Binding of the Cationic Dye, Janus Green B, as a Measure of the Specific Surface Area of Crystalline Silica in Aqueous Suspension

Twelve preparations of crystalline silica, with a wide range of particle sizes, were assayed by a new method, which measures surface adsorption of the cationic dye Janus green B to crystalline silica samples in a buffered aqueous suspension. The same samples were also assayed for total surface area by the Brunauer-Emmett-Teller (BET) method of surface adsorption of nitrogen gas. A strong linear correlation was found between the two methods of measurement (r = 0.977). Reproducible specific surface area measurements by the Janus green B adsorption method were made on 2-mg samples using ordinary visible wavelength spectrophotometric equipment, whereas the BET method necessitated sample sizes in excess of 100 mg and more specialized instrumentation. Five size-fractionated preparations from the same Min-U-Sil α-quartz sample showed an increase in BET surface area and Janus green B binding per unit weight with decreasing particle size. Among four standard α-quartz samples tested, Min-U-Sil 5 and F600 had the lowest specific surface areas, whereas DQ-12 and Chinese standard α-quartz had much higher surface areas. The synthetic silica preparations cristobalite and tridymite had intermediate surface areas. Binding by the cationic dye Janus green B is consistent with a surface charge mechanism and provides a useful new technique for the assessment of surface characteristics of crystalline silica samples. Its linear relationship to surface area suggests that the ratio of aqueous surface charge to surface area is constant for different crystalline silica preparations. Comparison of surface areas for different preparations of crystalline silica is important in understanding the relative activities of these preparations in studies on mechanisms of silicosis and silica-induced lung cancer.