Fluorspar apparatus

Moissan finally used as electrolyte a solution of dry potassium acid fluoride in anhydrous hydrofluoric acid. His apparatus consisted of two platinum-iridium electrodes sealed into a platinum U-tube closed with fluorspar screw caps covered with a layer of gum lac (42, 49, 59). The U-tube was chilled with methyl chloride, the gas now used in many modem refrigerators, to a temperature of —23°. 

Hydrofluoric acid is the most basic common precursor of most fluorochemicals. Aqueous hydrofluoric acid is prepared by reaction of sulfuric acid with fluorspar (CaF2). Because HF etches glass with formation of silicon tetrafluoride, it must be handled in platinum, lead, copper. Monel (a Cu-Ni alloy developed during the Manhattan Project), or plastic (e.g. polyethylene or PTFE) apparatus. The azeotrope contains 38 % w/w HF and it is a relatively weak acid (pfC 3.18, 8 % dissociation), comparable with formic acid. Other physicochemical properties of hydrofluoric acid are listed in Table 1.2. 

Scheele made a number of other extremely valuable contributions to chemistry. He added oil of vitriol to fluorspar (CaF ) and distilled it. He found extensive corrosion of the retort containing the reaction mixture all the luting used to seal the apparatus was now brittle and friable, and a white deposit was formed on all interior surfaces of the apparatus. Scheele had formed hydrofluoric acid (HE), which attacked the glass vessel (silicon dioxide) and formed gaseous SiF. When he duplicated the experiment but added water to the receiver, he was surprised to find a layer of gelatinous silica on top of the water. 

The laboratory equipment of the fluorine chemist is unusual in that ordinary chemical glassware cannot be used in most cases. Apparatus made of nickel, iron, copper, lead, silver, platmum, fluorspar or sintered alumina is used instead. Where transparency is indispensable, quartz equipment is utilized. 

An apparatus similar to that described for the preparation of SFs (p. 169) is used for the fluorination of pulverized electrolytic Cr or CrClg contained in a fluorspar or small alumina vessel. The fluorination temperature is 350-500°C. Some CrF, as well as most of the CrFs, migrates into the receiver. The main portion of the CrF4 is deposited in the reaction tube beyond the boat in varnish-like, glittering brown beads. When the fluorination is complete, the apparatus is flushed out with Ng or CO and the CrF4 is immediately sealed o in glass ampoules. 

After this, anhydrous HF at 300°C is passed over the CoClg, contained in a small fluorspar boat placed inside an iron tube, until evolution of HCl is no longer detectable at the end of the tube. The apparatus is then flushed with dry nitrogen to remove excess HF. 

The fluorspar tube is connected to the quartz sections of the apparatus with ground joints, which are externally sealed with an asbestos-waterglass mixture. 

The mineral fluorspar had been used as a flux since the middle ages (the name comes from the Latin meaning yZow), and in the Traite Lavoisier had included the fluoric radical as a simple substance (element) as yet unknown. Scheele obtained hydrofluoric acid in 1771. In 1813 Ampere pointed out the many similarities between the fluoric and muriatic (chlorine) compounds. He suggested the unknown element be called fluorine, and in the same year Humphry Davy tried to prepare it by the electrolysis of hydrofluoric acid. The corrosive nature of the acid presented insuperable problems, and it was to be another 73 years before the isolation of the element was achieved. During that time the toxic nature of fluorine compounds was to be responsible for the deaths of at least two chemists and was to ruin the health of many more. Success was finally achieved by Henri Moissan (1852-1907), who electrolysed potassium fluoride dissolved in anhydrous hydrofluoric acid using platinum apparatus. 

Oxidized chalcogens. Hydrogen fluoride is often prepared by treating fluorspar, Cap2, with H2SO4 and distilling off the HF, which is absorbed in water. The operation is carried out in lead-lined apparatus. 

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