GOLD NUMBER

GOLD NUMBER, a value characterizing the protective action of an emulsional colloid. The color of colloidal solutions, particularly metal hydrosols, very often depends not on the chemical nature of the colloid or its dispersion medium, but on the degree of dispersion, on the size of the colloidal particles, changing appropriately with the latter. This is clearly shown, for example, by gold hydrosols. With relatively large particles, they have a blue or violet color, whereas in a very finely divided state they impart a pure bright red color to the solution. Thus, as the degree of dispersion increases, the color of the sol changes from blue to red. The action of various substances, for example salts, that cause the coagulation and precipitation of gold hydrosols is therefore manifested first in the reverse change of its color (from red to blue). It is a direct indicator of the decrease in the degree of dispersion, which may precede the gross precipitation of the colloidal metal. Zsigmondy applied this color reaction, with its clarity resembling the color change of an indicator, to measuring the value of the protective action. According to his proposal, the gold number of a protective colloid is defined as the amount of it (expressed in milligrams) that is just insufficient to prevent the color change of 10 cm³ of an intensely red gold hydrosol when 1 cm³ of a 10% NaCl solution is added to it. This number, however, depends on the method of preparation of the gold sol and the size of its particles. In view of this, when determining the gold number, one should use the same sol, prepared by Zsigmondy's method; the average particle size of such a sol is approximately 25 mµ (fluctuations from 22 to 28 mµ are permissible). The following table gives the gold numbers of several protective colloids. Gold Colloid, Rubinstein.