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Youblob · CC0
Precipitated Silica: Hot Water Glass Carbonated, Then Acidified to pH 6.75 (Allen 1957)
Bubble carbon dioxide through hot dilute water glass and the silica comes down as an extremely fine white powder rather than a gel. Edward Allen of Columbia-Southern made it this way as a white reinforcing filler for rubber; this follows his Example XI, which gave a powder of 148 m² per gram.
Source: E. M. Allen (Columbia-Southern), US 2,805,955 (1957), Example XI. The percentages are our arithmetic; the filter, mill and indicator are ours. Picture drawn by Youblob.
Intermediate
2 days
Instructions
1
1
Dilute the water glass
Dilute the water glass
Take 12,850 g of sodium silicate solution holding 976 g Na2O and 3115 g SiO2 (7.6 % and 24.2 %, a silica to soda ratio of 3.19 by weight; our arithmetic). Dilute it with water to 48 litres in a stirred kettle with a heating coil and thermometer, and heat to 95 °C.
Materials for this step:
Water Glass (Sodium Silicate Solution)12850 g
Water1 as requiredTools needed:
Pressure Reactor
Stirrer
Thermometer
Balance Scale2
2
Carbonate for an hour
Carbonate for an hour
Bubble in pure carbon dioxide fast enough to add the theoretical amount in about 30 minutes, and keep going at that rate for about 1 hour. The slurry ends at pH 9.85.
Tools needed:
Carbon Dioxide GasPressure ReactorStirrerThermometer
pH Indicator Paper3
3
Filter and wash
Filter and wash
Filter the slurry and wash the cake twice with hot water.
Materials for this step:
Water1 as requiredTools needed:
Buchner Funnel (Porcelain)
Filter Paper4
4
Acidify to pH 6.75
Acidify to pH 6.75
Stir the cake back into water and bring it to pH 6.75 with 400 ml of 3.5 N hydrochloric acid.
Materials for this step:
Water1 as requiredTools needed:
Hydrochloric Acid
Glass VesselStirrerpH Indicator Paper5
5
Wash out the chloride
Wash out the chloride
Filter again and wash the cake with hot water until it is nearly free of chloride.
Materials for this step:
Water1 as requiredTools needed:
Buchner Funnel (Porcelain)Filter Paper6
6
Dry at 105 °C and grind
Dry at 105 °C and grind
Dry it at 105 °C in a fan oven and grind it to a fine powder. Allen's had a pH of 8.2, 0.61 % sodium and a surface area of 148 m² per gram.
Tools needed:
Drying Oven
Mortar and Pestle
Glass Jars with LidsMaterials
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Tools Required
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