Determination of the Molar Substitution of Cellulose Ethers Using Proton Magnetic Resonance Colin J. Clemett
Unilever Research, Port Sunlight Laboratory, Wirral, Cheshire, L62 4XN, United Kingdom CELLULOSE ETHERS containing methyl, ethyl, hydroxyethyl, and hydroxypropyl groups are widely used in industry, and it is necessary to be able to determine the number of moles of substituent per anhydroglucose unit, Le., the molar substitution (MS). A number of methods are available for doing this. These include the Zeisel method ( I ) and its combinations with GLC(2-4), and oxidation coupled with titration ( 5 ) or GLC (6). Recently (7), PMR has been exploited to provide a measure both of the MS and the degree of substitution (Le., the average number of cellulose hydroxyls substituted) of hydroxypropyl cellulose. However, none of these methods is general in that it will deal with all the groups mentioned above, and none will deal with the case of mixed ethers, such as ethylhydroxyethyl or methyl-hydroxypropyl celluloses, which are frequently encountered. We have found that a useful measure of MS can be obtained from the PMR spectrum of the cellulose ether after hydrolysis with concentrated (-36%) hydrochloric acid. The hydrolysis is necessary for two reasons, first to “depolymerize” the cellulose and so sharpen the spectral bands, thus obviating overlap of bands whose intensities must be accurately measured, and second to provide a solution of sufficient concentration to yield a useful spectrum. The method is fairly rapid (
