THE ANALYST. 23 ON THE DETERMINATION OF PHOSPHOltIC ACID AS PHOSPHO- MOLYBDATE. By OTTO HEHNEB, F.C.S. Read before the Society Of Public Analysts, on 15th January, 1079. IN bringing this communication before the Society of Public Analpts I am well aware that the subject has already been treated of by several chemists, and I also bear in mind that the method of determiniDg phosphoric acid as phosphate of magnesia is an excellent one when performed under the conditions and with the precautions which have been worked out of late years.Yet the precipitate of phospho-molybdate of ammonia is a particularly tempting one to found upon it a method for the quantitative determination of phosphoric acid, since it contains, in combination with a rery small quantity of phosphoric acid, an exceedingly large proportion of molybdic acid and other constituents; and because it is but very little soluble, and can be precipitated from acid media. Molybdic acid has long been employed both for the detection and for the separation of phosphoric acid for quantitative purposes, especially from such bases as are precipitated by ammonia or by phosphoric acid.But oming to the alleged variable com- position of the precipitate it has not hitherto been utilised with any measure of auccess for the direct determination of P,O,, although several attempts have been made.In going into this matter I have carefully perused everything accessible to me which has been written on the subject, and I will as briefly as possible give a list of the more important papers treating on it.Svanberg and Struve (Jahresb, 1847, p. 412) first made the observation, that molybdate of ammonia produces a yellow colouration or precipitate in the presence of phosphoric acid. They did Dot, however, recognise that the P20, was an essential constituent of the precipitate, to which they assigned the formula (NHJ2 0, 5 MOO, + H20. It was Sonnenschein (Jahresb, 1851, p.349) who first proved that the P,O, was a normal constituent, and he utilised the precipitste for the quantitative determination of the P206, and essentially 'his method is still employed at the present time. It consists in the precipitation of the P205 in concentrated solution, by an acid solution of MOO, mashing the precipitate with dilute molbydic solution, dissolving it in ammonia, and, * Owing to the pressure on our space this month we are compelled to hold over therte papers until our next number.24 THE ANALYST.~ precipitating the P2iI5 by mc:ins of insgnesia mixture. According to Sonneaschein the precipitate contains 3 per cent of P205. According to Nutzinger (Jahresb, 1855, p. 374) the composition of the dried precipitate (at 100°C) is 3.82 per cent.PzO,, and 92.70 per cent. MOO,. Scligsohn (Journ. f. Priict. Chem., vol. 67, p. 470) shows, in a rery careful examination, that the precipitate dried at 100°C contains 3.142 per cent. P205, 90.744 MOO,, 3.570 (NH4j20 and 2.544 H23. 4.91 H20. He first attempted the direct determination of the P,O, by measuring tho precipitate in graduated tubes. Lipowitz (Ibid) explains the variab!e composition of the precipitate, as apparent from the varying statements quoted above, by saying that MOO, is invariably mixed with the phospho compound, and he therefore prepares a molybdic solution, by the addition of tartaric acid t o Sonnenschein’s solution, and boiling, which would not deposit free MOO,.He collected the precipitate on a weighed filter, washed with dilute HKO, and dried over oil of vifroil. He statcs the average amount of P2O5 in tho precipitate to be 3.607 per cent.Even if this may of procedure were capable of furnishing accurate results it would be quite impracticable, on account of the drjing over H2S0, i t entails. Tartrates, morcovcr, act most inimically to the complete precipitation of the P205, more so than any othcr salts.Boussingault, however, asserts, without proof, as far as I can ascertain, that P20, can be more correctly determined by the direct weighing of the phospho-molybdate