Chemistry International
Vol. 24, No. 6
November 2002
IUPAC Projects
New
SIT Software Makes Speciation Calculations Easier
Metal
ion speciation in environmental systems is a hot topic because speciation
is strongly linked to bioavailability. For labile systems, the speciation
can be determined by equilibrium modelling calculations to give the
well known "speciation diagram," a plot of composition (species
concentration) vs. pH or composition vs. metal/ligand ratio.
Speciation
calculations require values for the stability constants of all the species
that may form from the system components. These values must be valid
at the ionic strength of the medium concerned. Here is a major problem:
the vast majority of equilibrium constants have been determined at a
fixed ionic strength that is typically much higher (ca. 0.1-1.0 M) than
applies in freshwater environmental systems (<0.02 M).
To correct
the stability constants to the required ionic strength one must be able
to calculate the activity coefficients of all species in the equilibrium
reaction. These corrections may be quite large (e.g., log K1
for the reaction Fe3+ + nta3- is 16.26 for I=0.10
M NaClO4 and the value calculated for I=0.001 M is 17.89).
The second
problem is that not all equations for activity coefficient calculations
(e.g., Debye-Hückel and Davies [eqtn. 1]) are valid to such high
ionic strengths:
log
gI = -AzI2{|1/2/(1+|1/2)-0.3|}
(1)
Specific
Interaction Theory, or SIT (and Pitzer parameters), provides
a valid tool for calculating stability constants over a wide range of
ionic strengths. The calculations are not trivial; they require a database
of ionic coefficients, eM,X, for
each ion-ion interaction:
log gM = -Azi2{|1/2/(1+1.5|1/2)+e(M,X)mX}
(2)
Now for
the easy and exciting part! An IUPAC project, undertaken by L. D.
Pettit of Academic Software, has produced the SIT program that will
calculate:
- ionic
activity coefficients in a user-specified medium, up to 5.0 molal
- stability
constants in the range 0-5.0 m, based on published values at a single
ionic strength
-
molarity from molality, and vice versa
This is
an enormous asset for those involved in speciation calculations. It
is a user-friendly program that is now in the public domain at <www.iupac.org/projects/
2000/2000-003-1-500.html>. Interested persons are invited to
test the SIT program and send comments to the author. The program includes
a comprehensive, but not exhaustive, file of SIT parameters. The file
can be edited, but if users have access to reliable SIT parameters that
are not included, please send these to the author at <www.acadsoft.co.uk>
so that the release version of the program can be updated for all to
benefit.
The SIT
program can be readily applied to industrial processes for which stability
constants may be required at very high ionic strengths.
Two other
IUPAC products will assist in the understanding and application of stability
constants. The IUPAC Stability Constant database, SC-database1
should be a starting point for stability constant data. It links data
directly to the program SPECIES that will calculate speciation
curves. The HELP files in the database provide definitions and information
about temperature, ionic strength, and solvent effects. For a much wider
discourse, the program Sol-Eq2 explains the principles
of solution equilibrium and explores many applications in environmental,
biological, and industrial systems. It includes a primer on how to do
your own speciation calculations.
- SC-database,
Stability Constants Database; IUPAC, Academic Software. 2001.
- Sol-Eq.
Solution equilibria: Principles and Applications. Academic Software.
2001.
Both of
these programs are available to order via <www.acasoft.co.uk>.
Reviewed
by Kip Powell,
vice president, Analytical Chemistry Division.
www.iupac.org/projects/2000/2000-003-1-500.html