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Addition of 1ml solution of 10%NaCl to 10ml of gold sol in the presence of 0.02g of starch,the coagulation is just prevented.The gold number of starch is a)0.025 b)0.25 c)2.5 d)25
ANS:C 
Gold number is defined as the number of milligrams of a hydrophilic colloid which just prevents the precipitation
of 10 ml of a gold sol on the addition of 1 ml of ten per cent sodium chloride solution.The onset of precipitation of gold sol is indicated by change in color from red to blue.The smaller the gold number,greater is the protective power of the given protective colloid.The gold numbers of a few protective colloids are as follows:
Substance              Gold number
Gelatin                 0.005–0.01
Casein                            0.01
Hemoglobin           0.03–0.07
Albumin                      0.1–0.2
Sodium Oleate           0.4–1.0
Gum Arabic            0.15–0.25
Dextrin                       6–20
Potato Starch              20–25


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