Chemical Properties
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]
Uses
Corrosion inhibitor in lubricants, concrete.
Description
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)
Physical properties
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
Preparation
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.
Purification Methods
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.]
General Description
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.
Flammability and Explosibility
Notclassified
