Gold numbers of some colloids are Gelatin: 0.005 - 0.01, Gum Arabic: 0.15 - 0.25, Oleate: 0.04 - 1.0, Starch: 15 - 25. Which among the following is a better protective colloid?

First, let us recall what the gold number means. According to Grahame’s definition, the gold number of a lyophilic (protective) colloid is the minimum mass, in milligrams, of that colloid which just prevents the coagulation of 10 mL of a standard gold sol when 1 mL of 10 % sodium chloride solution is added. In simple words, it tells us how much of the protective colloid is required to guard the gold sol against coagulation.

A smaller mass needed implies stronger protection. Hence the smaller the numerical value of the gold number, the greater is the protective power of the colloid. Mathematically we can write this inverse relation as

$$\text{Protective power} \;\propto\; \frac{1}{\text{Gold number}}.$$

Now we list the given ranges of gold numbers:

$$\begin{aligned} \text{Gelatin} &: 0.005 \text{ to } 0.01,\\ \text{Gum Arabic} &: 0.15 \text{ to } 0.25,\\ \text{Oleate} &: 0.04 \text{ to } 1.0,\\ \text{Starch} &: 15 \text{ to } 25. \end{aligned}$$

We compare these numbers one by one:

• For Gelatin the value lies between $$0.005$$ and $$0.01$$.
• For Gum Arabic it is at least $$0.15$$, which is already much larger than that of Gelatin.
• For Oleate the lowest end is $$0.04$$, still higher than Gelatin’s highest end, $$0.01$$.
• For Starch the smallest value is $$15$$, enormously larger than all the above.

Because the protective power increases as the gold number decreases, the colloid with the lowest gold number acts as the best protective colloid. Gelatin clearly has the lowest range, so it requires the least quantity to protect the gold sol.

Therefore, Gelatin is the most efficient (better) protective colloid among the choices.

Hence, the correct answer is Option A.