*B 323 EXCHANGE A Study of tlie Composition of an Ammonium Pkospkomolykdate and tke Determination ox Phosphorus. DISSERTATION SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIRE- MENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY * IN THE FACULTY OF PURE SCIENCE IN COLUMBIA UNIVERSITY BY NORMA E. JOHANN, A.B., A.M. New York City 1921 ^ The Jackson Press, Kingston 1921 A Study oi tlie Composition of an Ammonium PkosphomolyLdate and tne Determination or Phosphorus. DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Faculty of Pure Science, Columbia University BY NORMA E. JOHANN, A.B., A.M. n New York City 1921 The Jackson Press, Kingston 1921 * * . * * * 5 *" * ACKNOWLEDGMENT The author wishes to express her appreciation of the interest and advice given throughout this investigation by Professor H. T. Beans, at whose suggestion it was undertaken. N. E. J A STUDY OF THE COMPOSITION OF AN AMMONIUM PHOSPHOMOLYBDATE AND THE DETERMINA- TION OF PHOSPHORUS In view of the conflicting statements found in the literature regarding the composition of the ammonium phosphomolybdate precipitate, it was decided to try once more to analyse this precipi- tate and to determine, if possible, the conditions for obtaining a precipitate of constant composition. Following are given some of the analyses of ammonium phos- phomolybdate for phosphorus pentoxide and molybdic oxide found in the literature together with the method of treating the precipitate. In many cases the methods of analysis are not given. Author Method of treating ammonium Ratio phosphomolybdate precipitate P 2 5 Mo0 3 P 2 5 :Mo0 3 Nutzinger 1 dried at 100 C 3.82% 92.70% 1:23.92 Seligsohn 2 dried at 100 C 3.142 90.744 1:28.48 Sopp 3 3.20 86.0 1:26.50 Struve & Svanberg 4 3.63 86.88 1:23.60 Sonnenschein 5 3.03 86.87 1:28.27 Macagno 6 dried at 100C 3.142 90.744 1:28.48 Rammelsberg 7 dried at 100 C 3.90 86.45 1:21.85 Gibbs 8 dried by pressing with woolen paper 3.70& 89.00 1:23.72 3.83 Hundeshagen 9 washed with dilute HN0 3 dried at 130-150 C 3.72& 91.86 1:24.35 Babbitt 10 washed with water, dried at 3.77 92.11 24.10 85-90C. for 3 days 3.724 90.315 1:23.92 Doolittle & Eavensen 11 dried to constant weight at 130 C. 3.76 91.65 1:24.00 iJahresb, 1855, 374. 2 J. f. Prakt. Chem. 67, 470. 3 Pogg. Annal. 109, 136. 4 Jahresb, 1847, 412. 5 J. f. Prakt. Chem. 53, 342 (1851). 6 Chem. News 31, 197 (1875). 7 Ber. 10, 1776 (1877). 8 Am. Chem. Jour. 3, 317 (1881). Z. f. Anal. Chem. 28, 141, (1889). 10 J. Anal, and Appl. Chem. 7, 165 (1893). "Jour. Am. Chem. Soc. 16, 234 (1894). 451720 Blair & Whitfield 12 dried at 100C. for 12 hours 3'59 88.06 1:24.19 dried over KOH for 3 months 3.59 88.06 1:24.19 Gladding 13 washed with 1% HNO 3 , dried to constant weight at 105 C 3.76 91.36 1:23.96 Baxter 14 washed with 10% NH 4 N0 3 , heated to constant weight at 300C 3.7422 92.16 1:24.29 Hundeshagen says that the composition of the precipitate after washing with cold dilute nitric acid and drying at 130-1 50 C. cor- responds to the formula (NH 4 ) 3 PO 4 . 12 MoO 3 whether precipitated in nitric, hydrochloric or sulphuric acid solutions containing ammo- nium salts. The precipitate obtained with excess acid, washed with cold dilute acid and dried in a desiccator contains, according to the acid, two molecules of HNO 3 or HC1 with one molecule of H 2 O probably in unstable chemical combination. This formula (NH 4 ) 3 PO 4 . 12 MoO 3 of the dried precipitate is the one now most generally accepted. It corresponds to 3.783% P 2 O 5 and 92.06% Mo0 3 . Finkener (Ber. 11, 1638 (1878) was one of the first to point out that the ratio of P 2 O 5 to MoO 3 is 1 : 24 and not 1 : 20 as given by Debray (Chem. News 17, 183 (1868) or 1 : 22 as given by Ram- melsberg. He and also Auld (Analyst 37, 130 (1912) state that the variation in composition is due to variation in the amount of ammonia and water in the molecule. The work of the present in- vestigation confirms this statement. Wolcott Gibbs believed that there are three classes of phosphomolybdates. He prepared and analysed three in which the ratio of P 2 O 5 to MoO 3 was 1 : 20, 1 : 22, 1 : 24. It may be possible to have a mixture of the three classes of salts. Hissink and von der Waerden (Chem. Weekblad 2, 179 (1905) have proposed a formula requiring 12.65 molecules of MoO 3 per molecule of phosphomolybdate and Wardlaw ( J. Pro. Roy. Soc. N. S. Wales-4S, Part I, 73 (1914), 12.75. A large number of investigators have determined phosphorus by weighing either the dried precipitate (NH 4 ) 3 PO 4 . 12 MoO 3 or the compound P 2 O 5 . 24 MoO 3 , obtained by ignition of the phos- phomolybdate under various conditions. Following are some of the factors given for these procedures. 12 Jour. Am. Chem. Soc. 17, 747 (1895). 13 Jour. Am. Chem. Soc. 18, 23 (1896). 14 Am. Chem. Jour. 28, 298 (1902). Factor Author Method of treating precipitate to P 2 O_ Finkener 15 washed with NH 4 NO 3 , heated to 160-180 C 0.03794 Hehner 16 washed with alcohol, dissolved in NH 4 OH evap- orated solution, dried at 100 C 0.03509 Tamm 17 washed with 1% HNO 3 , dried at 120 C 0.0376 Wood 18 washed with dilute HN0 3 , dried at 100C 0.0373 ignited 0.0396 Carnot 19 washed with H 2 O, dried at 100 C 0.0373 Villiers & Borg 20 dried at 100 C. for 6 hrs 0.03728 ,Meineke 21 washed with H 2 O, alcohol, ether, and ignited 0.03949 Gladding 13 washed with 1% HN0 3 , dried at 105C 0.0376 Woy2^ washed with 1%NH 4 N0 3 + HNO 3 , ignited - P 2 O 5 . 12MoO 3 0.03946 Lorenz 23 washed with 2% NH 4 NO 3 , alcohol, ether, dried in vacuum 0.03295 Baxter 14 washed with 10% NH 4 NO 3 , heated to constant weight at300C 0.03742 Raben 24 washed with alcohol, dried at 110-120C 0.0375 Christensen 25 ignited assumed P 2 O 5 . 24Mo0 3 0.0394 Jorgensen 26 heated, not too strongly to P 2 O 5 . 24MoO 3 0.0394 Chesneau 27 dried at 105C 0.0366 ignited 0.0387 Maude 28 ignited 0.039467 Auld 29 washed with 1% HNO 3 , dried, ignited to constant weight . 0.0396 Neubauer & Lucker 30 washed with 2% NH 4 NO 3 , acetone 0.03295 The wide variation in the factors proposed even when the con- ditions are nearly identical and the many conditions of treatment which have been suggested show conclusively that analysts have encountered difficulties in the application of those procedures in- volving the drying or ignition of the phosphomolybdate precipitate. With a view to overcoming some of the difficulties and uncer- tainties of the gravimetric method, various volumetric procedures 15 Ber. 11, 1638 (1878). "Analyst^, 23 (1879). 17 Chem. News 49, 208 (1884). 18 Jour. Anal. Chem. 1, 138 (1887). 19 Chem. News 67, 101 (1893). 20 Comptes Rendus 116, 989 (1893). 21 Chem. Ztg. 20, 108 (1896). 22 Chem. Ztg. 21, 441 (1897). 23 Z. f. Anal. Chem. 46, 193 (1907). 24 Z. f. Anal. Chem. 47, 546 (1908). 25 Z. f. Anal. Chem. 47, 529 (1908). 26 Z. f. Anal. Chem. 46, 370 (1907). 27 Comptes Rendus 146, 758 (1908). 28 Chem. News 101, 241 (1910). 29 Analyst 37, 130 (1912). 