The tri-sodium salt is not precipitated even in strong solution by magnesium chloride or magnesia mixture unless the latter contains much ammonia; in this case the resulting precipitate is easily soluble in ammonium salts, and hence is probably merely magnesium hydroxide. Barium chloride precipitates from a not too dilute solution microscopic spherules, which dissolve readily in a cold, strong solution of sodium chloride. On boiling this solution a crystalline precipitate is at once. formed, consisting of microscopic rhombohedra (?) resembling sodium barium trimetaphosphimate and probably a double salt.

2

Silver salts. Two of these were obtained, one having the formula P3N2OH,Agз, and the other P2O2OH2Ag. The tendency to formation of the latter is so strong that a solution of the tertiary sodium salt gives, with silver nitrate, a white amorphous precipitate, which, however, is mixed with some crystals of the former.

Tri-silver diimidotriphosphate, P3N2OH,Agз.-This salt is very characteristic. It can be obtained pure only by precipitating an acidified solution of the tertiary sodium salt by an excess of silver nitrate. To a solution of the sodium salt in 30 parts water, 1 to 2 molecular weights nitric acid are added, and then silver nitrate gradually; as soon as a milkiness, due to the formation of the penta salt, is observed, more acid is added, and then more silver nitrate, acid being added as often as necessary; the crystallization soon starts, but several times the theoretical amount of nitrate is required, and under no circumstances is the precipitation complete. Generally spherules with many rhombic facets are obtained, but with slow crystallization (from more dilute solutions) the forms shown in figs. 5 and 6 result.

The colorless crystals are monoclinic, and suggest those of tertiary silver trimetaphosphimate, but are shorter and thicker, and approach more nearly to the orthorhombic form. No other faces are ever seen. The angle a (between the edges), measured under the microscope, is very nearly 98°; the acute angle between the edges of B is, roughly speaking, 80°, so that when the form fig. 6 occurs, and this face is uppermost, it is apt to be mistaken under superficial observation for trimetaphosphimate (angle a, 78°).

The salt is practicably insoluble in rather difficultly in dilute nitric acid.

water, easily in ammonia, and

When pure it is not discolored

by light or by caustic soda. It contains no crystal water. Analysis

1. P: N: Ag=3: 2.01: 2.94. Direct from decomposed solution of trimetaphosphimic acid.

2. P: Ag=3:2.99. From sodium salt.

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Penta-silver diimidotriphosphate, P2N2O, H2 Ags. One molecular weight tertiary sodium salt is dissolved with addition of not less than 11⁄2 nor more than 4 molecular weights ammonia, and added to an excess of a solution of silver nitrate; the pure white amorphous voluminous precipitate is washed, sucked out, and dried in vacuo. When fresh it is colorless, but on drying it assumes a yellowish tint, and after standing for some time it is partially converted into crystals, the nature of which was not determined. On boiling with water it also becomes yellowish, and in each case the change of color is accompanied by decomposition, as its ammoniacal solution is partially precipitated in amorphous form by magnesia mixture, an effect which is not observed in the perfectly fresh white salt. Warming with ammonium nitrate destroys the yellow color, which is consequently not due to silver phosphate. The fact that it remains white on boiling with ammonium nitrate serves to distinguish it from the trimetaphosphimates, the more basic silver salts of which, as pointed out, become orange-red under these conditions. The salt is very difficultly, yet perceptibly, soluble in water, and is reprecipitated by adding considerable silver nitrate; it is easily soluble in ammonia, and nitric acid converts it partly into the tertiary salt before dissolving it.

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 of the decomposition was not investigated.

IMIDODIPHOSPHORIC ACID, P2NOH

This is probably the substance described by Gladstone under the name pyrophosphamic acid. It is moderately stable in cold, not too acid, solution, and hence occurs in large amount among the decomposition products of trimetaphosphimic acid.

Tri-sodium imidodiphosphate, obtained by decomposing the tertiary silver salt by sodium chloride, has alkaline reaction. Its solution, as well as that of the tetra sodium salt, is precipitated by alcohol as a sirup which can not be made to crystallize, and the aqueous solution of which dries to a transparent gummy mass. It is more soluble in dilute alcohol than sodium pyrophosphate.

The magnesium salt above described is amorphous, voluminous, and nearly, but not quite, insoluble in water, and somewhat more soluble in solutions of ammonium salts.

Tri-silver imidodiphosphate, P2NOH2Agз.-This is obtained as an amorphous, white precipitate, when the free acid, or a solution of a tertiary salt is precipitated by silver nitrate. Thus formed, it tends to collect in small lumps, a property which distinguishes it from any of the other silver salts referred to in this paper. When precipitated by acid from its ammoniacal solution it forms a powder that settles slowly. When slowly separating, or when precipitated from hot solutions, it forms semicrystalline granules, often united to crusts or dendritic forms. When formed in a moderately acid solution it usually shows a slight deficiency of silver (analyses 1-2), probably due to a trace of a more acid salt; but such a salt can not be obtained in pure form. Pure tertiary salt can be obtained by decomposing the crude salt with somewhat less than the required amount of sodium chloride, and reprecipitating. It is insoluble in water and is not affected by light.

Bull. 167——8

Tetra-silver imidodiphosphate, P2NO,HAg.-This salt exists in a white and in a yellow form. The former is obtained by precipitating a solution of silver oxide in ammonium nitrate by the sodium salt. It forms a voluminous, flocculent precipitate, which remains white only in presence of its mother liquor; on washing out it undergoes partial transformation into the yellow form, the same change occurring temporarily on boiling under the mother liquor. It shows a slight deficiency of silver (analysis 4).

The yellow form is obtained as an amorphous, flocculent precipitate by adding an ammoniacal solution of an imidodiphosphate to an excess of silver nitrate (analyses 1-2), or as an imperfectly crystalline powder, by evaporating such an ammoniacal solution, containing an excess of silver, over sulphuric acid (analysis 3). From this it appears that the remaining hydrogen atom is not replaceable by metal. The difference between the white and yellow forms is perhaps due to tautomerism. Dried at 100°, it gave:

Imidodiphosphoric acid is easily distinguished from diimidotriphosphoric acid by the insolubility of its magnesium salt, by the peculiar granular or lumpy appearance of its tertiary silver salt, by its sirupy

tertiary sodium salt, and by its ammoniacal 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 solution of the sodium salt redissolves in an excess of the latter, is again reprecipitated by an excess of cupric salt, but under no circumstances 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 midodiphosphate 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 precipitate repeatedly by alcohol, finally recrystallizing the pyrophosphate from water.

Decomposition of imidodiphosphoric acid.-When a soluble imidodiphosphate is boiled with acetic acid, it is converted for the most part into orthophosphoric acid, and to a much less extent into pyrophos phoric acid. The signficance of this fact is pointed out in the introduction to section 2 of this paper. A solution of the sodium salt (which analysis has proved to be free from pyrophosphate) in 30-50 parts water is weakly acidified with acetic acid, boiled seven to eight minutes, and cooled. If this solution be made alkaline with caustic soda, and alcohol added, an abundant crystallization of tertiary sodium phosphate is formed, which may be recognized by its crystalline form and by the usual reactions. In this the microscope shows a few crystals of sodium pyrophosphate. The latter acid may be isolated by making use of the