Most effective coagulant for a colloidal solution of arsenic sulphide in water is

Options

A. 0 . 1 M sodium phosphate
B. 0 . 1 zinc sulphate
C. 0 . 1 M zinc nitrate
D. 0 . 1 M aluminium chloride

Answer

To determine the most effective coagulant for a colloidal solution of arsenic sulfide (As₂S₃) in water, we need to analyze the properties of the colloidal solution and the nature of potential coagulants.

### Step-by-Step Solution:

1. **Identify the Nature of the Colloidal Solution**:
- Arsenic sulfide (As₂S₃) forms a colloidal solution where the dispersed particles are negatively charged. This is typical for sulfide sols.

2. **Understand the Coagulation Process**:
- Coagulation involves neutralizing the charge on the colloidal particles, allowing them to come together and form larger aggregates that can settle out of the solution. Since the particles are negatively charged, we need a coagulant that provides positive ions (cations).

3. **Select Potential Coagulants**:
- Common coagulants include:
- Sodium Phosphate (Na₃PO₄) - provides Na⁺ ions
- Zinc Sulfate (ZnSO₄) - provides Zn²⁺ ions
- Zinc Nitrate (Zn(NO₃)₂) - provides Zn²⁺ ions
- Aluminum Chloride (AlCl₃) - provides Al³⁺ ions

4. **Evaluate Coagulants Based on Charge**:
- According to the Hardy-Schulze rule, the effectiveness of a coagulant increases with the valency of the cation. This means that cations with higher positive charges are more effective at neutralizing the negative charges of the colloidal particles.
- The cations from the coagulants are:
- Na⁺ (1+)
- Zn²⁺ (2+)
- Al³⁺ (3+)

5. **Determine the Most Effective Coagulant**:
- Among the listed coagulants, Aluminum Chloride (AlCl₃) provides Al³⁺ ions, which have the highest charge (3+). Therefore, it will be the most effective coagulant for the negatively charged arsenic sulfide colloidal solution.

### Conclusion:
The most effective coagulant for a colloidal solution of arsenic sulfide in water is **Aluminum Chloride (AlCl₃)**.