3 Z. f. Anal. Chem. 51, 161 (1912). have been proposed. The method most generally used is an acidi- metric one in which the phosphomolybdate precipitate washed free of acid is dissolved by the addition of a measured excess of standard alkali and the excess is titrated back with standard acid. Here again wide variation is encountered in the statements of the authors as to the numbers of moles of alkali equivalent to one mole of phosphomolybdate precipitate. The values most frequently given are 23, 26 or 28 moles of alkali per mole of precipitate. Thilo (Cherri. Ztg. 11, 193 (1887) dissolved the yellow precipi- tate in ammonium hydroxide titrated back with sulphuric acid using litmus as indicator. His ammonium hydroxide solution contained 32 g. NH 3 per liter when 1 cc. was equivalent to .0669 g. of preci- pitate. Rothberg and Auchinvole (J. Anal, and Appl. Chem. 6, 243 (1892) washed the precipitate with 1% nitric acid solution, then w r ith potassium nitrate to remove the free acid, dissolved it in a standard sodium hydroxide solution and titrated back the excess with nitric acid using phenolphthalein as the indicator. Manby (J. Anal, and Appl. Chem. 6, 82 (1892) and Handy, (J. Anal, and Appl. Chem. 6, 204, (1892) used this same method, standardizing their alkali against the pure precipitate or a steel of known phos- phorus content. Pemberton (J.A.C.S. 15, 382 (1893), 16, 278 (1894) was the first to make an extended investigation of this method. He agrees with Hundeshagen that 23 molecules of alkali are required for one molecule of ammonium phosphomolybdate. Kilgore (J.A.C.S. 16, 765 (1894), 17, 950 (1895) modified Pem- berton's method in that he precipitated the phosphomolybdate at 60 C. instead of at boiling temperature as Pemberton did. Kilgore washed the precipitate with 1% nitric acid solution, then with a 3% potassium nitrate solution and finally with water. He dissolved the precipitate in standard potassium hydroxide solution and titrated back the excess with nitric acid using phenolphthalein as the indi- cator. Various other modifications of the Pemberton-Kilgore method have been suggested. Lagers (Z. f. Anal. Chem. 47, 561 (1908) believes that the formula for the ammonium phosphomolybdate precipitate is (NH 4 ) 3 PO 4 . 12.65 MoO 3 which requires 24.30 molecules of sodium hydroxide for neutralization. The existing uncertainty of the composition of the phospho- molybdate precipitate and the conflicting statements found in regard to the volumetric method, make this volumetric method unsatisfac- tory as well as the direct gravimetric method. One difficulty in the 7 volumetric method is that the end-point is not sharp. This may be due to several causes, as, for instance, the ammonia or phosphate present in the solution of the ammonium phosphomolybdate preci- pitate. The presence of ammonia was believed to cause interference with the end-point when phenolphthalein is used as the indicator by Neumann (Z. Physiol. Chem. 37, 115 (1903), who obtained cor- rect results only when he dissolved the precipitate in sodium hy- droxide solution, boiled out the ammonia and then titrated back the excess alkali with sulphuric acid using phenolphthalein as the indicator. In this case one molecule of phosphorus pentoxide was equivalent to 56 molecules of sodium hydroxide. Neumann's method has been found unsatisfactory by some authors among whom are Gregersen (Z. Physiol. Chem. 53, 453 (1907), Taylor and Miller (J. Biol. Chem. 18, 215 (1914), and Kleinmann (Biochem. Zeitschr. 99, 19 (1919). Falk and Sugiura (J.A.C.S. 37, 1507 (1915) also studied Neumann's method. They remove the ammonia either by boiling or by adding formaldehyde. Heubner (Biochem. Zeit. 64, 393 (1914) used Neumann's method with Gregersen's modification. These authors give the factor 1 cc. n/2 NaOH is equivalent to 0.553 mg. phosphorus (theoretical) while Heubner used 0.57. Jodidi (J.A.C.S. 37, 1708 (1915) also used Gregersen's modification giving 0.57 as the factor. Wardlaw (J. Pro. Roy. Soc. N. S. Wales 48, Part I, 73 (1914) makes the statement, which the present investigation confirms, that neither Neumann's method nor any one of the modifications gives correct results. The amount of phosphorus calculated from an acidimetric titration is in every case too high, the error increasing with the amount of phosphorus present. The object of the present investigation is, therefore, to make a more accurate determination of the composition of the ammonium phosphomolybdate precipitate, to determine the conditions for ob- taining a precipitate of constant composition, to establish the proper conditions for the acidimetric method and thus to propose the best method for the determination of phosphorus. The investigation will be presented under the following headings : Preparation of Ma- terials ; Composition of the Ammonium Phosphomolybdate Precipi- tate, (a) The Molecular Ratio of phosphorus pentoxide to molybdic oxide, (b) The Presence of Acid in the Molecule; New Method for the precipitation of Ammonium Phosphomolybdate; Volumetric Method. PREPARATION OF MATERIALS Potassium dihydrogen phosphate, KH 2 PO 4 . As a standard substance for obtaining a known weight of phos- phorus, potassium dihydrogen phosphate was used. This salt was chosen because it can easily be recrystallized, it contains no water hydration, and can be dried to constant weight giving a definite compound, KH 2 PO 4 . Its purity can also be tested by igniting a given weight to metaphosphate. The commercial salt was recrystal- lized several times and dried at 110C. Portions of known weight were then ignited to metaphosphate with the following results : Weight of KH 2 PO 4 Weight of KPO 3 Weight of KP0 3 Difference calculated found 4. 2054 g. 3. 6489 g. 3. 6480 g. .0009g. 3.5075 3.0434 3.0425 .0009 3.8746 3.3619 3.3613 .0006 Molybdic oxide, MoO z . Due to the uncertainty of the formula of ammonium molybdate and to the volatility of molybdic oxide at the temperature of the decomposition of ammonium molybdate, it is not possible to obtain a known weight of molybdic oxide from a given weight of ammo- nium molybdate. It was therefore necessary, for the purpose of mixing known weights of phosphate and molybdic oxide, to prepare the latter. This was done by treating small portions of pure ammo- nium molybdate with concentrated nitric acid in a porcelain crucible, evaporating to dryness and igniting gently with a Bunsen flame. More nitric acid was then added and the evaporation and ignition repeated whereupon pure MoO 3 was obtained. Molybdate precipitating reagent. The usual molybdate precipitating reagent was not used for reasons given later. In all the precipitations of phosphorus here described the precipitating reagent was prepared by dissolving 100 grams of ammonium para molybdate in 400 cc. of water and 80 cc. of concentrated ammonium hydroxide. A second solution was pre- pared consisting of 400 cc. of concentrated nitric acid and 600 cc. of water. The required quantity of solution I was poured into the corresponding quantity of solution II just before using. Ammonium phosphomolybdate. The