precipitate than by conversion into the magnesia compound. Vhen this statement was made the magnesia method was certainly far from perfect. According to Boussingault the precipitate contained 3-73 per cent.of PZO,. Eggerts (Journ. Pract. Chem., 1860, p. 498) finds in the precipitate dried a t 95QC 3-74 per cent. P20,. H e states that when dried above that temperature decom- position takes place, with the probable iormation of pjxophosphate. He washes the precipitate with water to which 1 per cent. of NHO, has been added and dries a t 95OC, or measures the prccipitate in a narrow tube.He dsserts, that the precipitate is never crystalliuo, even when viewed with high powers of the microscope, and he also states that with 0.0017 P205 in 15 C.C. of wator 4 C.C. of molybdic solution produce no precipitate. Both these stntemcnts, as well a8 his solubility determinations given below, are erroneom. Rammelsberg, (Berl. Ber., 1977, p. 1776) finds in the dried precipitate (at 100°C) 3.90 per cent.P2O5, 86.45 per cent. MOO,, 3.25 (NH,),O, and 5.77 H20. Apart from other and less reliable btatements we have therefore in the yellow precipitnte, according to He assigns to it the forniula 2 (3(NHJ20, P205) 4- 15 (H20, 4 Jiloc),). Sopp (Pogg. Annal. 109, p. 136) finds 3.20 P205, 86 0 MOO,, 5.91 (NH4),0 and n’utzinger . . . . . . . . .. . . . . . . . . 3.82 per cent. PZ05 Eeligsohn . . . . . . . . . . . . . . . . . . 3.14 ,, Lipowitz . . . . . . . . . . . . . . . . . . . . . 3.61 ,, Boussingault . . . . . . . . . . . . . . . . . . 3.73 ,, Rammelsberg . . . . . . . . . . . . . . . . . . 3.90 ,, Eggerts . . . . . . . . . . . . . . . . . . . . . 3.74 ,, sopp . . . . . . . . . . . . . . . . . . . . . 3.20 ), Or 1 part of Pz 0 5 furnishes from 26.64 to 31.84 pfWtS of JellOW precipitate.I had practically completed my investigation when I noticed in one of the mostTHE ANALYST. 25 recent numbers of the Berl. Berichte that Pinkener had worked out a method such as I had in view. I shall have occasion to refer to it further on. My object then was to find out the reason for that variation in the composition of the precipitate, and to study, if possible, the conditions under which it could be obtained of constant composition.On examining a very large number of phospho-molybdate precipitates obtained in the ordinary course of analysis, by means of the microscope, I very often discovered side by side with the yellow and distinct crystals of the phospho compound colourless acicular q-stals of molybdic acid, and I have therefore no doubt that Lipowitz’s explamtion is correct.The quantity of the MOO, crystals is the larger the higher the temperature a t which the precipitate is obtained. I may mention that phospho-molybdate crystallizee, usually, in wry distinct but minute and almost granular crystals, apparently belonging to the hexagonal system ; but sometimes beautiful six-rayed stars of great regularity are obtained.In all of my experiments I precipitated at the lowest possible temperature, at mod 30 or 35OC. I avoided all excess of acid, especially of free H C1, neutralising it, as far possible, with NH3. The molybdic solution employed was prepared according to Fresenius, namely, by dissolving 1 part of molybdic acid in 4 parts of ammonia of 0 96 sp.gr., and pouring the solution thus obtained slowly into 15 parts of HN@, of 1.2 sp. gr., avoiding all rise of temperature. It may be warmed, indeed, almost boiled, both in its concentrated state or when diluted, for many hours, without depositing any MOO,. I determined not to weigh the precipitate directly, as had hitherto been done, but to dissolve it in ammonia, to evaporate this solution and to ascertain the weight of the residue, I thus avoided the clumsy and troublesome scraping out of the precipitate from the beaker, to which, as a rule, it clings most obstinately, and substituted for that operation one involving practically no risk of loss.I also obviated the collection on a weighed filter. Thus I really weighed (at least in my earlier experiments) a mixture of molybdate of ammonia and of phosphate of ammonia, thus decreasing still further the proportion of the P,O, in the substance.It is plain that I could not employ for the washing of the precipitate the usual acid liquid containing molgbdate of ammonia, but was obliged to remom all acids, and, indeed, all soluble matter. But it is statEd that the yellow precipitate is soluble to some extent in any of the menstrua which were thus left to me, notably in water and in alcohol.Besides, I found, that on washing with water, the precipitate almost invariably ran through the filter as soon as all acid had been removed. According to Eggerts 1 part of the precipitate is soluble in 10,000 parts of water, in 6,600 parts of 1 per cent.nitric acid and in 620 of alcohol of 0.80 sp. gr, I digested some of the yellow precipitate, which had been shown by the microscope to be free from crystals of molybdic acid, with water, alcohol of 90 per cent. and alcohol of 45 per cent., for some weeks, shaking frequently. The temperature during that time varied, but was, when I ultimately filtered, about 14OC. I found on0 part of the precipitate to require for solution of This solution contains 5 per cent, of molyhdic acid.Water . . . . . . . . . . . . . . . . . . . . . . . . 21,186 park, Strong ,\!coh*d . . . . . . . . . . . . . . . . . . . . . 8,117 ,) Dilute Alcohol . . . . . . . . . . . . . . . . . . . . . 13,61326 THE ANALYST. Since about 28 parts of the precipitate contain 1 part of P205, no less than 593,200 parts of water, or 378,000 parts of dilute alcohol, are necessary to dissolve 1 part of P206 in that form.Or assuming that 100 C.C. of water or dilute alcohol were employed for washing a precipitate, and usually a smaller quantity would suffice, no more than 0.00017 or 0.00026 grm. P206 respectively could be dissolved, assuming that by filtration alone a saturated solution could be obtained, which is undoubtedly not the case.The solubility of the yellow precipitate in water or dilute alcohol may therefore be safely disregarded, and my experiments will show that no influence whatever can be traced to the employment of these liquids. I prefer to use dilute alcohol, because with it the precipitate is not apt to go through the filter, a t least not when the latter is of a sufficiently close and tight texture.I prepared a dilute solution of phosphate of soda, and determined in it, as carefully as possible, the amount of phosphoric acid, both by evaporation and by precipitation with chloride of magnesium mixture. I have entirely discarded sulphate of magnesia mixture, because absolutely exact results cannot be obtained when it is used, as has been shown over and over again-(see Abesser, Jani and Marcker, Zeitschr. f.Anal. Chem., 1873, p. 243 ; Fresenius’ Quantitative Analysis, 6th edition, p. 403, etc.)-the results being a8 a rule too high. Exp. 1. 60 C.C. phosphate solut,ion gave 0 0215 pyrophosphate of soda, containing 0-01147 grm. Exp. 2. 50 c,c. furnished 0,0213 pyrophosphate of soda or 0.01136 F m .P2 05. Exp. 3. 50 C.C. precipitated with magnesium mixture yielded 0.0184 grm. Mgl P2 07 = ESP. 4. Obtained from 60 C.C. 0.0182 grm. Mg2 P2 07 containing 0.01164 grm. P2 06. P2 0 5 . 0.01176 P!J 06. The P,O, found fluctuated therefore from 0.01136 to 0.01176, the averago being 0.01 15 P2 0, in 50 C.C. Exp. 6. 60 C.C. were concentrated to about 20 c.c., and precipitated, at a temperature of about 30 with 60 C.C.molybdate solution. After 10 hours the precipitate was separated by filtration, washed with alcohol, care being taken to pour a3 little as possible of the precipitate upon the filter, dissolved in NHs and the solution evaporated, the solution being kept strongly alkaline to the very end of the evaporation. The residue was dried at 1OOQC until the two last weighing8 agreed within 1 Mgrm.Obtained 0.3436 grms. residue, or 1 part of P2 Oa yielded 29.87 parts residue. Exp. 6. As above. Obtained 0,3426 residue, 1 P2 0 6 therefore gave 29.79 parts residue. Exp. 7. As above. Obtained 0.3536 residue, 1 P2 0s = 30-75 parts residue. Exp. 8. As above. 