Title: Ionic Strength Corrections for Stability Constants
Members: V. Buzko,
I. Sukhno, and H.
Completion Date: 2005 - project completed
To facilitate the calculation of reliable stability constants for applications
in an assortment of natural media (e.g. seawater, biological fluids).
To do this reliable activity coefficients must be available. This will
be accomplished by preparing a computer based method for correcting
stability constants for ionic strength changes in the range 0 - 5 mol
dm-3 (with routines for converting to molal) using activity
coefficients calculated from the Pitzer equation and Specific Interaction
Theory (SIT). A database of published coefficients will be prepared
and, where values are not available, they will be calculated using SIT
approximations. The software will be made freely available on the web.
It will not contain a critical evaluation of existing parameters.
Currently the only general methods for calculating activity constants
(and correcting stability constants) for changes in ionic strength are
based on the Davies equation and Debye-Huckel theory. This equation
is applicable only at low ionic strengths, particularly when ionic charges
are above one. Hence it has very limited reliability in biological and
some environmental systems (e.g. seawater).
Semi-empirical methods that have been used to model complexation reactions
include the Pitzer models and Specific Ion Interaction Theory (SIT),
and these have been shown to be equivalent for all practical purposes
(I. Grenthe and A. Plyasunov, Pure Appl. Chem., 69, 951-8,
1997). Both depend on selecting and applying interaction coefficients
and parameters applicable to the system being modeled.
Compilations of recommended values for many common ions, and possible
approximations for calculating unknown values, have been published.
However a worker wishing to apply corrections in a particular system
can be faced with time-consuming research. The proposal is that a computer-based
method of calculating activity coefficients (and hence stability constants
corrections) is developed. This will contain a database of parameters
for systems that have been studied, together with a subroutine for calculating
values for which recommended values are not available. The database
will not be a critical evaluation of literature values but will be designed
to allow new values to be added (or existing values corrected) by the
user as necessary.
The format of the program will facilitate linking to speciation software
to allow modeling under a range of ionic strengths and conditions.
In May 2002, version 1.1 of SIT was released; the program uses specific
interaction theory to render ionic strength corrections for stability
constants, and includes activity coefficient calculation molar-molal
Version 1.2, released in Sep 2002, includes improved graph drawing
routines and the list of SIT parameters have been extended to include
those for many carboxylic acids. For a review of SIT, read about it
Version 1.5 was released in September 2003
- for Windowns 9x, NT, 2000, and XP, download the following package
> SIT-v1.5-Sep03.zip (zip file -
- for a Russian version, download the following package > SIT-v1.5-Sep03_Russian.zip
(zip file - 495KB)
For a review, read about it in Chem.
> Nov 2004 report update (pdf
file - 12KB)
A suite of inter-linked programs -The Adjustment, Estimation
and Uses of Equilibrium Constants in Aqueous Solution- has
been developped and includes:
- SIT program - Version 2.0 (2004)
- Electrolytes - to calculate activity coefficients and water
activity using Pitzer and Lin-Tseng-Lee equations.
- Acid-base - to calculate acid-base equilibria in electrolytes
and seawater for over 260 acids.
- Temperature Effects - to calculate temperature dependence
of log K values using 8 thermodynamic equations of increasing complexity.
- Oxygen solubility - to calculate oxygen solubility in 22
electrolytes, natural fluids and seawater as a function of temperature,
- Speciation - to calculate and display species distribution
curves for complexes (including insolubles).
- Titration and speciation simulations - to simulate titration
curves in real time and display speciation dynamically.
For a review, read about it in Chem.
> access/download the entire suite, see follow-up project
The current version of the suite of programs may also be downloaded
and select 'Projects'
> see related project 2006-010-1-500
titled 'Adjustment, estimation and uses of equilibrium reaction constants
in aqueous solution'
Last update: 30 April 2008
<project announcement published in