View Article Online / Journal Homepage / Table of Contents for this issue

GLADSTONE ON PYROPHOSPHORIC . 435 Published on 01 January 1867. Downloaded by University of Windsor 22/10/2014 03:36:55.

XL.- On Pyropl~osphoricAcid.

By Dr. J. K. GLADSTONE,Ph.D., F.R.S.

IN the classic research of Graham, pyrophosphoric acid was viewed as a bibasic acid, having the conetitution 2H0.P05; but under the present view, that the atomic weight of oxygen is 16, we are compelled to recognize in it the exktence of two atoms of , and to assign to it the formula 2H,O.P2O,, or’rather P,H,O,. This makes it a quadribasic acid, and that

* Chem. SOC.Jour., six, 238. 2n2 View Article Online

436 GLADSTONE ON PYROPHOSPHORIO ACID. it actually is such, is indicated by its giving some double salts, in which the two metaJs exist in the proportion of 3 atoms of the one to 1 atom of the other. A still better proof of the above formula may be found in the fkt that amides of pyro- exist., in which 1, 2, and 3 molecules of HO are replaced by NH,. It is my intention to treat in this paper of-1st. Some normal ; 2ndly. An allotropic modification of these salts ; and, 3rdly, the formation of pyrophosphoric acid, and the theory of its constitution.

Normal Salts. If solutions of a silver-salt and of of eodium be mixed in any proportion, a permanent white precipitate is the result ; but if a solution of ferric chloride be added in small quantity to a solution of pyrophosphate of sodium, the white precipitate, produced at the moment of contact, dissolves in the excess of sodium-salt. If more ferric chloride be added, a per- manent, flocculent precipitate is obtained, If the addition be continued, the solid gyrophosphate is again dissolved, but now by the excess of ferric salt. On performing this experiment with solutions of known strength, it was found that the pyrophosphate of sodium con- tinued to dissolve the iron-salt, till the proportion of two atoms of ferric chloride, feC1," to one of pyrophosphate of' sodium, P2Na4O7,was arrived at ; then, the permanent precipitate in-

Published on 01 January 1867. Downloaded by University of Windsor 22/10/2014 03:36:55. creased, with a milky appearance, until the proportion became four to one, at which time no more iron was found in solution, and the ferric salt' separated readily in flocculent masges. To dissolve this up again, many atoms of ferric chloride were found to be necessary. This suggests the existence of a soluble double salt, P2Na,fe20, ; and an insoluble normal pyrophosphate, P,fe,O,. Schwarzenbergt, long ago, gave the above, plus 3 atoms of water, as the composition of ferric pyrophosphate, and the following analysea confirm the conclusion :- I. 0.330 grrn., dried at loo* C., and heated to low redness,

it To avoid great complexity of formulEe, I have adopted Williamson's fenicum with the atomic weight 18'66, and the symbol fe. t Ann. Ch. Pharm., lxv, 163. View Article Online

GLADSTONE ON PYROPHOSPHORIC ACID. 437

gave off water, losing 0.058 grm. The residue boiled long with hydrochloric acid, and precipitated by magnesium-salt in the presence of citric acid, gave 0-234 grm. of pyrophosphate of magnesium. 11. 0.349 grm. gave 0.245 of pyrophosphate of magnesium.

Calculated. Found. I. IT. 20.48 19.78 19.60 Phomhorus4 ...... Iron...... 24.65 - - Oxygen ...... 37.03 - - Water ...... --17.84 17-57 100~00 The supposed double salt, P2Na,fe,07, being in solution, and &xed with chloride of aodium, a.nd, possibly, other combina- tions, could not be satisfactorily examined. An attempt wa8 made to separate it by dialysk. It was soon evident that the double salt remained in the dialyser, while the chloride of sodium passed through ; but when the chlorine had practically disap- peared, the salt partially gelatinized. The whole contents of the dialyser were, nevertheless, evaporated down, and the phos- phorus, iron, and water were estimated. There wa8 also a considerable amount of sodium, which could be determined by difference; but the numbers obtained did not agree well with the above formula. In the reaction8 described above, have spoken of ferric

Published on 01 January 1867. Downloaded by University of Windsor 22/10/2014 03:36:55. I chloride and pyrophosphate of sodium, but the iron may be combined with other , or potassium may be substituted for the other alkali, without affecting the results. If a cupric salt be mixed with a soluble pyrophosphate, the precipitate first produced dissolves in excess of the phosphorus- compound, giving a deep-blue solution. On the further addi- tion of copper, a permanent green salt separates, which was found, on analysis, to be the normal pyrophosphate, P2Cu,07, combined with two atoms of water. I. 0.359 grm., dried at looo C., and heated to low redneas, gave off water, and lost 0.041 grm. The residue was boiled long with hydrochloric acid, and gave 0.238 pyrophosphate of magnesium. The copper had previously been separated by hydroaulphuric acid, and yielded 0.170 grm. of oxide, View Article Online

438 GLADSTONE ON PYROPHOSPHORIC ACID. 11. 0.344 grm. lost 0-038 grm. when hea€ed. 111. 0.378 gm. of a different preparation, lost 0941 p. when heated, and gave 0.230 grm. of pyrophosphate of mag- nesium, and 0.177 grm. of oxide of copper. These numbers indicate the composition P,Cu40,.2E,0.