26 C.C. of alcohol were used for washing. Obtained 0.3549 residue, or 1 P2 0 5 Ezp.9, A8 above. 50 C.C. of alcohol used for washing. Residue weighed 0.3643 grms. or Exp. 10. As before. 100 C.C. alcohol used for washing. Residue 0.3487; or 30032 residue from Ex$. 11. 100 C.C. phosphate solution evaporated to about one half, and precipitated with 75 C.C. Exp. 12. 100 C.C. phosphate solution, precipitated without concentrating it first, with 75 C.C.Exp. 13. 25 C.C. phosphate solution, 25 molybdic solution and 25 C.C. dilute nitric acid, jielded Exp. 14. 2 C.C. phosphate, containing no more than 0-00046 P2 05, 50 C.C. water, and 10 C.C. I n all these cases, I took care not t o allow the bulk of the liquid to become too small, as by concentration MOO, would become precipitated. I obtained, on the average, from 1 part of phosphoric acid 30.35 parts of residue, or 100 parts of residue contain - - 30.86 residue.1 l‘~ 0s =30.81. 1 PI 06. molybdic solution. Result 0.6877 residue, or 1 1’2 05 =29*90 parts residue. molpbdic solution. 0’6971 grm. residue. 1 P2 06 ~ 3 0 . 3 1 residue. 0.1741 residue, or 30.28 residue from 1 P2 05. MOOS solution gave 0.0141 grm. residue or 30.65 parts from 1 P2 0 6 .THE ANALYST.27 3.295 of P,O,. following amounts of phosphoric acid :- Dividing the quantities of residue obtained by 30.35, we obtaiu the Taken. Found. 6 6 7 8 9 10 11 12 13 14 ... ... ... ... ... ... ... ... ... ... ... ... ... ... 1. ... ... ... ... ... ... ... ... ... ... ... ... ... ... 0.0115 0.0115 0°0115 0.0115 0.0115 0.01 15 0.0230 OnO230 0.00575 0.00046 ... ... ...... ... ... ... ... ... ... 0.0113 0.0113 0.0116 0.0117 0.0117 0.0115 0.0227 0*0230 0.0057 0.00046 It will be observed, that without adopting any special precautions against admixtux of Moos with the yellow precipitate taking place, except avoiding high temperatures or great contentration, very tolerably constant results were obtained, more constant in fact, than could have been yielded by the magnesia process.The following experiments however will show clearly that the proceca as hithcrto used by me was somewhat defective. A solution was made of 1.9763 grm. of a phosphorite in 250 C.C. after due separation of the silica. Ezp. 16. 50 C.C. were precipitated, and the phosphornolybdate washed with alcohol until nitrate of silver solution proved the complete absence of chlorine; obtained 2,5625 grm.residue. Divided by 30.35 this indicates 0-0844 Pa 06. The residue dissolved in NHs and the solution precipitated with Mg. mixture gave 0.1376 Mg2 P2 0.1 = 0.0880 P2 06. Ezp. 16. 50 C.C. precipitated as above, with addition of 25 C.C. dilute HNOs ; obtained 2,5283 grm. residue, or 0.0833 P2 05, From this residue 0.1367 Mg2 P2 0 7 were obtained, contaicing Ezp.17. 10 C.C. phosphate solution, 10 C.C. dilute HN03 and 25 C.C. molybdic solution gave 0,5376 grm. residue, or 0,0177 P2 0 6 ; or from 30 C.C. 0.0885 P2 06. Ezp. 18. 10 C.C. phosphate, 10 C.C. diluted HNC3, 25 C.C. water and 26 C.C. molybdic solution gave 0.5378 rekidue, containing (divided by 30.35) 0,0177 P2 0s (or from 60 C.C. 0.0885 P2 05) Taken. P2 Oa calculated from Ps Oh in 50 C.C.60 C.C. ... 0.0880 O G 4 4 60 C.C. ... 0.0874 0.0833 - 0.0886 10 C.C. ... 10 C.C. ... - 0.0885 0.0874 P2 05. Obtained therefore- w 2 p2 07. calculated from Residue. Two of these four determinations are therefore unsatisfactory, whilst two, those in which but 10 C.C. phosphate solution were employed, agree well with the phosphate of magnesia determinations.On investigation I noticed that the residues obtained from smaller quantities are always perfectly solublo in water, showing that they really consisted of molybdate of ammonia and of phosphate of ammonia, whilst in cases where I employed a large quantity of phosphoric acid the residue was not entirely soluble in water, a separation of an insoluble molybdate of ammonia and loss