Hydrofluoric acid

“ The salts of hydrofluoric acid are known as fluorides and are easily obtained by the action of the acid on metals or their oxides, hydroxides or carbonates. The fluorides of the alkali metals, of silver, and of most of the heavy metals are soluble in water; those of the alkaline earths are insoluble. A characteristic property of the alkaline fluorides is their power of combining with a molecule of hydrofluoric acid and with the fluorides of the more electro-negative elements to form double fluorides, a behaviour not shown by other metallic halides. ”

“ The strong, aqueous, hydrofluoric acid obtained by the action of oil of vitriol on fluor-spar, is a densely fuming, volatile, colourless liquid, which boils at 60° F., and remains unfrozen at 4°. It combines with water so greedily, and evolves so much heat in doing so, as to give rise to a hissing noise like that produced when a red-hot iron is plunged into cold water. ”

“ Moreover, an acid need not contain a fraction of oxygen, as there is a numerous class of hydracids, in which the acidifying principle is hydrogen instead of oxygen, such as the hydrochloric, hydriodic, hydro-bromic, and hydrofluoric acids. ”

“ Hydrofluoric acid is used for etching glass and opening up silicates. It attacks silica, forming fluoride of silicon (SiF4) , which is volatile. Silica is by this means eliminated from other oxides, which, in the presence of sulphuric acid, are fixed. The commercial acid is seldom pure, and generally weak; and the acid itself is dangerously obnoxious. The use of ammonium fluoride (or sodium fluoride) and a mineral acid is more convenient. Determinations of this kind are made in platinum dishes enclosed in lead or copper vessels in a well-ventilated place. ”

“ When fluorine is delivered into an aqueous solution of hydriodic acid, each bubble as it enters produces a flash of flame, and if the fluorine is being evolved fairly rapidly there is a series of very violent detonations. A curious reaction also occurs when fluorine is similarly passed into a 50 per cent. aqueous solution of hydrofluoric acid itself, a flame being produced in the middle of the liquid, accompanied by a series of detonations. ”

“ The hydroxide, Sc (OH) 3, is thrown down by alkalies as a bulky white gelatinous mass; the oxide is a white powder, less readily soluble in dilute acids than most of the rare earths. The fluoride is important on account of its insolubility in mineral acids, which exceeds that of all the other rare earth fluorides, and approaches that of thorium. It is thrown down from neutral or acid solutions by addition of hydrofluoric acid or a soluble fluoride; if the solution be boiled, a soluble silicofluoride will also precipitate scandium fluoride, though no precipitate is obtained in the cold. ”

“ Hydrofluoric acid and its derivatives both in their manufacture and use and in the superphosphate industry affect the health of the workers.
If hydrogen fluoride or its compounds escape into the atmosphere they attack the respiratory passages and set up inflammation of the eyes; further, workers handling the watery solutions are prone to skin affections (ulceration) . ”

“ For deep etching, hydrofluoric acid is diluted with 11⁄2 vols. of water and stored for twenty-four hours before use. The objects are immersed in the baths for thirty to fifty minutes, and kept quite still the while. If the etching is to be left clear, the acid is neutralized by boiling the glass in soda, but if to be frosted afterward it is coated with the first named etching liquid while still damp. Finally, the ink is washed off with turpentine, the glass rubbed over with sawdust, washed in hot lye and rinsed with water. ”

“ If heated, the liquid acts more rapidly. The work should always be conducted in the open air, and care should be taken not to inhale the fumes, which are highly injurious to the health, and not to get any liquid on the skin, as hydrofluoric acid is one of the most dangerous poisons. Hydrofluoric acid must be kept in earthen or leaden vessels, as it destroys glass.
Removing Paint And Varnish From Wood.—The following compound is given as one which will clean paint or varnish from wood or stone without injuring the material:
This mixture is applied to the surface and left on for some time. ”

“ Soc., 1889, 55, p. 163) determined the vapour density of hydrofluoric acid at different temperatures, and showed that there is no approach to a definite value below about 88° C. where it reaches the value 10.29 corresponding to the molecular formula HF; at temperatures below 88° C. the value increases rapidly, showing that the molecule is more complex in its structure. (For references see J.N. Friend, The Theory of Valency (1909) , p. 111.) The aqueous solution behaves on concentration similarly to the other halogen acids ”