yellow precipitate, ammonium phosphomolybdate, was pre- pared by precipitating the phosphorus in a solution of potassium dihydrogen phosphate with ammonium molybdate solution in the presence of ammonium nitrate, washing with 1 % nitric acid solution, dissolving the precipitate in ammonium hydroxide and reprecipitat- ing with nitric, hydrochloric or sulphuric acid three times. The precipitates after the first were washed with the corresponding acid and finally twice with cold water and then dried. Washing solution. The washing solution found most satisfactory for washing the yellow phosphomolybdate precipitate contained 10 grams of ammo- nium nitrate and 5 cc. of concentrated nitric acid per liter of solu- tion. Magnesia Mixture. In order that both phosphorus and molybdenum could be deter- mined in the same solution it was necessary to avoid the presence of chloride in the magnesia mixture. This reagent was therefore pre- pared containing acetates and an amount of magnesium equivalent to that contained in the magnesia mixture ordinarily used. For the preparation of this reagent 5.5 grams of magnesium oxide were dis- solved in a slight excess of acetic acid solution. To this solution ammonium acetate was added in sufficient quantity to prevent the subsequent precipitation of magnesium in the alkaline solution finally obtained. The solution was then neutralized with ammonia and an excess equivalent to 25 cc. of concentrated ammonia was added. The volume of the solution was now brought up to 500 cc. by the addition of water. Lead acetate solution. For the precipitation of molybdenum as lead molybdate, a lead acetate solution was prepared by dissolving 20 grams of lead acetate in 500 cc. of water and adding sufficient acetic acid to prevent hydrolysis. COMPOSITION OF THE AMMONIUM PHOSPHOMOLYB- DATE PRECIPITATE (a) The Molecular ratio of phosphorus pent oxide to molybdic oxide. Method of Procedure : The yellow precipitate, prepared as de- scribed above, was analysed by precipitating the phosphorus with the magnesium acetate solution and the molybdenum in the filtrate with lead acetate. The method was first tested by mixing known 10 weights of potassium dihydrogen phosphate and molybdic oxide approximately in the proportion found in the yellow precipitate. The molybdic oxide was dissolved in ammonium hydroxide, the potassium dihydrogen phosphate added, the solution made slightly acid with acetic acid and magnesium acetate solution added. 10 cc. of the latter were used for 20 to 25 mg. of phosphorus and the volume of the resulting solution made about 250 cc. This was heated to about 70C, ammonium hydroxide added slowly with constant stirring until the solution was alkaline, and then 5 cc. of concentrated ammonium hydroxide added in excess. After standing six hours or longer the precipitate was filtered off, the filtrate and washing solution being caught in volumetric flask which had pre- viously been calibrated. The precipitate was washed with \% ammonium hydroxide solution and then dissolved in 10 to 20 cc. of 6M HC1 solution. To this solution 2 cc. of magnesium acetate solution were added and the magnesium ammonium phosphate re- precipitated as above described by adding ammonium hydroxide slowly and with constant stirring. Sometimes the precipitate when first formed was gelatinous but it changed over within a short time into the crystalline form. After standing it was filtered on a proce- lain Gooch crucible, washed free of chlorides, ignited to constant weight and weighed as magnesium pyrophosphate. By drying the precipitate in the crucible slowly on a quartz plate over a burner, igniting slowly by means of a Bunsen burner and finally with a Meker burner a' residue was obtained which was light gray. In very few cases only was the residue pure white. The filtrate from the first precipitation was made up to a defi- nite volume at 25 C. and portions weighed out by means of a weight burette. The density of the solution was determined at 25 C. Thus the volume of the portion taken for analysis could be calculated. This portion was taken of such a size as not to give more than 0.25 grams of precipitate. It was made slightly acid with acetic acid, 10 cc. of an ammonium nitrate solution containing 5 grams of the salt were added, and the whole diluted to 300 cc. The solution was then heated almost to the boiling point and 10 cc. of lead acetate solution added slowly with stirring. After standing twelve hours the precipitate was filtered on paper, washed with hot water, ignited gently with the Bunsen flame in a porcelain crucible and weighed as PbMoO 4 . It was found necessary to add ammonium nitrate to the solution in order to form a precipitate of such character that it could be filtered. The following results were obtained : 11 P calculated P calculated Weight Mo0 3 Mo0 3 found from fromKH 2 P0 4 fromMg 2 P 2 O 7 used PbMoO 4 . 01634 g. . 01622 g. 0.8348g. 0.8349g. .03910 .03941 1.6124 1.6125 .03233 .03264 1.8045 1.8059 .02389 .02394 1.9520 1.9501 .02487 .02475 0.9698 0.9709 The average deviation of the weight of P found from the weight used is 6 parts per 1000. The values found for MoO 3 check those used within 1 part per 1000. Experimental Data Portions of precipitates prepared as described above were weighed out, dissolved io ammonium hydroxide and analysed by the above method. In each case the analysis was made on a different phosphomolybdate precipitate. Acid used for Method of treat- Ratio reprecipitation ing precipitate P 2 g MoO 3 P 2 O 5 :MoO 3 HNO 3 dried with alcohol and ether, 3.56% 87.63 ' 1:24.25 then at 110 for 10 min. 3 . 56 87 . 33 1 :24.16 HC1 dried with alcohol and ether, 3.47 85.20 1:24.22 then at 110 for 10 min. 3.58 88.02 1:24.23 H 2 SO 4 dried with alcohol and ether, 3 . 50 86 . 02 1 : 24.25 then at 110 for 10 min. 3.51 86.11 1:24.20 HNO 3 dried at 110C. for 2 hrs. 3.67 89.84 1:24.16 3.69 90.57 1:24.20 3.73 92.05 1:24.34 3.73 92.30 1:24.38 HNO 3 dried over H 2 SO 4 in the 3.72 91.17 1:24.16 dark, to constant weight 3 . 74 91 . 28 1 : 24.09 Mean molecular ratio. . . . 1:24.22 Although the actual percentages found do not agree it is seen that the ratio of phosphorus pentoxide to molybdic oxide is con- stant, whether the precipitate is dried at 110C., with alcohol and ether, or over H 2 SO 4 , and whether formed in the presence of nitric, hydrochloric or sulphuric acid. The average deviation of the above ratios is 2.5 parts per 1000. The variation in composition is there- fore due to variation in the amount of other constituents in the molecules, as for instance ammonia and water, due to the method of preparing the precipitate for weighing. In most cases where the precipitate was dried at 110C. the ammoniacal solution was slightly blue showing reduction of molybdenum to a small extent. The molecular ratio of phosphorus pentoxide to molybdic oxide was found when a given weight of phosphorus was precipitated and the precipitate obtained was analysed. 12 Method of Procedure: A number of precipitates formed under varying conditions were dried at 110 and analysed. 