SED & PREDOM / PREDOM

PREDOM: Predominance Area Diagrams

How it works

PREDOM uses a point to point technique, which is slow, and has a plot resolution that depends on the interval between points. The advantage is that log(Tot.Conc.) can be used as the variable in an axis.

• The concentration area to be studied is divided with 100 steps in each axis, giving a grid of 10201 chemical compositions to solve. Larger grids (smaller steps) may be selected by the user when using the “advanced” program level.
• For every point in the grid, the mass-balance equations are solved (if the Tot. Conc. is given for at least one component) by calling the HALTAFALL algorithm, and then the species distribution is determined.
• If no solid is present, the predominant soluble species is the one with a highest fraction of the main component. For example, for Fe₂(OH)₂⁴⁺, the fraction is 2[Fe₂(OH)₂⁴⁺]/[Fe]_TOT while for Fe(OH)₂⁺, the fraction is [Fe(OH)₂⁺]/[Fe]_TOT. Therefore, the first complex will dominate over the second only if [Fe₂(OH)₂⁴⁺] > ½[Fe(OH)₂⁺].
• If some solids are formed by the main component, then the predominating species in that point of the diagram will be the solid with the largest fraction of the main component. In this case the predominant species in the plot is the solid, even in a case where the solid's concentration (mol/L) is smaller than the predominating aqueous species.

Lines separating predominance areas

Once the chemical composition of all the points in the grid has been calculated, and the predominant species in each point has been found, PREDOM calculates where and how to draw the lines that separate the predominance areas, as well as where to put the labels for each predominance area.

This is not always straightforward, and in som complex situation the diagram may appear “scrambled”. Usually the diagram is useful anyway: it indicates clearly under which conditions aqueous species and solids predominate.

One way to obtain a better looking diagram, is by slightly changing the number of steps in each axis in the grid, selecting the “advanced” program level before Making a Diagram.