stokes.] PRODUCTS OF TRIMETAPHOSPHIMIC ACID. 109 
products. If, therefore, we have ascertained -the time required for all 
the trimetaphosphimic acid to be decomposed under given conditions, 
while diimidotriphosphoric acid still remains, we can isolate the latter 
in a condition of purity. Since the latter is merely a transition product, 
the amount present at any time is small, and the actual yield is only 
about 10 per cent of the theoretical. 
The following method leads to the desired result: 1 molecular weight 
a-sodium trimetaphosphimate is dissolved in 15 parts cold water, .'> 
molecular weights nitric acid (strength about 15 per cent) added, and the 
solution allowed to stand for seven days at the ordinary temperature. 
Enough ammonia is then added to give a faint odor and an excess of 
magnesium nitrate mixture l added, which throws down imidodiphos- 
phoric, pyrophosphoric, and orthophosphoric acids, while diimidotri- 
phosphoric acid remains wholly in solution. The liquid is removed as 
far as possible from the semi gelatinous precipitate by suction and press- 
ing out in the filter, 2 without washing. The filtrate is approximately 
neutralized with nitric acid and precipitated by silver nitrate, a consid- 
erable excess of which should be used, as penta-silver diimidotriphos- 
phate is much less soluble in dilute silver nitrate than in water. The 
slightly washed silver salt still contains some imidodiphosphate, as the 
magnesium salt of this acid is not wholly insoluble. To remove this it 
is dissolved in the smallest possible volume and amount of ammonia and 
a little magnesium nitrate mixture added, which throws out the remain- 
ing imidodiphosphoric acid. The liquid is again filtered by suction, with 
but little washing, and the silver salt again precipitated by neutralizing 
with nitric acid, with addition of a little silver nitrate. The moist silver 
salt is treated with an excess of moderately strong solution of sodium 
chloride, acidified with acetic acid, the silver chloride filtered off with 
but little washing and an equal volume of alcohol added to the filtrate, 
which precipitates tri-sodium diiruidotriphosphate. This is purified by 
dissolving in a little water and reprecipitating by alcohol. 
The magnesium precipitate, containing imidodiphosphoric, orthophos- 
ihoric, and a little pyrophosphoric acids, is pressed out strongly under 
i screw press, pulverized, and dissolved in the least possible amount'of 
lilute nitric acid.' Ammonia is added drop by drop till a slight perma- 
lent precipitate forms, and after filtering, silver nitrate added as long 
is the precipitate is white. Nearly all the orthophosphoric acid remains 
n solution, even after adding much silver nitrate; but, if too much be 
idded, or if the amount of orthophosphoric acid be relatively great, the 
atter portions of the precipitate are yellow. Exact quantitative data 
san not be given, but in general it is necessary to use an amount of sil- 
ver nitrate at least ten times the weight of the mixed magnesium salts. 
Silver pyrophosphate is partly precipitated toward the end, and partly 
] Made by dissolving 100 grains crystallized magnesium nitrate and 35 grams ammonium nitrate in 
r ater, adding enough ammonia to give a faint odor and diluting to I liter. 
2 Schleicher and SchuH's hardened niters, No. 575, have pn>\ ed indispensable in this operation. 
3 This is best done by rubbing up with water in a glass mortar, adding the acid gradually. 
