ON THE ESTIMATION OP POTASH AND PHOSPHORIC ACID. 33 



Mohjbdic-Acid Method. 



Konnenschein's process of precipitation with molybdic acid, with subsequent 

 treatment with magnesia mixture, and weighing as magnesium pyrophosphate, 

 is probably the most uniformly accurate of all known processes for deter- 

 mining phosphoric acid. It appears always to be employed when great 

 accuracy is desired, and some chemists use it habitually. In some respects, 

 however, the process is not well fitted for general uso, for the following 

 reasons : — 



A very large excess of molybdic acid above that which is actually precipi- 

 tated as " phospho-molybdate of ammonium " is re .pared for' the complete 

 separation of the phosphoric acid of the solution. The reagent is somewhat 

 expensive, and there is no simple process of recovering the molybdenum 

 from the filtrate. 



The yellow precipitate contains le.33 than four per cent, of anhydrous 

 phosphoric acid, and thus becomes very balky and unmanageable when the 

 quantity of phosphoric acid present exceeds -1 or -2 of a gramme. This fact 

 leads to the employment of very small quantities of the material ; and as the 

 yellow precipitate has to be subsequently redissolvod and precipitated with 

 magnesia mixture in the ordinary way, the error liable to occur from the 

 use of an unusually small weight of the sample detracts greatly from the 

 value of the method. 



The above considerations, together with the loss of time and expense in- 

 cident to the use of the process, prevent the Committee from recommending 

 it for general adoption, though it is of opinion that in many instances the 

 method may be used with great advantage, and that in some cases it is 

 invaluable. 



Pisani has described a method of determining molybdic acid by reducing 

 its acid solution with zinc, and titrating the brown liquid with standard 

 permanganate. 



J. Macagno has proposed to apply this process to the determination of 

 phosphoric acid, by first precipitating the latter with " molybdate solution " 

 and then titrating the molybdic acid in the precipitate in the above manner. 



The Committee has instituted some experiments on this process, but the 

 results were very unsatisfactory 



Reduced Phosphates. 



It is well known that the soluble phosphate of somo superphosphates has 

 a tendency to pass back into the insoluble condition. It is plausibly argued 

 that the finely divided insoluble phosphate thus produced is equal in mano- 

 rial value to the soluble phosphate originating it, and therefore that in 

 judging of the value of the manure tho insoluble " reduced " phosphate 

 should be stated separately, and regarded as of equal manurial value to the 

 actual soluble phosphate. 



The methods which have been employed for the determination of " re- 

 duced " phosphate are based on the ready solubility of such precipitated 

 phosphate, in certain liquids, or on its easy decomposition by certain alkaline 

 salts. For its solution, citrate of ammonium has been employed, and for its 

 decomposition with formation of a soluble j)hosphate, oxalate of ammonium* 

 or bicarbonate of sodium f is used. 



A series of very suggestive experiments on Chesshire's bicarbonato-of- 

 sodium and Gibson's oxalate-of-ammonium methods have been communicated 



* Cheui. News, Sept. 10, 18G9, p. 123. 



t Cheui. News, Sept. 3, 1869, p. Ill ; Church's ' Laboratory Guide,' 3rd edition, p. HO. 



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