The spontaneous decomposition of lecithin and its bearing on the determination of the isoelectric point

Abstract

Electrotitrations of lecithin in 90% alcohol and indicator titrations in benzene-alcohol (19/1) demonstrate a spontaneous decomposition (rapid when the substance is kept exposed to the air, less rapid when kept in a desiccator) consisting of the formation of an alkali-titrable group, and due to the liberation of fatty acids, which were separated. No choline separation could be detected. The decomposition is likely to cause large errors in the determination of the isoelectric point, an acid being formed. To find the probable magnitude of the isoelectric point, the dissociation constant of choline was detd. approximately and found to be that of a very strong base (pK ca. 14 referred to pH). From this value and the dissociation constant of phosphoric esters (ca. 1), the isoelectric point of lecithin should be expected at pH 7.5. The more acid values generally found are due to decomposition.