PHOSPHORIC ACID. 179 



this somewhat tedious and costly process, by the addition of 

 citric acid to the original solution, followed by magnesia mix- 

 ture and ammonia, it is possible to keep the iron, aluminium, 

 and lime in solution and obtain all the phosphoric acid as mag- 

 nesium ammonium phosphate. The method is only suitable 

 when the quantity of phosphoric acid is fairly large and that 

 of iron and aluminium not considerable. 



The phosphoric acid of a manure may be present in three 

 states of combination, as already stated : 



1. Water-soluble phosphates. 



2. Citrate-soluble phosphates. 



3. Insoluble phosphates. 



The methods of determining the first and third require little 

 or no explanation. For the second the residue after washing 

 with water is used. It is extracted with a solution of ammo- 

 nium citrate of specified strength, for a specified time, and at 

 a specified temperature. 



The ammonium citrate solution employed is usually of 

 sp. gr. 1'09 at 20 and contains about 370 grammes of crystal- 

 lized citric acid, neutralised with ammonia, in two litres of 

 water (American), or 333 grammes citric acid in two litres 

 (Petermann), or about 360 grammes (Halle process). 



The temperature and time of digestion recommended by 

 different workers varies 30 or 40 for half an hour (Frank- 

 land) ; 50 for half an hour (Halle station) ; 35 to 40 for one 

 hour (Norway stations) ; 65 for half an hour (American 

 official chemists). 



In any case the liquid is filtered and washed, and the phos- 

 phoric acid in the residue determined as usual. 



Thus direct determinations are made of 



(1) Phosphoric acid soluble in water. 



(2) Total phosphoric acid. 



(3) Phosphoric acid insoluble in ammonium citrate. 

 The difference between (2) and (3) gives the citrate and 



water-soluble phosphoric acid, and by subtracting (1) the 

 citrate-soluble or "reverted " phosphoric acid is obtained. 



N2 