of ammonia having taken place, although the solution during evaporation had been kept strongly ammoniacal.I therefore abandoned the plan of weighing, what I may, for brevity’s sake, call an ammoniacal residue, so liable to decomposition, and converted it, by repeated evaporation with small quantities of water to dryness, into what I will call an aqueous residue, incapable of losing by drying any further quantity of ammonia, and consisting of phosphate of ammonia and of an acid molybdate of ammonia, This kind of residue moreover hae In further experiments I found a similar difference.28 THE ANALYST.the advantage of drying most rapidly to weight absolutely constant. I found that I had to divide the aqueous residue by 28.5 in order to obtain the amount of P2 0, oon- tained in it.2.1932 grammes of a sample of superphosphate were dissolved and the solution made up to 250 C.C. Exp. 19. 60 C.C. were precipitated and the precipitate after dissolving in NH, converted into Mgz P2 Or, obtained 0.0908 Mgz PZ 07, or 0.0581 Pz 05, or from 10 C.C. 0.0117 P2 Or,. Ezp. 20. 10 C.C. were precipitated with 80 C.C. Moo3 solution, and the ammoniacal residue prepared.This amounted to'0.3532 grm., or 0.0116 P2 05. Zxp. 21. 10 C.C. phosphate solution, 10 C.C. HN03 and 30 C.C. Moo3 solution yielded 0,3519 grrn. ammoniacal residue = 0.0116 Pz 0 5 . Exp. 22. 10 C.C. phosphate solution, 20 C.C. HN03 and 40 C.C. Moo3 solutiongave ammoniacal residue 0.3525 grm. = 0.0116 PZ 05. Exp. 23. 10 C.C. precipitated with 25 C.C. molybdate ; aqueous residue 0.3405, divided by 28.6 = 0.0119 PZ 05.I t was dissolved in ammonia and the solution precipitated with magnesia mixture. Xzp. 24. Same quantities taken as before. Aqueous residue 0.3350 grm. 2.*T = 0.0117 Pz 06. It yielded 0*01S6 grm. Mgz Pz 0 7 or 0.0119 Pz 06. Exp. 25. Quantities as before. Aqueous residue 0.3347 = 0.0117 PZ 06. Thie gave 0.0184 Mgz PZ 0 7 or 0*0118 P2 0 6 .Ezp. 26. 10 C.C. phosphate solution, 50 C.C. water) 26 C.C. molybdic solution, yielded aqueous residue 0.3322, or 0'0116 P2 0s. This furnished 0.0179 Mg2 PZ 0.1 or 0.0114 PZ Oa. Exp 27. 10 C.C. phosphate, 10 C.C. dilute HWO3, and 25 molybdic solution gave 0,3194 aqueous residue = 0.0112 Pz Oa. 0.0183 Mgz Po 07 reanltad = 0*0117 P2 06. From it 0.0177 Mgz Pz 07 or 0.0113 P2 05.Tabulated, these results are as follows :- PZ Or, calculated from Mgz PZ 07. Pz 0 5 from aqueous residue. 10 C.C. phosphate . . . . . . 9 9 9 9 . . . . . . . . . . . . Average Solution of another phosphorite, ... 0.01 17 . . . . . . . . . - ... 0,0117 . . . . . . . . . 0.0119 ... 0.01 19 . . . . . . . . . 0.01 15 ... 0.0118 . . . . . . . . . 0.0117 ... 0.0114 . . .. . . . . . 0.0116 .. 0.0113 . . . . . . . . . 0*0112 ... 0.0116 . . . . . . . . 0.0116 2.5151 grms. per 250 C.C. Exp. 28, 10 C.C. phosphate and 20 C.C. solution of another phosphate, 2.5161 grms. per 260 C.C. molybdic solution yielded 0,6308 grm. aqueous residue or 0.0221 F'z 06, Converted into phosphate of magnesia, it furnished 0.0335 Mgz P2 07, or 0.0214 Pz Or,. E q . 29. 10 C.C.phosphate, 10 C.C. of a 20 per cent. nitrate of ammonia solution, and 20 C.C. molybdic solution yielded 0.6442 aqueous residue, equal to 0.0226 P2 0 6 , this furnished 04353 Mg2 P2 0 7 or 0*0226 PZ Oa. Exp. 30. Same as 29. Aqueous residue 0*6360=0*0223 P2 06* From this 0,0349 Mg2 PZ 07 equal to 0-0223 Pz 0s. Ezp. 31. 10 C.C. precipitated with an excess of molybdic solution, the precipitate washed with 'LO per cent.NH4 NOS, and then dissolved in NH3 and straight converted into magnesia precipitate. Exp 32. 10 C.C. phosphate, 30 C.C. molybdate. Residue 0.6197 equal to 0.0218 PZ 06. Thia furnished 0*0349 M 2 P2 O7 or 0.0222 P2 Or,. Exp. 33. 10 C.C. phosphate solution, 25 C.C. molybdate. Residue weighed 0.6452 grme. = 0.0226 P2 0 6 . Exp, 3 i . 60 C.C. phosphate solution were precipitated with 125 molybdic solution, the precipitate washed with 20 per cent.NE4 NOS, dissolved in NH3, and the solution at once precipitate with magnesia mixture. Obtained 0.1707 grm. Mg2 Pz 0 7 containing 0*1092 grm. P2 06, or from 10 C.C. phosphate solution 0.0218 grm. PI 05. Exp. 35. 