' $A$ ' is a protecting colloid. The following data is obtained for preventing the coagulation of 10 mL of…
' $A$ ' is a protecting colloid. The following data is obtained for preventing the coagulation of 10 mL of gold sol to which 1 mL of $10 \% \mathrm{NaCl}$ is added. What is the gold number of 'A'?
$\begin{array}{|c|c|c|}\hline \text{Expt. No.} & \begin{array}{c} \text{Wt (in mg) of A} \\ \text{added to gold sol} \end{array} & \text{Coagulation} \\ \hline 1 & 40 & \text{Prevented} \\ \hline 2 & 35 & \text{Prevented} \\ \hline 3 & 25 & \text{Not prevented} \\ \hline 4 & 32 & \text{Not prevented} \\ \hline 5 & 33 & \text{Prevented} \\ \hline\end{array}$
32
33
35
40
Solution
Gold number is defined as the minimum amount of lyophilic colloid in milligrams which prevents the flocculation of 10 ml gold sol by the addition of 1 ml of $10 \% \mathrm{NaCl}$ solution. More the gold number, lesser will be the protective power of lyophillic colloids.
So among the 1,2 and (5), (5) have the lowest amount of ' $A$ ' added to gold sol so the gold number of ' $A$ ' is 33 .