Potassium nitrite, KNO₂

The Potassium nitrite, KNO₂, is made by the reduction of potassium nitrate by heating it alone, or with metals such as lead and iron, or with substances containing sulphur or carbon. It is also formed from potassium nitrate by electrolysis with a silver cathode and a copper anode, the yield being almost quantitative. The pure salt can be obtained by precipitating the aqueous solution with methyl alcohol. Another method for the production of the nitrite depends on the double decomposition of silver nitrite and lithium chloride.

The pure, anhydrous nitrite is not hygroscopic, but as usually prepared the substance is a very deliquescent, crystalline solid, its aqueous solution having a slight alkaline reaction. At 15.5° C. the solubility is 300 grams per 100 grams of water. The heat of formation in aqueous solution from the elements is 88.9 Cal. The products of electrolytic reduction are hyponitrite, ammonia, and hydroxylamine.

At 350° C. potassium nitrite begins to decompose in accordance with the equations

3KNO₂ = K₂O + KNO₃ + 2NO;
5KNO₂ = K₂O + 3KNO₃ + 2N.

Heating with nitrogen peroxide oxidizes the nitrite to nitrate:

KNO₂ + NO₂ = KNO₃ + NO.

Potassium nitrite readily reacts with cold sulphuric acid of 17 per cent, strength, yielding a mixture of nitric oxide and higher oxides of nitrogen. If the mixed gases are passed through a solution of an alkali-metal hydroxide, pure nitric oxide is isolated. For laboratory purposes this method affords a convenient means of preparing the gas, 10 grams of potassium nitrite producing about 2.5 litres of the oxide.