Analytical Chemistry I: Qualitative Analysis - SS3 Chemistry Past Questions and Answers - page 4

The systematic identification of cations in an unknown sample involves a series of tests that are based on the unique chemical properties of each cation. Here is an outline of the procedure for identifying cations:

1.    Preliminary Test: The first step is to perform a preliminary test to determine the pH of the solution. This helps to identify if the sample is acidic, neutral, or basic, which can provide initial clues about the presence of certain cations.

2.    Group Separation: The next step is to perform group separation. Cations are often classified based on their behaviour with specific reagents. The most common groups include Group 1 cations (Ag⁺, Hg₂²⁺, and Pb²⁺), Group 2 cations (Cu²⁺, Cd²⁺, Bi³⁺, Sb³⁺, and As³⁺), Group 3 cations (Al³⁺, Fe³⁺, and Cr³⁺), and Group 4 cations (Zn²⁺, Mn²⁺, Ni²⁺, Co²⁺, and Mg²⁺).

3.    Specific Tests for Cation Groups: Within each group, specific reagents are added to the solution to perform tests and identify the cations present. For example, in Group 1, adding dilute HCl will form a white precipitate with Ag⁺, while no precipitate will be formed with Hg²⁺ and Pb²⁺.

4.    Confirmatory Tests for Individual Cations: After identifying the cation group, confirmatory tests are conducted to identify individual cations. These tests involve reactions specific to each cation. For example, to confirm the presence of Cu²⁺ in Group 2, adding NH₃ (ammonia) will form a deep blue precipitate.

5.    Final Confirmation: To ensure accuracy, further tests may be conducted to confirm the presence of specific cations. These may involve complexometric titrations or specific colourimetric tests.

By following this systematic procedure, chemists can identify the cations present in the unknown sample and establish their concentrations.

Identifying anions in an unknown sample requires a systematic approach based on the unique properties of each anion. Below are the steps involved in identifying anions:

1.    Preliminary Test: As with cations, the first step is to conduct a preliminary test to determine the pH of the sample. This information can provide initial clues about the potential anions present.

2.    Group Separation: Anions are typically classified based on their behaviour with specific reagents. Common groups include Group 1 anions (Cl⁻, Br⁻, and I⁻), Group 2 anions (SO₄²⁻ and CO₃²⁻), and Group 3 anions (NO₃⁻ and ClO₃⁻).

3.    Specific Tests for Anion Groups: Within each group, specific reagents are added to the solution to perform tests and identify the anions present. For example, in Group 1, adding AgNO₃ (silver nitrate) will form a white precipitate with Cl⁻, a cream precipitate with Br⁻, and a yellow precipitate with I⁻.

4.    Confirmatory Tests for Individual Anions: After identifying the anion group, confirmatory tests are conducted to identify individual anions. These tests involve reactions specific to each anion. For example, to confirm the presence of SO₄²⁻ in Group 2, adding BaCl₂ (barium chloride) will form a white precipitate of BaSO₄.

5.    Final Confirmation: Additional tests may be performed to ensure the accuracy of the results. These could involve pH tests, redox reactions, or specific colourimetric tests.