Basics: coordination water, structural water
Water molecules can be coordinatively bound to ions ( coordination water , e.g. in the case of copper sulphate ), be present as structural water bound to molecules via hydrogen bonds , or - as with the minerals of the zeolite group - not be involved in the crystal lattice. The bonding relationships of water in protein crystals are most diverse .
The water of crystallization is usually only loosely bound and escapes when heated, which in the case of copper sulphate can lead to discoloration, in the case of other substances it can even dissolve in its own water of crystallization.
Hydrates and anhydrates
So there are z. B. Anhydrous sodium sulfate Na 2 SO 4 and the decahydrate ( Glauber's salt ) with 10 ( oldgr . : deca = ten) water molecules per formula unit Na 2 SO 4 . The bond is symbolized by a superscript point (sometimes also an x), but this notation does not provide any information about the type of bond: Na 2 SO 4 · 10 H 2 O.
The salts that are free of water of crystallization are called anhydrates (e.g. sodium sulfate anhydrate, copper sulfate anhydrate, not to be confused with anhydride and anhydrite ). Anhydrates are mostly highly hygroscopic substances and are therefore used to dry solvents and gases.
Differentiation from gas hydrates
Gas hydrates (like methane ice ) are actually not hydrates, but inclusion compounds ( clathrates ) . Here the gases are stored in the structural cavities of crystalline water, one does not speak of crystalline water because the water molecules themselves create the structure. The trapped atoms or molecules are trapped in the structure like in a cage, hence the name cage compounds . When the water melts, the gases are then released again.
Medicinal substances and other organic substances
Medicinal substances and other organic substances are often basic and are used as hydrohalides . In some cases the hydrohalides also crystallize as hydrates. Examples: Tirofiban and Ziprasidon . Estradiol hemihydrate is another example of a basic drug that is used in the form of its hydrohalide.
Technically, the ability of gypsum is used to absorb the crystal water that has been partially or completely lost through heating ( burning ) when mixed with water and to harden it in the process. When the dihydrate CaSO 4 · 2 H 2 O is heated to about 110 ° C, baked gypsum (called hemihydrate or hemihydrate , CaSO 4 · ½ H 2 O) is formed, at 130 to 160 ° C stucco (mixture of a lot of hemihydrate and little anhydrite ).
Anhydrite also occurs as a mineral in salt deposits and consists of calcium sulfate (CaSO 4 ) free of water of crystallization . A further technical application is placing the desiccant silica gel with cobalt chloride , which is colored blue in the anhydrous state and in the crystal water-containing pink. In the case of silica gel treated in this way, the pink discoloration indicates that the silica gel can no longer absorb any more moisture and therefore has to be regenerated by heating.
Salt hydrates can serve as heat stores. The hydrate is melted and the heat can be stored. If necessary, it reacts again to form a hydrate and gives off heat in the process. Work according to this principle:
- Brockhaus ABC Chemie , VEB FA Brockhaus Verlag Leipzig 1965, pp. 746-747.
- Brockhaus ABC Chemie , VEB FA Brockhaus Verlag Leipzig 1965, p. 456.