Gold Number
Gold number was defined by Richard Adolf Zsigmondy in 1901. It is a scale used to measure the protective power of colloids. The extent of protective action of lyophilic sols on lyophobic sols is different from substances to substances. It is measured in terms of gold number. It is the number of milligrams of the protecting colloid which must be added to 10 millilitre of a given gold sol to protect the precipitation of the gold sol on adding 1 millilitre of a 10% solution of NaCl, the colour of the gold sol changing from red to blue or violet. The greater the gold number of a protective colloid (lyophilic sol), the lesser the protective action of the colloid.
Significance of Gold Number
Protective Power
The gold number indicates how effective a colloid is at stabilizing a gold sol against coagulation. Gold number and protective power are inversely related to each other. Smaller the gold number, greater the protective power which means less colloid is needed to prevent coagulation. similarly, higher gold number indicates lower protective ability which means require more colloid is needed to prevent coagulation.
Coagulation Process
Coagulation occurs when the addition of an electrolyte, such as sodium chloride, disrupts the electrical double layer around the colloidal particles, leading to their aggregation. The presence of a protective colloid helps to maintain the stability by preventing this aggregation.
Colour Change Indicator
The stability of the gold sol can be observed visually through colour changes that occur during coagulation. When coagulation occurs, the colour shifts from red to blue or purple which indicates an increase in particle size due to aggregation.
Comparison of Colloids
Different protective colloids exhibit different gold numbers. For example, gelatin shows the highest protective power due to its low gold number (see table below).
Gold numbers and protective colloids are given below in the table
| Protective Colloids | Gold Number(mg) |
|---|---|
| Gelatin | 0.005 - 0.01 |
| Haemoglobin | 0.03 - 0.07 |
| Egg Albumin | 0.15 - 0.25 |
| Gum Arabic | 0.15 - 0.25 |
| Potato Starch | 20 - 25 |
| Dextrin | 125 - 150 |
Which one of the following has minimum gold number ?
A. Starch
B. Sodium oxalate
C. Gum arabic
D. Gelatin
Gold number gives
A. the amount of gold present in the colloid
B. the amount of gold required to break the colloid
C. the amount of gold required to protect the colloid
D. the measure of protective power of a lyophillic colloid
Gold number is associated with
A. Electrophoresis
B. Purple of cassius
C. Protective colloid
D. Amount of pure gold