1. To a volume of 150 cc. containing about 47 mg. of phos- phorus pentoxide, 10 cc. of concentrated sulphuric acid and 30 grams of ammonium nitrate were added. The solution was heated to 75 and 40 cc. of molybdate solution (20 cc. of the above described ammonium molybdate solution poured into 20 cc. of 8M nitric acid) added. After standing over night, the precipitate was filtered off, washed with the ammonium nitrate, nitric acid solution, dissolved in ammonium hydroxide and reprecipitated from a hot solution with nitric acid after adding 2 cc. of the molybdate solu- tion. After standing the precipitate was filtered on a porcelain Gooch, washed with the ammonium nitrate, nitric acid solution five to six times, then twice with \% nitric acid solution and dried to constant weight at 110C. This precipitate was dissolved and analysed as described above. 2. Same as 1 except that the molybdate solution contained 10 grams of the salt in 100 cc. of water. Also the first precipitate was filtered on a Gooch, finally washed twice with ice cold water, dried at 110 and weighed. This precipitate was then dissolved and re- precipitated with nitric acid adding 2 cc. of the ammonium molyb- date solution, filtered on a Gooch, washed with the ammonium ni- trate, nitric acid solution, finally twice with ice cold water, dried at 110C. and weighed. This precipitate was dissolved and ana- lysed. 3. Same as 2 except that the first precipitate was dissolved and analysed. 4. In a volume of 40 cc. containing about 47 mg. of phosphorus pentoxide and 5 grams of ammonium nitrate the phosphorus was precipitated in the cold with 60 cc. of ammonium molybdate solution (20 cc. of the above described solution I poured into 40 cc. of solu- tion II). After standing 12 hours the precipitate was filtered, washed with the ammonium nitrate, nitric acid solution, then twice with \% nitric acid solution, dried at 110 and weighed. This was dissolved, reprecipitated with nitric acid, washed, dried and weighed as before. This precipitate was dissolved and analysed. 5. Same as 4 except that the first precipitate was dissolved and analysed. 6. Same as 4 except that 5 grams of ammonium chloride were used in place of ammonium nitrate and hydrochloric acid in place 13 of nitric. After standing the precipitate was filtered, washed with an ammonium chloride solution, then twice with \% hydrochloric acid solution. After drying at 110 it was dissolved and reprecipi- tated with nitric acid. This precipitate was treated as in 4 then dssolved and analysed. 7. Same as 6 except that the first precpitate was dissolved and analysed. Experimental Data Wt. of yellow Exp. Method ppt. per 47 mg. of P 25 P 2 5 taken 1 1 1.2755g. 3.56% 2 1 1.2850 3.55 3 2 1.2718 3.63 4 3 1.2930 3.59 5 3 1.2383 3.56 6 4 1.1693 3.62 7 5 1.2583 3.60 8 6 1.2152 3.62 9 7 1.2390 3.60 Mo0 Ratio ).:Mo0 3 1:25.37 1:25.47 1:24.95 1:25.17 1:25.46 1:25.19 1:25.36 1:25.12 1:25.32 1:25.27 91.60% 91.57 91.74 91.65 91.82 92.52 92.51 92.25 92.49 Mean molecular ratio The above results were obtained by starting with definite known weights of phosphorus pentoxide. The yellow precipitate formed in each case was dried at 110* and weighed before analysing. The wide variation in the weight of ammonium phosphomolybdate pre- cipitate obtained from a given weight of phosphorus arises either in a variable decomposition of the precipitate during drying or in the difference in the conditions of precipitation. In all cases the surface of the dried precipitates were greenish and the ammoniacal solutions were blue. The intensity of the blue color varied. The conditions of precipitation described except in the case of experiments 8 and 9 are similar to conditions of precipitation found in the literature. In experiments 8 and 9 chloride ion was intro- duced. In experiments 1, 2, 3, 6 and 8 the precipitate obtained upon reprecipitation was the one which was analysed^ while in experi- ments 4, 5, 7 and 9 the original precipitate was analysed. Upon comparing the molecular ratio of phosphorus pentoxide to molybdic oxide it will be noticed that there is no difference obtained whether the first or second precipitate is analysed. For instance, in experi- ment 1 the ratio is 1 : 25.37 while in 7 it is 1 : 25.36 and in experi- ment 4 the ratio is 1 : 25.17 while in experiment 6 it is 1 : 25.19. Reprecipitation, therefore, does not appear to effect the composition of the precipitate. 14 It will also be noticed that although the percentages of phos- phorus pentoxide in the yellow phosphomolybdate precipitates found are fairly constant, the precipitates vary in molybdenum content. The average deviation of these ratios, namely 5.6 parts per 1000, is larger than that found in the case of the previous precipitates formed by precipitating large amounts of phosphorus and analysing portions of the precipitates obtained. This second set of precipitates ob- tained from known weights of phosphorus and the whole precipitate analysed are higher in molybdenum content than the first. It was thought that this might be due to molybdenum occluded on the above precipitates. In preparing large quantities no molybdate solution was added before successive reprecipitations, while in precipitating the phosphorus from a known weight of potassium dihydrogen phosphate, 2 cc. of molybdate solution were added before precipi- tation. An experiment was therefore tried where the phosphorus was precipitated according to method 4 above except that on reprecipi- tation no molybdate was added. The following results were ob- tained: 3.73% P 2 O 5 , 92.34% MoO 3 , ratio P 2 O 5 :MoO 3 1:24.41. This ratio checks the first set obtained. From this it is evident that the precipitates obtained according to the above 7 methods contained more molybdenum than the other precipitates. If this excess of molybdenum is constant, however, no error would be introduced in a determination of phosphorus depending on the composition of the ammonium phosphomolybdate precipitate. The conditions for obtaining a precipitate of constant composition will be discussed later. (b)The Presence of acid in the Molecule. A great variety of formulas have been proposed for the yellow phosphomolybdate precipitate, some of which are (NH 4 ) 3 PO 4 . 10MoO 3 , (NH 4 ) 3 PO 4 . 10MoO 3 . 14H 2 O, (NH 4 ) 3 PO 4 . llMoO 8 , (NH 4 ) 3 PO 4 . HMoO 3 . 12H 2 O, (NH 4 ) 3 PO 4 . 12MoO 3 , (NH 4 ) 3 PO 4 . 12MoO 3 . 3H 2 O, (NH 4 ) 3 PO 4 . 12MoO 3 + (NH 4 ) 2 HPO 4 . 12MoO 3 +8H 2 O, (NH 4 ) 2 'HPO 4 . 12MoO 3 , (NH 4 ) 3 PO 4 . 12.65MoO 3 , (NH 4 ) 3 PO 4 . 12.75MoO 3 , (NH 4 ) 3 PO 4 . 12MoO 3 . 2HNO 3 . H 2 O, 4[(NH 4 ) 3 PO 4 . 12MoO 3 ] + (NH 4 ) 2 SO 4 . 5Mo0 3 , 8(NH 4 ) 3 P0 4 . 12Mo0 3 + (NH 4 ) 2 S0 4 . 8MoO 3 . The last two formulas are given for precipitates formed in solutions contain- ing large amounts of sulphate ion. -15- Hundeshagen (Z. f. Anal. Chem. 28, 141 (1889), Neumann (Z. Physiol. Chem. 37, 115 1903), Richardson (J.A.C.S.. 29, 1314 (1907), Artmann and Brandis (Z. f. Anal. Chem. 49, 1 (1910), Falk and Sugiura (J.A.C.S. 37, 1507 (1915) and Posternak (Compt. Rend. 170, 930 (1920) consider that the phosphomolyb- date (NH 4 ) 3 PO 4 . 