[Analytical study of anticholinergics from the group of carboxylic acid esters. VI. Study of the stability of the chloride of the beta-diethylaminoethylester of diphenylhydroxyacetic acid (benactyzine), of the chloride of the beta-diethylaminoethylester of the phenyl-n-propylacetic acid (prospasmine), of the 2-(phenylcyclohexylhydroxyacetoxy)-ethyl-dimethyldulfonium iodide (hydroxythiospasmine) and of the 2-(phenylcyclohexylacetoxy)-ethyldimethylsulfonium iodide (thiospasmine)]