Characterisation of Precipitated Silica

As an adsorbent, the surface area of precipitated silica is required. This can be measured by gas adsorption (Sec.2.1.5). Since the commercial samples also contain impurities, chemical analysis is useful for characterisation. 

The main impurities in precipitated silica are the alkali metals Na and to a smaller extent K. Besides, the chloride content is relevant if the powder is to be used for studying the adsorption of cations. The water content can be simply determined by the loss of mass at 120°C. 

Weigh out accurately about 12 g of the powder and disperse in water, stir well and filter through a G4 sinter. Wash with water on the filter. Combine the filtrate and washings and make up to 100 cm in a volumetric flask. Add 1.0 cm of 5% potassium chromate indicator and titrate against standard 0.03 M silver nitrate. Repeat and calculate from the average titre the % Cl in your sample. Use a similar volume of the solution to determine the concentration of Na and K by atomic absorption or flame photometry and express your results kg of silica. 

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