The oxygen acids of silicon are called silicic acids . The simplest silica is monosilicic acid (orthosilicic acid) Si (OH) 4 or also H 4 SiO 4 . It is a weak acid ( pK s1 = 9.51; pK s2 = 11.74) and tends to (poly) condensation . Dehydration leads to compounds such as disilicic acid (pyrosilicic acid) (HO) 3 Si – O – Si (OH) 3 and trisilicic acid (HO) 3 Si – O – Si (OH) 2 –O – Si (OH) 3 . Cyclic (ring-shaped) silicas are e.g. B. Cyclotric silicic acid and cyclotetric silicic acid with the general empirical formula [Si (OH) 2 –O–] n . Polymers are sometimes referred to as metasilicic acid (H 2 SiO 3 , [-Si (OH) 2 -O-] n ). If these low molecular weight silicas condense further, amorphous colloids ( silica sol ) are formed. The general formula of all silicas is H 2n + 2 Si n O 3n + 1 . SiO 2 · n H 2 O is often given as the empirical formula ; In the case of silicas, however, the water is not water of crystallization , but can only be split off by a chemical reaction and is formed from constitutionally bound hydroxyl groups .
In general, the lower water content of orthosilicic acid products are grouped under the term polysilicic acids . The formal end product of dehydration is silicon dioxide , the anhydride of silicic acid. The salts of the acids are called silicates . Technically used or manufactured alkali salts are often called water glasses . The esters of silicic acids are called silicic acid esters .
Technically obtained colloids ( pyrogenic silicas ), fossil sediments ( kieselguhr ), the natural armor of diatoms , as well as certain glasses ( silica glass ) can all be described by the general empirical formula SiO 2 , i.e. they are silicon dioxides and can formally be the product of a chemical reaction from monosilicic acid be understood. For this reason, amorphous silicon dioxide (= silicic acid anhydride) is historically often and incorrectly referred to as silicic acid in German-speaking countries. B. fumed silica instead of fumed silicon dioxide .
Monosilicic acid occurs in all bodies of water, including drinking water, as well as in all animal and vegetable body fluids. In terms of quantity, diatoms mainly use silica to build their armor. The consumption of silicic acid through biological processes is balanced out by rain or seepage water that runs through soil layers and dissolves silicic acid from the silicates of the soil minerals (see silicate weathering ). Silica (often of volcanic origin) can lead to the formation of fossils through silicification , such as B. for the formation of petrified forests (see also quartz and fossilization ).
Monosilicic acid is slowly formed in aqueous suspensions of amorphous, disperse SiO 2 . At 25 ° C, about 120 mg SiO 2 dissolve per liter:
Under acidic or basic conditions, monosilicic acid is subject to an exothermic , intermolecular condensation reaction to form disilicic acid (1), tri-silica and subsequently to polysilicic acids. The elimination of water is initiated catalytically at pH> 2 by deprotonation (2) and at pH <2 by protonation (3). At pH = 8 to 9 the reaction is fast and the condensation takes place in minutes to seconds.
Each Si – OH group tends to form an Si – O – Si bond. In addition to ring-forming and chain-branching condensations, three-bonded and four-bonded Si units lead to crosslinking of the chains. The condensation reactions take place in a disorderly manner. Spherical, non-crystalline ( amorphous ) polysilicic acids are formed. These particles are no longer in the form of a solution, but form a colloid in the aqueous phase with a particle diameter between 5 and 150 nm. If the polysilicic acid particles are sufficiently large, the suspension becomes milky. Certain technical processes allow these particles to be stabilized before further crosslinking. The product is called silica sol . Without stabilization, the polysilicic acid particles tend to adhere to one another in a disordered manner; the particles form porous aggregates with (water-filled) cavities. In an aging process, the polysilicic acid particles “fuse” into one another through new Si – O – Si bonds. Such stable structures are called silica gels , of which silica gel is a technically perfected product of this structure. In general, the products are precipitated silicas or precipitated silicas mentioned. The aggregates are powdery, have a low bulk density and a high specific surface . If a water content is specified for these aggregates, it is - in contrast to silicas - physically (by adsorption ) bound water.
Silicas are very poorly soluble in water and only slowly soluble. At a pH of 7 and a temperature of 25 ° C, a maximum of 0.12 g (calculated as silicon dioxide) of silicic acids dissolve in one liter of water. This corresponds to a value of 120 ppm. The solubility increases with increasing temperature and increasing pH. For example, 330 ppm silicon dioxide dissolve in water at 75 ° C.
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