Silica Gel

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Silica gel is a polymerized form of silica (silicic acid) and is of a neutral pH. It has a negative charge and is absorbant so has many important uses including filtration and dessication.

Synthesis

Silica gel is created by:

  1. reacting silicon dioxide with an alkali or alkaline mineral
  2. dissolving it in acid
  3. precipitating the gel

Reacting with alkali(ne)

This step converts the dioxide form of silicon (silicon dioxide) to a silicate. It can be dissolved in this process, like in the case of an alkali (like sodium or potassium hydroxide) or not, like in the case of alkaline (like calcium hydroxide) mineral. This is done by heating a liquid slurry of alkali(ne) mineral with silicon dioxide. Liquid does not need to be used, especially in the case of an alkali, the alkali can be melted and a molten solution formed with the silicon dioxide.

Alternatively if the silicon is in a pre-reacted form like it is in plant matter or is a reactive ash, you may not need to react with alkali(ne) and can proceed straight to acid extraction.

Dissolving in acid

Now either the silicon dioxide is fully dissolved in the alkali(ne) or at least the surface is reacted with it creating a silicate. In this step that silicate is dissolved with an acid. Many use hydrochloric acid or sulfuric acid but I prefer phosphoric acid. The acid should be dilute to prevent precipitation if so desired.

Precipitating the gel

Multiple methods can be used here, but the goal is reducing the solubility of the silica from the solution (silicic acid) so it polymerizes into a gel. The most common way is to neutralize the pH. Also "salting out" can work. Evaporation or dilution is another way. Likely a combination of methods would be designed to give the best yield.

Alternative methods

Any method that can dissolve silicon dioxide can be used directly to make silica gel. Another option that does not require conversion to silicate first, would be hydrofluoric acid which is one of the most dangerous and deadly chemicals known.

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