230 AGRICULTURAL ANALYSIS 



pared, is not contaminated with weighable amounts of the other 

 substances found in the original solution. 88 The ease with which 

 oxalic acid can be determined volumetrically with potassium per- 

 manganate solution aids greatly in the time-saving advantages 

 of the process. 



In a hydrochloric acid solution of a mineral phosphate an 

 aliquot part of the filtrate representing about 250 milligrams of 

 calcium oxid, usually about 100 cubic centimeters, is used for 

 the analysis. Ammonia is added in slight excess and then the 

 acid reaction restored with hydrochloric until shown plainly by 

 litmus. The solution is then heated and the lime thrown down 

 by adding a solution of ammonium oxalate in excess. In order 

 to secure a greater dilution of the hydrochloric acid after the 

 precipitation has been made, water is added until the volume is 

 half a liter. Before filtering, the whole is cooled to room tem- 

 perature. The precipitate is washed first with cold and after- 

 wards with warm water. The well washed precipitate is dis- 

 solved in hot dilute sulfuric acid and the solution, while hot, 

 titrated with a standard solution of potassium permanganate set 

 by a solution of ammonio-ferrous sulfate. 



If one cubic centimeter of the permanganate represent 0.007 

 gram of iron it will correspond almost exactly to 0.0035 gram of 

 calcium oxid. The presence of iron in the original solution does 

 not seem to affect the results. 



Example. Sample of mineral phosphate, five grams in half a 

 liter. Strength of potassium permanganate, one cubic centimeter, 

 equivalent to 0.00697 gram of iron and to 0.003484 gram of 

 calcium oxid. 



Twenty-five cubic centimeters of the solution, representing one 

 quarter of a gram, in which the lime was precipitated as above 

 described, required 38.4 cubic centimeters of the potassium per- 

 manganate to saturate the oxalic acid. Then 38.4X0.003484= 

 0.133786 gram, or 53.51 per cent, of calcium oxid. The method 

 is also applicable to basic slags. 



203. Estimation of Iron and Alumina in Mineral Phosphates. 

 When mineral phosphates are to be used for the manufacture 

 "Die land wirtschaftlichen Versuchs-Stationen, 1887, 34 : 379. 



