Solubility Equilibria

Log Solubility Graphs

A nicely detailed treatment of chemical equilibria applied to environmental systems is provided by Benefield, et al., (1982). When dealing with equilibria that involve gases or solids, the construction lines for a graph of log concentration versus pH can be derived from the equilibrium expressions. Let us take some ferric ion equilibria and derive the equations. The following reactions take place when ferric ion is in water:

Fe(OH)3 = Fe+++ + 3 OH-

Fe+++ + OH- = Fe(OH)++

Fe(OH)++ + OH- = Fe(OH)2+

When we collect terms and calculate logarithms of the equilibrium constants, the plotting equations are:

log [ Fe+++ ] = 4.5 - 3 pH

log [ FeOH++ ] = 1.5 - 2 pH

log [ Fe(OH)2+ ] = - 1.9 - pH

log [ Fe2(OH)24+ ] = 4.9 - 4 pH

log [ Fe(OH)3 ] = - 9.0

log [ Fe(OH)4- ] = pH - 19

Note that ferric ion is amphoteric in being able to develop plus or minus charges for its complexes. These equations are for construction only; you must use logic to draw the lines for solubility. There is no way that you know which equations are important unless you can consult a chemistry book or a table of equilibrium constants. Water chemists have studied these reactions and have measured the various equilibria. While there are diagrams of the various equilibria in the research literature and in textbooks, temperature changes have a marked effect on solubility and on equilibrium constants. If the temperature is much different from that for a log concentration diagram that you find someplace, it may be too inaccurate for the system you are studying, and you need to plot a new graph

The equations from above are summarized in the following table:

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  • Plots of iron equilibria

    Benefield, L.D., J.F. Judkins, and B.L. Weand, Process Chemistry for Water and Wastewater Treatment, Prentice-Hall, Englewood Cliffs, NJ (1982)

    Thu Apr 6 11:52:34 EDT 1995