Gold number is:

Options

A. The number of mg of lyophilic colloid whichshould 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 whichshould 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.

Answer

To solve the question regarding the definition of Gold number, we will analyze the options provided and determine the correct definition step by step.

### Step-by-Step Solution:

1. **Understanding Gold Number**: The Gold number is a measure used in colloid chemistry to quantify the stability of colloidal solutions. It specifically relates to the amount of a protective colloid required to prevent the coagulation of a standard colloidal solution when an electrolyte is added.

2. **Analyzing the Options**:
- **Option 1**: Refers to the amount of lyophilic colloid needed to prevent coagulation of ferric hydroxide sol. This is not the standard gold sol.
- **Option 2**: States the amount of lyophilic colloid required to prevent coagulation of a standard gold sol upon addition of an electrolyte (10% NaCl). This aligns with the definition of Gold number.
- **Option 3**: Discusses the milligram of salt needed to coagulate a lyophilic colloid, which is not the definition of Gold number.
- **Option 4**: Mentions the milligram of an electrolyte required to coagulate a colloid, which again does not define Gold number.

3. **Selecting the Correct Option**: Based on the analysis, Option 2 is the correct definition of Gold number. It specifies that the Gold number is the milligram of lyophilic colloid that just prevents the coagulation of 10 ml of standard gold sol upon adding 1 ml of 10% NaCl solution.

### Final Answer:
The correct definition of Gold number is: **The number of milligrams of lyophilic colloid which should be added to 10 ml of standard gold sol to prevent its coagulation by the addition of 1 ml of 10% sodium chloride solution.** Therefore, the correct option is **Option 2**.

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