Moisture regulation 173 9.2 Silica gel 9.2.1 Origins The origins of silica gel lie on every beach and river bottom in the world. Sand is the raw material. Sand is relatively pure crystalline silicon dioxide. In order to manufacture silica gel, sand is first put into solution with a strong alkali. Then after filtration, precipitation, neutralisation, repeated rinsings and drying. amorphous silica is obtained. This is silica gel, needing only to be milled and classified to make it ready for use 9.2.2 Composition Silica gel manufactured in this way is completely amorphous, detectable fraction of crystalline silica, which is of concern as an There is still some residual salt, typically about 0.5% and mostly Na2S near neutrality. The usual specification is pH 4-8. There is little if any titratable acidity. Evaluation may routinely be accomplished by preparing a 10% slurry of silica gel in distilled water, extracting for two hours, and measuring the supernatant for conductivity, pH, and titratable acidity 9.2.3 Purity and compliance: EU, FCC, USP Silica gel is permitted for use as a desiccant with foods and pharmaceuticals under eu regulations. The US Food Chemicals Codex contains a monograph pecify ing silica gel for food use and the Us Pharmacopoeia describes silica gel for pharmaceutical use 9.2.4 Adsorption profile Silica gel adsorption, as with any sorbent, is proportional to the equilibrium relative humidity(ERH) and the temperature of its environment. In order to view adsorption characteristics it is customary to plot an adsorption isotherm at 25C as in Fig. 9. 1. As may be seen, the adsorptive capacity of silica gel is only 3-4% at an ERH of 10% rising to a capacity of over 30% at an ERH of 90% The rate at which silica gel approaches its capacity at differing ErH is illustrated in Fig. 9.2. Though capacity varies greatly, the rate at which silica gel approaches its capacity does not Regeneration gel may be regenerated and used indefinitely. With repeated adsorption and regeneration, some particle attrition occurs which eventually diminishes its usefulness. Complete regeneration is possible to 2% moisture at 150C for three hours. Moreover, 75-80% of capacity may be regained at 115-120oC for six hours. Microwave regeneration at low power(<400W) is also possible9.2 Silica gel 9.2.1 Origins The origins of silica gel lie on every beach and river bottom in the world. Sand is the raw material. Sand is relatively pure crystalline silicon dioxide. In order to manufacture silica gel, sand is first put into solution with a strong alkali. Then after filtration, precipitation, neutralisation, repeated rinsings and drying, amorphous silica is obtained. This is silica gel, needing only to be milled and classified to make it ready for use. 9.2.2 Composition Silica gel manufactured in this way is completely amorphous, lacking any detectable fraction of crystalline silica, which is of concern as an irritant. There is still some residual salt, typically about 0.5% and mostly Na2SO4. The pH is near neutrality. The usual specification is pH 4–8. There is little if any titratable acidity. Evaluation may routinely be accomplished by preparing a 10% slurry of silica gel in distilled water, extracting for two hours, and measuring the supernatant for conductivity, pH, and titratable acidity. 9.2.3 Purity and compliance: EU, FCC, USP Silica gel is permitted for use as a desiccant with foods and pharmaceuticals under EU regulations. The US Food Chemicals Codex contains a monograph specifying silica gel for food use and the US Pharmacopoeia describes silica gel for pharmaceutical use. 9.2.4 Adsorption profile Silica gel adsorption, as with any sorbent, is proportional to the equilibrium relative humidity (ERH) and the temperature of its environment. In order to view adsorption characteristics it is customary to plot an adsorption isotherm at 25ºC as in Fig. 9.1. As may be seen, the adsorptive capacity of silica gel is only 3–4% at an ERH of 10% rising to a capacity of over 30% at an ERH of 90%. The rate at which silica gel approaches its capacity at differing ERH is illustrated in Fig. 9.2. Though capacity varies greatly, the rate at which silica gel approaches its capacity does not. 9.2.5 Regeneration Silica gel may be regenerated and used indefinitely. With repeated adsorption and regeneration, some particle attrition occurs which eventually diminishes its usefulness. Complete regeneration is possible to < 2% moisture at 150ºC for three hours. Moreover, 75–80% of capacity may be regained at 115–120ºC for six hours. Microwave regeneration at low power (<400W) is also possible. Moisture regulation 173