Vol. 27 No. 3
May-June 2005

Equilibria in Solution: A Software Aid

Igor V. Sukhno, Vladimir Y. Buzko, and Leslie D. Pettit, members of the task group for the project titled Ionic Strength Corrections for Stability Constants (project #2000-003-1-500) are reporting the availability of Aq-Solutions, a software package of programs for the quantitative treatment of equilibria in solution.

Aq-Solutions consists of a controlling program (Aq_Solutions.exe). This gives access to a suite of programs which, it is planned, will be regularly extended and updated. Programs are included for the quantitative correction of stability constants for ionic strength changes and temperature changes, for the calculation of gas solubilities, for speciation and for titration simulations.

SIT corrects stability constants for changes in ionic strength using SIT (Specific Interaction Theory) and to estimate SIT parameters with full statistics. The program contains an editable database of published SIT parameters. It also provides routines to inter-convert molarities (c) and molalities (m), and lg K_c and lg K_m. A version of the SIT program to correct stability constants for changes in seawater salinity and in electrolyte mixtures (fluids) is in preparation.

Electrolytes calculates electrolyte activity coefficients, osmotic coefficients and the activity of water using Pitzer and the Lin-Tseng-Lee equations. The program can also be used to calculate ionic activity coefficients using the Millero-Pitzer method. A version of the Electrolytes program, which will calculate the ionic activity coefficients for the most important ions in seawater is in preparation.

Acid-base calculates the composition of acid-base systems at equilibrium in electrolyte solutions and seawater. It will also calculate H⁺ activity coefficients for correction of pa_H to p[H⁺] in different ionic media. The concept of apparent equilibrium constant is used to calculate equilibrium constants for more than 260 mono and polycarboxylic acids with background ions. A version of the Acid-base program to calculate the H⁺ activity coefficient in seawater at different salinity (chlorinity), and at different temperatures, is in preparation.

Temperature effects is an aid for the study and prediction of the temperature dependence of lg K using eight thermodynamic equations:
—Van’t Hoff equation
—Isobar equation
—Extended Isobar equation
—Clarke-Glew equation
—Extended Clarke-Grew equation
—DQUANT equation
—Valentiner equation
—Blandamer-Robertson-Scott equation

Full statistics are included.

Gas solubility calculates O₂ solubility in water, 22 electrolyte solutions, natural fluids, and seawater as a function of temperature, concentration, and salinity. A program to calculate the solubilities of other gasses is in preparation.

Speciation calculates and displays species distribution curves for complexes (including insoluble species). Speciation can be displayed as a function of pH, pL (any reactant), or total reactant concentration, and as either percentages or species concentrations (log values). When insoluble phases are present, lg S values may also be plotted. Both graphs and tables of numeric values may be displayed, printed, or saved to disk. The sensitivity of speciation curves to errors in lg K values can be demonstrated graphically.

Titration Simulations simulates M-L titration curves in real time and display speciation dynamically. Species L, HL, H2L, ML, ML2, MHL, and MH-1L may be included and stability constants adjusted in real time. Titration curves, speciation curves, and pi-charts are calculated in real time.

Other programs are in preparation and will be included at a later date. These include programs for electrode calibration, for properties of individual electrolyte solutions, and their mixtures (solubility, density, viscosity, ΔC_p, freezing-point, boiling-point, surface tension), as well as for water properties as a function of temperature, pressure, and concentration of individual electrolytes or electrolyte mixtures (a_w, pK_w, ΔH_w, ΔC_p, viscosity, density).

A preliminary version of the suite of programs may be downloaded from <www.iupac.org/projects/2000/2000-003-1-500.html> or from <www.acadsoft.co.uk>.

Please send questions and comments to the authors. Igor V. Sukhno <[email protected]> and Vladimir Y. Buzko <[email protected]> are both from the Department of Chemistry of the Kuban State University, in Krasnodar, Russia. Leslie D. Pettit <[email protected]>, Academic Software, Yorks, UK, is task group chairman.

www.iupac.org/projects/2000/2000-003-1-500.html