Analytical Methods in Rubber Chemistry, Part I. General Analytical Considerations

One of the consequences of the high-molecular character of rubber is the smallness of the effect on ultimate quantitative analysis of the presence of one or even several heteroatoms or groups in the long hydrocarbon chains. With rubber molecules possessing an average chain-length of about 4,500 isoprene units, the association of, say, ten nitrogen atoms wtih each chain would provide a nitrogen content of only about 0.0007 per cent, and the incorporation of about 0.005 per cent of ethereal oxygen in pure rubber hydrocarbon would be theoretically capable of providing sufficient linking material for an infinite degree of hetero-polymerization, or, if the oxygen were applied instead to produce the maximum possible degree of degradation, it would be capable of severing from one-quarter to one-third of the rubber chains. In view of these dimensions, and the fact that natural rubber normally contains difficultly removable hetero components, and is ready to take up additional amounts of oxygen, little progress can be made in the fundamental investigation of the complex molecules without a suitably accurate technique of chemical analysis, and this applies particularly to the determination of total oxygen and total nitrogen, and also to the determination of the six types of combined oxygen likely to be present in rubber, viz., hydroxylic, oxido-, ethereal, peroxidic, carbonyl and carboxylic. Unfortunately, great precision in estimation cannot be achieved without increasing greatly the rigour of the analytical technique, and perhaps it is not surprising that Midgley, Henne and Renoll's well-known high-precision method for determining carbon, hydrogen and, by difference, oxygen, still remains after five years the sole such chemical analytical method reported in the literature.