PHOSPHORIC ACID DETERMINATION 85 



may be found in this operation, their separation is recommended 

 by the method described by Neubauer. 58 



For the estimation of phosphoric acid the insoluble residue 

 obtained in the estimation of potash above described is used. 

 After the whole of the filtrate has run through a small filter upon 

 which the reddish brown precipitate has been collected, replace 

 in the flask and treat with a dilute sulphuric acid corresponding 

 to five cubic centimeters of the concentrated acid; then boil the 

 half-filled flask for about half an hour. A small quantity of oxid 

 sometimes remains very firmly attached to the platinum dish, and 

 for this reason the acid is conveniently used for washing out the 

 dish. It is then certain that no trace of phosphoric acid is lost. 

 If any stains remain upon the platinum dish due to the iron they 

 are easily removed by treatment with zinc and hydrochloric acid. 



The phosphoric acid which has passed into solution corre- 

 sponds to 20 grams of the original solution (soil) which has been 

 extracted and is conveniently determined by the molybdate method. 

 This molybdate method is recommended because, even with all 

 the care which has been exercised, the solution usually still con- 

 tains a little salicylic acid. Neubauer, however, highly recom- 

 mends in this case, for the estimation of small quantities of phos- 

 phoric acid, the direct weighing of the yellow precipitate. In 

 this case only 25 cubic centimeters of the solution are employed, 

 corresponding to five grams of the soil, and this is treated with 

 25 cubic centimeters of nitric acid of 1.2 specific gravity, and the 

 rest of the process is carried out as described in the volumetric 

 method. 



For the estimation of calcium and magnesium another portion 

 of the solution, corresponding to 25 grams of soil, except in the 

 case of marly soil, where a smaller quantity is used, is treated as 

 above described, except, of course, without the addition of cal- 

 cium carbonate. The residue is treated with water and the liquid 

 should give no trace of an acid reaction. Otherwise it is evident 

 that the deposition of the chlorid has not been sufficiently se- 

 cured. 



According to the estimated quantity of calcium from about two 



58 Zeitschrift fiir analytische Chemie, 1904, 48: 14. 



