Sodium nitrite, NaNO₂

The Sodium nitrite, NaNO₂, is obtained by reducing sodium nitrate with metals such as lead or iron, with sulphur or carbon, or with material containing these substances. In Dittrich's process the nitrate is heated with slaked lime and sawdust, the yield being almost quantitative, whereas the action of coal and graphite is too energetic:

2NaNO₃+Ca(OH)₂+C=2NaNO₂+CaCO₃+H₂O.

Other processes for the preparation of the nitrite are the electrolytic reduction of the nitrate; the action of nitrous fumes containing excess of nitric oxide on a solution of sodium hydroxide or carbonate; and the action of oxygen on ammonia in presence of platinized asbestos as catalyst, the ammonium nitrite produced being transformed by treatment with sodium hydroxide, and the evolved ammonia being available for further oxidation.

Sodium nitrite is a white, crystalline salt. Its melting-point is given by Divers as 213° C., but by Matignon and Marchal as 276.9° C. (corr.), and by Bruni and Meneghini as 284° C. At 15° C. 100 grams of water dissolve 83.3 grams. The heat of solution is -3.52 Cal., and the heat of formation in solution from the elements is 88.52 Cal.

In aqueous solution at 100° C. and between 50 and 55 atmospheres of pressure, sodium nitrite is not oxidized by prolonged contact with oxygen, even in presence of a catalyst. When heated in an atmosphere of oxygen for nine hours at 175 atmospheres of pressure, the temperature being gradually raised from 395° to 530° C., the solid is almost completely oxidized to sodium nitrate, but the reaction is too slow for practical application:

2[NaNO₂]+(O₂) =2[NaNO₃] + 45 Cal.

When heated, sodium nitrite decomposes in accordance with the equation:

3NaNO₂ =Na₂O+NaNO₃+2NO.

Heating in an atmosphere of nitrogen peroxide yields nitric oxide and sodium nitrate:

NaNO₂+NO₂=NO+NaNO₃.

At 60° C. in an atmosphere of carbon dioxide free from air, a 5 per cent, solution of sodium nitrite is decomposed by metallic copper, with evolution of nitrous fumes, but there is not sufficient evidence to enable the course of the reaction to be indicated by means of an equation.

When an anolyte of sodium nitrite dissolved in twice its weight of water is electrolyzed with a silver anode, a complex salt of the formula NaAg(NO₂)₂ is formed at the anode. On evaporation of the solution over sulphuric acid in vacuo, it separates in bright yellow crystals.

The nitrite finds extensive application in the manufacture of certain synthetic dyestuffs.

References are appended to the solubility in alcohol, and such properties of the aqueous solution as density, vapour-pressure, and electric conductivity.