12MoO 3 combines in various ways with the acids or salts present in the solution to form further complexes containing nitric acid, sulphuric acid or sulphomolybdates. The arguments advanced by these authors for the formulas which they propose are as follows : Hundeshagen states that the precipitate obtained with excess acid, washed with cold dilute acid and dried in a desiccator contains two molecules of acid with one molecule of water probably in un- stable chemical combination. Both acid and water are expelled completely at a temperature exceeding 130C. for on heating the precipitate to this temperature it lost weight giving off moist strongly acid vapors. In the case of the nitric acid compound the loss in weight was 7.13% or 7.67% on the substance dried at 150, the hydrochloric acid compound 4.78% or 5.02% on the substance dried at 150C. The precipitates dried in a desiccator required for neutralization two equivalents more alkali than those dried at 150 corresponding to as many equivalents of nitric or hydrochloric acid in combination with the phosphomolybdate. In point of weight two molecules of nitric acid would amount to 6.71% and two mole- cules of hydrochloric acid to 3.89% if referred to phosphomolybdate free from acid and water. If the numbers found as loss of weight on heating to 150 the salts previously dried in a desiccator are diminished by these amounts there remain for water 0.96% and 1.13%, of which figures the former represents exactly and the latter approximately a molecule of water. From these facts he writes the formula of the phosphomolybdate precipitate (NH 4 ) 3 PO 4 . 12MoO 3 . 2HNO 3 . H 2 O or (NH 4 ) 3 PO 4 . 12MoO 3 . 2HC1 . H 2 O when formed in a solution containing chlorides. Neumann accepts Hundeshagen's formula for the ammonium phosphomolybdate precipitate. The latter has shown that the two molecules of nitric acid can easily be replaced by salts or other acids present in solution. Neumann determined phosphorus in organic matter. Since in ashing sulphuric and nitric acids were present and a large amount of ammonium nitrate was added to insure complete precipitation, he believes that there can easily be a change in that part of the molecule. Such changes in composition, 16 however, he found have no effect on the amount of sodium hy- droxide required for titration. Richardson found that in the presence of sulphates, sulphuric acid or a sulphate was always found by analysis in the yellow phosphomolybdate precipitate. He states that it is altogether pro- bable that a complex ammonium sulphomolybdate is formed which has the acid nature of ammonium phosphomolybdate and therefore reacts with alkali similarly to the latter. Artmann and Brandis performed two experiments to show the effect of sulphate on the ammonium phosphomolybdate precipitate. In the first the phosphorus was precipitated in the presence of am- monium nitrate and potassium sulphate at 40 C, in the second in the presence of ammonium sulphate. In both cases the precipitates were dissolved, the ammonia distilled off and titrated, with high results. After acidifying the residues with nitric acid and adding barium chloride, precipitates of barium sulphate were obtained. Falk and Siguira precipitated phosphorus as ammonium phos- phomolybdate in the presence of sulphuric acid according to Neu- mann. They found that the precipitate thus formed contained sulphate as an essential part of the molecule together with excess molybdic oxide and no nitric acid. They found that the titration values were the same whether or not the precipitate was dried at 120 thus showing that no nitric acid was contained in the precipi- tate formed in a solution containing sulphuric acid and ammonium nitrate. They determined the amount of sulphate in the precipitate by dissolving it in ammonium hydroxide, reprecipitating with nitric acid, filtering, evaporating the filtrate to dryness after adding hydro- chloric acid, taking up the residue with water, filtering and adding barium chloride to the filtrate. They thus obtained .0179 - .0214 grams of barium sulphate from precipitates from phospate solu- tions containing 47.1 mg . P 2 O 5 . Similar treatment of a corre- sponding solution without sulphuric acid gave .0001 and .0004 grams of barium sulphate. Posternak prepared ammonium phosphomolybdate under vary- ing conditions and analysed the precipitates for phosphorus, in some cases transforming them into the barium salts. He found that phosphoric acid fixes 12, sulphuric acid 8 and nitric acid 4 mole- cules of molybdic oxide and concludes that in acid solution a tetra- molybdate is present which can combine with all mineral acids present in solution. 17 Other investigators do not agree with the above authors re- garding the presence of acid or sulphate in the molecule of ammo- nium phosphomolybdate. Blair and Whitfield (J.A.C.S. 17, 747 (1895) found that the amount of alkali required to titrate a pre- cipitate when washed with water or dilute acid and dried over potas- sium hydroxide for three months was less than that required to titrate a precipitate which was dried at 150C. from which, accord- ing to Hundeshagen, the acid would be removed. Further Frese- nius (Z. f. Anal. Chem. 3, 446 (1864) and Richters (Z. f. Anal. Chem. 10, 469 (1871) found that sulphuric acid does not interfere with precipitation of ammonium phosphomolybdate. The work of the present investigation shows that the presence of sulphuric acid or hydrochloric acid in solution has no effect on the precipitation of phosphorus as ammonium phosphomolybdate and that neither sulphate nor chloride is contained in the molecule under the conditions herein adopted. Method of Procedure: The presence of sulphate was tested for in precipitates obtained as follows : Enough potassium dihydrogen phosphate was weighed out to furnish from 46 to 68 mg. of P 2 O 5 . This was dissolved in water, 10 grams of ammonium nitrate added in solution, and diluted to 140 cc. To this cold solution were added 45 cc. ammonium molyb- date solution, (15 cc. of the ammonium molybdate solution I poured into 30 cc. 6MH 2 SO 4 ) and the solution stirred for a few minutes. The precipitate does not form as rapidly as in a nitric acid solution and tends to stick on the beaker. After standing over night the pre- cipitate was filtered off and washed with the ammonium nitrate, nitric acid solution. After washing carefully with about 400 cc. of wash solution, the funnel being allowed to drain between washings, it was still possible to obtain a test for sulphate by adding barium chloride to a small amount of washings and shaking for a few min- utes. The yellow precipitate was then dissolved in 10 cc. of con- centrated ammonium hydroxide, 2 cc. of molybdate solution added and reprecipitated by adding nitric acid. After standing again this