Particle Size and Surface Area by Titration

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 

However, in sols of such small particle size, there is an appreciable concentration of monomer at equilibrium. Also, in alkaline sols at pH 9-10.5, there is an appreciable amount of ionic silica which is converted to monomer before the titration. Since monomer reacts with base at pH 9 it is therefore necessary to correct the titration for the effect of soluble silica in order to obtain a reliable value for the specific surface area of the polymer. The term soluble silica is used to include the ionic silica and dimer which react with alkali-like monomer. 

The soluble silica can, of course, be removed at pH 2 either by washing the silica in a filter or ultrafilter or by centrifuging. Also the sample can be adjusted to pH 8 and let stand a few hours until the soluble silica has been polymerized upon the colloidal material. 

However, for colloids of extremely small particle size or dilute sols where an appreciable fraction of the silica is present as soluble silica, it is preferable to determine the amount of monomer by suddenly acidifying a sample and determining it by the molybdate method, after which the corresponding correction can be made. 

In the Sears procedure, titration with 0.1 N NaOH is begun with a silica sample in 150 ml of solution containing 30 g NaCl at pH 4.0. 

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