Tools
for the potentiometric determination of stability constants
In
spite of the fact that stability constants are continuously being
needed in many different fields, their determination has not
been a fashionable research subject for many years.One of the immediate
result is that an important body of expertise in the field is not being
adequately preserved and transmitted. The poor quality of articles that
are often submitted for publication where stability constants are
determined gives a direct measurement of the magnitude of the problem.
Key points are systematically ignored and, in the best case, the wheel
is often reinvented. The aim of this webpage is to be a point
of practical knowledge accumulation. Any contribution is welcome and
will be acknowledged. If enough feedback exists, we can think about
developing another structure such as, for example, a wiki.Experimental resources
- Classical recipees and advice: KTH, Some Laboratory Methods (1959). They will never get outdated... I just scanned an old photocopy of mine. Has anybody a better quality copy to offer?
- Peter Gans' homepage is one of the best sources of information about stability constant determination: http://www.hyperquad.co.uk/
Calculations
About thirty years ago the fashion was to develop new programs to calculate stability constants from potentiometric data. Anybody in the field with some knowledge of a programing langage developed, and published, his/her own program. Most of these programs did more or less the same thing, had a limited diffusion and are nowadays "lost". Keeping "alive" any piece of software over the years (= suviving operating system changes) has proved very difficult, and not only in this field.Two recommendations:
- HYPERQUAD. See http://www.hyperquad.co.uk/
- ESTA. My favourite option. You can get a copy from Peter May (p.may at murdoch.edu.au). The philosophy behind this calculation program can be found in a series of publications appeared in Talanta: P.M. May, K. Murray, D.R. Williams, The use of glass electrodes for the determination of formation constants—III Optimization of titration data: The ESTA library of computer programs. Talanta, 35, 825-830 (1988), doi:10.1016/0039-9140(88)80197-8; P.M. May and K. Murray, The use of glass electrodes for the determination of formation constants—IV Matters of weight. Talanta, 35, 927-932 (1988), doi:10.1016/0039-9140(88)80223-6; P.M. May and K. Murray, The use of glass electrodes for the determination of formation constants—V Monte Carlo analysis of error propagation. Talanta, 35, 933-941 (1988), doi:10.1016/0039-9140(88)80224-8
Advice
- Are you new in the field? Why not to start by working on a system that has been studied many times and used for comparison purposes? Just try the system nickel-glycine and check your results against previously published ones: E. Bottari et al. Annali di Chimica, 68, 813 (1978). If you cannot find this publication, ask me for a copy.
- Ready to publish your constants? Then read first: M. Filella and P.M. May, Reflections on the calculation and publication of potentiometrically determined formation constants.Talanta, 65, 1221-1225 (2005), doi:10.1016/j.talanta.2004.08.046. I hope you find it useful.