Reversible And Protective Colloids And Gold Number

Reversible And Protective Colloids And Gold Number

Lyophilic colloids like gelatin, gum arabic, starch, albumin, agar−agar etc. when merely shaken or warmed with the dispersion medium (or solvent) readily yield colloidal solutions. If such a colloidal solution is coagulated by certain means the coagulate (or coagulated mass) can easily be redissolved in to colloidal form. Substances like gelatin etc., which have great affinity for the dispersion medium and which even after coagulation can be directly and easily redissolved to colloidal from are called reversible colloids.

The reversible and lyophilic colloids are found to be equally stable and their stabilities are very little and not affected by the addition of small quantity of electrolytes. In fact these reversible colloids when added in small quantities to a comparatively unstable lyophobic sol imparts stability to the later thereby making the later much less sensitive towards electrolytes which ordinarily coagulate it. The process by which the lyophobic sols are protected from coagulation by electrolytes due to the previous addition of a lyophilic colloid is called protection and the lyophilic colloid employed for achieving this protection, is called protective colloid. Thus if a little gelatin is added to As₂S₃ sol the later becomes very stable towards electrolytes and the resulting sol is not coagulated until a large excess of electrolyte is added. Hence gelatin here acts as a protective colloid.

The protective action of the lyophilic colloid is due to the formation of a protective and very thin shell surrounding each particle of the lyophobic colloid, which in oppositely charged ions of added electrolyte cannot easily penetrate in a bid to neutralize the charge. Thus the electroyte will have first the coagulation. The extent to which different lyophilic colloids exert the protective action differ. Zsigmondy introduced the the term Gold Number to compare quantitatively the protective power of different lyophilic colloids. Gold number is defined as the least quantity of the protective colloid in miligrams, which is just sufficient to prevent the coagulation of 1 ml of standard gold sol. (i.e., containing 0.0053 to 0.0058% gold) by the rapid addition of 1 ml of 10% NaCl solution. The coagulation of gold sol is indicated by the change in colour from red to violet. Thus smaller the gold number the greater is the protective power of lyophilic colloid. Gelatin has the least value of Gold number.