Sears method, surface area measurement

Important experimental conditions used to obtain the data considered in this paper are given in Tables I and II. The various methods used to determine silica solubility are described in detail in the primary references. The surface area measurements were made using nitrogen adsorption two measurements were made for comparison using a Sears titration with 0.1 N NaOH. 

Average particle size as used herein is calculated from specific surface area as estimated by measuring the amount of alkali adsorbed from solution as pH of the sol is raised from 4-9 according to a modification of the method of Sears, [6]. The method is set out in the Analytical Procedures infra. 

Specific Surface Area. Sears titration method [6] was used to measure specific siuface area of the silica in the electrolyte-sol. 

The plots of Figures 11.5, 11.6, 11.7 show the effect of temperature on the growth of the silica particles by electrodialysis as the concentration of silica is increased by gradual addition of sodium silicate into the electrodialysis cell. In Figure 11.5 the specific surface area of the silica in m /g, as measured by the Sears titration method is plotted versus silica concentration in g/100 ml. In Figure 11.6 the average silica particle diameter in nm, as calculated from the specific surface area value assuming that the sol is made of dense spheres of amorphous silica with a density of 2.2 g/cc is plotted, versus silica concentration. 

FIGURE 11.5 Specific surface area of silica in m /g versus silica concentration in g/100 ms as measured by sears titration method. 

At a relatively early stage in the polymerization it is possible to characterize the polymeric silica, or silica particles in terms of the specific area of the silica-water interface. This is done by measuring the adsorption of hydroxyl ions in the pH range 4.00-9.00 (Beckman Type E electrode) in a nearly saturated salt solution which permits the surface charge denstly to approach a maximum. This method was developed by Sears (85) to determine the specific surface areas of colloidal particles and gels. Then it was found that if carried out rapidly it could give reproducible... 

Her has carried out a hitherto unpublished investigation of the rates at which extremely small particles of colloidal silica depolymerize to monomer both directly in the molybdic acid reagent and in dilute alkali in which the monomer is determined on separate samples by reaction with molybdic acid. The size of the particles was estimated from the specific surface area, which was determined by the Sears alkali titration method corrected for monomer. The measurements were made as the particles grew in size at pH 8.5 and also as they became aggregated at pH 5.9. 

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