Subject: Re: CCL:mac program for solving chemical equilibria

From: "W. R. Smith" <[email-masked]>
Date: Fri, 25 Jul 1997 11:31:17 -0400

Hello,

At last, a "computational chemistry" issue that has nothing too do with QM!

>At 09:14 PM 7/24/97 -0800, Paul G. Tratnyek wrote:
>>Hi! Does anyone know of any program for the Mac which can do calculations
>>associated with simultaneous  chemical equilibria, titrations, etc. in
>>non-aqueous media ( where pH is not a factor)?
>
[The most sophisticated speciation programs]

>I have seen are for
>geochemistry. They can handle lots of species, speciation at solid
>surfaces, etc.
>
>Of course, the concern of geochemists is exclusively with aqueous systems.
>
>The most recent one I have seen running on a Mac is ChemEQL 2.0. Info at:
>
>	http://www.eawag.ch/soft/chemeql.html

I must respond to the comment in [brackets] concerning "sophistication".

IMHO, most existing "speciation programs", the one mentioned being an
example (the name itself even implies that such calculations are special to
the domain of geochemistry) are based on 30-year-old thermodynamic and
numerical analysis technology.  Lots has happened since then!

There abound DOS-based geochemistry programs for aqueous systems dating
>from the 60's and early 70's.  They generally contain tightly integrated
computational algorithms and aqueous-system databases (the latter of which
one must usually accept as realistic thermodynamic descriptions of the
system).  The program mentioned by Tratnyek, although it runs on a Mac
(using a little better hardware technology than DOS :-)), appears to one of
the more advanced in its class, in that it can accept user thermodynamic
data.  However, it is still very specific to aqueous systems - and it is
likely based on the old numerical analysis technology (but one can't tell
>from its description).

Most geochemists continue to believe that the computational technology
originally developed in the 60's and early 70's for aqueous systems
(equilibrium constant equations + Newton-Raphson numerical analysis
technology) is the best (and only) appropriate approach for such problems.

In reality, aquatic speciation problems are only a special case of general
reaction equilibrium problems.  Apart from appropriate thermodyamic models,
the non-domain-specific technology, in terms of the computational kernel
that is central to such calculations, has advanced considerably.  

This fact is not generally realized in the geochemical community.  For
example, on page 517 of "Thermodynamics in Geochemistry", G. M. Anderson
and D. A. Crerar, Oxford university Press, 1993, there is the following
sentences:

"There is an important fundamental distinction between the free energy
based programs and those using equilibrium constants, and this often
determines which method to use in specific applications.  The free energy
programs actually require much more fundamental thermodynamic information,
and this can restrict their usefulness"

And on p. 400 of "Geochemical Thermodynamics", 2nd edition, D. K. Nordstrom
and J. l. Munoz, Blackwell, 1994:

"The only difference worth underscoring here is that a program that accepts
only free energy values for the operating database makes it far more
difficult to obtain reliable values for aqueous species than it is with a
program that uses equilibrium constants.  Some free energy values of
aqueous species do not exist because the measurements have not been made,
whereas the equilibrium constants typically are available:.

Both the above statements are widely believed in the geochemistry community
- and both are DEAD WRONG.

The technology to utilize either type of data (species-specifc free
energies or reaction sets with their associated equilibroum constants) has
been available since 1982 (section 9.4 of "Chemical reaction Equilibrium
Analysis: Theory and Algorithms", W. R. Smith and R. W. Missen,
Wiley-Interscience, 1982; Krieger, 1991).  An algorithm based on simple
linear algebra can be used to convert back and forth between species and
reaction data,  (This algorithm is incorporated within Mathtrek System's
EQS4WIN software - URL below).

Comments/discussion on these points are most welcome - any geochemists
lurking on the list?  How about physical chemists - does anyone like the
NASA chemical equilibrium algorithms?

-- W. R. Smith, PhD, P. Eng., Senior Scientist, Mathtrek Systems --
3-304 Stone Road West, Suite 165, Guelph, Ontario CANADA N1G 4W4
EMail: [email-masked]       Tel:519-763-1356,FAX:519-763-4525
--------------------- http://www.mathtrek.com ---------------------
-Mathtrek Systems - Home of EQS4WIN Chemical Equilibrium Software -