Thallous nitrite, TlNO₂

Thallous nitrite, TlNO₂, is prepared by mixing solutions of thallous sulphate and barium nitrite, filtering, evaporating the filtrate in vacuum, and adding alcohol. It forms orange-red crystals. The concentrated aqueous solution is yellow, but dilute solutions are nearly colourless. The dry salt has a low melting-point; it is unchanged at 140°, but at higher temperatures decomposes into thallous oxide and oxides of nitrogen, its stability being much inferior to that of the alkali nitrites.