When there is two sources of the same ion, and if the ion is involved in an equilibrium reaction, the changes in the equilibrium are known as the common ion effect. The common ion effect is a result of LeChatelier's Principle.
A solution of formic acid is mixed with a solution of sodium formate. What is the reaction chemistry in each solution and the mixture?
Formic acid solution:
HCHO2(aq) + H2O(l) → ← H3O+(aq) + CHO2–(aq)
Formic acid is a weak acid so there will be partial ionization to form some hydronium ion and formate ion, but the predominant species in solution is formic acid. The pH will be below 7; how far below depends on the concentration of formic acid and the pKa.
Sodium formate solution:
NaCHO2(aq) → Na+(aq) + CHO2–(aq)
Sodium ion is neutral but formate ion is basic so can undergo base hydrolysis:
CHO2–(aq) + H2O(l) → ← HCHO2(aq) + OH–(aq)
The sodium formate solution has a high concentration of sodium ions and formate ions and a small concentration of formic acid and hydroxide ions. The pH is above 7 and depends upon the concentration of sodium formate and the pKa of formic acid.
Now, what happens when the two solutions are mixed?
Start with the formic acid equilibrium and add the sodium formate solution:
HCHO2(aq) + H2O(l) → ← H3O+(aq) + CHO2–(aq)
NaCHO2(aq) → Na+(aq) + CHO2–(aq)
The sodium formate solution adds sodium ions and formate ions. This disturbs the formic acid equilibrium, driving it towards reactants.
This removes some hydronium ion from solution, increasing the pH.
Conclusion: addition of the common ion (formate) to the formic acid equilibrium increases the pH. The final solution is composed of high concentrations of formic acid, formate ion, and sodium ion and low concentrations of hydronium ion and hydroxide ion.
Find the pH of the following solutions at 25 °C:
1. A 0.050 M solution of formic acid.
2. A 0.050 M solution of sodium formate.
3. A solution that is 0.050 M in both sodium formate and formic acid.