The Determination of Free Sulfur in Soft Vulcanized Rubber by a Volumetric Method

Abstract

Abstract The methods so far proposed for the determination of the free sulfur in vulcanized rubber depend upon the removal of the sulfur by extraction of the rubber with hot acetone, and subsequent oxidation to sulfuric acid of the sulfur in the extract. The extract is liable however to contain, in addition to sulfur in the elementary state, organic compounds containing sulfur derived from the rubber resins, accelerator, or antioxidant. In the subsequent oxidation this sulfur will be oxidized to sulfuric acid to an extent which depends on the method of oxidation used. In the present work, a method of analysis has been developed which is more rapid than the existing methods, and in which it is considered that the determination of the sulfur present in the elementary state is less likely to be affected by organic compounds containing sulfur. It has been found that when vulcanized rubber placed in contact with a metal, such as tin or aluminum, is boiled in hydrochloric acid, hydrogen sulfide is generated by the action of the nascent hydrogen produced. This reaction occurred with rubber which had been previously treated with acid alone to decompose metallic sulfides, but no hydrogen sulfide was obtained from rubber which had been previously extracted with acetone and was free from metallic sulfides. As these results showed that the reactions were not due to the presence of mineral sulfides, or of sulfur combined with the rubber, it was concluded that the hydrogen sulfide was formed by the reduction of the free sulfur.