Find the approximate pH range suitable for the separation of Fe3+ and Zn2+ by precipitation of Fe(OH)3 from a solution that is initially 0.048 M in both Fe3+ and Zn2+.

Answer 1

To separate Fe3+ and Zn2+ by precipitation of Fe(OH)3, we need to find the pH range where Fe(OH)3 precipitates, but Zn(OH)2 stays soluble. We'll do this by examining their solubility products (Ksp) and calculating the pH range needed for precipitation.

The solubility product (Ksp) of Fe(OH)3 is 2.79 × 10^-39, and the solubility product of Zn(OH)2 is 3.0 × 10^-17.

For Fe(OH)3, the balanced equation and the corresponding solubility product expression is:
Fe³⁺ + 3OH⁻ ⇌ Fe(OH)₃(s), Ksp = [Fe³⁺][OH⁻]³

For Zn(OH)2, the balanced equation and the corresponding solubility product expression is:
Zn²⁺ + 2OH⁻ ⇌ Zn(OH)₂(s), Ksp = [Zn²⁺][OH⁻]²

Initially, the concentrations of Fe3+ and Zn2+ are both 0.048 M. Since we want Fe(OH)3 to precipitate, but not Zn(OH)2, we need to find the pH at which the concentrations of the ions in the solution exceed their solubility product limits.

For Fe(OH)3:
2.79 × 10^-39 = [(0.048)][OH⁻]³
[OH⁻]³ = 2.79 × 10^-39 / 0.048
[OH⁻]³ ≈ 5.81 × 10^-38
[OH⁻] ≈ 3.86 × 10^-13

To calculate the pH where Fe3+ begins to precipitate, we can use the relation:
pH = 14 - pOH
pOH = -log10[OH⁻]
pOH ≈ -log10(3.86 × 10^-13) ≈ 12.41
pH ≈ 14 - 12.41 ≈ 1.59

Now, we need to make sure that at this pH, Zn(OH)₂ will remain soluble. So we'll find the maximum concentration of OH⁻ that allows Zn(OH)₂ to be soluble:

For Zn(OH)2:
3.0 × 10^-17 = [(0.048)][OH⁻]²
[OH⁻]² = 3.0 × 10^-17 / 0.048
[OH⁻]² ≈ 6.25 × 10^-16
[OH⁻] ≈ 7.90 × 10^-8

Comparing the OH⁻ concentrations for both precipitates:
[OH⁻]Fe(OH)₃ ≈ 3.86 × 10^-13
[OH⁻]Zn(OH)₂ ≈ 7.90 × 10^-8

Since [OH⁻]Fe(OH)₃ is lower than [OH⁻]Zn(OH)₂, and the pH at which Fe(OH)3 begins to precipitate is 1.59, we can say that the separation of Fe3+ and Zn2+ by precipitation of Fe(OH)3 can occur at a pH of approximately 1.59, aslong as[^2] the pH remains below the point where Zn(OH)₂ starts to precipitate.

Refining this value further may require additional calculations and experimentation, but this approximate pH value should provide an initial estimate for the separation process.