adding 6 molar ammonium hydroxide until the precipitate of calcium phosphate first produced fails to redissolve, and then adding a few drops of nitric acid until the solution clears up.

Warm the acidulated solution to 40° and add 90 c.c. molybdate reagent (30 c.c. solution I poured into 60 c.c. solution II, § 188), set aside for at least four hours or more, preferably overnight. After digestion, filter off the supernatant liquid through a 7 cm. filter and test it with molybdate reagent to see if precipitation has been complete.24 Granting that complete precipitation has taken place, retain as much of the ammonium phospho-molybdate as possible in its original beaker and wash by decantation (using as a wash solution one which is 0.1 molar with respect to nitric acid and 0.1 molar with respect to ammonium nitrate) until the washings give no turbidity when tested for calcium with ammonia and ammonium oxalate. When washing is complete place the beaker containing the main bulk of the precipitate under the filter paper through which filtration has been effected, and pour over the filter paper several 10 c.c. portions of 2.5 molar ammonium hydroxide to dissolve any precipitate that might have been collected, then wash the filter paper carefully with five 10 c.c. portions of hot water. The final volume of filtrate and washings must not exceed 100 c.c. By this treatment both the precipitate on the filter paper as well as the main portion in the beaker should be dissolved; if, however, the latter remains undissolved, add more ammonia cautiously until solution is effected.

To the clear ammoniacal solution add 3 molar hydrochloric acid cautiously until the solution is slightly acid to litmus paper, then add dropwise an amount of magnesia mixture calculated to be fifty percent in excess over that demanded by the phosphorus present (see § 189); next add cautiously and with constant stirring an amount of 15 molar ammonium hydroxide which will give a final concentration of ammonium hydroxide of about 1.5 molar. The solution is then allowed to stand at room temperature for at least four hours (twenty hours is preferable).

As pointed out in § 189, the precipitate of magnesium ammonium phosphate so obtained will always be contaminated some

24 If precipitation has not been complete rejoin filtrate with the precipitate, add more molybdate reagent and set aside for further precipitation.

what with molybdenum so that for precise work it is necessary to resort to reprecipitation in order to obtain a pure precipitate of MgNH4PO4.6 H2O. The details of this part of the procedure will be found in § 189, and should be followed accordingly.

From the weight of MgNH PO4·6 H2O or Mg2P2O7 found, calculate the percentage of P2O5 in the sample.

195. Exercise No. 17. Determination of Silica, Sulphur and Phosphorus in an Iron Ore. The constituents that are most frequently called for in the analysis of an iron ore are iron, silica, sulphur and phosphorus.25 The iron is most easily determined in a small separate portion, while a much larger portion is taken for the silica, sulphur and phosphorus. The ores of iron which are usually encountered are:

Procedure. It is presupposed that the sample furnished for analysis has been carefully taken in accordance with the principles of sampling, and that it has been ground to a fineness of 100 mesh, as both these steps are essential preliminaries to the analysis. A determination of moisture in the samples should be made and included in the report, and the temperature at which the moisture is driven off should also be stated (105°-110°). The percentages of the several constituents should be calculated to the dry basis and so reported.

Weigh accurately about 5 grams of the ore and transfer to a porcelain casserole. Cover the casserole with a watch-glass and add about 10 c.c. of 16 M HNO, and 50 c.c. of 12 M HCl. Digest on the hot-plate for about 15 minutes. Dilute with about 100 c.c. of water and digest again for about 15 minutes. Filter while hot, collecting the filtrate in another casserole. Wash a few times with hot water and transfer the residue to the filter. Place the casserole containing the filtrate on the hot-plate and concentrate by evaporation. Place the filter in a platinum cru

25 Occasionally in addition to these some or all of the following are required: aluminum, titanium, manganese, calcium and magnesium. For methods dealing with the determination of these elements in iron ores, see U. S. Steel Corporation, Methods for the Commercial Sampling and Analysis of Iron Ores, § 13; also Fay, p. 15, § 13.

cible and burn the paper at low temperature. (Care must be taken to avoid loss of sulphur.) Add anhydrous sodium carbonate (five or six times the weight of the residue) and about 0.2 gram of sodium nitrate, mix with a dry glass rod and fuse for about a half hour with the usual precautions. While the crucible is cooling, spread the contents over the sides and bottom as previously described and when cold place in a small beaker, cover with a watch-glass and add hot water. After digesting for a short time, add in small portions about 20 c.c. of 12 M HCl and digest until the fused mass is detached from the crucible. Remove the crucible, washing the solution and silica from it into the beaker. When the fused mass is decomposed, transfer the solution and silica from the beaker to the casserole containing the main solution and continue the evaporation to dryness. The temperature at which this evaporation takes place must not exceed 120°. Remove the casserole from the hot-plate and moisten the residue with 10 c.c. of 12 M HCl and then add about 50 c.c. of boiling water. Digest on the hot-plate for a few minutes and then decant the solution through a small filter. Again treat the residue with acid and water and repeat as long as the residue contains iron as shown by its color. Finally transfer the residue to the filter, removing the residue from the casserole with a rubber-capped rod in the usual way. Wash thoroughly with hot water. From 1 to 3% of the total silica present will pass into this filtrate and hence at this point it must be estimated whether a second evaporation of the filtrate is necessary. If the amount of silica present warrants this second evaporation, it is carried out exactly as the first and the filters containing the silica are then placed in an unweighed platinum crucible and the ignition to constant weight carried out as usual, using a blast or Meker burner to complete the ignition. Moisten the residue with water and add 2 drops of 9 M H2SO4 and then about 5 c.c. of conc. HF solution. Evaporate to dryness under the hood. Ignite, first with the Bunsen and finally with blast or Meker burner and weigh. If more than a very slight residue remains, repeat the treatment with H2SO4 and HF and ignition to constant weight. From the loss in weight calculate the percentage of silica in the ore.

