Gold number is

Options

A. The number of mg of lyophilic colloid which should be added to 10 mL of ferric hydroxide sol so as to prevent its coagulation by the addition of 1 mL of 10% sodium chloride solution
B. The number of mg of lyophilic colloid which should be added to 10 mL of standard gold sol so as to prevent its coagulation by the addition of 1 mL of 10% NaCl
C. The mg of gold salt to be added to a lyophilic colloid to coagulate it
D. The mg of an electrolyte required to coagulate a colloid

Explanation

**Step-by-Step Solution:**

1. **Understanding Protective Colloids**:
- Protective colloids are substances that stabilize colloidal solutions and prevent the aggregation of particles. They play a crucial role in maintaining the stability of colloids.

2. **Definition of Gold Number**:
- The gold number is defined as the minimum amount of protective colloid (in milligrams) required to prevent a color change in a colloidal solution of gold from red to violet.

3. **Color Change Explanation**:
- In the presence of a protective colloid, the colloidal gold particles remain dispersed, maintaining their red color. If the protective colloid is insufficient, the particles aggregate, leading to a color change to violet.

4. **Measurement**:
- The gold number is measured in milligrams of the protective colloid needed to stabilize a specific volume of colloidal gold solution.

5. **Importance of Gold Number**:
- The gold number is significant in colloid chemistry as it indicates the effectiveness of a protective colloid. A lower gold number means that a smaller amount of protective colloid is needed for stabilization, indicating higher efficiency.

**Final Definition**:
The gold number is the minimum amount of protective colloid (in milligrams) that prevents the color change of colloidal gold from red to violet.

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