§ 93. QUANTITATIVE METHODS. 155 



of the solution and estimated for the amount of solution 

 taken for this analysis, will give the Al^O^. 



2. In case there is not Fe^Og and Al^Og enough present 

 to combine with all the phosphoric acid, more iron must 

 be added, until a drop of the liquid, on a watch-glass, 

 gives a reddish precipitate with a little ammonia, and the 

 amount of iron so added is to be subtracted from the total 

 amount found subsequently. This addition of iron is 

 most conveniently made in tlie form of a carefully meas- 

 ured quantity of an accurately titrated solution of ferric 

 chloride (Fe^Clg), about ^1^ the strength of the reagent or- 

 dinarily used. Proceed then as in 1. 



B. The method of removing phosphoric acid by means 

 of metallic tin admits of the determination, in a conven- 

 ient manner, of this acid, and alumina, ferric oxide, man- 

 ganous oxide, lime, and magnesia, in the same portion of 

 the solution. 



On evaporating to dryness to remove silica, after moist- 

 ening the dried residue with concentrated hydrochloric 

 acid in the usual manner, add nitric acid, dilute Avith 

 water, filter, wash the insoluble silica on the filter, evap- 

 orate the filtrate and washings u early to dryness, or until 

 all the hydrochloric acid is expelled, dissolve the residue 

 in concentrated nitric acid, heat the solution to boiling in 

 a beaker covered with a large watch-glass or an inverted 

 funnel, and add pure tin in small grains, and in small 

 portions at a time, to an amount about six times as great 

 as that of the phosphoric acid supposed to be present, di- 

 gest the mixture 5 or 6 hours in a warm place, dilute and 

 decant the clear supernatant liquid on the filter, and wash 

 the precipitate, containing stannic oxide, stannic phos- 

 phate, and i^erhaps some alumina and ferric oxide, several 

 times by decantation with boiling dilute nitric acid, and 

 finally with a little water ; then digest it with ammonio 

 sulphide, wash the undissolved aluminic hydrate and fer- 

 rous sulphide first with hot ammonic sulphide, and then 



