PRELIMINARY CONSIDERATIONS 55 



of nitric acid. This method is particularly suited to samples con- 

 taining much iron or aluminum phosphate. 



From the above directions it is seen that in a purely mineral 

 phosphate a single strong acid or a mixture of acids is sufficient 

 to bring all the phosphoric acid into solution. Where organic 

 matter is present the use of strongly oxidizing solvents as in 

 4 and 5 is necessary. In substances containing phosphorus in 

 organic forms such as blood, tankage, oil cakes, seeds, etc., espec- 

 ial care is required to complete the oxidation and secure all the 

 phosphorus in the form of phosphoric acid. 



GENERAL METHODS FOR ESTIMATING PHOSPHORIC 

 ACID IN FERTILIZERS 



62. Preliminary Considerations. The chief sources of the 

 phosphoric acid in commercial fertilizers are the mineral phos- 

 phates and bones. In respect of the general analyses of mineral 

 phosphates detailed directions have been given in the preceding 

 volume. Bones are valuable for fertilizing materials, both because 

 of their content of phosphoric acid and of their organic nitrogen. 

 The method of treating bones for their phosphoric acid will be 

 found in the general methods for fertilizing materials, and their 

 nitrogen content can be determined by the processes to be de- 

 scribed hereafter. Other fertilizing materials also contain phos- 

 phorus, as ashes, tankage, oil cakes, and other organic products. 

 In general, the methods for determining the phosphoric acid is 

 the same in all cases, but the means of destroying the organic 

 matter precedent to the analysis vary in different cases. In most 

 cases a simple ignition is sufficient, while, if the phosphorus be 

 found in certain organic products, the oxidation must be accom- 

 plished by one of the methods described in the processes adopted 

 by the official chemists. In all cases of acid phosphates 

 and superphosphates, the water and ammonium citrate-soluble 

 phosphoric acid is to be determined as well as the total. In 

 basic slags the amount soluble in ammonium citrate or dilute 

 citric acid is also to be ascertained. 



In all cases where soluble or so-called reverted acid is to be 

 considered, the analysis must be performed without previous 

 desiccation or ignition. If water content or loss on ignition is 



