Which of the following is most effective in coagulating positively charged methylene blue sol?

Options

A. `Na_3PO_4`
B. `K_4[Fe(CN)_6]`
C. `Na_2SO_4`
D. `Al_2(SO_4)_3`

Explanation

To solve the question of which substance is most effective in coagulating positively charged methylene blue sol, we can follow these steps:

### Step 1: Understand the Charge of the Sol
Methylene blue sol is positively charged. To coagulate or flocculate a positively charged sol, we need a negatively charged ion.

### Step 2: Identify the Given Compounds
We are provided with four compounds:
1. Sodium Phosphate
2. Potassium Hexacyanidoferrate
3. Sodium Sulphate
4. Another Sodium Sulphate

### Step 3: Determine the Charge of the Anions
Next, we need to identify the anions present in each compound and their respective charges:
1. **Sodium Phosphate (Na₃PO₄)**: The phosphate ion (PO₄³⁻) has a charge of -3.
2. **Potassium Hexacyanidoferrate (K₄[Fe(CN)₆])**: The hexacyanidoferrate ion ([Fe(CN)₆]⁴⁻) has a charge of -4.
3. **Sodium Sulphate (Na₂SO₄)**: The sulphate ion (SO₄²⁻) has a charge of -2.
4. **Another Sodium Sulphate (Na₂SO₄)**: Again, the sulphate ion (SO₄²⁻) has a charge of -2.

### Step 4: Compare the Valency of the Anions
According to Hardy-Schulze rule, the coagulating power of an ion is proportional to its valency. We compare the valencies:
- Phosphate ion: -3
- Hexacyanidoferrate ion: -4
- Sulphate ion: -2
- Sulphate ion: -2

### Step 5: Determine the Most Effective Coagulating Agent
From the comparison, the hexacyanidoferrate ion has the highest negative charge (-4), making it the most effective in coagulating the positively charged methylene blue sol.

### Conclusion
Thus, the most effective substance in coagulating positively charged methylene blue sol is **Potassium Hexacyanidoferrate**.

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