“ Extreme caution must be observed in handling the acid, since when brought in contact with the skin it produces dangerous sores very difficult to heal. The vapor is also dangerously poisonous when inhaled.
Hydrofluoric Formulas.—I.—Dissolve about 0.72 ounces fluoride of soda with 0.14 ounces sulphate of potash in 1/2 pint of water. Make another solution of 0.28 ounces chloride of zinc and 1.30 ounces hydrochloric acid in an equal quantity of water. Mix the solutions and apply to the glass vessel with a pin or brush. At the end of half an hour the design should be sufficiently etched. ”

“ Pure and uncombined hydrogen is not employed in the arts. Inhalations of this gas have, however, been occasionally used in medicine. Dr Beddoes recommended them in phthisis. In combination, the uses of hydrogen are almost numberless. Combined with oxygen, it forms water; with chlorine, hydrochloric acid; with fluorine, hydrofluoric acid; with cyanogen, hydrocyanic acid; with carbon, innumerable hydrocarbons; with nitrogen, ammonia; with sulphur, sulphuretted hydrogen—in fact, an enumeration of the valuable compounds which it enters into would occupy many pages of this work. ”

“ Second, hydrofluoric acid will bite its way through glass. The thicker the glass, the longer the time required to set the acid free. Do you follow?"
"Yes," said Hillyard.
"Good! Make a glass tube of such thickness that it will take hydrofluoric acid four hours and a half to eat its way through. Then fill it with acid and seal it up. You have a time-fuse which will act precisely in four hours and a half."
"If it comes into contact with the necessary explosive," Hillyard added.
"Exactly. Now attend to this! Our workmen in our munition factories work three hours and a half. ”

“ Whilst the electrolysis is proceeding, the apparatus is kept at a constant temperature of −23° C. by means of liquid methyl chloride. The fluorine, which is liberated as a gas at the anode, is passed through a well cooled platinum vessel, in order to free it from any acid fumes that may be carried over, and finally through two platinum tubes containing sodium fluoride to remove the last traces of hydrofluoric acid; it is then collected in a platinum tube closed with fluor-spar plates. ”

“ Hydrogen.—As just described, hydrogen combines with fluorine, even at -23° and in the dark, with explosive force. This is the only case in which two elementary gases unite directly without the intervention of extraneous energy. If the end of the tube delivering fluorine is placed in an atmosphere of hydrogen, a very hot blue flame, bordered with red, at once appears at the mouth of the tube, and vapor of hydrofluoric acid is produced. ”

“ During the first absorption of silicon fluoride by water, silicic acid is separated in the form of a jelly, but a certain quantity of the silicon fluoride also remains in the liquid, because the hydrofluoric acid formed dissolves the other part of the silica [9] and forms the so-called hydrofluosilicic [106] acid: H2SiF6 = SiF4 + 2HF = SiH2O3 + 6HF - 3H2O. That is to say, a metasilicic acid, SiH2O3, in which O3 is replaced by F6. This view of the composition of hydrofluosilicic acid may be admitted, because it forms a whole series of crystallisable and well defined salts. ”

“ When you try this chemical experiment, do not inhale the vapors of the hydrofluoric acid, for they are very injurious, and burn the skin very badly. Always experiment in a room to which plenty of fresh air has free access. ”

“ It was first isolated by H. Moissan in 1886 by the electrolysis of pure anhydrous hydrofluoric acid containing dissolved potassium fluoride. The U-shaped electrolytic vessel and the electrodes are made of an alloy of platinum-iridium, the limbs of the tube being closed by stoppers made of fluor-spar, and fitted with two lateral exit tubes for carrying off the gases evolved. ”

“ After a short account of the main properties of the silicated fluoric acid gas (silicon fluoride) , discovered by Scheele, fluoric acid (hydrofluoric acid) , discovered by Scheele but first obtained pure by Gay Lussac and Thenard, and fluoric acid (boron fluoride) , discovered by Gay Lussac and Thenard, he states that, on the hypothesis of M. Ampère—
“the silicated fluoric acid is conceived to consist of a peculiar undecompounded principle, analogous to chlorine and oxygen, united to the basis of silica, or silicum ”