The residue in the platinum crucible after the removal of the silica may contain, among other compounds, barium sulphate and titanium phosphate. Add about one gram of anhydrous sodium carbonate and fuse. Leach the fused mass with hot water and filter through a small paper. Wash with hot water. Reject the residue. Add the filtrate to the main solution containing the sulphate and phosphate. Pour this main solution, cooled to room temperature, into an excess of ammonium hydroxide solution (20 c.c. 15 M NH4OH diluted with water to 400 c.c.). Heat to boiling, filter and wash with hot water until the filtrate shows only a slight test for chloride. Concentrate the filtrate to a volume of 150 to 200 c.c., make very slightly acid with hydrochloric acid and determine the sulphate as barium sulphate exactly as described in § 185. From the result thus obtained, calculate the percentage of sulphur in the ore.

The precipitate of ferric hydroxide, etc., contains all the phosphorus of the ore as ferric phosphate. Transfer as much of this precipitate as possible to a 250 c.c. beaker by means of a small porcelain spoon or spatula. Place a clean beaker under the funnel and dissolve the precipitate remaining on the filter in hot 3 M HNO3. If the precipitate does not dissolve in a moderate amount of the acid, replace the beaker under the funnel by a clean beaker, heat the solution which has run through the filter and return it through the filter again and again till all the precipitate has dissolved from the filter. Wash the filter with water until most of the iron has been removed. Pour this acid solution into the beaker containing the major part of the ferric hydroxide precipitate. Wash out the beaker and add the washings to the main solution and then heat until the precipitate is completely dissolved. If necessary add more acid, but a large excess should be avoided. The volume of this solution should not exceed 150 c.c. If greater than this, concentrate by evaporation. Carefully neutralize this solution with ammonium hydroxide until a faint but permanent precipitate forms, then add about 5 or 10 c.c. 16 molar nitric acid until the deep red color of the solution changes to an orange yellow; after this change add 10 grams ammonium nitrate. Heat the solution to about 40° and add 90 c.c. molybdate reagent (30 c.c. solution I poured into

60 c.c. solution II, § 188); then set aside for 14-24 hours. Filter the "yellow precipitate" through a 9 cm. filter and wash with a solution consisting of 0.5 molar ammonium nitrate and 0.5 molar nitric acid. (This solution is most easily prepared by mixing 3 molar nitric acid and 3 molar ammonium hydroxide in the proportion of two moles of the former to one of the latter and diluting down to the desired molar concentration.) Wash the precipitate and filter until the successive portions of the washings show only a slight brown color when treated with hydrogen sulphide. Notice that it is not necessary to transfer the precipitate from the filter to the beaker but merely to wash it thoroughly. When this has been done, place the beaker in which the precipitation was made under the funnel and pour hot 3 molar ammonium hydroxide through the filter until the precipitate has dissolved. Wash the filter with water until the total volume of the solution is about 100 c.c. Now add 12 molar hydrochloric acid to the solution until the latter is faintly acid and cool to room temperature (18°-25°). To this slightly acid solution add dropwise an amount of "magnesia mixture" calculated to be fifty percent in excess over that demanded by the phosphorus present (see § 189). Next add cautiously, and with constant stirring, an amount of 15 molar ammonium hydroxide which will give a final concentration of ammonium hydroxide of about 1.5 molar. Allow the solution to stand at room temperature for at least four hours (twenty hours is preferable). Filter the precipitate through a weighed Gooch crucible and wash with 1.5 molar ammonium hydroxide until free of chlorides,26 then with three or four 15 c.c. portions of 95% alcohol, and then with two 15 c.c. portions of ether; suck dry at filter pump for ten minutes, place in desiccator for twenty minutes and weigh as MgNH4PO4 6 H2O. From the weight of the hexahydrate calculate the percentage of phosphorus in the ore.

196. Use of Factor Values in Gravimetric Determinations. By an application of the principle developed in § 125 it is possible to choose such a weight of sample for analysis that each milligram of precipitate shall correspond to a definite percentage of constituent in the sample. Where a series of determinations of the * If desired, the precipitate after being washed free of chlorides can be ignited to Mg2P207.