§ 93. QUANTITATIVE METHODS. 153 



um, Calcium^ Magnesium^ Aluminium, Iron, and Man- 

 ganese, and Phosphoric and Sulphuric acids. 



This is one of the most frequently recumng separations 

 in agricultural chemical analysis. 



For the best general method of separation in each par- 

 ticular case, the analyst will be referred tg one of the ta- 

 bles at the end of this section, in which the whole course 

 to be followed will be marked out in a few words, while 

 more detailed descriptions will be given in the following 

 paragraphs of some of the necessary manipulations men- 

 tioned in the table. 



A. Precipitation of ahmiina, A]„0^, /err Ic oxide, Fefi^, 

 Sind phosphoric acid (anhydride), PjO^., and estimation of 

 the two bases. 



If the substance contains a notable proportion of or- 

 ganic matter, this should first be destroyed in the solu- 

 tion, and the iron completely oxidized to ferric oxide at 

 the same time, by treatment with an active oxidizing 

 agent. 



This oxidation may be effected by passing chlorine gas 

 through the solution until it is nearly saturated ; if this 

 course is followed, the solution should be heated after- 

 wards, until the excess of chlorine is entirely expelled. 



Or, instead of using chlorine, the solution may be evai> 

 orated nearly to dryness, and sodic or potassic hydrate 

 added in slight excess, and sodic carbonate and a little so- 

 dic or potassic nitrate ; then dry the mixture completely 

 in a platinum dish, and ignite the residue gently until the 

 organic matter is destroyed ; exhaust the mass with water, 

 treat it with dilute hydrochloric acid, add this solu- 

 tion and the washings to the aqueous one, and proceed as 

 directed below for the estimation of ferric oxide, etc. If 

 a residue remains that is insoluble in hydrochloric acid, 

 dry, ignite, and weigh it, and add the amount to the 

 silicic acid already obtained. 

 7"* 



