METAL-ION COMPLEXATION IN AQUEOUS-SOLUTIONS OF 1-THIA-4,7-DIAZACYCLONONANE-N,N'-DIACETIC ACID, 1-THIA-4,8-DIAZACYCLODECANE-N,N'-DIACETIC ACID AND 2,5-DIAZAHEXANE-N,N'-DIACETIC ACID.