What is the minimum concentration?

Question

An aqueous solution at room temperature is 0.25 M in both Ba²⁺ and Ca²⁺ ions. You want to separate these two ions by selective precipitation using carbonate. What is the minimum concentration of carbonate required to start the first compound precipitating? (BaCO₃ Ksp = 2.58 × 10⁻⁹ M, CaCO₃ Ksp = 3.36 × 10⁻⁹ M).

Answer 1

Since both cations are +2 charged, we look to the Ksp values to see which will precipitate first as the carbonate salt.

Ksp for BaCO₃ is less than that for CaCO₃, so Ba₂₊ will precipitate first.

BaCO₃(s) ==> Ba²⁺(aq) + CO₃^2-(aq)

Ksp = [Ba²⁺][CO₃^2-]

2.58x10^-9 = (0.25)(x)

x = 1.03x10^-8 M = minimum concentration of CO₃^2- to begin precipitation

If you want to compare this to precipitation of CaCO₃, we have...

CaCO₃(s) ==> Ca²⁺(aq) + CO₃^2-(aq)

Ksp = [Ca²⁺][CO₃^2-]

3.36x10^-9 = (0.25)(x)

x = 1.34x10^-8 M which is greater than 1.03x10^-8 M determined for BaCO₃.