METAL CHELATE FORMATION BY<i>N</i>-PHOSPHONOMETHYLGLYCINE AND RELATED LIGANDS

Abstract

Abstract Thirteen selected divalent and trivalent metal ions were equilibrated with N-phosphonomethylglycine(PMG) at 1:1 and 1:2 molar ratios of metal ion to ligand at 25.0° and ionic strength μ=0.100 M. Similarly, the same metal ions were also studied with iminobismethylenephosphonic acid (IDP), which has not been previously investigated, as well as the well known ligand, iminodiacetic acid (IDA) (to obtain equilibrium data for the latter not available in the literature). The complexes formed were studied as a function of −log[H+]. All of the appropriate equilibrium stability constants were calculated with the aid of the interactive computer program BEST. The constants obtained generally reproduce the entire experimental potentiometric equilibrium curves to within 0.01 log[H+] units. The protonation constants 10.142, 5.460, 2.229 reproduce the potentiometric equilibrium curve of free PMG to within a σ value of 0.004 log[H+] units while 10.79.6.08, 5.04 and 0.86 are the protonation constants for IDP.