The gold number in chemistry refers to the minimum amount (in milligrams) of a protective colloid needed to prevent the coagulation of 10 ml of a standard gold sol when 1 ml of a 10% sodium chloride solution is added.
Understanding Gold Number
The gold number is a measure of the protective power of a lyophilic colloid (a colloid that readily forms a stable solution in a solvent). It was introduced by Richard Adolf Zsigmondy in 1901. Here's a breakdown:
- Lyophobic colloids: These colloids are unstable and easily coagulated by the addition of small amounts of electrolytes (like NaCl). Gold sols are a type of lyophobic colloid.
- Lyophilic colloids: These colloids are more stable due to a strong interaction between the dispersed phase and the dispersion medium. Examples include gelatin, starch, and albumin.
- Protective action: Lyophilic colloids can protect lyophobic colloids from coagulation. When a lyophilic colloid is added to a lyophobic colloid, it forms a protective layer around the lyophobic particles. This prevents them from aggregating when an electrolyte is added.
- Gold Number Definition: The gold number quantifies this protective ability. A lower gold number indicates a better protective colloid because less of it is needed to prevent coagulation.
Determining Gold Number
The gold number is determined experimentally. The procedure involves:
- Preparation of a standard gold sol: This is a stable dispersion of gold nanoparticles in water.
- Addition of the protective colloid: Varying amounts of the lyophilic colloid are added to separate samples of the gold sol.
- Addition of sodium chloride solution: A fixed amount (1 ml of 10%) of sodium chloride solution is added to each sample.
- Observation of coagulation: The samples are observed for signs of coagulation, typically indicated by a color change (from red to blue/purple).
- Calculation of gold number: The minimum amount of the protective colloid (in milligrams) that prevents the color change is the gold number.
Significance of Gold Number
The gold number provides a practical way to compare the effectiveness of different protective colloids. It is useful in various applications, including:
- Pharmaceutical formulations: Stabilizing drug suspensions and emulsions.
- Cosmetics: Preventing the aggregation of particles in creams and lotions.
- Industrial processes: Controlling the stability of colloidal dispersions.
Examples of Gold Numbers
Here are a few examples of approximate gold numbers for different protective colloids:
| Colloid | Gold Number (mg) |
|---|---|
| Gelatin | 0.005 - 0.01 |
| Casein | 0.01 - 0.02 |
| Gum Arabic | 0.15 - 0.25 |
| Potato Starch | Around 25 |
Gelatin, with its low gold number, is a significantly better protective colloid than potato starch.