“ Recovery of gaseous HF is reduced at high humidity.
NIOSH Manual of Analytical Methods (NMAM) , Fifth Edition PARTICULATE FLUORIDES and HYDROFLUORIC ACID: METHOD 7906, Issue 2, dated 20 May 2014 - Page 2 of 6
OTHER METHODS: This method updates NIOSH 7906 [7] by providing full method accuracy information, and specifies the use of an impregnated filter instead of a treated cellulose pad. The procedure, which is consistent with ISO 21438-3 [6] , replaces NIOSH 7903 [8] for the determination of particulate fluorides and gaseous HF in workplace air samples. ”

“ Place a 10 mL aliquot of fluoride standard solution into a 100 mL volumetric flask, dilute to the mark and mix thoroughly. The solution is stable for at least 4 weeks at room temperature. 9.
To prepare the impregnated filter, evenly distribute 120 μL 0.75 M Na2CO3 over the surface of the filter and allow to dry at room temperature for several hours.
See Special Precautions 1.
Sampler: 37-mm diameter cellulose nitrate pre-filter, 0.8 μm pore size, 37-mm impregnated cellulose nitrate filter. The untreated filter collects particulate fluorides, while the impregnated filter collects HF vapor. ”

“ Acids, particularly HF, are extremely corrosive to skin, eyes, and mucous membranes. HF will attack glass. Plastic labware is recommended. Wear gloves, lab coat, and safety glasses while handling acids. All work should be performed with adequate ventilation for personnel and equipment. It is imperative that acid be added to water in order to avoid a violent exothermic reaction [11,12] . SAMPLING, SAMPLE TRANSPORT AND STORAGE: 1.
Prior to sampling, load each clean sampler, first with a sampling (impregnated) filter, then with a prefilter, separating the filters with a spacer. ”

“ It is readily decomposed by water anti) alkaline hydroxides, yielding a mixture of nitrite and chloride. On treatment with silver fluoride it yields nitrosyl fluoride, NOF (O. Ruff, Zeit. anorg. Chem., 1905, 47, p. 190) . Nitroxyl fluoride, NO2F, is formed by the action of fluorine on nitric oxide at the temperature of liquid oxygen (H. Moissan and P. Lebeau, Comptes rendus, 1905, 140, pp. 1573, 1621) . It is a gas at ordinary temperature; when liquefied it boils at −63.5° C. and on solidification melts at −139° C. Water decomposes it into nitric and hydrofluoric acids. ”

“ Using 5- or 10-mL syringes, filter each sample extract solution through a PTFE filter and a styrenebased sulfonic acid resin cartridge (follow the manufacturer’s instructions) , discard the first two milliliters and place the remaining solution in clean plastic vessels. CALIBRATION AND QUALITY CONTROL: 12.
With dilution of the calibration stock solution in eluent solution, prepare calibration working standard solutions covering the range of approximately 0.4 to 8 μg/mL of fluoride. Store working standards in tightly sealed polyethylene bottles. Prepare fresh working standards biweekly. 13. ”

“ To obtain it disengaged from that combination, fluor spar, or fluat of lime, is put into a leaden retort, with a proper quantity of sulphuric acid, a recipient likewise of lead, half full of water, is adapted, and fire is applied to the retort. The sulphuric acid, from its greater affinity, expels the fluoric acid which passes over and is absorbed by the water in the receiver. As fluoric acid is naturally in the gasseous form in the ordinary temperature, we can receive it in a pneumato-chemical apparatus over mercury. ”

“ The Sixth Division comprehends certain bodies (the Fluoric Principle, and the Ammoniacal Amalgam) which present some extraordinary and anomalous results. It is worthy of remark, that, at the period at which this work was written, Davy considered the peculiar acid developed from fluor spar, by the action of sulphuric acid, as a compound of an acid unknown in a separate state, and water; whence he proposed to call it Hydro-fluoric acid,—a term extremely objectionable from its ambiguity, since it would indicate either hydrogen or water as one of its constituents. ”

“ Potassium and sodium readily dissolve in the anhydrous acid with evolution of hydrogen and formation of fluorides. The aqueous solution is strongly acid to litmus and dissolves most metals directly. Its most important property is that it rapidly attacks glass, reacting with the silica of the glass to form gaseous silicon fluoride, and consequently it is used for etching. ”