preparation ,properties, uses of silicates
Answers
preparation :
In chemistry, a silicate is any member of a family of anions consisting of silicon and oxygen, usually with the general formula [SiO(4−2x)−
4−x]
n, where 0 ≤ x < 2. The family includes orthosilicate SiO4−
4 (x = 0), metasilicate SiO2−
3 (x = 1), and pyrosilicate Si
2O6−
7 (x = 0.5, n = 2). The name is also used for any salt of such anions, such as sodium metasilicate; or any ester containing the corresponding chemical group, such as tetramethyl orthosilicate.[1]
properties:
Solid silicates are generally stable and well characterized.
Silicates with alkali cations and small or chain-like anions, such as sodium ortho- and metasilicate, are fairly soluble in water. They form several solid hydrates when crystallized from solution. Soluble sodium silicates and mixtures thereof, known as waterglass are in fact important industrial and household chemicals. Silicates of non-alkali cations, or with sheet and tridimensional polymeric anions, generally have negligible solubility in water at normal conditions.
Reactions Edit
Silicate anions are formally the conjugate bases of silicic acids. For example, orthosilicate can be viewed as the fourfold deprotonated orthosilicic acid Si(OH)
4. Silicic acids are generally very weak and cannot be isolated in pure form. They exist in water solution, as mixtures of condensed and partially p anions, in a dynamic equilibrium.[2] The general processes in this equilibrium are hydrolysis/condensation
≡Si–O–Si≡ + H
2O ↔ ≡Si–OH + HO–Si≡
=Si=O + H
2O ↔ =Si(–OH)
2
and protonation/deprotonation
≡Si–OH ↔ ≡Si–O−
+ H+
.
The equilibrium can shifted towards larger anions by increasing the silicate concentration and/or the acidity of the medium. The orthosilicate anion, for example, is assumed to be the predominant form of silica naturally dissolved in seawater, whose concentration is below 100 parts per million; and also when silica is dissolved in an excess of sodium oxide at pH 12 or more.[2][3] In higher concentrations or low pH, polymeric anions predominate.
In the limit, the silicate anions merge into a tridimensional network of silicon tetrahedra, with most oxygen atoms occurring as covalent bridges; which is some form of silica, such as silica gel, mixed with water.
uses:
Detection
Silicate anions in solution react with molybdate anions yielding yellow silicomolybdate complexes. In a typical preparation, monomeric orthosilicate was found to react completely in 75 seconds; dimeric pyrosilicate in 10 minutes; and higher oligomers in considerably longer time. In particular, the reaction is not observed with suspensions of colloidal silica.[3]
Zeolite formation
The nature of soluble silicates is relevant to understanding biomineralization and the synthesis of aluminosilicates, such as the industrially important catalysts called zeolites.[2]