Qualitative Analysis of Cations

In analytical chemistry, chemicals are analyzed qualitatively or quantitatively. Simply, qualitative analysis is checking whether there is a chemical compound present or not. Quantitative analysis is the determination of how much that compound is present in the sample.

Qualitative analysis of cations using the precipitation method

To identify cations that are dissolved in water, a precipitation method can be used. In this method, cations are classified into five groups.

Qualitative Analysis of Group 1 cations

Group 1 cations are identified by precipitating as their chlorides. When an aqueous solution of cations is added to diluted HCl acid,  a white color precipitate is formed if group 1 cations are present.

• AgCl – White color precipitate
• PbCl₂ – White color precipitate
• Hg₂Cl₂ – White color precipitate

In the analysis of group 1 cations, concentrated HCl cannot be used. Because PbCl₂ will dissolve in concentrated HCl and form an ion complex as follows.

PbCl₂ also dissolves in hot water. Therefore, hot water cannot be used in the analysis of group 1 cations. If a cations solution is added diluted HCl and there is no precipitate formed, it concludes that there are no group 1 cations in that solution.

Qualitative Analysis of Group 2 cations

If there is a precipitate formed in the group 1 analysis, the filtrate is taken for the analysis process for group 2 cations. If there is no precipitate from the group 1 analysis, the solution is directly taken for the group 2 analysis.

The filtrate of group 1 analysis is bubbled Hydrogen sulfide (H₂S) gas followed by diluted HCl acid. The reason for adding HCl is to reduce the concentration of sulfide ions. The H⁺ ions released from HCl shift the equilibrium of H₂S backward. Thus, the concentration of the sulfide ion is decreasing.

Group 2 cations are precipitated as the respective sulfides. These sulfides have relatively low K_sp values. Therefore, they will precipitate at low concentrations of sulfide ions. (S^2-).

Qualitative Analysis of Group 3 cations

The filtrate of the group 2 analysis is taken for the group 3 analysis. If there is a precipitate formed in group 2, it is separated first. In group 3 there are three cations which are Cr³⁺, Al³⁺, and Fe³⁺. These cations will be precipitated as their respective hydroxides at low concentrations of hydroxide ions (OH^-)

Qualitative analysis of group 3 cations is done in 3 steps.

1. First, the solution is heated until boiling. The purpose of boiling is to remove all the H₂S dissolved in the solution from the group 2 analysis.
2. Then, the solution is acidified using conc. HNO₃ acid in order to convert Fe²⁺ ions into Fe³⁺ ions if there are any.
3. Finally, the solution is cooled to room temperature, and added excess NH₄OH solution is followed by an NH₄Cl solution. The purpose of adding NH₄Cl is to reduce OH^- ions in the solution.

Due to the NH₄⁺ ions coming from NH₄Cl, the dissociation of the NH₄OH is reduced. Therefore, the concentration of the hydroxide ions in the solution is reduced.

Thus, the cations that are precipitated at low concentrations of OH^- ions will be precipitated in group 3 analysis. If the concentration of OH^- ions is high, these precipitates will dissolve in the solution.

Qualitative Analysis of Group 4 cations

If there is any precipitate formed in group 3 analysis, it is separated and the filtrate is taken for the group 4 analysis. There are four cations in group 4 which are Zn²⁺, Mn²⁺, Ni²⁺, and Co²⁺. These cations will precipitate as their respective sulfides at high concentrations of sulfide ions (S^2-).

The solution is added Ammonium hydroxide solution (NH₄OH) and bubbled with Hydrogen sulfide (H₂S) gas. The purpose of adding NH₄OH is to increase the concentration of sulfide ions (S^2-) ions in the solution.

According to Le Chatelier’s principle, the equilibrium will shift to the right and it will result in a high concentration of S^2- ions in the solution.  

These precipitates will dissolve in dilute acids.

Qualitative Analysis of Group 5 cations

If there is any precipitate formed in the group 4 analysis, it is separated and the filtrate is taken for the group 5 analysis. There are three cations in group 5, which are Ca²⁺, Sr²⁺, and Ba²⁺. These cations will be precipitated as their respective carbonates.

The qualitative analysis of Group 5 is done in three steps.

1. First, the solution is heated until boiling to remove the H₂S gas from the group 4 analysis.
2. The solution is cooled to room temperature and added small amount of NH₄Cl solution is followed by an excess amount of NH₄OH solution.
3. Finally, the solution is added (NH₄)₂CO₃ solution and heated until boiling.

When considering the filtrate of the group 5 qualitative analysis, it may present any other cations except those cations from groups 1 – 5.