Abstract
The reducing effect of compounds containing the sulphydryl group has been tested with sodium selenite and with sodium tellurite as indicators. The following test solutions were prepared: Reagent (1), an acid reagent made up of 100 cc. of 2% sodium selenite or sodium tellurite plus 20 cc. concentrated hydrochloric acid; reagent (2), 2% sodium selenite or sodium tellurite solution, which is slightly alkaline; reagent (3), a more alkaline reagent containing 2% sodium selenite or sodium tellurite in 10% sodium carbonate. The compounds studied were the thio compounds, ethyl mercaptan, thioacetic acid, thioglycollic acid, thiocresol, thiourea, glutathione, cysteine hydrochloride, and the sulphur containing compounds not possessing the SH group—cystine, and thiophene.
On reduction sodium selenite yields free selenium in the form of a brick red precipitate or a red to brick red colloidal solution depending upon the intensity of the reaction and the quantity of the free element liberated. Sodium tellurite under similar conditions yields a brownish black precipitate of free tellurium or a black brown colloidal solution of this element. Compounds that reduce in the cold give more profuse reduction under the application of heat. The test is carried out by allowing 3 cc. of the compound in aqueous solution to interact with 3 cc. of the selenite or tellurite reagent. The reduction of sodium selenite or tellurite by the compounds is of special value, since the reaction is irreversible and the reduced selenium or tellurium does not display a tendency to undergo re-oxidation.
All the thio compounds examined with the exception of thiourea have very marked reducing capacity. The limits of sensitivity are given in Table I.
The acid selenite reagent is not reduced in the cold by any of the thio compounds. The acid tellurite reagent is reduced in the cold by ethyl mercaptan thioacetic acid and thiocresol, but not by thioglycollic acid, glutathione, and cysteine. On heating, however, all the thio compounds reduce the acid selenite and the acid tellurite reagents. All the thio compounds reduce at room temperature and at the temperature of the water-bath the slightly more alkaline selenite and tellurite reagents.
Reduction in the cold takes place after a slight lapse of time. Two minutes we found the maximum time necessary for a precipitate to form. As the concentration of the thio compound is lowered a reddish colloidal solution forms but the time of its appearance is prolonged. A series of reaction mixtures may be made with different concentrations of thio compound as a classroom demonstration of reaction time.
The thio compounds are the only organic compounds that we have thus far studied with the exception of pyruvic acid, glucoreductone, and ascorbic acid that have such marked reducing properties that they reduce even at room temperature. Pyruvic acid reduces in the cold the 2% sodium selenite or sodium tellurite reagent containing 10% sodium carbonate. On the application of heat pyruvic acid reduces all the other selenite reagents and the acidified tellurite reagent. Pyruvic acid may be differentiated from the thio compounds by failure to reduce in the cold the slightly alkaline or the acidified selenite solution.
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