PHO.il'l~ORIt: ACID AS MAGNESIC PYROPHOSPHATE
XIX.--OS
135
TIIE ES'L'IJIATION O F PHOSYIIORIC ACID AS MAGN FXT(' PYROPHOSPHATE. l < Y T H O M A S S. (;LADDIN(%,
In a valuable paper on the determination of phosphoric acid as magneqio y)yropliosphate ( - 1NPY. Clterr~..7ov?*?iul,vol. 1, 391 j G O O C , ~ has demolistrated t,hat this niethod, far from being as accurate as is generally siipposctl, i s peculiarly liable to s ~ u r i ' e sof error, which cause a consider:ible overestimatioii of the pliosplioric acid actually pres71it. The great import nnce of this analytical determination from a cominei*ci:il point of view, gives enhanced interest to any inqniry as to its avciirate csecution. In view of the vast cal'ital at present engaged iri tlie cwnversioii of p1iosl)Iiatic mnteria1s into artificial fertilizers, i i i i t l iri view of the f a c t tlt;it vhemists freqnently differ SO widely i t 1 this importniit tletc.rniiiiiititiii, the necessity, both for t h e interests of cwnini~weand the cwdit of seieiice, of a11 actwrate and rigid metliotl can 1a, vlcarly iiiiderstood. 111 tlie case of large and valiiable v a i y ~ sw I ( 1 upon c~lieiiiicalaiinlysis, tlie slight error of even & of :L per eelit. i l l the cstiniatioii of pliosplioric acid causes a differerioe i i r valiir? ranging fro111t,\venty-tive i i p to two hundred dol Inrs. I Iinve heeii led by these eoiisillei.ixtioits to a carefol examination of t h e inodes o f proc+edureat preseiit, reconinieiicletl by the best authorities and \vliicli :ire in cuinnion use among cheniists. I n the latrst Aiiioricaii edition of / ~ ' w . ~ e i t i u sQ' ~ n td.n a l y s i s the diwctioiis are, to :idd to the nent.ral o r inocleixtcly aninionical solution of tlic pliospli:ite, inngnesia mixture in slight excess, and a f t e r letting stand for some time, its volume of sti'oiig nnimonia Sotu. tion. I n tlie case of a previous precipi tation as ,l)hospho-molybdate of ammonia, this precipitate is dissolved in weak ammonia and tlie tieiitral or sliglitly amrnoniacd solution, t,i*eatedas above. T h e method recoinmended by tlic convention of chemists of the different experimental stations of Gerinaiiy (Iietti in Dec. 1581) is translated as follows : (Zeit. fur. Anal. Chem. 21, p. 290) "The beaker glass is now placed undcr the fiinnel, the tilter pierced with a platiriiini wire, the precipitiite washed into the beaker glass wiili %$ per cent. ammonia liquor, and dissolved by stirritlg with a glass rod, and then eriongh of the 2+ per cent. ammonia liquor added, to bring the volume of the liquid up to about 75 cc. To 0.1 gram phosphoric wid, 10 cc. m:ignesia niixtuw ai'e now dropped in ac-
136
PHOSl’tiORIC ACI1) AS MAGNRSIC PYROPHOSPHATE.
companied by a constant d r r i n g up.” I n a foot note is added “ a g1adual addition of the magnesia mixture is advisable under all circumstances even when the ammoniacal solution of the molybdate preaipitate is previously nearly neutralized by addition of hydro chloric acid.” This mode of procedure differs from that given in “Fresenius ” princiyilly i i i the one point of imistiiig oi( ( 1 9 trd/(az addition of tlie magnesia mixture. In both methods a strongly ammoniacal magnesia mixture is employed and this is added to a more or less strongly ammoniacal solution of the phosphate. On the other hand the nietliod giving the Lest results in the hands of Gooch, consisted in the addition of a ueutrul magnesia mixture t o a neutrul solution of the phosphate or still better, especially in the case of a previous precipitation as molybdic salt, in a double pr.ecipitation of the animoriia-triagnesin-pliospliat~~. I n testing the above methods great (:nw was exercised in the preparation of the standard soliitions. These were made by the uye of chemically pure niicrocosniic Ralt Upon careful ignition of this salt in a covered platinum capsule 5.1415 g r a m gave 2.5110 grams of phospliat,. of soda as residue or 34.00 per cent. of phosplioric acid against 33.38 per cent. required by theory. With this salt, two solutions were prepared, tho one cotttaiiring4.121 grams of tlie salt in a liter or 0.035 grams P2 0 6 in every 25 cc., the other 20.605 grams to a liter or 0.175 grams of P, 06 in every 25 cc. 25 cc. were measured off with great accuracy by means of a pipette holding exactly 25 cc. by cat.eful weight test, when tilled to the mark. l t i i s was tticbn carefully rinsed out. The small neck Jf the pipette, ntakeb it easy to measure a liqnid to within -& cc b y t h i n method. In addition to the experiinents made with these solutions, others were made by direct weighings of the microcosmic salt.
METHOD I. To the solution of the phosphate, diluted to 50 cc. and made slightly tsZAcrZine by the addition of 3 cc. of ammonia liquor, the regular animonincal niagnesiutn chloride mixture was, added, 4 cc. t o every 25 cc. of solution I. and 18 co. to every 25 cc. of solution 11. After letting stand for 15 minutes, 25 cc. of strong ammonia solution were added, and the liquid tiltered off after about 30 miutes. T h e wacliing was performed with strong ammonia fiolution and was continued until 10 dvops of t h e washings gave no precipitate with acid of silver nitrate when acidified. I n these and all subsequent experiments the filter papers were each weighed, that
137
PIIOSPHORIC ACID AS MASNEOIC PYROPHOSPHATE.
an accurate weight allowance m i g i t be made for filter ash, and the precipitate was strongly ignited over a blast lamp for several min. utes, until white. T W ~ Heries J of experiments were made, the one without a previous precipitation as phospho-molybdate of nrnmoliia, the other with such previous precipitation. A. Without previoris precip. as molybdate salt. Amount taken. Mg,P,O, fouud.
1 25 cc. YOl. 2 "
I.
