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If a solution of sodium trimetaphosphimate, to which exactly 3 molecular weights caustic soda, or an excess of ammonia, have been added, be dropped into an excess of dilute silver nitrate, a light sulphur-yellow, voluminous, and amorphous precipitate is formed, which may be washed with cold water without change, and does not change color on drying. This likewise contains 6 atoms of silver, and is insoluble in ammonium nitrate. If boiled with a trace of silver nitrate it remains unchanged; but if boiled with pure water it changes slowly, or if sodium trimetaphosphimate or ammonium nitrate be present, in a few moments, into the same orange-red, crystalline powder above mentioned.1 On one occasion a deep red crystalline powder was obtained, but the conditions. for its formation could not be determined. The following figures resulted from the analyses of these salts, in every case dried at 100°:
RED SALT (CRYSTALLINE).
Mean ratio P: Ag=3:5.79.
Ratio P: Ag=3:5.93.
This is seen to crystallize from the liquid, so that temporary solution must take place.
Unsatisfactory as these figures are, they appear to indicate the exist ence of:
(1) A yellow salt, usually amorphous, occasionally crystalline, approximating in composition to P,NÒ,H2Ag. As this is only formed in the cold from a trimetaphosphimate to which alkali has been added, it may be a salt of the supposed open-chain acid.
(2) A red crystalline salt of the formula PзNОAg.
(3) A white amorphous salt of nearly the same composition. The orange-red salt, invariably containing too little silver, and varying in depth of color, may be a mixture or an intermediate form.
As the white salt is formed only in the presence of silver ammonia compounds, and is quite soluble in ammonium nitrate, it is clearly dif ferent in nature from the others. Probably it contains silver united to oxygen only, while in the red salt one-half the metal is associated with nitrogen. We may regard them as salts of the two forms of trimetaphosphimic acid:
The action of silver nitrate in preventing the transformation may be ascribed to its opposing dissociation. No deep decomposition occurs during the transformation, as the ammoniacal solution is not precipitated by magnesia mixture, and sodium trimetaphosphimate is regenerated by treatment with sodium chloride and acetic acid.
Ferric trimetaphosphimates.-Gladstone' states that when pyrophosphodiamic acid is added to ferric sulphate, acidified with sulphuric acid, a clear solution results, which, on boiling, deposits white ferric pyrophosphamate, which is insoluble in acids but soluble in ammonia. The ferric trimetaphosphimates seem to vary considerably in composition and properties and present no characteristic features. The following points were noted: On adding ferric chloride to a moderately strong solution of sodium trimetaphosphimate an amorphous white precipitate forms which redissolves in an excess of the sodium salt, probably with formation of a ferric-sodium salt, but which becomes permanent on adding more ferric chloride. This is scarcely soluble in dilute hydrochloric acid, but readily soluble in ammonia with faint red color. Sodium hydroxide regenerates sodium salt. If, however, the solution of the sodium salt be dilute, no precipitate forms in the cold, even with an excess of ferric chloride, but on boiling, after acidifying,
Quart Jour. Chem. Soc. London, Vol. III, p. 135; Jour. Chem. Soc., London, , Vol. II, p. 229; , Vol. IV, p. 290; , Vol. VI, p. 67.
a white amorphous precipitate is produced, which, like the above, is nearly insoluble in dilute acid and easily soluble in ammonia and consists of trimetaphosphimate mixed with decomposition products.
The solubility of these salts in ammonia, which they show in common with other ferric amidophosphates and pyrophosphates, may be ascribed to the formation of complex ions containing iron; in other words, of ferri-trimetaphosphimic acids, analogous to ferri-oxalic acid and similar bodies.
Free trimetaphosphimic acid.-Silver trimetaphosphimate, decomposed under cold water by hydrogen sulphide, gives a solution from which, when fresh, the characteristic salts may be prepared, and from which much alcohol gradually throws out an amorphous sticky mass. The solution, on evaporation in vacuo, leaves a transparent, gummy residue from which alkalies evolve much ammonia, and which, after redissolving, gives a precipitate with magnesium mixture, indicating partial decomposition. The acid seems therefore to be incapable of crystallization.
In marked distinction from metaphosphoric acid, trimetaphosphimic acid does not coagulate albumen.
