glass co tokyo japan but other commercially available cation selective membranes can be used in order to minimize the fouling of cation selective membranes by multivalent cations monovalent cation selective membranes e g selemion csv asahi glass co or neosepta cms trademark tokuyama soda co tokyo japan can be used these are prepared by synthesizing a thin positively charged layer on their surface t yawataya dechema monogr 47 501 514 1962 t sata et al j membr sci 45 197 208 1989 and t sata and r izuo j membr sci 45 209 224 1989 the anion selective membranes useful in the process of the present invention are strongly basic anion selective membranes such as neosepta am 1 trademark tokuyama soda and in particular monovalent anion selective membranes such as the selemion asv membrane trademark asahi glass co and the neosepta acs membranes trademark tokuyama soda co the selemion asv membrane is composed of a polystyrene matrix with quaternary ammonium groups which provide it with its anion selective properties in addition this membrane incorporates a thin highly cross linked surface layer composed of both strongly nr 3 and weakly nr 2 basic anion exchange groups the thickness of the highly cross linked layer is optimized so as to reduce divalent ion transport without unacceptably increasing the electrical resistance of the membranes under neutral or mild ph conditions monovalent anions such as cl are preferentially passed through the.