Systematic Equilibrium
Equations
Charge Balance
The charge balance equation states that there are as many positive charges as negative charges in solution. The total charge of a solution must be zero.
S Cizi = 0
In this equation, the Ci are concentrations and the zi are charges.
Mass Balance
The mass balance equations state that the sum of all molar (M) concentrations must equal the formal (F) concentration of the 'parent' species. For example, the mass balance for a triprotic acid would be
F H3A = M H3A + M H2A- + M HA2- + M A3-
There may be more than one mass balance equation.
Equilibrium Expressions
The equilibrium expressions are those which relate concentrations and/or activity coefficients. Remember to include Kw in acid/base calculations. For example, the triprotic acid would include the Ka1, Ka2, and Ka3 acid equilibrium expressions and Kw. Activities can be used if known. Generally, these calculations do not use activities unless a computer is availible.
Prescription Used to Calculate Concentrations
Step 1. Write all pertinent chemical reactions.
Step 2. Write charge balance equation.
Step 3. Write mass balance equations.
Step 4. Write all equilibrium expressions. Use activities if known.
Step 5. Count equations and unknowns. There must be as many or more equations than unknowns or else the problem cannot be solved.
Step 6. Use algebraic expressions to solve for species concentrations using 'standard methods. Several examples of this step are given in the link to Distribution of Equilibrium Species.
This page was created by Professor Stephen Bialkowski, Utah State University.
Last Updated Monday, August 28, 2006