196 
ME. G. GORE ON HYDROFLUORIC ACID. 
cipitated by ammonia ; the filtrate was evaporated and ignited in a platinum dish to 
expel the sulphate of ammonium ; the ignited residue was dissolved in water and pre- 
cipitated by carbonate of ammonium ; the precipitate contained thallium distinctly, and 
the filtrate contained thallium, sodium, lithium, and potassium salts. 
The following is the process employed by me for purifying ordinary hydrofluoric acid. 
Pass sulphuretted hydrogen through it until it contains the gas in excess ; this separates 
the sulphurous acid (and also the arsenic if the acid is not too strong). Stir thoroughly. 
Find the amounts of free sulphuric and hydrofluosilicic acids present by the means 
already described (see page 195), and add a little more than sufficient carbonate of potas- 
sium to neutralize them, i. e. 138 parts of anhydrous carbonate to 98 parts of H 2 S0 4 , 
or 60 parts of Si 0 2 *. Subside until quite clear and cold, and decant the clear 
liquid. The effervescence, stirring, and exposure to the air expels nearly all the excess 
of sulphuretted hydrogen ; careful addition of carbonate, oxide (or fluoride) of silver will 
remove the remainder. Distil the clear liquid in a leaden retort immersed in a bath of 
linseed-oil at a temperature varying from about 320° Fahr. to about 460° Fahr., using 
a Liebig’s condenser, containing a platinum tube to condense the vapourf. The acid 
requires to be redistilled once to render it quite pure. If the liquid product contains 
more than about 40 per cent, of anhydrous acid it will probably contain arsenic ; this 
may be precipitated by diluting the liquid and passing a prolonged current of sulphu- 
retted hydrogen gas through it. Avoid contact of the pure acid with leaden vessels. 
(Or the following modification of the process might probably be employed: — Find 
amounts of sulphuric and hydrofluosilicic acids present ; add more than sufficient car- 
bonate or sulphide of potassium to neutralize them. Subside, decant the liquid, and 
dilute it with water until it contains not more than about 40 per cent, of anhydrous 
acid. Pass an excess of sulphuretted hydrogen through it, filter, and distil.) I have 
made several gallons of the pure acid on various occasions by the first of these processes. 
Hot junctions in the distillatory apparatus are best made gas-tight by vulcanized india- 
rubber washers, or by binding them round with ribbon of the thinnest and best quality 
of vulcanized rubber secured by tarred string. Strong hydrofluoric acid containing 
traces of lead yielded no precipitate on addition of sulphuretted hydrogen water ; nor 
did I succeed in removing the lead by electrolysis ; it was partly precipitated by hydro- 
chloric acid and by sulphuric acid. 
The acid prepared by the foregoing process was very pure ; it yielded only a trace 
of fixed matter (probably derived from the gutta-percha bottle, see pages 193 & 194) 
on evaporation to dryness in a bright platinum dish ; it evolved no sulphuretted hydrogen 
or sulphur by addition of zinc or tin ; it gave no precipitate or coloration by addition of 
the following substances : — excess of aqueous ammonia or of pure anhydrous carbonate 
* One gallon of mixed samples of ordinary hydrofluoric acid required 22| ounces of carbonate of potassium 
of 92 per cent, to neutralize its sulphuric and hydrofluosilicic acids. 
t The tube employed by me was 6 feet long and | inch diameter, and was kindly lent to me by Messrs. 
Matthey and Co. 
