PHOSPHORUS AND PHOSPHORIC ACID 95 



What evidence is there of chemical combination ? Note the 

 formation of transparent glacial phosphoric acid, V^r^.Yif) 

 or HPO3. Boil the mixture for some time. A solution 

 of ordinary phosphoric acid, P2O5.3H2O or H3PO4, will 

 thus be obtained. Note that the solution is sour in taste 

 and acid to litmus. Neutralise a little with caustic soda, and 

 evaporate to obtain crystals of the soluble phosphate of sodium. 



Place 10 c.c. of the phosphoric acid solution, previously 

 largely diluted, in a small flask and run in lime water from a 

 burette, constantly shaking the flask, until the precise point is 

 reached when the precipitate at first formed no longer redis- 

 solves. Note the quantity of lime water used {x c.c). Test 

 the phosphate solution with litmus, and note that it is nearly 

 neutral. 



Place 10 c.c. of the diluted acid in another flask, and run 

 in lime water, constantly shaking, until, after filtering, no 

 further precipitate is produced by the addition of more lime 

 water. Note that the quantity of lime used is rather more than 

 double that required by the first experiment {2% c.c). On 

 testing with litmus the Uquid will be found to be still neutral. 



Into a third flask run three times the quantity of hme water 

 used in the first experiment (3:^ c.c), and, having filled the 

 burette with the diluted phosphoric acid, run it into the lime 

 water, with constant shaking, until, after filtering, the liquid 

 ceases to be alkaline, and becomes just neutral. Note that 

 approximately 10 c.c of the acid have again been used. 



There are obviously three stages in the reaction between 

 phosphoric acid and lime, corresponding to the formation of 

 three distinct compounds which may be expressed thus : — 



P2O5.3H2O + CaO = P2O5.2H2O.CaO + H2O 

 PA-SHgO + 2CaO = P2O5.H2O.2CaO + 2H2O 

 P A.3H0O + 3CaO = P205.3CaO + 3H2O 



and of these the first compound is clearly the only one soluble 

 in water. The three compounds are distinguished as mono- 



