stokes.] IMIDODIPHOSPHORIC ACID. 115 
tertiary sodium salt, and by its ainmoniacal solution giving a yellow 
salt with silver nitrate, while the silver salt of the latter, found under 
the same conditions, is either white, or, when too much ammonia has 
been used, discolored by silver oxide. 
Ferric imidodiphosphate. — No analysis of this was made. It is thrown 
down on boiling an acidified solution with a ferric salt as a white, 
amorphous precipitate, difficultly soluble in acids and easily soluble in 
ammonia. 
Cupric imidodiphosphates. — Several of these appear to exist. The 
amorphous, light-blue precipitate formed by cupric sulphate in a solu- 
tion of the sodium salt redissolves in an excess of the latter, is 
again reprecipitated by an excess of cupric salt, but under no •cir- 
cumstances completely. It dissolves in caustic potash to a violet 
solution. The soluble sodium double salt is partially precipitated on 
heating its aqueous solution, the precipitate redissolving on cooling. 
The precipitate with excess of cupric sulphate dissolves readily in cold 
acetic or sulphurous acid, and is temporarily reprecipitated on heating 
the solution, unless too dilute and too strongly acid, in which case some 
pyrophosphate crystallizes out on long heating. The same temporary 
precipitation occurs in a less degree on heating its solution in very 
dilute nitric acid. 
The property of being precipitated from an acid solution on boiling 
is one which cupric imidodiphosphate shares with the pyrophosphate, 
and the same is observed with the magnesium salts, but with the dif- 
ference that the pyrophosphates do not redissolve on cooling, while 
the imidodiphosphates redissolve either at once or in a short time. 
It is not possible to effect a complete separation of the two acids in 
this way, however, for if pyrophosphoric acid be present, more or less 
imidodiphosphate remains in the permanent precipitate, apparently 
because of the formation of complex salts containing both acids. The 
only method for separating the two acids when mixed in approximately 
equal proportions is to convert them into the sodium salts, and to pre- 
sipitate repeatedly by alcohol, finally recrystallizing the pyrophosphate 
from water. 
Decomposition of i midodiphosphoric acid. — When a soluble imidodi- 
phosphate is boiled with acetic acid, it is converted for the most part 
nto orthophosphoric acid, and to a much less extent into pyrophos- 
jhoric acid. The signticance of this fact is pointed out in the intro- 
luction to section 2 of this papei. A solution of the sodium salt (which 
malysis has proved to be free froin pyrophosphate) in 30-50 parts water 
s weakly acidified with acetic acid, boiled seven to eight minutes, and 
;ooled. If this solution be made alkaline with caustic soda, and alcohol 
idded, an abundant crystallization of tertiary sodium phosphate is 
ormed, which may be recognized by its crystalline form and by the 
isual reactions. In this the microscope shows a few crystals of sodium 
>yrophosphate. The latter acid may be isolated by making use of the 