precipitate was filtered off, washed with the same solution as above, and the filtrate plus washings evaporated to dryness adding 5 cc. concentrated hydrochloric acid when the solution was almost dry. The residue was taken up with dilute hydrochloric acid, the molyb- denum compound which separated filtered off, and 5 cc. of m/4 barium chloride solution added. After standing 24 hours the result- ing barium sulphate was filtered off and weighed. Blank deter- 18 minations were made in a similar manner on precipitates obtained from solutions containing nitric acid in place of sulphuric acid. Experimental results P 2 5 cal- culated from KH 2 P0 4 BaSO, Wtof BaSO x P 2 O 5 calcu- BaSO gms. per 50 mg. lated from gins. KH 2 P0 4 .0468g. .0474 .0136 .0111 .0145g. .0117 .0536g. .0543 .0199g. .0040 .0480 .0234 .0244 .0557 .0246 .0484 .0242 .0250 .0574 .0154 .0516 .0155 .0150 .0578 .0158 .0526 .0054 .0051 .0624 .0122 .0534 .0108 .0101 .0810 .0264 Wt. of BaSO 4 per 50 mg. of P 25 .0186g. .0037 .0221 .0134 .0137 .0098 .0163 Blank Determinations .0534 .0047 .0044 .0641 .0042 .0033 .0646 .0033 .0026 It will be seen that the blanks are much higher than those given by Falk and Sigiura. It is very probable that all the barium sulphate precipitates were obtained from sulphate which had not been washed out. The irregularity of results can certainly be explained by assum- ing the presence of varying amounts of sulphate left in or adsorbed upon the precipitate, for, as stated above, it seemed impossible to wash the precipitate free of sulphate. From these facts the con- clusion was drawn that sulphate is not a part of the ammonium phosphomolybdate molecule when it is formed in a solution con- taining sulphate. The presence of molybdenum was tested for in some of the above barium sulphate precipitates by fusing with sodium carbonate, dissolving in hydrochloric acid and passing hydrogen sulphide into the solution. A brown precipitate of molybdenum sulphide was obtained. A few experiments were performed to test for the presence of hydrochloric acid in the molecule of the precipitate formed in a chloride solution. Known weights of potassium dihydrogen phos- phate were dissolved in water, 5 grams of ammonium chloride added and the whole diluted to about 120 cc. To this cold solution were added 45 cc. of molybdate solution (15 cc. of the ammonium molyb- date solution poured into 30 cc. of dilute hydrochloric acid containing 12 cc. of concentrated acid and 18 cc. of water) and the solution stirred. After standing 12 hours, the precipitate was filtered ofif 19 and washed with the ammonium nitrate, nitric acid solution. The precipitate was dissolved in ammonium hydroxide and reprecipi- tated by adding 2 cc. of molybdate solution and 15 cc. of concen- trated nitric acid. The filtrate from the second precipitation was evaporated to a pasty mass, to avoid losing any hydrochloric acid which might be present, taken up with dilute nitric acid, filtered and silver nitrate added to the filtrate. Of four tested qualitatively, two gave precipitates and two none. Of the two which gave precipitates neither was as large as the blank. Another precipitate obtained by adding silver nitrate stood in the light for several weeks without showing any discoloration. The precipitates obtained were un- doubtedly not silver chloride and probably contained molybdenum from the excess molybdate solution added in reprecipitation of the yellow precipitate which is shown by the large blank obtained. A number of authors consider that either the nitric acid or the ammonium radical in ammonium phosphomolybdate or both are held in such unstable combination as to be readily replaced by vari- ous ions used in washing solutions. Artmann (Z. angew. Chem. 26, Aufstatzteil 203 (1913) agrees with Hundeshagen that the two molecules of nitric acid in the mole- cule of ammonium phosphomolybdate can be replaced by two molecules of ammonium nitrate when washing with a neutral solu- tion of that salt. Baxter and Griffin (A. Chem. Jour. 34, 204 (1905) found that the ammonium was replaced by potassium. In determining ammo- nia, in order to decide whether molybdic oxide or ammonium molybdate was occluded on the ammonium phosphomolybdate pre- cipitate, they washed the precipitate with potassium nitrate solution. They state "it was evident that either ammonium phosphomolybdae contains very much less ammonium than was usually supposed, or else the ammonium in the precepitate had been replaced by potas- sium of the washing solution. The latter explanation proved to be the correct one." Still the authors give the formula for the pre- cipitate as (NH 4 ) 2 HPO 4 . 12MoO 3 , only two hydrogens of phos- phoric acid replaced by ammonium, and also decide that ammonium molybdate is occluded. The authors believe that the third hydrogen is replaced upon heating the yellow precipitate to 300C. by ammo- nium from the ammonium nitrate which remains on the precipitate after washing with ammonium nitrate solution. This matter was not considered worthy of investigation in the present research because since we are dealing with a crystalline pre- 20 cipitate such replacement, if it occurs at all, can occur only upon the surface of the crystal particles and is therefore without signi- ficance in a determination of the composition of the yellow precipi- tate. In view of the negative results of the experiments conducted in this research seeking to produce phosphomolybdates containing hydrochloric or sulphuric acid under the most favorable conditions for their formation it is not thought that the reactions claimed by Hundeshagen, Artmann, and Baxter and Griffin occur even at the surface of the precipitated phosphomolybdate. NEW METHOD FOR THE PRECIPITATION OF AMMONIUM PHOSPHOMOLYBDATE The factors which must be considered in determining the con- ditions for complete precipitation of phosphorus and at the same time obtaining a precipitate of constant composition are, (1) the molybdate solution, (2) the temperature of precipitation, (3) the volume and acidity of the solution before precipitation, (4) washing of precipitate. Some authors also believe that the rapidity with which the precipitating solution is added has an effect on the com- position of the precipitate and also the length of time the precipitate is allowed to stand before filtering. Molybdate solutions in great variety have been proposed. Those which have been tested in this investigation have been found unsatisfactory largely because of their instability or erratic be- havior in the precipitation of ammonium phosphomolybdate. In order to overcome these defects in the reagents a radical change has been made in its preparation. By preparing this reagent in two solutions as above described, it is perfectly stable and its use in the manner described leads to definite and consistent results. Further it is sufficiently sensitive to low concentrations of phosphate ion to admit of precipitation in cold solution as a general procedure. How- ever in the determination of phosphorus in steel, the abnormally high concentration of ferric ion interferes with the precipitation of the phosphomolybdate probably due to the formation of complexes of ferric ion and phosphate. In this case therefore it is necessary to heat the mixture to 60 C. and agitate by shaking for a few minutes under which conditions the precipitate readily forms. The recommendations in the literature as to the temperature at which the precipitation of ammoinum phosphomolybdate is to be carried out are as varied as are those concerning the other condi- tions of the procedure. Woy (Chem. Ztg. 21, 441 (1897) and 21 also Kleinmann ' (Biochem. Zeitschr. 