Journal of Solution Chemistry, Vol. 31, No. 6, June 2002 ( C  2002) An NMR-Based Approach to the Measurement of High Log K HL Values at Low Ionic Strength. 31 P NMR Study of 1,2-Diaminoethane- N , N , N , ′ N ′ - tetra(methylenephosphonic Acid) Protonation Equilibrium at pH > 12 Konstantin Popov, 1,∗ Alexei Popov, 2 Hannu R¨ onkk¨ om¨ aki, 3 Andrey Vendilo, 2 and Lauri H. J. Lajunen 3 Received September 20, 2001; in final form May 2, 2002 A sequential NMR based approach is proposed for measurements of high log K values at low ionic strength. [ 31 P] NMR technique is used to determine the protonation constants of 1,2-diaminoethane- N, N, N, ′ N ′ -tetra(methylenephosphonic acid) (EDTPH, H 8 L) at 25 ◦ C in 0.1 mol-dm −3 KNO 3 and at 37 ◦ C in 0.15 mol-dm −3 NaCl at pH 11–14. For equilibrium L + H ⇔ HL log K are found to be 13.3 (0.1) and 12.9 (0.1), respectively. KEY WORDS: Stoichiometric dissociation constants; polyaminopolyphosphonic acids; NMR. 1. INTRODUCTION In practical studies of electrolyte solutions, e.g., in biochemistry, analytical and coordination chemistry, the stoichiometric dissociation constants of weak acids are commonly used. In coordination chemistry, they are needed for determination of stability constants of metal complexes. For organophosphonates, they are usually determined by a potentiometric glass electrode or NMR titration at a constant ionic strength I = 0.1 M KNO 3 or 0.15 M adjusted by NaCl to physiological conditions. 1 Department of Physical and Colloid Chemistry, Moscow State University of Food Technologies, Volokolamskoye Sh.11, 125080, Moscow, Russia. 2 State Research Institute of Reagents and High Purity Substances (IREA), Bogorodskii val 3, 107258 Moscow, Russia. 3 Department of Chemistry, University of Oulu, P.O.Box 3000, FIN-90014 University of Oulu, Oulu, Finland. 511 0095-9782/02/0600-0511/0 C  2002 Plenum Publishing Corporation