Activity-Ratio and Predominance Diagrams

The construction of an activity-ratio diagram can be summarized in the following four steps  

Identity a set of solid phases that contain a chemical element of interest and may be controlling its solubility. Write a dissolution reaction for each solid, with the free ionic species of the element as one of the products. Be sure that the stoichiometric coefficient of the free ion (metal or ligand) is 1.0. 

Compile values of Kdis for the solid phases. Write an algebraic equation for each log Kdis in terms of log [activity] variables for the products and reactants in the corresponding dissolution reaction. Rearrange the equation to have log[(solid phase)/(free ion)]—the log activity ratio—on the left side, with all other log[activity] variables on the right side. 

Choose an independent log[activity] variable against which log[(solid)/ (free ion)] can be plotted for each solid phase. A typical example is pH = -log(H% 

Select fixed values for all other log [activity] variables, corresponding to an assumed set of soil conditions. Use these values and that of log Kdis to develop a linear relation between log[(solid)/(free ion)] and the independent log [activity] variable for each solid phase considered. Plot all of these equations on the same graph. 

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For an introduction to the two types of diagram, see, for example, Chaps. 5 and 6 in G. Sposito, The Chemistry of Soils, Oxford University Press, New York, 1989. Activity-ratio and predominance diagrams are discussed with many examples in Chaps. 5 and 7 of W. Stumm and J. J. Morgan, Aquatic Chemistry, Wiley, New York, 1981... 

Prepare activity-ratio and predominance diagrams for the Al(III) minerals whose dissolution reactions are described in Eqs. 3.18-3.20. Set pH = 6 for the activity-ratio diagram, but otherwise use fixed activity data as given in connection with Figs. 3.5 and 3.7. Repeat your calculations for (H20) = 0.5 instead of unit water activity. What is the effect on mineral stability (Answer. 

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