DETERMINATION OF THE FREE ACID IN SUPERPHOSPHATES. 



73 



These figures show that mono-basic posphates of sodium 

 and calcium are not quite neutral to methyl orange, but 

 slightly acid. They are more nearly neutral to the alizarin 

 derivative. The di-bassic salt of sodium is alkaline to all 

 indicators, but the di-basic calcium phosphate is acid 

 towards phenolphthalein and turmeric. The sharpness of 

 the end reaction of the different indicators used towards 

 these salts is shown in the following table. 





NaH 2 P0 4 



Na 2 HP0 4 



Mono- calcium 

 phosphate. 



Di-caleium 

 phosphate. 



1. Methyl orange 



fair, not 

 very sharp 



fair 



not very 

 sharp 



fair 



2. Phenolphthalein 



fair 



sharp 



fair 



fair 



3. Cochineal 



fair 



not sharp, 

 purple colour 

 produced and 

 difficult to 

 tell end point 



fair 



fair 



4. Litmus 



not good, 

 purple colour 

 produced and 

 transition to 

 blue is long 



not sharp 



not at all 

 sharp 



not sharp 



5. Congo red 



not sharp 



very bad 

 (dirty black 

 colour pro- 

 duced before 

 the blue) 



not sharp 



end reaction 

 very bad 



6. Lacmoid 



not sharp 



not sharp, 

 (purple pro- 

 duced) . 



fairly sharp 



not good 



7. Eesorcin deriva- 

 tive 



not sharp 



very bad 



not sharp 



not good 



8. Turmeric 



not sharp 

 very bad 



very peculiar 

 (see below) 



not at all 

 sharp 



not at all 

 sharp 



9. Sodium alizarin 

 sulphonate 



very sharp 



very sharp 



very sharp 



not very 

 sharp 



The behaviour of turmeric with di-sodium phosphate is 

 most peculiar. If 20 cc of the original solution (10 grms. 

 per 1000) is taken, a drop or two of tumeric gives a brown 

 coloration (indicating alkalinity) and about *3 cc. of N/10 

 H 2 S0 4 are required to neutralize it. If, however, the 20 cc. 



