Nitrilotris(methylenephosphonates) in aqueous solution and solid state – dilatometric, potentiometric and NMR investigations

Dissociation and alkali complex formation equilibria of nitrilotris(methylenephosphonic acid) (NTMP, H6L) have been studied by dilatometric, potentiometric and 31P NMR-controlled titrations. Dilatometry indicated the formation of alkali complexes ML (M=Li, Na, K, Rb, Cs) at high pH with a stability decreasing from Li to Cs. An efficient combination of potentiometric and NMR methods confirmed two types of alkali metal complexes MHL and ML. Stability constants for the equilibria following M+ + HL5− ⇌ MHL4− and M+ + L6− ⇌ ML5−, respectively, were determined: logKNaHL=1.08(0.07), logKKHL=0.86(0.08), logKNaL=2.24(0.03). Systematic errors are introduced by using alkali metal hydroxides as titrants for routine potentiometric determinations of dissociation constants pKa5app and pKa6app. Correction formulae were derived to convert actual dissociation constants pKa into apparent dissociation constants pKaapp (or vice versa). The actual dissociation constants were found: pKa5(H2L4− ⇌ H+ + HL5−)=7.47(0.03) and pKa6(HL5− ⇌ H+ + L6−)=14.1(0.1). The anisotropy of 31P chemical shifts of salts MnH6 − nL (M=Li, Na, n=0–5) is more sensitive towards titration (n) than isotropic solution state chemical shifts.