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Analytical Chemistry Division (V)


Number: 2001-038-2-500

Title: Recommendations for NMR Measurement of pK values in strongly basic and highly acidic aqueous solutions

Task Group
K. Popov

Members: L.H.J. Lajunen and H. Rönkkömäki

Completion Date: 2006 - project completed

To formulate the general rules and technique that should be used when high pK values are measured by NMR with a special emphasis on the procedure which should be followed if pK> -log[H+] available at a given ionic strength. To give the guidelines for analytical, inorganic, physical and co-ordination chemists in fundamental constants measurements, which in turn could be used in industrial, medical and environmental applications.

A large amount of practically useful chelators, such as organophosphonates, hydroxyacids, macrocyclic complexones remove the last proton in strongly basic solutions, such as at pH 12-15. Therefore the corresponding pK-values could not be measured accurately by potentiometric titration. Generally more accurate NMR technique in particular case is recently used with some serious methodological errors. This was mentioned in IUPAC project 560/36/93.

The knowledge of high HL dissociation constants is needed for both ML stability constants calculations and Speciations in some important technologies, such as basic detergent solutions, basic hydroxyperoxide solutions, basic leaching of contaminated soils and minerals with chelating agents.

An error free NMR approach was developed and demonstrated by the applicants during IUPAC Assembly in Brisbane (see corresponding poster). The methodology permits to eliminate most of errors associated with NMR applications for quantitative equilibrium studies.

Recent project intends to formulate the general rules that should be accounted when high pK values are measured by NMR with a special emphasis on the procedure which should be followed if pK> -log[H+] available at a given ionic strength and to summarise errors which could arise when equilibrium is studied by NMR (e.g. chem. Shift dependence on ionic strength, pD/pH scale problems, etc). The recommendations are to be illustrated by a comparison of data measured with different technique.

(Nov 2004) A manuscript has been submitted and reviewed by ICTNS; final revisions are being completed prior publication in Pure and Applied Chemistry.

Project completed - IUPAC Technical Report published in Pure Appl. Chem. 78(3), 663-675, 2006

Last update: 14 March 2006

<project announcement published in Chem. Int. 24(6) 2002>

Page last modified 14 March 2006.
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