99, 19 (1919) precipitate at boiling temperature, Kilgore (J.A.C.S. 16, 765 (1894); 17, 950 (1895) at 60, Wood* (J. Anal, Chem. 1, 138 (1887) at 30-40, Meineke (Chem. Ztg. 20, 108 (1896) at 50-60, Gladding (J.A. C.S. 18, 23 (1896), also Jorgensen (Z. f. Anal. Chem. 46, 370 (1907) at 50, Chesneau (Compt. Rend. 146, 758 (1908) precipi- tates at 65-70 to avoid the formation of ammonium tetramolyb- date. Artmann and Brandis (Z. f. Anal. Chem. 49 f 1, (1910) use 50 cc. of solution, precipitate at 40-50, a higher temperature caus- ing adsorption of molybdenum as ammonium tetramolybdate. Hib- bard- (J. Ind, Eng. Chem. 5, 998 (1913) uses 65 to prevent the separation of molybdic acid. Artmann and Brandis believe that the rapidity with which the molybdate solution is added has a large influence. They add the solution dropwise. Gladding makes a point of adding the solution slowly and stirring. Rapid addition causes adsorption of molybdic acid. Artmann and Brandis also allow the precipitate to stand in contact with the solution for an hour and a quarter only at 40, not longer, in order to avoid adsorption of molybdenum. Washing: Gladding washes with \% nitric acid solution then water. Chesneau, Woy, Jorgensen, and Christensen (Z. f. Anal. Chem. 47, 529 (1908) use an ammonium nitrate, nitric acid solution. Chesneau also uses water. Isbert and Stutzer (Z. f. Anal. Chem. 26, 583 (1887) wash with ice cold water. Artmann and Brandis observed turbidity after washing with water which they claim was white ammonium tetramolybdate and contained no phosphorus. In the present investigation the most satisfactory wash solution was found to be one containing 10 grams of ammonium nitrate and 5 cc. of concentrated nitric acid per liter. A 1 % nitric acid solution can also be used. It was, however, not possible to wash the pre- cipitate free of electrolyte with water or even ice cold water without causing it to pass through the filter. The portion which ran through was not white but yellow. Ice cold water is more satisfactory than water at room temperature. When it is desirous to wash the pre- cipitate free of acid as in the acidimetric method dilute potassium nitrate or potassium chloride solutions proved satisfactory. The following method is proposed for- the precipitaiton of phosphorus as ammonium phosphomolybdate, and for the deter- mination of phosphorus : To a pure phosphate solution containing 20-35 mg. phosphorus in a volume of 120 cc., 5 g. of ammonium nitrate are added. To 22 the cold solution are added rapidly 45 cc. of molybdate solution (15 cc. of solution I poured into 30 cc. of solution II). After stir- ring the solution for a few minutes the precipitate forms and settles rapidly. This is allowed to stand from 4 hours to 24 hours. It is often convenient to filter the day after precipitating. For less phos- phorus less molyddate solution may be used. The precipitate is dissolved in 10 cc. of ammonium hydroxide and the phosphorus precipitated with magnesia mixture under the usual conditions and weighed as magnesium pyrophosphate. The completeness of precipitation of the phosphorus as ammo- nium phosphomolybdate was tested by dissolving the precipitate in ammonium hydroxide and precipitating the phosphorus with mag- nesium acetate solution as described above. P calculated from P calculated from Difference KH 2 P0 4 taken Mg.P.,0, found . 0251 g. " . 0248 g. . 0003 g. .0232 .0229 .0003 .0281 .0281 .0000 .0241 .024CT .0001 .0490 .0489 .0001 .0457 .0457 .0000 Average deviation 0001 To test the effect of chloride and sulphate in solution on the precipitation of phosphorus the following experiments were per- formed : 1. To a pure phosphate solution as above 5 grams of ammonium chloride were added and 45 cc. of molybdate solution (15 cc. of the ammonium molybdate solution I poured into 30 cc. of dilute hydro- chloric acid 4:6). After filtering and washing the precipitate it was dissolved and the phosphorus precipitated with magnesia mix- ture. 2. Same as 1 except that nitric acid was used in the molybdate precipitating reagent. 3. In this case the phosphate solution contained 5 grams of ammonium sulphate and the precipitating reagent was the one de- scribed in this paper and the same as in 2. The following results were obtained: Method P calculated from P calculated from Difference KH,P0 4 Mg 2 P,0 7 obtained 1 .0240g. ' .0239g. .0001 2 .0244 .0243 .0001 3 .0259 .0258 .0001 3 .0240 .0239 .0001 -23- , In view of the great variations in the results of precipitates that were dried it is useless to weigh the ammonium phosphomolybdate precipitate. It is therefore necessary to determine phosphorus in another' way. Hence analyses were made on precipitates without drying them. The precipitates were obtained from known weights of phosphorus as follows: 1. To the pure phosphate solution 5 grams of ammonium ni- trate were added in a volume of 120 cc. To this cold solution 45 cc. of molybdate reagent (15 cc. of solution I poured into 30 cc. of solu- tion II) were added rapidly. After the precipitate stood it was filtered, washed with the ammonium nitrate, nitric acid solution, dissolved in ammonium hydroxide and reprecipitated with nitric acid after adding 2 cc. of ammonium molybdate solution in the same volume as in the first precipitation. After standing again it was filtered and washed as before, then dissolved in ammonium hydrox- ide and analysed as described previously in this paper. 2. Same as 1 except that 5 grams of ammonium chloride were added in place of ammonium nitrate. 3. Same as 1 except that 5 grams of ammonium sulphate were added in place of ammonium nitrate. 