Analytical Chemistry

Separation by precipitation relies on significant differences in the solubilities of the ions with a common precipitating agent (e.g., sulfate, hydroxide, oxalate). These solubility differences often arise from differences in ionic charge (oxidation state) or, to a lesser extent, ionic radius. Let's analyze the pairs:

• (A) Eu(II) and Dy(III): Europium has a stable +2 oxidation state, while Dysprosium's stable state is +3. Ions with different charges generally form compounds with vastly different solubilities. For instance, EuSO₄ is sparingly soluble, whereas Dy₂(SO₄)₃ is much more soluble. Similarly, their hydroxides would have different solubility products. This difference allows for separation by selective precipitation. (Correct)
• (B) Gd(III) and Dy(III): Both Gadolinium and Dysprosium are typically in the +3 oxidation state. As adjacent lanthanides, their ionic radii and chemical properties are very similar. Their corresponding salts (e.g., hydroxides, oxalates) tend to have similar solubilities, making separation by simple precipitation difficult. (Incorrect)
• (C) Eu(II) and Yb(II): Both Europium and Ytterbium can exist in the +2 oxidation state due to stable f⁷ (for Eu²⁺) and f¹⁴ (for Yb²⁺) configurations. While both are in the +2 state, there is still a difference in their ionic radii and positions within the lanthanide series, which can lead to differences in the solubility of their salts (e.g., sulfates, carbonates). Separation might be possible through fractional precipitation, exploiting 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 normal aqueous conditions. If we consider these ions as potentially existing, the large 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, if considering the stable forms, one would try to separate Eu(II) from Gd(III), which is similar to case (A). (Correct)

Thus, pairs involving ions with different stable oxidation states (A) or potentially sufficient solubility differences even within the same unstable/less common state (C, D) can be separated by precipitation methods.

Kjeldahl's method is suitable for estimating nitrogen in:

• Amino acids (A)
• Amines (B)
• Amides
• Other compounds where nitrogen is in -3 oxidation state

It cannot be used for:

• Azo compounds (C) -N=N-
• Nitro compounds (D, H) -NO₂
• Nitriles (E) -CN
• Diazonium salts