194 Gooch and Austin — Constitution of the 



ammonia comes to view. The results discussed seem certainly 

 to point to a general tendency on the part of free ammonia, 

 ammonium chloride and excess of the phosphate to produce a 

 salt rich in ammonia and deficient in magnesium, which for a 

 definite amount of magnesia precipitated must leave upon 

 ignition a residue weighing more than the normal phosphate. 

 "If it be assumed that a salt of the symbol (NH 4 ) 4 "Mg(P0 4 ) 2 — 

 the next natural step to the normal salt, NH 4 MgP0 4 — is present 

 in the precipitate, the residue which such a salt would leave 

 upon ignition would be the metaphosphate Mg(P0 3 ) 2 . From 

 the relations of the symbols for magnesium pyrophosphate 

 and magnesium metaphosphate the weight of the residue 

 obtained, and the weight of the pyrophosphate theoretically 

 derivable from the weight of magnesium salt used, it is possi- 

 ble, of course, to calculate the proportionate amounts of pyro- 

 phosphate and metaphosphate present in any ignited residue. 

 Proceeding in this manner, it appears, that, in order to account 

 for the variations noted, it is necessary to assume the presence 

 in many cases of very considerable amounts of the metaphos- 

 phate. Thus, in the case of those results obtained according 

 to the usually accepted method of precipitating and washing 

 with strongly ammoniacal liquids, viz. in experiments (1) and 

 (2) of Table IV, the proportion of metaphosphate needed to 

 account for the observed error reaches ten per cent. 



Precipitation by Excess of the Magnesium Salt. 



The relations which obtain in the reverse process of precipi- 

 tation — the action of an excess of the magnesium salt upon a 

 soluble phosphate — were studied in experiments to be described. 

 A solution of pure hydrogen disodium phosphate was prepared 

 by carefully recrystallizing the pure salt of commerce five 

 times from distilled water in a platinum dish, dissolving the 

 crystals, and diluting to definite volume. The standard of the 

 solution was established by evaporating to dryness in a weighed 

 platinum crucible known volumes of the solution, igniting the 

 residue and weighing the sodium pyrophosphate. Magnesia 

 mixture, the precipitant, was prepared by dissolving fifty-five 

 grams of magnesium chloride in as little water as possible and 

 filtering, mixing with this solution twenty-eight grams of 

 ammonium chloride purified by treating it in strong solution 

 with bromine water and a slight excess of ammonia, filtering, 

 diluting to one liter, and, after standing for some hours, filter- 

 ing again. 



The tests of the following table show that the precipitation 

 of a soluble phosphate by the magnesia mixture is practically 

 complete in faintly ammoniacal solutions even when very 

 dilute and charged with large amounts of ammonium chloride, 



