CrossRef enabled

PAC Archives

Archive →

Pure Appl. Chem., 2006, Vol. 78, No. 3, pp. 663-675


Guidelines for NMR measurements for determination of high and low pKa values (IUPAC Technical Report)

Konstantin Popov1,2*, Hannu Rönkkömäki3 and Lauri H. J. Lajunen4

1 Institute of Reagents and High Purity Substances (IREA), Bogorodsky val-3, 107258, Moscow, Russia
2 Moscow State University of Food Production, Volokolamskoye Sh. 11, 125080 Moscow, Russia
3 Finnish Institute of Occupational Health, Oulu Regional Institute of Occupational Health, Laboratory of Chemistry, Aapistie 1 FIN-90220 Oulu, Finland
4 Department of Chemistry, University of Oulu, P.O. Box 3000, FIN-90014 Oulu, Finland

Abstract: Factors affecting the NMR titration procedures for the determination of pKa values in strongly basic and strongly acidic aqueous solutions (2 > pH > 0 and 14 > pH > 12) are analyzed. Guidelines for experimental procedure and publication protocols are formulated. These include: calculation of the equilibrium H+ concentration in a sample; avoidance of measurement with glass electrode in highly acidic (basic) solutions; exclusion of D2O as a solvent; use of an individual sample isolated from air for each pH value; use of external reference and lock compounds; use of a medium of constant ionic strength with clear indication of the supporting electrolyte and of the way the contribution of any ligand to the ionic strength of the medium is accounted for; use of the NMR technique in a way that eliminates sample heating to facilitate better sample temperature control (e.g., 1H-coupled NMR for nuclei other than protons, GD-mode, CPD-mode, etc.); use of Me4NCl/Me4NOH or KCl/KOH as a supporting electrolyte in basic solution rather than sodium salts in order to eliminate errors arising from NaOH association; verification of the independence of the NMR chemical shift from background electrolyte composition and concentration; use of extrapolation procedures.