4. Same as 1 except that 5 grams of ammonium cholride were added in place of ammonium nitrate and hydrochloric acid was used in the molybdate reagent in place of nitric acid. 5. Same as 1 except that sulphuric acid was used in the molyb- date reagent in place of nitric acid. The following results were obtained : P calculated P calculated Exp. Method from KH.,PO 4 from Mg 2 P,,O 7 Molecular obtained Ratio 1 1 .0280g. .0277g. 1:24.61 2 1 .0233 .0230 1:24.63 3 2 .0239 .0237 1:24.43 4 2 .0241 .0238 1:24.64 5 3 .0354 .0351 1:24.35 6 4 .0229 .0228 1:24.41 7 4 .0239 .0237 1:24.54 8 5 .0234 .0232 1:24.33 9 5 .0243 .0240 1:24.54 Mean molecular ratio 1:24.50 24 These experiments show that the presence of chloride or sul- phate in solution has no effect on the completeness of the precipita- tion of phosphorus and also that the yellow precipitate obtained under these conditions is one in which the ratio of phosphorus pentoxide to molybdic oxide is constant. The average deviation of the above ratios is 5 parts per 1000. The results, however, do not agree with the generally accepted formula where the ratio of phos- phorus pentoxide to molybdic oxide is 1 : 24. THE VOLUMETRIC METHOD By referring to the introduction it will be seen that the acidi- metric titration method for the determination of phosphorus has always been surrounded by considerable uncertainty. These uncer- tainties involve the choice of factor which of course depends upon the composition of the ammonium phosphomolybdate precipitate, whether or not the ammonia is to be removed before making the titration and the choice of the indicator for the determination of the end-point. Furthermore, when phenolphthalein, which is the indicator most generally proposed, is used it is always a difficult matter to decide when the end-point is reached as the change in the indicator is very gradual. In order to proceed intelligently in establishing the conditions for the acidimetric titration, electrometric titrations were made on solutions containing 5 mg. of phosphorus. The precipitate was dis- solved in a known volume of standard sodium hydroxide, free of carbonate, placed in a cell and the hydrogen ion concentration measured at 25 C. by the Saturated Potassium Chloride Calomel Cell Method as described by Fales and Mudge (J.A.C.S. 42, 2434 (1920). After equilibrium was established, 0.25M sulphuric acid was added in successive portions and the potentiometer read after stir- ring the solution. About five minutes elapsed between readings. It was not possible to wait for the solution to come to equilibrium after each addition of acid, for the voltage changed continuously for sev- eral hours and the solution finally turned blue. The curves given below were obtained from the following data by plotting cubic centimeters of acid added as abscissa and voltages as ordinates. These titrations were repeated on similar solutions from which the ammonia had been removed, before titration, by boiling in a current of hydrogen. (See curve II). 25 Data for Curve I (Solution contained 5 mg. of Phosphorus, titration made in presence of the ammonia.) cc. acid added voltage cc. acid added voltage 0.0 .9605 4.0 .8269 0.5 .9540 4.2 .7979 1.0 .9490 4.4 .7628 1.5 .9436 4.6 .7019 2.0 .9374 4.8 .6630 2.5 .9290 5.0 .6395 3.0 .9160 5.2 .6275 3.2 .9088 5.4 .6196 3.4 .8986 5.6 .6110 3.6 .8832 6.0 .5919 3.8 .8574 Data for Curve II (Solution contained 5 mg. of Phosphorus, titration made after removing the ammonia.) cc. acid added voltage cc. acid added voltage 0.0 .9440 3.0 .7932 0.4 .9372 3.2 .7530 1.0 .9253 3.4 .6871 1.5 .9102 3.6 .6607 2.0 .8823 3.8 .6458 2.2 .8654 4.0 .6341 2.4 .8488 4.2 .6285 2.6 .8322 4.4 .6203 2.8 .8150 4.8 .6020 Data for Curve III (Solution contained very little phosphorus.) cc. acid added voltage cc. acid added voltage 0.0 .9080 1.6 .6713 0.2 .8995 1.8 .6548 0.4 .8872 2.0 .6367 0.6 .8710 2.2 .6229 0.8 .8480 2.4 .6120 1.0 .8225 2.6 .6040 1.2 .7857 2.8 .5944 1.4 .7027 3.0 .5356 None of the curves shows a sufficiently pronounced point of inflection to yield a satisfactory color change in the indicator except in the case of Curve III. The rather indefinite point of inflection does occur in the phenolphthalein range and therefore shows that this is the best indicator for this titration. By comparing the curves made in the presence (Curve I) and absence of ammonia (Curve II), 26 it is seen that there is no advantage in its removal. The unsatisfac- tory form of the titration curve is certainly due to the presence of phosphoric acid. In the case of Curve III where the solution con- V0 5" 6 JL'TO A tained very little phosphorus a more definite point of inflection was obtained showing that the method is more satisfactory for very small amounts of phosphorus. That the difficulties encountered in titration were really due to the presence of phosphoric acid rather than ammonia, as is generally 27 assumed, was suggested by Fairchild (J. Ind. Chem. Eng. 4, 520 (1912), who proposed its removal by precipitation with barium. It should also be noted that Hibbard (J. Ind. Eng. Chem. 5, 998 (1913) attributed difficulties to the presence of both phosphoric acid and ammonia. As a result of this work it has been shown that the determina- tion of phosphorus in quantities of 10 mg. or over is best accom- plished by precipitation with the ammonium molybdate solution which has been described and under the conditions which have been given. The precipitate obtained is dissolved and the phosphorus precipitated by magnesia mixture and weighed as Mg 2 P 2 O 7 . For quantities of phosphorus less than 10 mg. the weighing of such small quantities of Mg 2 P 2 O 7 becomes inaccurate and the use of the acidi- metric method is therefore advocated. SUMMARY 1. A new determination of the ratio of phosphorus to molyb- denum in the yellow phosphomolybdate precipitate has been made by a new method of analysis. 2. It has been shown that the composition of the ammonium phosphomolybdate precipitate after drying is so variable as to render the gravimetric determination of phosphorus by this method most unsatisfactory. 3. A new method of the precipitation of phosphorus as ammo- nium phosphomolybdate has been devised. When precipitated by this method the yellow phosphomolybdate precipitate as formed in solution shows a constant ratio of phosphorus pentoxide to molybdic oxide which is 1 : 24.50 with a mean deviation of 5 parts per 1000. 4. It has been shown that this ratio remains constant even when large concentrations of chloride or sulphate are present in the solu- tion from which the phosphomolybdate forms. 5. It has been shown that under the conditions given for the formation of ammonium phosphomolybdate no complexes with sul- phates or chlorides are produced. 6. Electrometric titrations of yellow phosphomolybdate precipi- tates have been made. The curves plotted from these results have not thus far appeared in the literature and have furnished a rational method for the acidimetric estimation of phosphorus with a deter- mination of its limitations. 7. Accurate methods for the determination of phosphorus have been described with the limitations under which they can be used. FIT A. Norma E. Johann was born in New York City, January 9, 1893. In 1914 she received the degree of A.B. from Hunter College where she taught in the Department of Chemistry two years following graduation. She has held the position of Assistant in Chemistry at Columbia University since 1917. She was a graduate student in the Department of Chemistry, Columbia University, during the sum- mers of 1915 and 1918 and during the academic years 1914-15, 1915-16, 1916-17, 1917-18, 1918-19, 1919-20, and 1920-21. In 1917 she received the degree of A.M. 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