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Concept
Common-ion effect
Summary
The common-ion effect refers to the decrease in solubility of an ionic precipitate by the addition to the solution of a soluble compound with an ion in common with the precipitate. This behaviour is a consequence of Le Chatelier's principle for the equilibrium reaction of the ionic association/dissociation. The effect is commonly seen as an effect on the solubility of salts and other weak electrolytes. Adding an additional amount of one of the ions of the salt generally leads to increased precipitation of the salt, which reduces the concentration of both ions of the salt until the solubility equilibrium is reached. The effect is based on the fact that both the original salt and the other added chemical have one ion in common with each other.
Hydrogen sulfide (H2S) is a weak electrolyte. It is partially ionized when in aqueous solution, therefore there exists an equilibrium between un-ionized molecules and constituent ions in an aqueous medium as follows:
H2S H+ + HS−
By applying the law of mass action, we have
Hydrochloric acid (HCl) is a strong electrolyte, which nearly completely ionizes as
HCl → H+ + Cl−
If HCl is added to the H2S solution, H+ a common ion and creates a common ion effect. Due to the increase in concentration of H+ ions from the added HCl, the equilibrium of the dissociation of H2S shifts to the left and keeps the value of Ka constant. Thus the dissociation of H2S decreases, the concentration of un-ionized H2S increases, and as a result, the concentration of sulfide ions decreases.
Barium iodate, Ba(IO3)2, has a solubility product Ksp = [Ba2+][IO3−]2 = 1.57 x 10−9. Its solubility in pure water is 7.32 x 10−4 M. However in a solution that is 0.0200 M in barium nitrate, Ba(NO3)2, the increase in the common ion barium leads to a decrease in iodate ion concentration. The solubility is therefore reduced to 1.40 x 10−4 M, about five times smaller.
solubility equilibrium
A practical example used very widely in areas drawing drinking water from chalk or limestone aquifers is the addition of sodium carbonate to the raw water to reduce the hardness of the water.
Official source
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