50 C.C. were precipitated as above, the precipitate sashed with alcohol (dilute).Obtained 3.1807 grm. aqueous residue, or 0.1 116 P' 0 5 , or from 10 C.C. 0 0223. This residue dissolved in NH3 furnished 0.1719 grm. Mgn P2 0 7 or 0.1099 P2 Or, from 10 C.C. therefore 0.0220 P1O5. Obtained 0.0346 Mgz P2 0 7 = 0.0221 PZ 0s. From it Mg2 P2 07 0.0361 grms. = 0,0231 Pz Oa.THE ANALYST. 29 Exp. 36. 1 C.C. of the phosphate solution were mixed with 10 C.C.NH4 Nos, and precipitated with 6 C.C. MOO, solution, Aqueous residue 0,0692 or 0-0024 PZ Oa instead of 0*00223. Thus the results are- P2 0s calculated from Mg2 P2 0,. '' '' from acmeous residue. 10 C.C. phosphate solution ... 0.0214 . . . . . . . . . 0,0221 ), 7 ) 0.0226 0.0226 ... 0.0223 . . . . . . . . . 0.0223 ... G-0221 - 9 9 Y 9 0.0222 0.0218 ... 0.0231 . . . . . . .. . 0.0226 ... 0-0218 . . . . . . . . . 9 , 9 9 ... 0.0'220 . . . . . . . . . 0.0223 Average ... 0.0222 . . . . . . . . 0.0223 ... . . . . . . . . . 9, 9 , 9 , 2 ) . . . . . . . . . ... . . . . . . . . . Y9 9 9 Y ) 9 9 These results are, I may safely say, highly satisfactory. They speak to the accuracy both of the magnesia method and of the molybdic method. The latter is, however, far less troublesome, requiring less labour, time and attention than the precipitation as phosphate of magnesia and ammonia.The precipitation does not take longer than two or three hours, and hence a determination of phosphoric acid can readily be completed within a day. The expense of the rnolybdic acid is trifling, and it is a very simple matter to recover it from the solutions obtained on filtering the Sellow precipitate and on dissolving the aqueous residue, after weighing.I prefer to work up the molybdic residue as follows :-I render the solution acid, if not already so, by means of nitric acid, add phosphate of soda, and heat ; collect the precipitate, wash it superficially, dissolve in NK, and precipitate the P, 0, by means of magnesia mixture. The filtrate is made slightly acid with nitric acid, when molybdic acid at once separates, is collected on a filter, washed and dried.I will now refer, in 8 few word?, to the method recently proposed by Finkener, alluded to above (Berl. Ber. 13, 1878). I became acquainted with it only after I had practically completed my investigation as detailed in this paper. Finkcner states that the yellow precipitate can readily be obtained of constant composition, provided theTe is alwajs a sufficient quantity of free nitric acid, that at least one third of the total MOO, taken remains in solution after precipitation of the phosphoric acid, and that 100 C.O.of the liquid contains no less than 25 grm. NE, NO,. He washes with a 20 per cent. solution of NH4 NO, slightly acid with HNO,, then with a little water removes the precipitate, partly by washing, partly by dissolving in ammonia with a porcelain basin, concentrates, adds HNO,, evaporates and by careful heating drivee off the nitrate of ammonia. The residue thus obtained contains 3.794 per cent. P, 0, (1 P2 0,=26.36 parts residue.) To this is to be remarked that his precautions for obtaining a precipitate of constant composition are quite superfluous and clumsy. For although there is no doubt that nitrate of ammonia greatly increases the delicacy of the molybdic reaction, it evidently produces separation of molybdic acid or of a molybdate of ammonia. For according to Richters (Dingl. Polyt. Jourii., Vol. 199, p. 183) the precipitates produced in liquids containing much NH, NO, dz@r from those obtained under ordinary circumstances by their greater bulk and their Ziglzter colour. Furthermore, on heating the residue to get rid of the nitrate of ammonia, reduction of the molybdic acid always take8 place, asd I obtained whencrer I followed Finkener's instructions, a residue dissolving in NU, with a dark blue colour, showing the presence of molybdic oxyde.