Question

Which of the following protective colloids has a high gold number?

• A. Acacia
• B. Tragacanth
• C. Albumin
• D. Gelatin

Hint:

- The gold number measures the protective power of a colloid, with higher values indicating better stability of colloidal dispersions. 
- Tragacanth has the highest gold number among common protective colloids, making it highly effective in stabilizing colloidal solutions.

Answer 1

The Correct Option is B

The gold number is a measure of the protective power of a colloid, defined as the minimum amount (in mg) of colloid required to prevent the precipitation of 10 mL of gold sol when added to the sol. A higher gold number indicates a greater protective power, meaning the colloid is better at stabilizing the sol. Among the options: 
- Tragacanth has the highest gold number, indicating it has the greatest ability to protect a colloidal gold sol from precipitation. It is widely used in various pharmaceutical formulations for this reason. 
- Acacia, Albumin, and Gelatin are also used as protective colloids, but their gold numbers are generally lower compared to Tragacanth. Thus, the correct answer is Tragacanth, which has a high gold number. 

Why Other Options Are Incorrect: 
- (A) Acacia: While Acacia is commonly used as a protective colloid, its gold number is not as high as Tragacanth. 
- (C) Albumin: Albumin is a protein with protective colloidal properties, but it has a lower gold number compared to Tragacanth. 
- (D) Gelatin: Gelatin has protective colloidal properties but does not have as high a gold number as Tragacanth.

Answer 2

The gold number measures how well a protective colloid stops a colloidal gold sol from flocculating when electrolyte is added: it is defined as the smallest weight, in milligrams, of the colloid needed to just prevent the colour change of 10 mL of a standard gold sol on adding a fixed amount of sodium chloride. A larger gold number means a weaker protective colloid, since more of it is needed to give the same protection. Let's rank the four options.

1. Acacia: Acacia (gum arabic) is a commonly used protective colloid with a moderate gold number, generally quoted in the range of about 0.15 to 0.25, making it a fairly good, though not the best, protector.
2. Tragacanth: Tragacanth gum has a distinctly higher gold number than the other three, typically cited around 2, which places it well above acacia, albumin, and gelatin. That higher number means tragacanth needs a much larger amount to achieve the same protective effect, making it comparatively the weakest protective colloid of the group and the one with the highest gold number.
3. Albumin: Egg albumin generally shows a low to moderate gold number, often in the range of about 0.08 to 0.10, indicating it is a reasonably strong protective colloid.
4. Gelatin: Gelatin is known as one of the most efficient protective colloids available, with a very low gold number, typically about 0.005 to 0.01, meaning only a tiny amount is needed to protect the gold sol.

Since a high gold number signals weaker protective ability, and tragacanth requires far more material than acacia, albumin, or gelatin to achieve the same protection, it is the option with the highest gold number among the four.

Therefore, the correct answer is Tragacanth.