THE ACIDIMETRT OF HTDROöEN FLUORIDE. 257 



transferring- the solution, when almost neutral, to a glass vessel, in 

 order to observe the shade of colour more accurately. 



The strength of the acid was determined irra vi metrically by 

 mixing in a platinum crucible a weighed quantity of the solution 

 with excess of slaked lime made each time from a weighed quantity of 

 precipitated calcium carbonate, letting the mixtiu'e stand for a night, 

 drying, and then igniting till the weight became constant. The 

 solution of the acid used for the trials recorded in Table I was also 

 assayed by digesting it with precipitated and finely divided silica, 

 and then igniting with a little sulphuric acid. The results of the 

 two methods agreed well ; they are given in the table. That used 

 in trials recorded in Table III was assayed also by gravimetrically 

 estimating the acid as calcium fluoride. The silica method gave in 

 this case a low result and was rejected. 



Jjitinna as indicator. Only historically and because of its universal 

 employment f<jr testing neutrality do we give consideration first to 

 litmus used in titrating hydrogen fluoride. On the addition of potash 

 or soda to dilute solution of liydrofluoric acid coloured by litmus, the 

 red colour rapidly deepens to violet and by the time «:)ne molecule of 

 alkali has been added the litmus has become an almost pure blue, that 

 is, blue almost free from any violet tinge. In fact the eifect upon 

 litmus in the saturation of hydrogen fluoride with potassium hydrox- 

 ide is much like that reversed of the saturati(jn ol jjotassium carbonate 

 by sulphuric acid. However, by practice and ])v keeping before one a 

 second vessel of water coloured by litmus of the right tint, — full bine, 

 as it would be called — it is possible to titrate hydrofluoric acid by 

 means of litmus. But its use is not recommended. 



Blue litmus paper reddened with a solution of potassium fluoride 

 containing a. small quantity oi hydrogen fluoride becomes blue again 

 when left ex|)osed to the air. If the paper be wetted soon after it has 



