Question

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

Short answer

(a) Add sodium carbonate (Na2CO3) to the solution to form calcium carbonate (CaCO3) precipitate. Filter the solution to separate the precipitate, leaving behind Na+ ions. (b) Add sodium sulfide (Na2S) to the solution to form copper sulfide (CuS) precipitate. Filter the solution to separate the precipitate, leaving behind Mg2+ ions. (c) Adjust the pH of the solution containing Pb2+ and Al3+ ions to around 5, then add a sulfide like sodium sulfide (Na2S). This forms a lead sulfide (PbS) precipitate. Filter the solution to separate the precipitate, leaving behind Al3+ ions. (d) Add sodium chloride (NaCl) to the solution containing Ag+ and Hg2+ ions to form silver chloride (AgCl) precipitate. Filter the solution to separate the precipitate, leaving behind Hg2+ ions.

Step-by-step solution

(Separating Na+ and Ca2+)

: 1. Add Sodium Carbonate: Add a solution containing the sodium carbonate (Na2CO3) to the mixture. This will selectively cause a reaction with the calcium ion (Ca2+), forming calcium carbonate (CaCO3) precipitate. \[Ca^{2+} + CO_3^{2-} \rightarrow CaCO_3 \downarrow\] 2. Filtration: Filter the solution to separate the precipitated calcium carbonate from the solution. The remaining filtrate will contain Na+ ions.

(Separating Cu2+ and Mg2+)

: 1. Add Sodium Sulfide: To the solution containing Cu2+ and Mg2+ ions, add a solution containing sodium sulfide (Na2S). This will selectively cause a reaction with the copper ion (Cu2+), forming copper sulfide (CuS) precipitate. \[Cu^{2+} + S^{2-} \rightarrow CuS \downarrow\] 2. Filtration: Filter the solution to separate the precipitated copper sulfide from the solution. The remaining filtrate will contain Mg2+ ions.

(Separating Pb2+ and Al3+)

: 1. Adjust pH: Adjust the pH of the solution to around 5 by adding an acid or a base, depending on the initial pH of the solution. 2. Add Sulfide: To the pH-adjusted solution containing Pb2+ and Al3+ ions, add a solution containing a sulfide, such as sodium sulfide (Na2S) or hydrogen sulfide (H2S). This will selectively cause a reaction with the lead ion (Pb2+), forming lead sulfide (PbS) precipitate. \[Pb^{2+} + S^{2-} \rightarrow PbS \downarrow\] 3. Filtration: Filter the solution to separate the precipitated lead sulfide from the solution. The remaining filtrate will contain Al3+ ions.

(Separating Ag+ and Hg2+)

: 1. Add Chloride: To the solution containing Ag+ and Hg2+ ions, add a solution containing a chloride, such as sodium chloride (NaCl). This will selectively cause a reaction with the silver ion (Ag+), forming silver chloride (AgCl) precipitate. \[Ag^+ + Cl^- \rightarrow AgCl \downarrow\] 2. Filtration: Filter the solution to separate the precipitated silver chloride from the solution. The remaining filtrate will contain Hg2+ ions.

Key concepts

Precipitation Reactions

Precipitation reactions play a crucial role in separating different ions in a solution. When two solutions containing different ions are mixed, sometimes a solid, or precipitate, can form if the combination of certain ions results in an insoluble compound. This process is known as a precipitation reaction.
For example, when sodium carbonate (Na\(_2\)CO\(_3\)) is added to a solution containing calcium ions (Ca\(^{2+}\)), calcium carbonate (CaCO\(_3\)) precipitate forms. The chemical equation for this reaction is:
\[Ca^{2+} + CO_3^{2-} \rightarrow CaCO_3 \downarrow\]
This reaction is highly useful in separating calcium ions from other ions that remain dissolved, like sodium ions (Na\(^+\)). One key characteristic of precipitation reactions is that they allow for the selective removal of specific ions by forming insoluble compounds.
All we need is to ensure that the conditions, like proper concentration and pH, are just right to favor the formation of the desired precipitates. This technique is especially effective in cation separation when a particular ion forms a unique and easily separable compound. Always consider solubility rules to predict which compounds can form precipitates.

Selective Precipitation

Selective precipitation is a technique where specific reagents are added to a solution to precipitate out, or solidify, one particular ion while leaving others in solution. This is particularly useful in mixtures of ions, where one wants to isolate and remove a specific cation from a combination.
For instance, adding sodium sulfide (Na\(_2\)S) to a mixture containing copper ions (Cu\(^{2+}\)) and magnesium ions (Mg\(^{2+}\)) will lead to the precipitation of copper sulfide (CuS), while magnesium ions stay dissolved. The reaction is shown as:
\[Cu^{2+} + S^{2-} \rightarrow CuS \downarrow\]
This is possible because copper sulfide has a lower solubility than magnesium sulfide, hence it forms a solid first. Similarly, silver ions (Ag\(^+\)) can be selectively precipitated using a chloride source, forming insoluble silver chloride (AgCl), due to its characteristic low solubility.
The key to successful selective precipitation lies in choosing the right reagent that reacts distinctly with only one of the cations in the solution. It's about knowing which compounds have low solubility and carefully controlling the conditions like temperature and pH to avoid forming unwanted precipitates.

Filtration Techniques

Filtration techniques follow precipitation reactions and are vital in isolating solid precipitates from liquid solutions. Once a precipitation reaction occurs and a solid precipitate forms, effective separation requires the use of filtration.
In the laboratory and industrial settings, filtration separates the solid from the liquid by trapping the solid on a porous material, often using a filter paper in a funnel. The liquid, known as the filtrate, passes through the pores and leaves the precipitate behind.
For instance, when lead sulfide (PbS) is precipitated from a mixture, filtration is used to capture the solid lead sulfide, allowing aluminum ions (Al\(^{3+}\)) to remain in the solution. A similar step occurs when separating silver chloride (AgCl) by filtration, ensuring it doesn't remain mixed with other cationic substances in the filtrate.
By properly selecting filter media and controlling factors such as vacuum pressure, filtration can be optimized to ensure clear separation. This step is crucial in the practical application of cation separation, enabling a pure recovery of intended compounds from a complex mixture. Remember, the efficiency of filtration often directly affects the purity of the final product.