1 88 AGRICULTURAL ANALYSIS 



The sulfates of iron and aluminum disturb the accuracy of the 

 reaction, since they also react with carbonates. Inasmuch as the 

 mineral phosphates entering the factory have been examined for 

 iron and alumina, the magnitude of this disturbance can be as- 

 certained and due allowance made therefor, or the iron may be 

 thrown out previous to the determination by potassium ferrocya- 

 nid. The process is conducted as follows : 



Twenty grams of the sample are shaken for half an hour in a 

 liter flask with water, and one gram of ferrocyanid of potash dis- 

 solved in water added thereto, the flask filled to the mark, shaken, 

 and the contents poured on a filter. To 100 cubic centimeters of 

 the filtrate a known weight of calcium carbonate is added, stirred 

 for half an hour, the undecomposed carbonate separated by filtra- 

 tion, washed with a small quantity of water, dried, ignited gently 

 and weighed. The quantity of calcium carbonate thus determined 

 deducted from the whole amount used, represents the quantity de- 

 composed by the free acids and the iron and aluminum compounds 

 above noted. The constant error, due to the last named source, 

 is applied as a correction and the quantity of free acid thus ap- 

 proximately determined. 



The carbon dioxid contained in the residue above mentioned 

 is determined more rapidly and with greater precision by decom- 

 posing it with an acid and weighing or measuring the evolved 

 gas. 



The carbonate remaining in the residue may also be determined 

 by titration as follows : The residue is placed in a flask of 200 

 cubic centimeters capacity and decomposed with 25 cubic centime- 

 ters of normal hydrochloric acid, filled to the mark, shaken and the 

 contents poured through a dry filter. One hundred cubic centi- 

 meters of the filtrate are titrated with half-normal soda-lye, using 

 methyl orange as indicator. The correction for iron and aluminum 

 must again be made, since any iron and aluminum phosphate which 

 is found with the residue of calcium carbonate decomposes cor- 

 responding quantities of the hydrochloric acid. Since the fresh 

 superphosphate always contains some free sulfuric acid, it is ad- 

 visable to report the result as degree of acidity, comprising therein 

 the free phosphoric acid, the free sulfuric acid and all other com- 



