Sodium Cobalti-nitrite, Na₃Co(NO₂)₆

Sodium Cobalti-nitrite, Na₃Co(NO₂)₆, is best obtained by the action of nitrous acid upon a suspension of cobalt carbonate in the requisite quantity of sodium nitrite solution. The liquor, which at first becomes reddish brown, after several hours assumes a deep brown, almost black colour, is filtered, and the sodium cobaltinitrite in solution is precipitated by the addition of absolute alcohol. Cunningham and Perkin prepare the salt by dissolving 150 grams of sodium nitrite in hot water and, after cooling to 45-50° C., adding 50 grams of cobalt nitrate until all has dissolved. Glacial acetic acid is now added, and a rapid current of air is aspirated through to remove oxides of nitrogen. On addition of alcohol the sodium cobalti-nitrite separates out.

The salt may also be prepared by adding 113 grams of cobalt acetate in dilute acetic acid to a solution of 220 grams of sodium nitrite in 400 c.c. of water.

Sodium cobalti-nitrite, as obtained by the foregoing methods, is an orange-yellow powder, readily soluble in water, yielding an intense brownish yellow solution, which decomposes on heating. Dammer gives the salt as having 2.75 molecules of water. Either the salt to which he refers was not dry, or else the alcohol used in preparing Cunningham's specimens dehydrated them. A characteristic reaction of the solution is the precipitation by it of potassium, ammonium, barium, or lead cobalti-nitrite when added to a solution containing a soluble salt of one of these metals.