DECOMPOSITION PRODUCTS OF TRIMETAPHOSPHIMIC ACID, Orthophosphoric acid and ammonia are the ultimate products of the action of acids on trimetaphosphimic acid, as well as of the decomposition of the free acid alone in aqueous solution when heated or kept for a long time. I have, however, been able to isolate three well-marked intermediate bodies, viz:
Diimidotriphosphoric acid, PzN.O.H;;
If sodium trimetaphosphimate solution (1-15) be boiled with 2-3 molecular weights nitric acid, the transformation into ammonia and orthophosphoric acid is complete in about ten minutes, but on interrupting the boiling after three to four minutes and cooling, the presence of these products can be detected; the same change occurs slowly in the cold, many days being required for complete decomposition. From this solution silver nitrate throws down first the silver salt of unchanged trimetaphosphimic acid, then the others in the order named, and finally silver phosphate, which is most easily soluble in nitric acid.
Although these products were first detected in this way, many experiments have convinced me that it is not a practical method of separation. The separation by fractional precipitation is not sharp; the properties of trimetaphosphimic and diimidotriphosphoric acids are so similar and the stability of the latter so slight that the results are very unsatisfactory. In the absence of trimetaphosphimic acid, however, diimidotriphosphoric acid is readily separated from the later decomposition
See Rusen beim, Zeitschr. anory, Chem. Vol. XI, p. 214, etc.
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, 3 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 magnesiam nitrate mixturel added, which throws down imidodiphosphoric, pyrophosphoric, and orthophosphoric acids, while diimidotriphosphoric acid remains wholly in solution. The liquid is removed as far as possible from the semigelatinous precipitate by suction and pressing out in the filter, without washing. The filtrate is approximately neutralized with nitric acid and precipitated by silver nitrate, a considerable excess of which should be used, as penta-silver diimidotriphosphate 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 remaining 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 diimidotriphosphate. This is purified by dissolving in a little water and reprecipitating by alcohol.
The magnesium precipitate, containing imidodiphosphoric, orthophosphoric, and a little pyrophosphoric acids, is pressed out strongly under a screw press, pulverized, and dissolved in the least possible amount of dilute nitric acid." Ammonia is added drop by drop till a slight permanent precipitate forms, and after filtering, silver nitrate added as long as the precipitate is white. Nearly all the orthophosphoric acid remains in solution, even after adding much silver nitrate; but, if too much be added, or if the amount of orthophosphoric acid be relatively great, the latter portions of the precipitate are yellow. Exact quantitative data can not be given, but in general it is necessary to use an amount of silver 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 grams crystallized magnesium nitrate and 35 grams ammonium nitrate in water, adding enough ammonia to give a faint odor and diluting to 1 liter.
? Schleicher and Schüll's hardened filters, No.575, have proved indispensable in this operation. * This is best done by rubbing up with water in a glass mortar, adding the acid gradually.
remains in solution. To get rid of it entirely, as well as of traces of orthophosphoric acid and of a substance of relatively high percentage of nitrogen, the tri-silver imidodiphosphate must be dissolved in ammonia, reprecipitated as magnesium salt, pressed out, and reconverted into the silver salt in the same manner. The mother liquor of the second silver precipitation, if neutralized with ammonia, gives a mixture of silver imidodiphosphate and pyrophosphate. This, after conversion into the sodium salt by sodium chloride, addition of a little caustic soda, and precipitating by alcohol, gives a sirup which is converted by vigorous rubbing into a pasty mass. This, by repeated solution and reprecipitation by alcohol, gives the characteristic flat prisms of sodium pyrophosphate. After conversion into the silver salt this proved to be free from nitrogen and an analysis gave:
Tedious as the above method is, many experiments have failed to lead to the discovery of a better one, and have convinced me that it can not be abbreviated in the least without injuriously affecting the quality of the products.
DIIMIDOTRIPHOSPHORIC ACID, P3NO,H,.
This acid forms two series of salts in which 3 and 5 atoms of hydrogen are replaced. The alkali salts with 3 atoms of metal are of neutral; those with 5 atoms of alkaline reaction. It seems impossible to replace the 2 remaining atoms, as is pointed out under the silver salts.
Tri-sodium diimidotriphosphate, P3N2O,H,Na3.-The preparation of this salt is described above. It is easily soluble in water, this solution leaving on evaporation a mass of indistinct prisms. Precipitated from its aqueous solution by alcohol, it usually forms granules without evidence of crystalline form; but when very slowly precipitated it may be obtained as flat, very obtusely pointed prisms or rhombic plates, visible only when magnified 400 diameters. It contains no crystal water. The substance dried at 100° gave:
PN: Na 3: 2.02: 2.92.
A penta-sodium salt probably exists, being obtained by decomposing the penta-silver salt with sodium chloride. It has alkaline reaction and is precipitated by alcohol as an uncrystallizable sirup.