226 AGRICULTURAL ANALYSIS 



most important for factory use. The carbonates present in a 

 sample indicate the loss of an equivalent amount of acid in the 

 process of conversion into superphosphate. 84 



The apparatus employed for estimating carbon dioxid may be 

 any one of those in ordinary use for this purpose. The principle 

 of the process depends on the liberation of the gas with a mineral 

 acid, its proper desiccation, and subsequent absorption by a caustic 

 alkali, best in solution. The methods described for soils in Vol- 

 ume I are generally applicable to this class of materials. 



The weight of the sample should be regulated by the content 

 of carbonate. When this is very high, from one to two grams 

 will be found sufficient ; when low, a larger quantity must be used. 

 Hydrochloric is preferred as the solvent acid. Those forms of 

 apparatus which are weighed as a whole and the carbon dioxid 

 determined by reweighing after its expulsion, are not as reliable 

 as the absorption apparatus mentioned. 



196. Soluble and Insoluble Matter. To determine the insolu- 

 ble and by difference the soluble and volatile contents of a min- 

 eral phosphate, five grams of the finely ground phosphate are put 

 into a beaker, 25 cubic centimeters of nitric acid (specific gravity 

 i. 20), and 12.5 cubic centimeters of hydrochloric acid (specific 

 gravity 1.12) are added. The beaker, covered with a watch- 

 glass, is placed upon the water bath for 30 minutes. The con 

 tents of the beaker are well stirred from time to time, and at the 

 end of the period the beaker is removed from the bath, filled with 

 cold water, well stirred, and allowed to settle. The solution is 

 filtered into a half-liter flask, and the residue is thoroughly 

 washed with cold water, partially dried, and then ignited (finish- 

 ing with the blast-lamp), and brought to constant weight. The 

 figures thus obtained will, however, be incorrect, because the 

 fluorin liberated during the solution of the phosphates dissolves 

 a portion of the silica. Hence the results are too low. Never- 

 theless, as the same action would occur in the manufacture of a 

 superphosphate from the same material, the determination may 

 be considered as a fair approximation to commercial practice. 

 The ignited residue must be tested for phosphorus pentoxid. 

 84 Phosphates of America, 4tb Edition, 1892 : 145. 



