Separation of Cations

Precipitation Step 1

If you have a mixture of metal cations in solution, add chloride ions (in the form of HCl) to precipitate Ag⁺, Pb²⁺, and Hg₂²⁺ as their insoluble chlorides. The precipitate is removed by filtration and the rest of the cations stay in solution.

Note: the lack of a precipitate in any of these precipitation steps confirms the absence of all cations in that set.

In contrast, formation of a precipitate indicates the presence of at least one of the ions in that set, possibly more.

Precipitation Step 2

To the filtered, acidic solution from above, add sulfide ions (in the form of H₂S). If present, Bi³⁺, Cd²⁺, Cu²⁺, Hg²⁺, Pb²⁺, Sb³⁺, Sn²⁺, and Sn⁴⁺ precipitate as their insoluble sulfides, and are removed by filtration. The rest of the cations stay in solution.

Precipitation Step 3

Increase the pH of the filtered solution from above and add more H₂S. If present, Al³⁺, Co²⁺, Cr³⁺, Fe²⁺, Fe³⁺, Ni²⁺, Mn²⁺, and Zn²⁺ precipitate as hydroxides, or sulfides (or both), and are removed by filtration. The rest of the cations stay in solution.

Precipitation Step 4

To the filtered solution from above, add carbonate ions (or phosphate ions). If present, Ba²⁺, Ca²⁺, Mg²⁺, and Sr²⁺ precipitate as their carbonates (or phosphates) to be removed by filtration. The rest of the cations stay in solution and cannot be precipitated.