Ammonium chloride materially increases the solubility of magnesium ammonium phosphate even in the presence of considerable concentrations of ammonium hydroxide, but exact figures on this score seem to be lacking. While the presence of ammonium chloride is necessary during the precipitation of magnesium ammonium phosphate to prevent the precipitation of magnesium hydroxide, its concentration should not much exceed 0.4 molar.

As regards its stability the hexahydrate does not suffer any change in weight at room temperatures (18°-30°) and under ordinary laboratory atmospheric conditions; if placed in a desiccator over anhydrous calcium chloride, it will lose weight at the rate of about 1 mg. in 24 hours for each gram of sample, the temperature being 18°-30°. At 50° in a drying oven the loss in weight is very much greater, amounting to about 28 mg. in 24 hours for each gram of sample.

Advantage can be taken of the foregoing behavior of MgNH4 PO4.6 H2O as a basis for weighing it in this form, and for such purpose the following procedure has been found entirely satisfactory and recommendable.16 The precipitate of MgNH1PO4·6 H2O is collected on a Gooch crucible and washed free of salts, with 1.5 M NH4OH; it is then treated two or three times with 15 c.c. portions of 95% alcohol and finally twice with 15 c.c. portions of ether;17 after this it is sucked dry at the filter pump for ten minutes. The Gooch crucible is then allowed to stand around in the air for fifteen or twenty minutes, or placed in a desiccator for a no greater length of time and then weighed at once as MgNH4PO4·6 H2O.

If it is desired to effect the onversion of the hexahydrate to the pyrophosphate, according to the reaction:

2 MgNH PO4-6 H2O + A→ Mg2P2O7 + 2 NH3 + 7 H2O

MgNH4PO4·6

a temperature of 750° and prolonged heating of an hour or so are necessary. In practice it is customary to employ higher temperatures (1100°-1300°) and a shorter time for this conversion, making use of a Meker burner or a blast lamp for thirty

16 Dissertation of William A. Worsham, Jr., A Study of the Stability of Magnesium Ammonium Phosphate (Mg NHẬPO4‍6 H2O) and a Method of Determining Phosphorus as Magnesium Ammonium Phosphate (Mg NH4PO4‍6 H2O), Columbia University (1923).

17 A good grade of ether with the usual 3% alcohol in it is perfectly satisfactory.

minutes. The heating is always repeated until constant weight is obtained.

There is one point in particular about the conversion of the hexahydrate (or the monohydrate) to the pyrophosphate which deserves mention.

192. "Black Precipitate." If magnesium ammonium phosphate has been collected on a filter paper, then in the subsequent ignition of the filter paper some of the resulting carbon seems to become enclosed by the precipitate and it is often a very difficult matter to burn off this carbon. It seems further that this carbon exerts a reducing effect on the magnesium pyrophosphate, giving rise to the so-called "black precipitate" which, once it has been obtained, baffles all attempts to make it white again, either by prolonged ignition or treatment with oxidizing agents like nitric acid or ammonium nitrate. Undoubtedly there is some error introduced by accepting the results based upon "black precipitates" of magnesium pyrophosphate, but it is generally believed that the error is not large and it has been more or less the custom to accept such results. One thing, however, is imperative with respect to the ignition of magnesium pyrophosphate; namely, consecutive weighings, each preceded by at least thirty minutes strong ignition, should agree within 0.2 mg. If the ignited precipitate continually loses weight, it indicates the presence of magnesium meta-phosphate Mg(PO3)2 which is losing P2O5 by volatilization,

2 Mg(PO3)2 + A→ Mg2P2O7 + P2O5

in which event the results are valueless. Owing to the high temperatures which must be used in the ignition of magnesium pyrophosphate, it is not advisable to use porcelain crucibles, as these are likely to crack. This makes the choice fall to the use of platinum crucibles, but great care must be employed in using them for this purpose to insure that they are kept entirely out of the reducing zone of the flame, as otherwise some of the pyrophosphate will be reduced to phosphorus with consequent ruination of the crucible.

Precision. The precision which is obtainable for small amounts of phosphorus up to 2-3 mg. by precipitating phosphorus as

ammonium phospho-molybdate and then titrating the precipitate alkalimetrically is probably not better than 5 parts per 100 because of the small quantities of phosphorus involved and the further fact that the end point of the titration is somewhat uncertain. The precision for larger amounts of phosphorus from 3 to 50 mg. where the phosphorus is first precipitated as ammonium phospho-molybdate and then reprecipitated as magnesium ammonium phosphate depends almost entirely on the second part of the method because in the first part of the determination the phosphorus is quantitatively precipitated, generally speaking.18 Assuming due care in the precipitation of the magnesium ammonium phosphate hexahydrate MgNH,PO4·6 H2O and weighing it as such, we can probably claim a precision of 5 to 20 parts per 1000 for amounts of phosphorus ranging between 3 and 50 mg., provided of course that the magnesium ammonium phosphate has been redissolved and reprecipitated to free it of the molybdate which is dragged down the first time.

