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

To separate the cations in each solution mixture: (a) Add sodium sulfide (Na2S) to form insoluble cadmium sulfide (CdS) precipitate, filter it out, leaving Na+ in the solution. (b) Add excess ammonia to form soluble [Cu(NH3)4]2+ complex ion, separate it, and recover Cu2+ and Mg2+ ions from their complexes and salts. (c) Add sodium hydroxide (NaOH) to form insoluble lead hydroxide (Pb(OH)2) precipitate, filter it out, and recover Al3+ and Pb2+ ions from their respective hydroxide salts. (d) Add sodium chloride (NaCl) to form insoluble silver chloride (AgCl) and mercury(I) chloride (Hg2Cl2) precipitates, separate them using ammonia, and recover Ag+ and Hg2+ ions from their respective chloride salts.

Step-by-step solution

(a) Separating Na+ and Cd2+ ions

To separate Na+ and Cd2+ ions from a solution mixture, we can take advantage of the difference in solubility of their respective sulfide salts. Step 1: Add a source of sulfide ions, such as sodium sulfide (Na2S), to the solution containing the Na+ and Cd2+ ions. This will cause the formation of insoluble cadmium sulfide (CdS) in the presence of Cd2+ ions while sodium sulfide (Na2S) will remain soluble in the solution. Step 2: Filter out the cadmium sulfide (CdS) precipitate, which will separate the Cd2+ ions from the solution. The remaining solution will contain Na+ ions. Step 3: If necessary, evaporate the water from the remaining solution to obtain solid sodium ions in the form of a sodium salt, such as sodium chloride (NaCl).

(b) Separating Cu2+ and Mg2+ ions

To separate Cu2+ and Mg2+ ions from a solution mixture, we can make use of the difference in their complexation tendencies with ammonia (NH3). Step 1: Add excess ammonia solution to the solution containing Cu2+ and Mg2+ ions. This will cause the Cu2+ ions to form a soluble complex ion, [Cu(NH3)4]2+, while Mg2+ ions will remain as free ions in the solution. Step 2: Separate the [Cu(NH3)4]2+ complex ion from the Mg2+ ions by filtration, if there is a change in color or a formation of a precipitate. In some cases, you may need to use an ion-exchange column to perform this separation step. Step 3: If necessary, recover the Cu2+ and Mg2+ ions from their respective complexes and salts by evaporating the water or by performing a chemical reaction that releases the free metal ions.

(c) Separating Pb2+ and Al3+ ions

To separate Pb2+ and Al3+ ions from a solution mixture, we can use the difference in solubility of their hydroxide salts. Step 1: Add a concentrated solution of sodium hydroxide (NaOH) to the solution containing Pb2+ and Al3+ ions. This will cause the formation of insoluble lead hydroxide, Pb(OH)2, while aluminum hydroxide, Al(OH)3, will remain soluble in the solution. Step 2: Filter out the lead hydroxide (Pb(OH)2) precipitate, which will separate the Pb2+ ions from the solution. The remaining solution will contain Al3+ ions. Step 3: If necessary, recover the Al3+ and Pb2+ ions from their respective hydroxide salts by performing a chemical reaction that releases the free metal ions, such as by acidification with hydrochloric acid (HCl).

(d) Separating Ag+ and Hg2+ ions

To separate Ag+ and Hg2+ ions from a solution mixture, we can utilize the difference in solubility of their chloride salts. Step 1: Add a solution of sodium chloride (NaCl) to the solution containing Ag+ and Hg2+ ions. This will cause the formation of insoluble silver chloride (AgCl) and insoluble mercury(I) chloride (Hg2Cl2). Step 2: Filter out the silver chloride (AgCl) and mercury(I) chloride (Hg2Cl2) precipitates, which will separate the Ag+ and Hg2+ ions from the solution. Step 3: To separate AgCl from Hg2Cl2, add aqueous ammonia solution to the mixture of the two precipitates. This will cause the AgCl to dissolve into [Ag(NH3)2]+ complex ion, while Hg2Cl2 will remain insoluble. Filter out the insoluble Hg2Cl2. Step 4: If necessary, recover the Ag+ and Hg2+ ions from their respective chloride salts by performing a chemical reaction that releases the free metal ions, such as by acidification with nitric acid (HNO3).

Key concepts

Cation Separation

Cation separation involves distinguishing and removing specific positive ions (cations) from a solution mixture. This technique relies on the unique physical and chemical properties of each ion.
In a solution, cations may exist in different forms. Some may form precipitates with added chemicals, while others remain in solution. Through strategic addition of reagents like sodium sulfide or hydroxide, ions such as cadmium or lead can be made to form solid precipitates and be filtered out.
Sometimes, cations can be separated due to their tendency to form complex ions. For instance, by adding ammonia, copper ions can transform into a complex that remains in solution, allowing for the filtration of other cations, like magnesium, which do not form such complexes easily. This ability to manipulate chemical reactions for ion separation is crucial in qualitative analysis.

Solubility Principles

Solubility principles are focused on understanding how different substances dissolve in solvents like water. It is the foundation for techniques that separate ions based on their solubility differences in solutions.
Every compound has a unique solubility in water, which depends on temperature and the nature of ions involved. For instance, salts like cadmium sulfide or lead hydroxide have low solubility, meaning they will form a solid precipitate when mixed with sulfide or hydroxide ions, respectively.
Conversely, sodium salts tend to be highly soluble, remaining dissolved even when other cations may precipitate out. Typically, the application of solubility principles allows us to predict which compounds will remain in solution and which will precipitate, a technique often used to separate ion mixtures.

Complex Ion Formation

Complex ion formation is a process that occurs when metal ions bind with several ligands, such as ammonia, to form a more stable complex ion. This process changes the chemical behavior and solubility of the original ions.
When ammonia is added to a solution with copper ions, for instance, a complex ion, \([Cu(NH3)4]^{2+}\), is formed. This complex is water soluble and has different solubility dynamics compared to Cu ions alone, influencing how these ions interact in the solution.
Such formations can be pivotal in an ion separation process, as they allow targeted separation of ions based on which can form soluble complexes. The rest can typically be isolated or filtered out through changes observed, like color changes or precipitate formations.

Chemical Precipitation

Chemical precipitation is a method used to separate ions through the formation of solid particles from a solution. It utilizes the differences in solubility of various ionic compounds.
By introducing a precipitating agent, such as sodium chloride, ions like silver or mercury can form insoluble chloride salts that precipitate out of the solution. This principle is used to separate ions effectively by making them aggregate into a solid form which can be removed by filtration.
Another example is the addition of ammonia to develop complex ions to subsequently isolate ions, enhancing the separation process. Understanding how and when to use these methods ensures effective analysis and separation of components in a mixture through chemical precipitation.