Example: What is solubility of Ni(OH)2 in: i) pure water; and ii) a solution adjusted to pH = 10 by addition of NaOH.?
For M = Ni, Table 10.1 gives Sj — 0.23 V and S2 — 0.66 V. From these standard electrode potentials, Ksp = 2.6xJ0"J5. Irrespective of the nature of the aqueous phase, the nickel ion and hydroxide concentrations must satisfy:
i) pure water
The water dissociation equilibrium is:
and electrical neutrality of the solution requires that:
Substituting the two mass action equations into the condition of electrical neutrality yields:
Because is << 1, the first term on the left hand side of this equation is negligible, and the hydroxyl ion concentration is:
or pH = - log(10"14/1.73x10"5) = 9.2 From the solubility-product equilibrium, the solubility of nickel in pure water is:
ii) NaOH solutions with a pH = 10
For this solution, the hydroxyl ion concentration is 10-4 M and the solubility of nickel from solid Ni(OH)2
Nickel solubility from Ni(OH)2 in pure water is 30 times larger than in alkaline water.
This is an example of Le Chatelier's principle: because the product c^2+ cOH- is constant, increasing c decreases c„,.2+ .
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