78 AGRICULTURAL ANALYSIS 



ness, nitric acid may be used, but not more than one or two 

 drops. 



B. Total Phosphoric Acid. (i) For bone and flesh-meal, 

 fish-guano, and similar fertilizers the reagents necessary are the 

 same as before. 



Carefully burn five grams to ash, boil the ash for half an hour 

 with nitric acid of 1.32 specific gravity, dilute with water, and, 

 after cooling, dilute to- 500 cubic centimeters. Filter through a 

 dry filter and add 100 cubic centimeters of the molybdate solution 

 for each 100 milligrams of phosphorus pentoxid present to 50 

 cubic centimeters of the filtrate. Treat further as before described. 



(2) Phosphates, guanos, bone-black, etc. 



One gram of substance, after powdering, and, if necessary, 

 igniting, is covered with four cubic centimeters of hydrochloric 

 acid of 1.13 specific gravity and a little water and heated for an 

 hour and a half. Evaporate to dryness without filtration, making 

 repeated additions of nitric acid until no more vapors of hydro- 

 chloric acid are evolved. Boil the residue with nitric acid, cool, 

 make up to 100 cubic centimeters with water, and shake. Filter 

 and treat 50 cubic centimeters of the resulting solution by the 

 molybdate method and proceed further as before described. 



84. Sources of Error in the Molybdate Method. When con- 

 ducted with proper care, the gravimetric molybdate method is one 

 of the most exact processes known to analytical chemistry. 



There are, however, some sources of error in the process which 

 should be avoided as carefully as possible or taken into account. 



i. Hrror Due to Occluded Silica. When silica passes into 

 solution in the original sample, and this may be the case especially 

 with mineral phosphates, it may appear both in the yellow pre- 

 cipitate and in the final magnesium pyrophosphate. In all such 

 cases the residue, after ignition, should be dissolved in hydro- 

 chldric acid and any insoluble residue weighed as silica and de- 

 ducted from the first weight. If the silica be removed by evap- 

 orating the solution of the original material to dryness, and 

 igniting to destroy organic matter, care must be taken to recon- 

 vert all phosphoric acid into the ortho form by long boiling with 

 nitric acid before precipitation. 



