Question

Suggest how the cations in each of the following solution mixtures can be separated: (a) \(\mathrm{Na}^{+}\) and \(\mathrm{Cd}^{2+},(\mathbf{b}) \mathrm{Cu}^{2+}\) and \(\mathrm{Mg}^{2+},(\mathbf{c}) \mathrm{Pb}^{2+}\) and \(\mathrm{Al}^{3+},(\mathbf{d}) \mathrm{Ag}^{+}\) and \(\mathrm{Hg}^{2+} .\)

Short answer

Precipitate with appropriate reagents: a) \( \text{Cd}^{2+} \) with sulfide, b) \( \text{Cu}^{2+} \) with sulfide, c) \( \text{Pb}^{2+} \) with chloride, d) \( \text{Ag}^{+} \) with chloride.

Step-by-step solution

Collision of Chemical Properties

Understanding the chemical properties of the cations involved is crucial. For each pair of cations: a) Both \( \text{Na}^{+} \) and \( \text{Cd}^{2+} \) are water soluble, but \( \text{Cd}^{2+} \) forms precipitates with sulfides.b) \( \text{Cu}^{2+} \) can be precipitated using sulfides in an acidic medium, whereas \( \text{Mg}^{2+} \) remains in solution.c) \( \text{Pb}^{2+} \) forms insoluble precipitates with chloride ions, while \( \text{Al}^{3+} \) does not.d) \( \text{Ag}^{+} \) forms a precipitate with chloride ions, similar to \( \text{Pb}^{2+} \), but \( \text{Hg}^{2+} \) needs different conditions.

Selection of Separation Method

Choose a chemical reagent that will selectively react with one of the cations, forming a precipitate and leaving the other in solution:a) Add \( \text{H}_2\text{S} \) or sodium sulfide to precipitate \( \text{CdS} \), removing \( \text{Cd}^{2+} \) from the solution.b) Introduce a sulfide such as \( \text{H}_2\text{S} \) to form \( \text{CuS} \), a black precipitate, isolating \( \text{Cu}^{2+} \).c) Add hydrochloric acid, or a source of chloride ions, to precipitate \( \text{PbCl}_2 \).d) Introduce a chloride source, forming \( \text{AgCl} \) to separate \( \text{Ag}^{+} \).

Post-Precipitation Filtration

Filter the solution after precipitation.Following reagent addition, use filtration to separate the solid precipitate of the first cation from the liquid containing the second cation:a) Filter to separate \( \text{CdS} \) from the solution with \( \text{Na}^{+} \).b) \( \text{CuS} \) is filtered out, leaving \( \text{Mg}^{2+} \) in the solution.c) Remove the \( \text{PbCl}_2 \) precipitate through filtration; \( \text{Al}^{3+} \) remains.d) Similarly, filter out \( \text{AgCl} \) precipitate, with \( \text{Hg}^{2+} \) still in solution.

Key concepts

Chemical Precipitation

Chemical precipitation is a process used to separate ions in a solution, exploiting their varying solubility properties. It involves adding a reagent that reacts with specific ions to form an insoluble solid, known as a precipitate. This technique is fundamental in separating different cations based on their unique chemical behaviors.
For example, in the separation of

1. Na⁺ and Cd²⁺, cadmium ions (Cd²⁺) can be precipitated as cadmium sulfide (CdS) by introducing a sulfide ion source like hydrogen sulfide (H₂S).
2. Similarly, copper ions (Cu²⁺) react with sulfide ions in a sulfuric acid environment to form copper sulfide (CuS), leaving magnesium ions (Mg²⁺) soluble.
3. For lead ions (Pb²⁺), chloride ions can be added to form lead chloride (PbCl₂), an insoluble precipitate.
4. Silver ions (Ag⁺), when mixed with chloride ions, create silver chloride (AgCl), effectively separating it from mercury ions (Hg²⁺).

The insolubility of these precipitates ensures that only the targeted cations are selectively removed from the solution.

Filtration

Filtration is a mechanical or physical operation used to separate solids from liquids by using a filter medium that allows only the fluid to pass through. This is crucial in the context of chemical precipitation, as it allows the solid precipitates formed in a reaction to be isolated from the liquid phase containing other ions.
Once a precipitate is formed from the reaction of cations with a specific reagent, the solution is passed through a filter. The solid residue left on the filter can then be collected, analyzed, or disposed of, ensuring that the cations remaining in the solution are free from contamination by the precipitated cations.
For instance:

• After the precipitation of CdS from a Na⁺ and Cd²⁺ solution, filtration separates the CdS precipitate, leaving a clear solution containing only Na⁺.
• The black CuS solid is similarly filtered out from a solution with Mg²⁺.
• Lead chloride (PbCl₂) and silver chloride (AgCl) solid residues are filtered out from their respective solutions.

This makes filtration an efficient method for cleaning solutions and obtaining pure samples.

Cation Reaction with Sulfides

Cations can interact with sulfide ions to form compounds that are typically insoluble in water. This reaction is especially useful in separating certain cations from a mixture. Sulfide ion sources, such as hydrogen sulfide (H₂S), are commonly used for this purpose.
For example, copper ions (Cu²⁺) in solution will form an insoluble black precipitate of copper sulfide (CuS) upon the addition of sulfide ions. This allows them to be effectively separated from other ions, such as magnesium ions (Mg²⁺), that do not form a precipitate with sulfides.
Similarly, cadmium ions (Cd²⁺) will also react with sulfide ions to form cadmium sulfide (CdS), a yellow precipitate, which can be differentiated and filtered from ions like sodium ions (Na⁺) that remain soluble. These reactions rely on the unique interactions between specific cations and sulfide ions to achieve selective precipitation.

Insolubility of Chloride Precipitates

Certain cations form insoluble precipitates when they react with chloride ions, making this a useful method for their separation from mixtures. Adding chloride ions to a solution can lead to the formation of solid precipitates, provided that the cation involved is capable of forming such a compound.
Lead ions (Pb²⁺) are a typical example; they react with chloride ions to produce lead chloride (PbCl₂), an insoluble precipitate. This property is advantageous because it allows lead ions to be separated from ions like aluminum ions (Al³⁺) that do not form an insoluble chloride precipitate.
Similar behavior is observed with silver ions (Ag⁺), which also form a precipitate of silver chloride (AgCl) upon the addition of chloride ions. This method offers a straightforward and effective way to isolate silver ions from other ions in solution, such as mercury ions (Hg²⁺), which require different conditions for precipitation. The inability of these specific chlorides to dissolve in water under standard conditions ensures they can be easily removed via filtration.