Inaccuracies in Determination of Acidity of Raw Rubber by Water Extraction

Abstract

From the results obtained it is concluded that the boiling water hydrolyzes some of the esters in the acetone extract. In the case of direct extraction of the rubber with water, hydrolysis of esters must take place in the rubber, forming water-soluble acids; or else, although the esters are practically insoluble in the water, it extracts a little ester each time the extraction thimble empties (Bailey-Walker apparatus), carrying the ester down to be hydrolyzed in the boiling water below. Temperature and time of heating play a prominent part. The temperature must be close to the boiling point and the acidity increases progressively with time of heating. The fact that temperature is so important in producing this effect accounts for the erratic values obtained by direct water extraction. On the electric hot plates used water did not always boil steadily, and that contained in the thimble might vary considerably in temperature, much of the time not being hot enough to hydrolyze or extract any ester. Therefore, to avoid any difficulty in the determination of water-soluble acids in rubber, extract first with acetone, then digest this extract with water on a boiling water bath until no increase in acidity of the water extract is obtained. This is a part of the procedure developed at the Netherlands Government Institute by Van Rossem and Dekker.