Water of crystallization

Crystal water or water of hydration is the name for water, which is found bound in the crystalline solid state. Crystal water -containing substances are also referred to as hydrates.

Basics: coordination water, structural water

Water molecules can be coordinated to ions ( coordination of water, for example, when copper sulfate) are present as bound water structure via hydrogen bonds to molecules, or - as in the minerals of the zeolite group - not be involved in the crystal lattice. At the most diverse are the bonding of water in protein crystals.

The crystal water is usually only loosely bound and escapes on heating, which can lead to the resolution in its own water of crystallization upon copper sulphate to discoloration, even with other agents.

Nomenclature

Hydrates and anhydrates

There are, for example, of anhydrous sodium sulphate Na2SO4 and the decahydrate ( Glauber's salt ) with 10 ( altgr.: deca = ten) water molecules per formula unit of Na2SO4. The binding is by a superscript dot (sometimes called an x ) symbolizes this notation, there is no information on the type of bond: Na2SO4 · 10 H2O.

The crystal anhydrous salts are called anhydrates (eg, sodium sulfate anhydrous, copper sulphate anhydrate, not to be confused with anhydride and anhydrite). Anhydrates are usually highly hygroscopic substances and are therefore used for drying solvents and gases.

Delineation of gas hydrates

Gas hydrates (such as methane ), no hydrates, but inclusion compounds are actually ( clathrates ). Here the gases are incorporated into the structure cavities of crystalline water, you do not speak here of crystal water, because the water molecules generate the structure itself. The trapped atoms or molecules are trapped in a cage in the structure, hence the name cage compounds. The melting of the water, the gases are then released again.

Drugs and other organic compounds

Drugs and other organic compounds are often basic, and are used as hydrohalides. In some cases, the hydrohalides crystallize simultaneously as hydrates. Examples: tirofiban and ziprasidone. Estradiol hemihydrate is another example of a drug.

Technical Applications

Technically, one uses the assets of the gypsum by heating (firing) to take partially or completely lost crystal water when mixed with water again and to corroborate it. By heating the dihydrate CaSO4 · 2 H2O at about 110 ° C calcined gypsum produced ( hemihydrate or hemihydrate called, CaSO4 · ½ H2O), at 130 to 160 ° C stucco (a mixture of a lot of hemihydrate and anhydrite little ).

Anhydrite also comes as a mineral in salt deposits and consists of crystal of anhydrous calcium sulfate ( CaSO4 ). Another industrial application is the displacement of the desiccant silica gel with cobalt chloride, which is blue and colored in pink hydrated in the anhydrous state. In this way treated silica gel, the pink coloration indicates that the silica gel can absorb any more moisture and needs to be regenerated by heating.

Salt hydrates can be used as heat storage. In this case the hydrate is melted and the heat can be stored. If necessary, it responds back to a hydrate and releases heat from. According to this principle work:

• Heat Packs
• Central heating systems with solar panels
• Waste heat recovery in cars