Calculated. Found, I. JI. 111.

Phosphorus 18.43 18.51 7 18-47 Copper,. . . 37.57 37.78 - 37.36 Oxygen .. 33-29 - - -. Water . , . . 10.71 11.42 11.04 lOd34 100.03 An attempt was made to isolate the doubIe pyrophosphate of copper and sodium by dialysis, but it Famed through the parch- ment paper about as readily as the sulphate of sodium did. Excess of pyrophosphate of sodium is also capable of dis- solving the precipitates produced by it with mercurous, ferrow, zinc, aluminium, and lead salt,s, but apparently not those with mercuric or chromic salts* These compounds are capable of being decomposed by free acids; 60 that, on the addition of a little sulphuric acid, for instance, to the sodio-ferric salt, the complete ferric pyrophosphate Reparates.

Allotropic Xatts. Published on 01 January 1867. Downloaded by University of Windsor 22/10/2014 03:36:55.

Many of these metallic pyrophoaphates are capable of existing in a different condition. Thus, the ferric salt already described is very soluble in acida. A few drops of sulphuric acid will dissolve a considerable quantity ; but if this solution be heated, it becomes turbid, and a white flocculent salt makelit its appearance, which is just like the original ferric compound, but no longer soluble either in pyrophosphate of sodium, ferric chloride, or dilute acids. Like the normal salt, however, it is easily dis- solved by ammonia. Analysis showed that this salt has the =me ultimate compo- sition as that from which it is prepared, namely, P2fe,0,.3H,0. I. 0.377 grm., dried at looo C., lost 0.065 grm. when strongly heated, and gave 0.277 grin. of pyrophosphate of magnesium. View Article Online

GLADSTONE ON PYROPHOSPHORIC ACID. 439

11. 0.320 grm. gave 0.232 grm. of pyrophosphate of mag- nesium. 111. 0.255 grm. lost 0-044 grm. when strongly heated. IV. 0.200 grm. gave 0.071 grm. of ferric oxide.

Calculated, Found. I. 11. 111. IV. PhorJpkrcrus 20.48 20.51 20.24 - - Iron ...... 24.65 - - - 24.85 Oxygen.. . . 37-03 - - - - Water . . . . 17-84 17.24 I 17.25 c -- 100~00

Other aci& may be used in the place of sulphuric acid to effect this change, but free acid is necessary. The normal salt may be boiled with water for a couple of hours without altera- tion, nor does the presence of neutral salts appear to have any effect. The temperature at which this allotropic salt forms depends upon the comparative amount of the different sub- stances present. If the acid and water be nearly saturated with the ferric compound, a temperature of 30" or 40" C. suf- fices. A means of testing for pyrophosphoric acid may be founded on this property. If the presence of that acid be suspected in a neutral solution, it is only necessary to add a drop or two of an iron-salt, sufficient sulphuric acid just to dissolve the pre- Published on 01 January 1867. Downloaded by University of Windsor 22/10/2014 03:36:55. cipitate, if any, and boil. The allotropic salt will reveal itself by a cloudiness appearing first in the warmer currents, and then extending itself a13 a gelatinous or flocculent white pre- cipitate throughout'the whole liquid. If it is necessary to employ acid in order to dissolve the substance to be tested, cold aulphuiic acid should be employed in as small a quantity as possible. An allotropic pyrophosphate of copper may be prepared in a way precisely analogous to that given above for the ferric salt. The following analyses confirm its formula as P,Cu,O,. 2H,O :- I. 0.407 grm., dried at 100°C., lost 0.044 grm. when strongly heated, and gave 0.193 of oxide of copper, and 0.269 of pyro- phosphate of magnesium. View Article Online

440 GLADSTONE ON PYROPHOSPHORIC ACID.

Calculated. Found. Phosphorus ...... 18-43 18.45 Copper...... 37-53 37-84 Oxygen ...... 33.29 - Water ...... 10.71 10.81 1oo*oo SimiIar precipitates are obtained when solutions of ferrous or zinc pyrophogphate in dilute sulphnric acid are heated, but the lead, chromium, and mercurous salts did not give precipitates under such circumstances. This allotropism does not Beem to extend to pyropholsphoric acid itself, or its compounds, with the alkalis. At least, when the allotropic ferric mlt was decomposred by potaBh, and the allotropic copper salt waB decomposed by hydrosnlphuric acid, the pyrophosphoric element in solution wag found in the normal condition.

