Precipitated silicon dioxide (also: precipitated silica) is an amorphous form of silicon dioxide (SiO2); it is a white, powdery material. Precipitated silicon dioxide is produced by precipitation from a solution containing silicate.
The three main classes of amorphous silica are pyrogenic silica, precipitated silica and silica gel. Among them, precipitated silica has the greatest economic importance. In 1999 more than a million tons were produced, half of which was used in tires and shoe soles.
Precipitated silica is prepared by condensing silica from dilute alkali silicate solution, which is accessible by digesting quartz sand with sodium carbonate or potassium carbonate. This alkali silicate solution is also called water glass. Silicic acid has a strong tendency to polymerize, converting silanol groups (Si-OH) to siloxane bonds (Si-O-Si). The reaction kinetics (reaction rate) strongly depend on the pH value.
Industrial production is carried out by acidifying an aqueous alkali silicate solution (e.g. sodium silicate) with sulfuric acid. Gelling is prevented by stirring, otherwise silica sols or silica gels would form.