Gold Number

GOLD NUMBER, a value characterizing the protective action of an emulsion colloid. The color of colloidal solutions, in particular 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 correctly along with the latter. This is clearly shown, for example, by gold hydrosols. With a relatively large particle size, they have a blue or violet color, while in a very finely divided state they impart a pure bright red color to the solution. Thus, as one transitions to higher and higher degrees of dispersion, the color of the sol changes from blue to red. The action of various substances, for example, salts that cause coagulation and precipitation of the gold hydrosol, will therefore manifest itself primarily in the reverse change of its color (from red to blue). It is a direct indicator of a decrease in the degree of dispersion, which can long precede the coarse precipitation of the colloidal metal. Zsigmondy applied this color reaction, which in its clarity resembles the color change of an indicator, to the measurement of the magnitude of protective action. According to his proposal, the gold number of a protective colloid is called the amount of it (expressed in milligrams) which is just insufficient to prevent the color change of 10 cm³ of an intensely red gold hydrosol upon the addition of 1 cm³ of a 10% NaCl solution. This number, however, depends on the method of preparing the gold sol and the size of its particles. In view of this, when determining the gold number, one should use one and the same sol, prepared according to the Zsigmondy method; the average particle size of such a sol is equal to approximately 25 mμ (fluctuations from 22 to 28 mμ are permissible). The following table shows the gold number of several protective colloids.

D. Rubinstein.