Нитрит кальция

Описание

Calcium nitrite has the formula of Ca(NO2)2 and the molecular weight of 132.0894 g/mol. It can be made by the usual double decomposition reaction of sodium nitrite and CaCl2. It forms a tetrahydrate that is isomorphous with the strontium and barium salts:
2Na2NO2 (aq) + CaCl2 ? Ca(NO2)2 (aq) + 4NaCl (aq)

Химические свойства

white or yellowish deliquescent, hexagonal crystal(s); prepared from nitric oxide and a mixture consisting of calcium ferrate and calcium nitrate; used to inhibit corrosion in lubricants and concrete [MER06]

Физические свойства

Calcium nitrite, Ca(NO2)2, has the CAS number of 13780-06-8 and occurs as yellow-white crystals. It is very hygroscopic. Its density is 2.23 g/cm³ and it decomposes at 392°C to form nitrogen oxides and CaO. It is slightly soluble in alcohol. Ca(NO2)2·4H2O loses two waters of hydration at about 80°C and one at 105°C to form the monohydrate. It loses the other at about 155°C. The anhydrate, if further heated, is unstable and oxidizes to the nitrate in air above 220°C:
Ca(NO2)2 + O2 + heat?Ca(NO3)2

Использование

Calcium nitrite has several industrial uses:
1. Calcium nitrite has high solubility, so can be used as a solution or powder in concrete as an “anti-freeze” during winter months as needed. Using this antifreeze at negative temperature can promote the hydration of minerals in cement, and the freezing point of operative temperature can be reduced to –25°C.
2. This salt is a steel rust inhibitor: calcium nitrite can clean steel and protect it from rust, and can protect steel material in concrete used to construct buildings and structures, to extend the useful life of concrete in buildings.
3. In concrete, Ca(NO2)2 acts as a coagulator: calcium nitrite is free of chlorine and alkali, capable to reduce alkali aggregation of concrete and improve its early strength and compressive resistance. The calcium nitrite protects steel reenforcing bars from chlorideinduced corrosion. One advantage to calcium nitrite is that its protection mechanism is well defined. Data are known that show the levels of chloride to which given levels of calcium nitrite will protect. Once corrosion initiates, the rates are lower with calcium nitrite present. These results have been used to produce reinforced concrete structures with durability in excess of 50–100 years.
4. Calcium nitrite can be used as corrosion inhibitor when added to lubricating agents.
5. Calcium nitrite can also be used in pharmaceutical, dyes and metallurgy industries, as a mild reducing agent.

Подготовка

Industrially, calcium nitrite is manufactured by reacting “NOX” gas with “lime”:
Ca(OH)2 + NO +NO2?Ca(NO2)2 +H2O
By carefully controlling calciumnitrite conditions, the selectivity on converted lime, on a molar basis, is over 95% pure. It is soluble in water (98.2 g/100 ml at 20 °C) and the solution must be evaporated at low temperature to obtain the tetrahydrate. If heated with NO2 gas, it also form the nitrate:
Ca(NO2)2 + NO2?Ca(NO3)2 + NO
Anhydrous calcium nitrite is unstable and slowly oxidizes in air to form the nitrate. It is also soluble in alcohol at 0.97 g/100 ml at 20 °C.

Общее описание

Calcium nitrite solution (Ca(NO₂)₂) is widely utilized as a corrosion inhibitor. It protects the metal surface against chloride attack and increases the threshold level and improves the overall life of the concrete. Ca(NO₂)₂ does not change the mechanical properties of the cement.

Методы очистки

Crystallise it from hot water (1.4mL/g) by adding ethanol and cooling to give the hydrate. It is deliquescent. [Ray & Ogg J Am Chem Soc 79 265 1957.]