Determination of Total Sulfur in Rubber

With the recent increase in research on natural and synthetic rubber, there has arisen a need for a rapid method for the determination of total sulfur in rubber samples where only a semimicrosample can be spared for analysis. The usual methods in which the rubber is oxidized by digestion with acid, followed in some cases by fusion to complete oxidation, and the sulfur is precipitated and weighed as barium sulfate, yield good results on semimicro-samples, but are time-consuming, because quantitative precipitation of the sulfate cannot be attained unless the solution is allowed to stand overnight before filtration. Several volumetric methods, most of which depend on the precipitation of sulfur as barium sulfate, have been described, but none is very satisfactory in the determination of small amounts of sulfur. St. Lorant has proposed a volumetric method for the determination of microsamples of sulfate, in which the sulfate is reduced to sulfide with hydriodic acid, and the hydrogen sulfide is distilled off and determined colorimetrically. The author has recently developed a modification of this method which has proved to be very satisfactory for the analysis of sulfur in small samples of rubber. The procedure consists essentially of oxidation of the rubber in the usual manner, reduction of the sulfate to sulfide, separation of the hydrogen sulfide by distillation, and titration of the latter iodometrically. The new method is very rapid (20 to 25 minutes for the distillation and titration), and is not subject to the errors of adsorption, occlusion, and solubility encountered in the gravimetric barium sulfate method, or to difficulties in detection of the end-point encountered in some volumetric methods. It possesses the distinct advantage over some of the gravimetric methods that it is applicable to the analysis of rubber containing barium, lead, and calcium. The only serious objections to the method from the standpoint of general use are that hydriodic acid is expensive and that the applicability of the method is somewhat limited because of the fact that no more than about 5 mg. of sulfur can be handled conveniently. This means that the sample size must be small when analyzing rubber high in sulfur; and when small samples are used there is always the danger of error due to unequal distribution of sulfur in the sample.