31P NMR Study of organophosphonate protonation equilibrium at high pH

Abstract

A ³¹P NMR technique was used to determine the protonation constants of the organophosphonates 1-hydroxyethane-1,1-diphosphonic acid (HEDPA, H₄L), nitrilotri(methylenephosphonic acid) (NTPH, H₆L) and 1,2-diaminoethane-N,N,N′,N′-tetra(methylenephosphonic acid) (EDTPH, H₈L) at 24°C and I=3.4–3.5 mol dm^-3 (CH₃)₄NCl over the range pH 11–14.5. For the dissociation of the aliphatic hydroxy group of HEDPA log K(H_-1L+HL)14.6 while for NTPH and EDTPH log K(L+HHL) was found to be 14.2(0.1) and 13.8(0.1), respectively. The influence of (CH₃)₄NCl content on ³¹P NMR chemical shifts was demonstrated.