Question

Which pair of ions among the following can be separated by precipitation method?

• A. Eu(II) and Dy(III)
• B. Gd(III) and Dy(III)
• C. Eu(II) and Yb(II)
• D. Eu(II) and Gd(II)

Hint:

Using a simple frame or just bolding for the box Key Points:
Precipitation separation relies on solubility differences. Large solubility differences often exist between ions of different charges (oxidation states), e.g., Ln(II) vs Ln(III).
Ln(II) sulfates (like EuSO$_4$, YbSO$_4$) are often less soluble than Ln(III) sulfates.
Ln(III) ions adjacent in the series (like Gd$^{3+$, Dy$^{3+$) have very similar properties and are hard to separate by precipitation.
Stability of oxidation states is crucial: Eu$^{2+$ and Yb$^{2+$ are relatively stable; Gd$^{2+$ is not.

Answer 1

The Correct Option is A

Precipitation-based separation exploits solubility disparities among ions sharing a precipitating agent (e.g., sulfate, hydroxide, oxalate). These differences often stem from variations in ionic charge (oxidation state) or, to a smaller extent, ionic radius. Let's examine these pairs:

• (A) Eu(II) and Dy(III): Europium favors a +2 oxidation state, while Dysprosium prefers +3. Differently charged ions typically form compounds with significantly different solubilities. For example, EuSO₄ is sparingly soluble, whereas Dy₂(SO₄)₃ is much more soluble. Similarly, their hydroxides will exhibit different solubility products. This allows separation via selective precipitation. (Correct)
• (B) Gd(III) and Dy(III): Both Gadolinium and Dysprosium usually exist in the +3 oxidation state. As adjacent lanthanides, their ionic radii and chemical characteristics are very similar. Their corresponding salts (e.g., hydroxides, oxalates) tend to have comparable solubilities, making separation by simple precipitation challenging. (Incorrect)
• (C) Eu(II) and Yb(II): Both Europium and Ytterbium can exist in the +2 oxidation state because of stable f⁷ (for Eu²⁺) and f¹⁴ (for Yb²⁺) configurations. Though in the +2 state, differences in ionic radii and position within the lanthanide series can lead to solubility variations in their salts (e.g., sulfates, carbonates). Separation might be achieved through fractional precipitation, leveraging these solubility differences. (Correct)
• (D) Eu(II) and Gd(II): Europium readily forms Eu(II). Gadolinium, however, strongly prefers the +3 oxidation state to achieve the stable half-filled [Xe]4f⁷ configuration. Gd(II) is not a common or stable oxidation state under typical aqueous conditions. If considering these ions as potentially existing, the significant difference in stability and electronic configuration between Eu(II) and the unstable Gd(II) would likely lead to different chemical behaviours and potentially different solubilities, allowing separation. Alternatively, separating Eu(II) from Gd(III) is similar to case (A). (Correct)

In conclusion, pairs featuring ions with different stable oxidation states (A) or potentially sufficient solubility differences even within the same unstable/less common state (C, D) are separable using precipitation techniques.