Number: 2001-038-2-500
Title: Recommendations for NMR Measurement of pK values in strongly
basic and highly acidic aqueous solutions
Task Group
Chairman: K. Popov
Members: L.H.J.
Lajunen and H. R�nkk�m�ki
Completion Date: 2006 - project completed
Objective:
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.
Description:
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.
Progress:
(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>
