stokes.] DIIMIDOTRI- AND IMIDODIl'HOSPIIORIC ACIDS. 113 
Attempts to replace one or both of the remaining hydrogen atoms 
by silver proved unsuccessful. If the tertiary sodium salt be dissolved 
with 1-2 molecular weights caustic soda, or even with 5-6 molecular 
weights ammonia, and added to silver nitrate, the resulting precipitate 
is discolored by free silver oxide. On standing several days it becomes 
yellow, but its ammoniacal solution is now precipitated by magnesia 
mixture, indicating decomposition. The magnesia precipitate gave a 
silver salt resembling tertiary imidodiphosphate, but the exact nature 
3f the decomposition was not investigated. 
IMIDODIPHOSPHORIC ACID, P 2 ISr0 6 H s . 
This is probably the substance described by Gladstone under the 
lame pyrophosphamic acid. It is moderately stable in cold, not too 
icid, solution, and hence occurs in large amount among the decomposi- 
ion products of trimetaphosphimic acid. 
Tri-sodium imidodiphosphate, obtained by decomposing the tertiary 
ilver salt by sodium chloride, has alkaline reaction. Its solution, as 
rell as that of the tetra sodium salt, is precipitated by alcohol as a 
imp which can not be made to crystallize, and the aqueous solution 
f which dries to a transparent gummy mass. It is more soluble in 
ilute alcohol than sodium pyrophosphate. 
The magnesium salt above described is amorphous, voluminous, and 
early, but not quite, insoluble in water, and somewhat more soluble 
l solutions of ammonium salts. 
Tri-silver imidodiphosphate, P 2 NO G H 2 Ag 3 .— This is obtained as an 
morphous, white precipitate, when the free acid, or a solution of a 
Iurtiary salt is precipitated by silver nitrate. Thus formed, it tends 
» collect in small lumps, a property which distinguishes it from any 
? the other silver salts referred to in this paper. When precipitated 
I acid from its ammoniacal solution it forms a powder that settles 
owly. When slowly separating, or when precipitated from hot solu- 
ms, it forms semicrystalline granules, often united to crusts or den- 
•itic forms. When formed in a moderately acid solution it usually 
lows a slight deficiency of silver (analyses 1-2), probably due to a 
ace of a more aeid salt; but such a salt can not be obtained in pure 
rm. Pure tertiary salt can be obtained by decomposing the crude 
It with somewhat less than the required amount of sodium chloride, 
d reprecipitatiug. It is insoluble in water and is not affected by 
fht. 
Bull. 167 8 
