THE ANALYST. 87 NOTE ON THE PREPARATION OF PURE HYDROFLUORIC ACID. By ALFHED H. ALLEN. ( R e a d at the Meeting, February 5, 1896.) COMMERCIAL hydrofluoric acid is liable to contain various impurities, including lead, iron, aluminium, calcium sulphate, organic matter, etc. I n the absence of a platinum still it is not easy to prepare a strong acid suitable for use in the analysis of silicates, but the necessity of obtaining a product free from fixed impurities led me, some years since, to adopt the following contrivance, which I have since used with very satis- factory results. The apparatus simply consists of the largest platinum crucible available. I n this is placed a mixture of ordinary hydrofluoric acid with about an equal measure of strong sulphuric acid. I n a smaller platinum crucible is placed a weighed quantity of the silicate to be treated, and this is moistened with three or four drops of strong sulphuric acid. The smaller crucible is then placed inside the larger, and is pre- ferably supported on a disc of plaster of Paris, or on a ring cut from a piece of one- inch leaden gas-pipe.The larger crucible is then covered with a platinum dish filled with cold water. On heating the arrangement on an iron plate, the hydrofluoric acid in the outer crucible is volatilized, and is condensed on the lower surface of the platinum dish, from which it drops on to the silicate contained in the inner crucible. After a time, when about 10 C.C. of distillate has collected in the inner crucible, the latter is removed, the contents evaporated off, and the treatment once or twice repeated, to ensure the complete decomposition of the silicate. By the foregoing means pure concentrated hydrofluoric acid is prepared at the time and in the exact quantity required, without the use of any special apparatus. I)ISCUSSION. Mr. BLOUNT said that he had distilled absolutely pure hydrofluoric acid in a leaden vessel, having an exit tube sloped upwards, and connected with a platinum condensing-tube sloped downwards, so that the condensed acid came into contact with platinum alone. This was a very convenient plan, as a fairly large quantity of the acid could be made at one operation. Mr. BLLZX said that when a small quantity of lead in the hydrofluoric acid was of no consequence, he had himself used a leaden retort constructed out of gas-pipe, but without the platinum tube mentioned by Mr. Blount.