193. Exercise No. 15. Determination of Phosphorus in Plain Carbon Steels or Iron.19 Alkalimetric Method. - The content of phosphorus to be expected in this class of products is a maximum of 0.05% for good grades of steels and 0.5 to 0.9% for pig and cast irons. The phosphorus in iron or steel exists as a phosphide so that the first step in analysis is the oxidation of the phosphorus to ortho-phosphate ion. This oxidation is best accomplished by dissolving the sample in 6 molar nitric acid, and as soon as solution is effected by adding 10 c.c. or so of 0.1 molar potassium permanganate and boiling to effect the oxidation of the phosphorus.

Method. Weigh out 2 grams of steel drillings or 1-2 grams of pig iron, place in a 400 c.c. beaker and add 50 c.c. 6 molar nitric acid, keeping the beaker covered with a watch-glass and warming gently until solution is completed (with pig irons some undissolved graphite will be left at this juncture and it is preferable to let it stay for the time being). Add 10 c.c. 0.1 molar potas

18 In the case of steels where the amount of phosphorus is very small and the amount of iron very large, it appears that phosphorus is not always quantitatively precipitated as ammonium phospho-molybdate, probably due to the fact that some of it is tied up as the complex or undissociated ferric phosphate.

19 Special Alloy Steels containing titanium, tungsten, vanadium require special treatment because of the presence of these interfering substances, see § 187.

sium permanganate and boil until the pink color disappears; the excess of permanganate is reduced by this procedure and the manganese precipitates out as hydrated manganese dioxide; this precipitate is dissolved by adding sodium sulphite solution or ferrous sulphate solution in small portions until the solution clears up. At this point with pig iron any undissolved graphite should be filtered off so as to have a clear solution for the precipitation of the ammonium phospho-molybdate. Cool the solution to about 50° and neutralize with 6 molar ammonium hydroxide until the solution is deep red in color (usually about 40 c.c. will be required); then add 3 molar nitric acid until the red color changes to orange. Adjusting the temperature of the solution to approximately 40°, add 30 c.c. molybdate reagent (10 c.c. solution I poured into 20 c.c. solution II, § 188) and set aside to digest for overnight or longer if no precipitate has appeared by morning.20 Filter on a 9 cm. filter paper and wash with 0.1 molar nitric acid until free of iron (thiocyanate test) and then with 0.1 molar potassium nitrate until free of acid (methyl orange test). Place the filter paper and its precipitate in a glass-stoppered Erlenmeyer flask and add 20 c.c. 0.1 molar standard sodium hydroxide solution, shake violently to disintegrate the filter paper and hasten the solution of the ammonium phospho-molybdate. If after agitation any of the precipitate remains undissolved, add 10 c.c. more of the standard sodium hydroxide solution and shake again. When the precipitate is dissolved add 50 c.c. water and 3 drops phenolphthalein solution and titrate back the excess of sodium hydroxide with 0.1 molar standard nitric acid. From the amount of sodium hydroxide used up in taking care of the precipitate according to the reaction 2(NH4)3PO412 M0O3 + 46 NaOH → 2(NH4)2HPO4 + (NH4)2M0O4 + 23 Na2MoO4 + 22 H2O calculate the amount of phosphorus.21

194. Exercise No. 16. Determination of Phosphoric Anhydride in Apatite. Apatite is a phosphate rock of the essential composition (CaF) Ca4(PO4)3 in which some of the calcium is usually

20 Some authors recommend shaking the solution violently for four to five minutes after the addition of the molybdate reagent and then filtering within thirty minutes, but it seems that about five percent of the phosphorus remains in solution by this procedure.

21 From the reaction given 1 P 23 NaOH; hence verify that 1 c.c. 0.1 M NaOH = 0.0001349 g. P.

replaced by small amounts of magnesium, iron, or aluminum, some of the fluorine by chlorine, and some of the phosphate by small amounts of silicate. Its theoretical composition as well as the composition of an actual specimen is given herewith:

As already mentioned in § 187 et seq., any silicic acid that is present must first be removed and then the phosphate precipitated as ammonium phospho-molybdate, in order to separate it from the elements which form insoluble phosphates, i.e., calcium, magnesium, iron, aluminum, etc.

Procedure. Assuming that the sample has been properly prepared with respect to its homogeneity (§ 34) and ground to a fineness of 50-70 mesh, weigh out into a beaker or casserole 0.180-0.200 g., add 15 c.c. 6 molar nitric acid, cover the vessel with a watch-glass and warm gently until solution has been effected; then evaporate to dryness on the water bath (see § 183) and maintain at 100°-115° for about an hour to dehydrate the silicic acid. Add 25 c.c. 6 molar nitric acid and heat for several minutes to dissolve the soluble portion, filter off the silica on a 7 cm. filter paper and wash with as little warm water as possible. After six washings test the next washing for the presence of calcium by collecting 2-3 c.c. of it in a test tube and making alkaline with ammonia, a turbidity indicates presence of calcium owing to the formation of calcium phosphate. If calcium is found present join contents of test tube with the main filtrate and continue washing until washings are free of calcium, joining all tests with main filtrate. The final volume of filtrate and washings must not exceed 100 c.c. After washing, the next step is to neutralize most of the excess of nitric acid in the filtrate by cautiously

22 Dana, loc. cit., § 3.

23 Hoffman, Report Geological Survey, Canada, 1 H, 1879.