Solubility Equilibrium - Dissolution With Reaction

Dissolution With Reaction

A typical reaction with dissolution involves a weak base, B, dissolving in an acidic aqueous solution.

B(s) + H+ (aq) BH+ (aq)

This reaction is very important for pharmaceutical products. Dissolution of weak acids in alkaline media is similarly important.

H_nA(s) + OH-(aq) H_n-1A-(aq) + H₂O

The uncharged molecule usually has lower solubility than the ionic form, so solubility depends on pH and the acid dissociation constant of the solute. The term "intrinsic solubility" is used to describe the solubility of the un-ionized form in the absence of acid or alkali.

Leaching of aluminium salts from rocks and soil by acid rain is another example of dissolution with reaction: alumino-silicates are bases which react with the acid to form soluble species, such as Al3+(aq).

Formation of a chemical complex may also change solubility. A well-known example, is the addition of a concentrated solution of ammonia to a suspension of silver chloride, in which dissolution is favoured by the formation of an ammine complex.

AgCl(s) +2 NH₃(aq) + (aq) + Cl- (aq)

Another example involves the addition of water softeners to washing powders to inhibit the precipitation of salts of magnesium and calcium ions, which are present in hard water, by forming complexes with them.

The calculation of solubility in these cases requires two or more simultaneous equilibria to be considered. For example,

Intrinsic solubility equilibrium	B(s) B(aq): Ks =
Acid-base equilibrium	B(aq) + H+(aq) BH+(aq) Ka = /

A number of computer programs are available to do the calculations. They include:

• CHEMEQL A comprehensive computer program for the calculation of thermodynamic equilibrium concentrations of species in homogeneous and heterogeneous systems. Many geochemical applications.
• Geochem-EZ (freeware) a multi-purpose chemical speciation program, used in plant nutrition and in soil and environmental chemistry research to perform equilibrium speciation computations, allowing the user to estimate solution ion activities and to consider simple complexes and solid phases.
• The Geochemist's Workbench geochemical modeling package for creating activity diagrams, calculating detailed reaction paths, and coupling chemical reaction with mass and heat transport in one or two dimensions.
• HySS (freeware) which was used to produce the diagram at the right.
• JESS All types of chemical equilibria can be modelled including protonation, complex formation, redox, solubility and adsorption interactions. Includes an extensive database.
• MINEQL+ A chemical equilibrium modeling system for aqueous systems. Handles a wide range of pH, redox, solubility and sorption scenarios.
• PHREEQC USGS software designed to perform a wide variety of low-temperature aqueous geochemical calculations, including reactive transport in one dimension.
• Visual MINTEQ A Windows version of MINTEQA2 (ver 4.0). MINTEQA2 is a chemical equilibrium model for the calculation of metal speciation, solubility equilibria etc. for natural waters.
• WinSGW A Windows version of the SOLGASWATER computer program.