PURPOSE: To improve current efficiency in the electrolysis of alkali metal halides, by a method wherein a sulfonyl halide group is substituted for sulfonic acid group in a fluorine-contg. ion-exchange membrane, the resulting ion-exchange membrane is treated with a tetra-substd. thiourea and then hydrolyzed.
CONSTITUTION: A sulfonyl halide group is substituted for sulfonic acid acid group in a fluorine-contg. ion-exchange membrane having sulfonic acid group as an ion- exchange group. The resulting ion-exchange membrane is treated with a tetra- substd. thiourea devivative, and then hydrolyzed to substitute a carboxyl group for the sulfonyl halide group. As said tetra-substd. thiourea devivatives, the compds. of the formula R 1 R 2 NC(=S)NR 3 R 4 (wherein R 1 , R 2 , R 3 , R 4 are each alkyl, aryl, aralkyl, acyl) are preferred. Among these compds, tetramethylthiourea is particulrly preferred. Preferred examples of other tetra-substd. thiourea derivatives are compds. of the formula R 1 SC(=NR 2 )NR 3 R 4 . Among these compds., S-methyl- N-phenyl-N', N'-dimethylisothiourea is particularly preferred.