MR. Gr. GORE ON HYDROFLUORIC ACID. 
195 
separated, equal 64 parts of sulphurous acid present. One gallon of a mixture of dif- 
ferent samples of commercial hydrofluoric acid of specific gravity U25 yielded by the 
above treatment 3^- avoirdupoise ounces of dry sulphur =1T45 per cent, of S0 2 . Another 
sample yielded sulphur equal to 2-72 per cent, of S0 2 thus. I have not determined the 
amount of hydrochloric acid in commercial hydrofluoric acid; it is, however, quite 
small ; nor have I ascertained the quantity of arsenic, which is also small. 
The total amount of all the free acids present in a sample was roughly determined by 
adding a known weight of pure precipitated dry carbonate of calcium to it, until the 
liquid exhibited a neutral reaction to test-paper after expulsion of the liberated carbonic 
acid by heat. A standard solution of carbonate of sodium is inapplicable for the purpose, 
because the fluoride of sodium produced has an alkaline reaction. 
To find the amount of free sulphuric acid, 100 grains of the acid was carefully evapo- 
rated in a platinum dish at a gentle heat until all sulphurous, hydrofluoric, hydrochloric, 
and hydrofluosilicic acid was expelled ; the residue was then exactly neutralized by a 
standard solution of carbonate of sodium in the usual manner, and the amount of sul- 
phuric acid calculated therefrom. The residuary acid liquid was in some cases diluted 
with water, filtered, acidified with a little hydrochloric acid, and the total amount of 
sulphuric acid in it determined by the ordinary gravimetric process with solution of 
chloride of barium. 
To determine the amount of hydrofluosilicic acid, an excess of dilute aqueous ammonia 
was added to 100 grains of the acid previously diluted in a large platinum dish, and the 
silica separated from the washed precipitate (which usually contains iron and sometimes 
lime) in the usual manner and weighed. 30 parts of Si 0 2 equal 72 parts of HF, SiF 4 . 
The most feasible method of determining the amount of HF in the impure acid con- 
sists in neutralizing it with hydrate of potash, evaporating the liquid to a small bulk, 
adding an excess of precipitated silica, heating the mixture with sulphuric acid, and 
ascertaining the amount of silica dissolved, and calculating it as fluoride of silicon (see 
pages 176, 177, & 178). Numerous attempts were made to determine the amount of HF 
by diluting the acid, digesting it with pure precipitated silica, and calculating from the 
amount of silica dissolved the amount of HF present ; but the results were not even 
approximately accurate, they were calculated too highly, apparently owing to the forma- 
tion of a variable mixture of Si F 2 and HF, Si F 2 . 
The sulphate-of-calcium residue from the manufacture of hydrofluoric acid is liable 
to contain minute amounts of potassium, sodium, lithium, thallium, and copper ; also 
iron freely (derived from the action of the ingredients upon the iron retort), and some 
free sulphuric acid. Upwards of fifty-six pounds weight of the residue was digested and 
washed with eight or ten gallons of distilled water, the clear liquid evaporated to small 
bulk in a leaden pan, and the greater portion of the free sulphuric acid expelled by 
heating in a platinum dish, the residue diluted, filtered, and precipitated by sulphuretted 
hydrogen (the yellowish-brown precipitate contained copper and thallium) ; most of the 
iron was then separated as sulphate by crystallization, and nearly all the remainder pre- 
