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Dissociation and alkali complex formation equilibria of nitrilotris(methylenephosphonic acid) (NTMP, H 6 L) have been studied by dilatometric, potentiometric and 31 P 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 + + HL 5− ⇌ MHL 4− and M + + L 6− ⇌ ML 5− , respectively, were determined: log K NaHL =1.08(0.07), log K KHL =0.86(0.08), log K NaL =2.24(0.03). Systematic errors are introduced by using alkali metal hydroxides as titrants for routine potentiometric determinations of dissociation constants p K a5 app and p K a6 app . Correction formulae were derived to convert actual dissociation constants p K a into apparent dissociation constants p K a app (or vice versa). The actual dissociation constants were found: p K a5 (H 2 L 4− ⇌ H + + HL 5− )=7.47(0.03) and p K a6 (HL 5− ⇌ H + + L 6− )=14.1(0.1). The anisotropy of 31 P chemical shifts of salts M n H 6 − n L (M=Li, Na, n =0–5) is more sensitive towards titration ( n ) than isotropic solution state chemical shifts. |
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