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Thermodynamics in GeochemistryThe Equilibrium Model$
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Greg M. Anderson and David A. Crerar

Print publication date: 1993

Print ISBN-13: 9780195064643

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780195064643.001.0001

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Speciation Calculations

Speciation Calculations

Chapter:
(p.505) 19 Speciation Calculations
Source:
Thermodynamics in Geochemistry
Author(s):

Greg M. Anderson

David A. Crerar

Publisher:
Oxford University Press
DOI:10.1093/oso/9780195064643.003.0023

This chapter focuses on one of the most common questions asked about natural chemical systems: what are the concentrations or activities of the different species present in a system at complete chemical equilibrium? We might be concerned, for example, with oxygen or sulfur fugacities, with the activities of complex ions, or activity ratios of reduced and oxidized species of the same component. In practice, these calculations range from trivially simple to enormously complex, depending on the number of species (and components) in the system. We will follow roughly this order—from trivial to complex—and outline some of the most common approaches used in performing speciation calculations. This simplest procedure is probably used most often, and works best with systems containing relatively few chemical species. As a general rule of thumb, you might try this if there are fewer than 10 species, but move on to another more sophisticated method for more complicated systems. As an example, we will solve for the equilibrium concentrations of all species in an acetic acid + water solution of a given concentration, m. Specifically, we might be interested in the pH of a 0.1 m HAc solution, but in calculating this we will also get the activities of all other species, whether we need them or not. This is one of the simplest examples imaginable, but the method works exactly the same way with more complicated systems. An excellent reference on this general approach is Butler (1964, Chap. 3). There are six steps to follow:… 1. Write all species of relevance or interest. Count the number of unknown species. You will need this many equations. 2. Write all known equilibrium constant equations. 3. If there are charged species, write a charge balance equation. 4. Write all known mass balance equations. 5. You should now have the same number of equations as unknown species. Reduce these by algebraic substitution to one (or two) equations that can be solved for the unknown concentrations. At first, assume all activity coefficients are 1.0.

Keywords:   progress variable, quasistatic processes, reaction path calculations, speciation calculations

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