Zsigmondy introduced a term called gold number to describe the protective power of different colloids. This is defined as, “weight of the dried protective agent in milligrams, which when added to 10 ml of a standard gold sol (0.0053 to 0.0058%) is just sufficient to prevent a colour change from red to blue on the addition of 1 ml of 10 % sodium chloride solution, is equal to the gold number of that protective colloid.”
Thus, smaller is the gold number, higher is the protective action of the protective agent.
Table: Gold numbers of some hydrophilic substances
|Hydrophilic substance||Gold number||Hydrophilic substance||Gold number|
|Gelatin||0.005 - 0.01||Sodium oleate||0.4 – 1.0|
|Sodium caseinate||0.01||Gum tragacanth||2|
|Hamoglobin||0.03 – 0.07||Potato starch||25|
|Gum arabic||0.15 – 0.25|