On heating they yield phosphine and leave a residue of pyrophosphate, or a mixture of metaand pyrophosphates, with a little phosphorus.
The metallic phosphites are stable both dry and in solution; when strongly heated they evolve hydrogen and yield a pyrophosphate, or, especially with the heavy metals, they give hydrogen and a mixture of phosphide and pyrophosphate.
When heated to 100° C., it loses five molecules of water of crystallization, and at a higher temperature loses the remainder of the water and also ammonia, leaving a residue of magnesium pyrophosphate, Mg 2 P 2 0 7.
The same reaction is made use of in the quantitative determination of magnesium, the white precipitate of magnesium ammonium phosphate being converted by ignition into magnesium pyrophosphate and weighed as such.
Crystals may be obtained by heating di-calcium pyrophosphate, Ca2P207, with water under pressure.