THE ACIDIMETEY OF HYDROGEN FLUORIDE. 259 



fluoride, giving a. very sharp colour change at the point of neutraHty. 

 It cannot of course he used with aTiimonia. 



Rosolic acid is ahnost equal to Phenolphthalein, i)ut not quite so 

 sharp in its indication. It can be used with ammonia as titrating 

 agent, although ammonia is a little slower than the fixed alkalis in 

 atfectinof this and other colours. In the case of this indicator and 

 Phenolphthalein the clutnge of colour is well adapted for being ob- 

 served in platinum vessels. 



Cochineal and hrazil irood behave alike. Either becomes violet 

 gradually and indefinitely before the acid is neutralised, but finally it 

 turns bluish \'iolet. Tliis change of colour is fairly shar]) and is 

 satisfactory for indicating neutrality. lîrazil-wood paper becomes 

 bluish violet before all the acid is neutralised and is therefore unsuit- 

 able for use. 



Turmeric paper is satisfactory but not quite as sensitive as some 

 of the other indicators and near the finishing point responds slowly. 



If will be seen that, as in the case of the ordinary vegetable acids, 

 the best indicator is ])henolphthaleïn or. when ammonia is the titrating 

 agent, rosolic acid. 



We have yet to mention that all these indicators give satis- 

 factory results only when the alkali solution is almost pure. The use 

 of an impure alkali solution or of a solution which has been kept long 

 enough in glass vessels to have taken u|) silica, carbonic acid, and 

 other impurities in appreciable quantity is liable to give ill-defined 

 and generally too-low results. 



In conclusion, we liave to return our thanks to Dr. Divers for 

 his valuable suggestions in working out this paper. 



