 n ; Curve 3), 2-mercaptoethanol cerol 10 5.01 27.1 27.0 (10 ml of 0.025 n ; Curve 4), mono-thioglycerol (10 ml of Thioglycollic acid 8 4.01 18.5 18.4 0.025 n ; Curve 5) and mono-thioglycerol in presence of so­ Thioglycollic acid 10 5.02 23.1 23.0 dium acetate (10 ml of 0.025 n ; Curve 6), with copper (I) /?-mercaptopro- chloride (0.05 n ) . pionic acid 8 4.00 21.2 21.2 /9-mercaptopro- pionic acid 10 5.01 26.6 26.5 Thiobenzoic acid 8 4.02 27.8 27.7 Thiobenzoic acid 10 5.02 34.7 34.6 * Thiosalicylic acid 8 3.99 30.8 30.8 * Thiosalicylic acid 10 5.02 38.7 38.5

* Titrations carried out in presence of 10 m l of 0.2 N sodium acetate. Table I. Potentiometric determination of some thio-com- pounds. an accuracy of + 0.4 per cent. The potential rise at the equivalence point is quite pronounced in all the cases (Figs. 1 and 2) except thioglycerol and thiosalicylic acid (Figs. 1 and 2 curves 5 and 4 respectively). The titrations in these cases could, however, be carried out successfully in presence of sufficient amount of sodium acetate (Figs. 1 and 2, curves 6 and 5 respectively). In all the cases, inflec­ tion occurs just after one mole of copper (I) chloride has been added. The utility of the proposed method of 0.025 n ; Curve 1), /?-mercaptopropionic acid (10 ml of becomes all the more as the corresponding oxygen 0.025 n ; Curve 2), thiobenzoic acid (8 ml of 0.025 n ; Curve derivatives, which are generally obtained as impuri­ 3), thiosalicylic acid (8 ml of 0.025 n ; Curve 4) and thio­ in presence of sodium acetate (8 ml of 0.025 N; ties in these thio-compounds, do not interfere. Curve 5), with copper (I) chloride (0.05 n ) .

The authors are thankful to Professor R. C. M e h r o t r a , viding all the laboratory facilities and to the C.S.I.R. Head of the Chemistry Department, University of for a fellowship to one of them (G.J.M.). Rajasthan, Jaipur, for helpful suggestions and pro-