Formation and Constitution. The methods hitherto described for the preparation of ppn>- phosphoric acid depend on the elimination of water fiom ortho- phosphoric acid, or orthophosphates, by meam of heat. It may, however, be formed synthetically. If phoraphoric anhydride be treated with a solution of hydrate of potassium in absolute alcohol, it forms the pyrophosphate. If oxychloride of phoa- phorus be dropped into a strong solution of potasb, or into a Published on 01 January 1867. Downloaded by University of Windsor 22/10/2014 03:36:55. moderately strong solution of ammonia, a violent reaction ensues, and it is a pyrophosphate that is found in solution. If either alkali be too dilute, the orthophosphate is produced; if the ammonia be the strongest, pyrophospho-diamic acid results, as shown in a previous communication. The formation of a pyrophosphate from phosphoric anhydride and an alkaline hydrate appears very simple : P,O, -+- 4KH0 = P,K40, + 2H20. It is more difficult to understand how an acid containing P, ... can be produced from the oxychloride, which is PCl,O, and why the pyrophosphoric amides, which also contain P, .... should result from the decomposition of the amidated chlorides by water. And this apparent difficulty will be increased when we View Article Online

GLADSTONE ON PYROPHOSPHORIC ACID. 441 come to consider the formation of compounds containing P, . . . from the same chlorides. The following considerations may furnish a probable explanation :- When a chloride and water act on one another, three Werent courses are open, each giving hydrochloric acid as one of the resultant bodies. In the first case, the chlorine combines with one of the atoms of , while the remaining HO (hydroxyl) takes its place in the original compound. Thus :- PCl, + 3H,O = 3HC1 + PH,O, (phosphorous acid). In the second case, two atoms of chlorine simultaneously attack the two atoms of hydrogen, and the liberated single atom of oxygen takes their place. Thus :- PCl, + H,O = 2HC1 + PCl,O (oxychloride of phosphorus). In each of these cases we may consider the new compound as formed on the same type as the original chloride, only in the one case Cl is replaced by HO, and in the other 2Cl are replaced by 0:- c1 HO Thus P C1 becomes P HO cl HO cl and P C1 c1 becomes P C1 0. cl c1

Published on 01 January 1867. Downloaded by University of Windsor 22/10/2014 03:36:55. But there is a third case, where the two atoms of hydrogen in water are attacked simultaneously by two molecules of the chloride, and the result is that the oxygen is left in combination with two molecules of the aubstance originally combined with the chlorine. Here it is the simplest to consider that it is the water type which is preserved. It is this third mode of action which explains the production of pyrophosphates fi-om the oxychloride. If we act on oxychloride of phosphorus with water, we find that a slow replacement of the chlorine takes place, each atom decomposing an atom of water, and the result is c1 HO P c1 0 + 3H,O = 3HCl + P HO 0. c1 HO View Article Online

442 GLADSTONE ON PYROPHOSPHORIC ACID. which is PH,O,, tm’basie or ortho-phosphoric acid, a, body which may be viewed a8 framed on the same type as the penta- chloride. If, instead of’ employing pure water in this reaction, we employ a dilute aqueous solution of ammonia or potash, the result is simply a chloride and an orthophosphate. If, however, we employ these alkaline hydrates in a stronger ~olution,the result is totally different. We now obtain a salt formed, not on the t’ype of the oxychloride, but on the type of the hydrate, one which we must view as a substitution-product of water rather than of pentachloride of phosphorus. To ex- plain this, the reaction must be broken up into two stages, though it is not improbable that these may occur simultaneously in nature. The first stage is- 2PC1,O + E}O = KCI + HC1 + Papcl;o}O. 0

and the second is-

or P,H,O,-pyrophosphoric acid. It thus a.ppears that, according to the strength of the hydrate, either the whole chlorine may be replaced by hydroxyl, or two molecules of the oxychloride of phosphorus may simultaneously attack one molecule of the alkali. There atill remains the other Published on 01 January 1867. Downloaded by University of Windsor 22/10/2014 03:36:55. possible mode of action, namely, the replacement of two atoms of chlorine by one of oxygen. Thus :- c1 P C10 + K,O = 2EC1 + P 0, c1 which, acted on by more potash, would give

P 0 + K,O = KCl + P K$ 0,

or PK03-rnetap7tosphte of potassium; and this I found to be actually formed when the oxychloride was dropped on oxide of potassium, or on sesquicarbonate of ammonium. At least, there is something produced along with the pyrophosphate which, View Article Online

BASSETT ON JULIN’S CHLORIDE OF CARBON. 44;: when set free by acetic acid, coagulates albumin. Oxychloride of phosphorus does not decompose the dry neutral carbonates of the alkalis, and so metaphosphoric acid cannot be prepared in this way from them. The production of a pyrophosphate fiom an orthophosphate by heat would appear to be analogous to the formation of an oxide from a hydrate by the same agency- 2CuHO = H,O + CU,O, and aimilarly, 2PNa2H04 = H20 + P,Na40,.

the rational formula of pyrophosphoric acid, may be drawn from the readinesa with which it forms amides, since HO and NH, are easily replaceable. The fuller consideration of these bodies must, however, be left for another communication. Published on 01 January 1867. Downloaded by University of Windsor 22/10/2014 03:36:55.