The Determination of Copper in Crude Rubber

Abstract

Abstract All types of rubber contain some copper. Although the proportion is in general extremely low, it is well not to disregard it, for this element acts as a catalyst which has a deleterious effect on the aging of rubber. For rubber to age satisfactorily it should contain not more than 1.5 mg. of copper per 100 grams of rubber; hence the interest in any method for determining the copper content of crude rubber. PRESENT METHOD The method used thus far is based on the color reaction which sodium diethyldithiocarbamate gives with a very dilute copper solution. According to the method of the British Standards Institution, the ash is treated with hydrochloric acid, and the metals of the iron group are then precipitated by ammonia. After solution of the precipitate in hydrochloric acid and reprecipitation by ammonia, the filtrate is brought to the boiling point and finally is diluted to a known volume. This solution is then used for a colorimetric determination. As a result of numerous experiments with crude rubbers from different sources in which the method described above was used, it has been found that the reagent called for in this method is not specific enough; in fact it gives a color reaction with metals other than copper, notably with iron, manganese, zinc and heavy metals which cannot be separated reliably by the ammonia method. There would seem to be two solutions to this problem; either to develop another method which employs a reagent having a more specific action, or to improve the currently used method of the British Standards Association in such a way as to eliminate the disturbing ions.