138 The Citric Solubility of Mineral Phosphates 



solved wlu'ii 40 gnuns of the fertiliser were taken. It is thus shown, whicli 

 is also apparent a jyriori, that citric solubility depends on the quantity 

 of mineral pliosphato or slag used. With iiuantities like .50 grams or 

 100 grams of mineral phosphate the citric solubihty would be extremely 

 small. On what theoretical grounds are quantities like 5 grams (official 

 test) in the case of slag and 1 gram (private test) in the case of mineral 

 phosphate used to determine the availability of pho.sphate in the soil? 



Hi 



\'II. JHK l.VCOMPLETEXESS OI' 'IHE REACTIONS. 



All the foregoing results have been considered from the standpoint 

 of the amount of phosphate dissolved per cent, of the weight of mineral 

 phosphate taken. It is desirable, however, to consider the amount of 

 phosphate dissolved in relation to the amount theoretically obtainable 

 if the reaction were complete. We cannot say definitely what the reaction 

 is but we know that the equation in Section 1 V' gives a suitable prediction 

 formula. Suppose we put the question : What proportion of the amount 

 indicated l)y this equation is obtained in each separate experiment? We 

 know from theory that the hydrogen ion concentration of an acid is 

 increased by dilution and we should therefore expect (/realer proportions 

 of the. possible total amounts at higher dilutions than at lower dilutions. 

 Let us consider the first series of experiments from this standpoint. If 

 the citric acid concentration at the beginning of the reaction was 3w gram- 

 molecules and the reaction went completely, according to the equation 

 given in Section IV we should have w gram-molecules of monocalcium 

 phosphate formed, equivalent to ic gram-molecules of tricalcium phos- 

 phate. Hence for the completed reaction we should have iv gram- 

 molecules of citric acid giving 1/3 «' gram-molecules of tricalcium phos- 

 phate. Suppose we find only y gram-molocules of tricalcium ])hosphatc 



then -^-^ is the percentage of tiie theoretical amount which has 



been found. Tlie following table (Table IX) shows the concentration of 

 the citric acid, the theoretical amount of phosphate obtainable in a 

 complete reaction according to the equation, the actual amount of phos- 

 phate obtained, as gram-molecules of tricalcium phosphate per litre, and 

 the percentage of the theoretical amount of phosphate obtainable. 



It is seen that this percentage is practically identical with the per- 

 centage in col, 5 of Table III. Indeed the numbers in col. 5 of that 

 table multiplied by 1-016 give the percentage in col. 4 of Table IX. This 

 arises from the following considerations. If a^ be the weight of phosphate 



