SUPERPHOSPHATE 129 



111. Examination of Rock Phosphates. 



(a) Samples of finely ground rock phosphates, such as 

 Florida or Aruba phosphates, are qualitatively examined 

 for calcium, aluminium, and iron. A separate quantity 

 is moistened with a little water and treated with strong 

 sulphuric acid. The gas formed should be tested for 

 hydrofluoric acid and also for carbon dioxide, hydro- 

 chloric acid, and HgS. The residual material on cooling 

 should set dry and friable, and may be tested for water 

 soluble phosphoric acid. 



(b) The solubility of the ground samples should be 

 tested, as previously described, in distilled water, 2 per 

 cent, citric acid solution, and ammonium citrate solution, 

 and also by passing carbon dioxide into a suspension of 

 the material in water. 



112. Qualitative Examination of Superphosphate. 



(a) A sample of superphosphate should be tested 

 for aluminium and iron, for soluble calcium, and for 

 soluble phosphoric acid. Its acid character will be 

 made apparent by testing with blue litmus paper. 



(J?) A small quantity of superphosphate is mixed 

 with some nitrate of soda or nitrate of lime, and the 

 mixture incorporated by rubbing in a mortar. A 

 portion of the mass is heated in a test-tube with a drop 

 or two of water, and the remainder is allowed to stand 

 for a day or so. Free nitric acid will be formed in both 

 cases. This emphasises the fact that nitrates and 

 superphosphate are incompatible manures. 



Analysis of Superphosphate of Lime. 



Superphosphate, being prepared chiefly from ground mineral 

 phosphates by the limited action of sulphuric acid, consists chiefly 

 of CaH4(P04)2 and Ca3(P04)2, together with calcium sulphate. It 

 also sometimes contains "reverted" or "retrograde" phosphate, 



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