| http://www.w3.org/ns/prov#value | - es of these salts can also be used.The phosphonocarboxylic acid compounds suitable for producing phosphonocarboxylic acid tin (II) chelates used in the present invention are represented by the following general formula (I): ##STR3## wherein R.sub.1 is --COOM' or --PO(OM).sub.2 ;R.sub.2 is a hydrogen atom, an alkyl group containing 1 to 4 carbon atoms (such as a methyl group, an ethyl group, an n-p
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