258 1'- H AG A, AND Y.JiÖSAKA. 



become blue the red colour i« generally restored. If, however, it is 

 let dry thoroughly, subsequent moistening with water generally fails 

 to bring back the red colour. The colouring matter seems to be 

 modified by drying up Avith the fiuoride. Dissolved litmus added to 

 such a solution is also coloured red when much water is present, but 

 becomes more and more blue on evaporating the solution, and when 

 it has become sufficiently concentrated the greater part of the colouring 

 matter separates out as a blue poMder. This change to blue is not due 

 to loss of hydrofluoric acid, for on addition of water the colouring 

 matter dissolves a^ain oiving a red-coloured solution as before. A 

 solution in which litmus paper was turned apparently permanently red 

 and only on drying became neutral or very faintly alkaline, was in one 

 trial fcnmd to c<:)rrespond very nearly to HK3F4. Litmus paper is 

 known to be reddened by monopotassium orthophosphate in solution 

 and to become bkie again when dried, from which it may Ije inferred 

 that hydrogen fluoride, like hydrogen phosphate, is a poly basic acid, 

 H,F, or H3F, or H,F,. 



Ijücuioid solution behaves essentially like Htmus, the diflerence 

 being that nmch less alkali is required U) produce a bluish violet 

 colour in a solution cohjured by lacmoid than in oue coloured by 

 litmus. By titrating to a distinct blue, good results may be got with 

 it, but it is not a desirable indicator. Lacmoid paper behaves also 

 like litmus paper 



Phenacetolin changes in colour somewhat gradually when near the 

 neutrality point, but by titrating to pure purple or rose-violet free from 

 any tinge of yellow it may be used successfully. 



Methfil oramje is quite useless for the ordinary titration of hydrogen 

 fluoride, although it seems to find neutrality in KoHF;.. The colour 

 changes are very indefinite. 



PheHolplifhaleÏH is most satisfactory as an indicator for hydrogen 



