Q1(g). Masking copper to determine nickel

Answer in brief

Masking makes an interfering ion unreactive without separating it; demasking releases it again. Mask copper with thiosulfate, then titrate Ni(II) with EDTA.

Key points

• Masking converts an interferent into a sufficiently stable complex or another form that does not consume the analytical reagent.
• Demasking deliberately destroys or removes the masking interaction so the previously protected ion can be determined.
• For Ni²⁺/Cu²⁺, add excess sodium thiosulfate: it reduces Cu²⁺ to Cu⁺ and stabilises Cu(I)-thiosulfate complexes, preventing copper consumption of EDTA.
• Titrate nickel with standard EDTA using murexide in a controlled ammoniacal medium near pH 8-9: Ni²⁺ + Y⁴⁻ → NiY²⁻.