1750 MANURES AND MANURING 



to repeated extractions with 2 per cent citric acid, the samples being hand 

 shaken for an hour at room temperature. It was found that the amount 

 of phosphoric acid dissolved in the consecutive extractions varied not 

 only according to the nature of the mineral and its lime content, but also 

 according to its state of division : 



i) Nodular and rock phosphates (I^and Pebble, Medulla, Angaur) 

 yielded their phosphoric acid less readily than the more friable minerals 

 of a sedimentary origin, nodular phosphates requiring 7 extractions and 

 rock phosphate 5 or 6 extractions for the complete removal of the phosphoric 

 acid. 



2) A Bir el Afou phosphate, ground to a degree of fineness which 

 allowed 85 per cent of it to pass through a sieve with 100 meshes to the 

 inch, 5delded 29 per cent of its phosphoric acid in the first extraction and 

 required 4 further extractions for the complete removal of the phosphoric 

 acid, whereas an Egyptian phosphate of 58 per cent fineness only yielded 

 21.71 per cent of its total phosphoric acid to the first extraction, and for 

 the complete removal of the phosphoric acid, 5 further extractions were 

 necessary. 



3) Calcium carbonate when present, neutralised the citric acid, 

 thereby reducing its solvent power in proportion to the amount of carbo- 

 nate present. In the first few extracts obtained from a calcareous sample, 

 a Gouraya phosphate containing 21.22 per cent of calcium, the phosphoric 

 acid increased whilst the carbonate decrea^sed. On the other hand with 

 Constantine, Gafsa and Bir-el-Afou phosphates containing only 13.27, 

 10.10 and 9.95 per cent of carbonate respectively the amounts of phosphoric 

 acid found in the extracts followed a regular descending curve from the 

 first to the last of the extractions. 



In a second series of experiments a study was made of the influence 

 of the other salts used as fertihzers (sulphates, nitrates and chlorides of 

 ammonium, potassium, sodium and magnesium) on the solubility of phos- 

 phoric acid. Citric acid solutions were prepared containing increasing 

 fractions of the equivalent weights of these salts and in each case 250 cc. of 

 the solution was shaken up with 2.5 gms. of the raw phosphate for an hour. 

 With the system ; tricalcic phosphate, phosphoric acid, calcium citrate and 

 citric acid it was found that the solvent action of citric acid ceases on the 

 establishment of equilibrium between the free phosphoric acid and the calcium 

 citrate which is formed. The presence of a salt of a strong, acid, such as 

 potassium sulphate, upsets this equiUbrium by causing a double decom- 

 position between the added salt and the calciiun citrate ; potassium citrate 

 is formed and acts further on the tricalcium phosphate until a new equi- 

 librium is established, this time between the two salts with a common 

 ion, potassiuni sulphate and potassium phosphate. Nitrates and chlorides 

 of ammonium, potassium, sodium and magnesium behave in an analogous 

 manner. On the other hand the presence of calcium salts (chloride and ni- 

 trate) inliibits the solvent effect. The general conclusion may be drawn 

 that the solubihty of phosphoric acid is increased by the addition of salts 

 of strong acids giving rise to soluble pho,sphates, and is decreased by the 



