94 CONTRIBUTIONS TO CHEMISTRY AND MINERALOGY. [bull. 167. 
Assuming formula (b) for trimetaphosphimic acid, its decomposition 
would be thus represented: 
PO.OH PO(OH) 2 \ 
HN NH NH +NTI3 
I I I > 
HO.PO PO.OH PO.OH PO(OH), 
\ / \ /. 
N NH 
H 
Trimetaphosphimic acid. Diimidotriphosphoric acid. 
1. 2. 
(NH 4 ) 2 HP0 4 /PO(OH) 2 
_* u ^PO(OH) 2 
PO(OH) 2 PO(OH) 2 -^^ -f NH, 
\ / ^ 2H,P0 4 
NH 
Imidodiphosphorlc acid. 
3. 4 and 5. 
The second acid is important as a connecting link, proving that 
trimetaphosphimic acid actually has the triple formula (PN0 2 H 2 )> 
Most remarkable is the formation of pyrophosphoric acid, which occurs 
directly from imidodiphosphoric acid. The facts are these: If sodium 
imidodiphosphate be weakly acidified with acetic acid and boiled from 
five to ten minutes, not more than 15 per cent is converted into 
pyrophosphoric acid and the remainder into orthophosphoric acid. 
Control experiments with sodium pyrophosphate showed that under 
the same conditions but a small portion is converted into orthophos- 
phoric acid, 90 per cent of the pyrophosphate being recovered. The 
pyrophosphoric acid is therefore not an intermediate product, but the 
imidodiphosphate is converted for the greater part directly into ortho- 
phosphoric acid, and in a much less degree into pyrophosphoric acid. 
The above formula of imidodiphosphoric acid containing the group 
P — NH — P, follows directly from that of trimetaphosphimic acid. In 
view of the ease with which the condensed phosphoric acids split up in 
aqueous solution into orthophosphoric acid, while the reverse never 
occurs, it is difficult to see how the action of water or acids on a body 
of the assumed formula could give pyrophosphoric acid. If we adopt 
PQ/NH 2 
/ ru ^OH 
Gladstone's formula 1 O , we can explain the formation of 
\pO(OH) 2 
pyrophosphoric acid, but must at the same time assume that some- 
where in the series the group P — N — P has been converted into 
P — O — P, for in the chloronitride, P : $N 3 C1 6 , phosphorus can be united 
only by nitrogen. If the acid were really the amide of pyrophosphoric 
acid, boiling with acetic acid would first convert it into pyrophos- 
phoric acid which, as above shown, is fairly stable under the observed 
'Jour. Chem. Soc. Loudon, [2], Vol. VI, p. 70. 
