IIILLEBRAND.) MAGNESIUM. 65 
precipitation is made. These points are only of importance when a 
single precipitation is to be made or in the final of two or more, as will 
be discussed later. 
Platinum sulphide usually strongly contaminates the separated 
phosphate, but this matters not, as it remains on the filter when the 
phosphate is redissolved in hydrochloric acid, of which not more than 
the amount really needed should be used. The solution thus obtained 
is united with that of the residue from evaporation and ignition of the 
second filtrate from calcium oxalate, and is diluted if accessary. A 
few drops of sodium-ammonium phosphate solution are now added, 
and ammonia in slight excess, with constant stirring till the crystalline 
precipitate has well formed. The large excess of ammonia of 0.96 
specific gravity (one-third the original volume) usually prescribed has 
been shown by Gooch and Austin 1 to be quite unnecessary, in fact, 
disadvantageous. 
Neubauer in the above-cited paper has shown, and Gooch and Austin 
(loc. cit.) have confirmed his statements, that it is only by working 
under these conditions — absence of any large excess of precipitant, of 
ammoniacal salts, and of ammonia — that a precipitate of normal com- 
position is obtainable. It usually differs from the normal in contain- 
ing relatively more ammonium and less magnesium— for instance, an 
admixture of such a molecule as Mg(NH 4 ) 4 (P0 4 ) 2 — the result being 
that when ignited in the ordinary way too much magnesium is found, 
because of formation of some metaphosphate. To obviate this error 
Neubauer considers it absolutely necessary to blast the precipitate for 
half an hour, and then to repeat the blasting for a second half hour to 
see if a constant weight has been reached. The phosphate is then 
entirely pyrophosphate, which is quite unaffected by further Masting. 
The intense heat has caused a decomposition of the metaphosphate 
with volatilization of P a 5 , as follows: 2 Mg(P0 3 ) 2 = Mg 2 P a 7 +PA. 
Neubauer worked with the usual excess of ammonia, and it remains 
to be seen whether by precipitating and working according to Gooch 
and Austin the composition of the precipitate is always close enough to 
the ideal MgNH 4 P0 4 to obviate the necessity for blasting. 
From the labors of Neubauer and of Gooch and Austin it is clear that 
the common way of adding the phosphate precipitant to the ammoniacal 
solution of the magnesium salt is not calculated to produce a precipi- 
tate of normal composition. The precipitant should be added to the 
acid solution of the magnesium, and ammonia should then be added in 
slight excess. 
Gooch and Austin call attention to a modification proposed some 
years ago by Wolcott Gibbs, 2 whereby the phosphorus and magnesium 
iAm.Jour.Sci., 4th series, Vol. VII, p. 187, 1899; Chemical News, Vol. LXXIX, pp.233, I I 
Zeitschr. fur anorg. Chemie, Vol. XX, p. 121 , 1899. 
2 Am. Jour. Sci., 3d series, Vol. V, p. 114, 1873. 
Bull. 170 5 
