164 THOMAS COOKSEY. 



of phenol-phthalein solution and noting the amount of 

 standard caustic potash required to produce a permanent 

 pink colour. Care must of course be taken that no carbon- 

 ates, silicates, or borates are present. Proceeding in this 

 way I found that one molecule of caustic potash KHO was 

 equivalent to one quarter molecule P 2 0. 5 or one half group 

 P0 4 , that is 1 cc. normal caustic potash was equivalent 

 to *0355 gm. P 2 Oo. I at first imagined that this method 

 of estimating phosphoric acid was a new one, but upon 

 looking up the literature of the subject, I found that this 

 process had been proposed and worked out by Emmerling 

 in 1880 and published in "Landwirthschaftliche Versuchs- 

 station," 188G, page 429. 



Emmerling, however, applied the principle somewhat 

 differently — The solution containing the phosphate, to 

 which an excess of calcium chloride had been added, was 

 run into a measured quantity of a standard caustic soda 

 solution, until the pink colour given by phenol-phthalein 

 had just disappeared. The number of cc. required was 

 noted; the same quantity of the phosphate solution (con- 

 taining the calcium chloride) was then titrated with 

 standard caustic soda until, with methyl orange as indicator, 

 the pink colour just disappeared. The number of cc. of 

 standard soda required was subtracted from the amount 

 used in the titration with phenol-phthalein. The difference 

 gave the amount of phosphate in solution. Emmerling 

 states that small amounts of iron and alumina do not inter- 

 fere with the estimation. This statement from my obser- 

 vation is not strictly true. Very small quantities, such as 

 may occur in super-phosphate of lime perhaps make little 

 difference, but if the quantities of iron and alumina are at 

 all considerable, phosphates or hydrates of these metals 

 are precipitated in arriving at the neutral point with 

 phenol-phthalein. I have used the previously described 



