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: logK_NaHL=1.08(0.07), logK_KHL=0.86(0.08), logK_NaL=2.24(0.03). Systematic errors are introduced by using alkali metal hydroxides as titrants for routine potentiometric determinations of dissociation constants pK_a5^app and pK_a6^app. Correction formulae were derived to convert actual dissociation constants pK_a into apparent dissociation constants pK_a^app (or vice versa). The actual dissociation constants were found: pK_a5(H₂L⁴⁻ ? H⁺ + HL⁵⁻)=7.47(0.03) and pK_a6(HL⁵⁻ ? H⁺ + L⁶⁻)=14.1(0.1). The anisotropy of ³¹P chemical shifts of salts M_nH_6 − nL (M=Li, Na, n=0-5) is more sensitive towards titration (n) than isotropic solution state chemical shifts.