Project Details Ionic Strength Corrections for Stability Constants

Project No.:
Start Date:
01 January 2000
End Date:
17 December 2005
Division Name:
Analytical Chemistry Division
Division No.:


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 specificinteraction theory to render ionic strength corrections for stabilityconstants, and includes activity coefficient calculation molar-molalinterconversion.

Version 1.2, released in Sep 2002, includes improved graph drawingroutines and the list of SIT parameters have been extended to includethose for many carboxylic acids. For a review of SIT, read about itin Chem.Int. Nov2002

Version 1.5 was released in September 2003
– for Windowns 9x, NT, 2000, and XP, download the following package> (zip file -445KB)
– for a Russian version, download the following package > file – 495KB)
For a review, read about it in Chem.Int. Nov2003

> Nov 2004 report update (pdffile – 12KB)

A suite of inter-linked programs -‘The Adjustment, Estimationand Uses of Equilibrium Constants in Aqueous Solution’- hasbeen developped and includes:

  • SIT program – Version 2.0 (2004)
  • Electrolytes – to calculate activity coefficients and wateractivity using Pitzer and Lin-Tseng-Lee equations.
  • Acid-base – to calculate acid-base equilibria in electrolytesand seawater for over 260 acids.
  • Temperature Effects – to calculate temperature dependenceof log K values using 8 thermodynamic equations of increasing complexity.
  • Oxygen solubility – to calculate oxygen solubility in 22electrolytes, natural fluids and seawater as a function of temperature,salinity etc.
  • Speciation – to calculate and display species distributioncurves for complexes (including insolubles).
  • Titration and speciation simulations – to simulate titrationcurves in real time and display speciation dynamically.
  • For a review, read about it in Chem.Int. May2005

    > download the entire suite > file – 6.97MB)

    For more information, visit
    questions/comments, e-mail L.D. Pettit <>

    The current version of the suite of programs may also be downloadedfrom: select ‘Projects’

    project completed

    > see related project 2006-010-1-500titled ‘Adjustment, estimation and uses of equilibrium reaction constantsin aqueous solution’


    <project announcement published inChem.Int.23(1) 2001>