8O AGRICULTURAL ANALYSIS 



The following course of procedure in the determination of 

 phosphoric acid can be recommended to avoid or correct this 

 error : 



Separate the phosphoric acid in the form of the yellow precip- 

 itate and wash this latter in the usual way. Too high a heat 

 should not be employed, nor should the solutions be allowed to 

 stand too long, lest excess of molybdic acid separate. Dissolve 

 the phosphomolybdate in 100 cubic centimeters of cold two and 

 five-tenths per cent, ammonia and add as many cubic centi- 

 meters of the usual magnesia mixture (55 grams magnesium 

 chlorid and 70 grams ammonium chlorid dissolved in a liter 

 of two and five-tenths per cent, ammonia) as there are centi- 

 grams of phosphorus pentoxid present. Addition should not be 

 made faster than 10 cubic centimeters per minute. Stir during 

 the addition. After the precipitation stir briskly once more and 

 allow to stand at least three hours. Wash with two and five- 

 tenths per cent, ammonia till the chlorin reaction disappears, dry 

 the filter, and introduce into a well cleaned crucible which has 

 been thoroughly ignited. Place the lid at an angle, carbonize 

 the filter, and gradually raise the heat, though not higher than 

 a medium red heat, till the pyrophosphate becomes completely 

 white. When this happens bring the blast into action and ignite 

 to constant weight. The weight finally accepted must not change 

 even after half an hour's ignition. Upon this requirement espe- 

 cial stress must be laid. Pure magnesium pyrophosphate does 

 not suffer any loss even after several hours' ignition, nor does 

 a good platinum crucible. To the weighed amount of pyrophos- 

 phate add the correction given in the table. For example, if the 

 weight be 250 milligrams, the correction to be added is four and 

 two-tenths milligrams, and the correct weight is then 254.2 milli- 

 grams. Multiplication of the sum by sixty-four gives the amount 

 of phosphorus pentoxid in the weight taken for analysis. 



When phosphoric acid is to be estimated as pyrophosphate it 

 must always be first separated as molybdate, even when the orig- 

 inal solution contains no bases capable of forming insoluble phos- 

 phates, as otherwise these corrections will not be applicable. 



