Current hydrofluoric acid

Toxicity. Sulfur tetrafluoride has an inhalation toxicity comparable to phosgene. The current OSHA standard maximum allowable concentration for human exposure in air is 0.4 mg/m (TWA) (54). On exposure to moisture, eg, on the surface of skin, sulfur tetrafluoride Hberates hydrofluoric acid and care must be taken to avoid bums. One case of accidental exposure of electrical workers to decomposed SF gas containing SF has been cited (108). 

Simons An electrochemical method for fluorinating organic compounds. First developed by J. H. Simons at Pennsylvania State College in 1941 but not announced until 1948 for reasons of national security. A direct current is passed through a solution of an organic compound in anhydrous hydrofluoric acid hydrogen is evolved at the cathode and the organic material is fluorinated at the anode. 

The same behaviour is observed in perchloric or hydrofluoric acid solutions. Unfortunately a quantitative analysis cannot be carried out because the unusual adsorption state current is merging into the current of oxygen adsorption at the surface defects, the former decreases Nhile the latter increases with surface defects density (17). 

Fig. 5.7 Time variation of titanium electrode etching current density for (a) Nitric acid/hydrofluoric acid electrolyte (b) Boric acid/nitric acid/hydrofluoric acid electrolyte.

According to H. Lose, the dry crystals are not deliquescent J. J. Berzelius says they are. The crystals have a negative double refraction. Ammonium hydrofluoride volatilizes completely when heated. According to H. Rose, when ammonium hydrolluoride is mixed with silica or a silicate and calcined, the silicate is more readily decomposed than if hydrofluoric acid had been used. On electrolysis with a current of 5 amperes, 0. Ruff and E. Geisel9 obtained fluorine. 

Another preparative method in which the rate of precipitation is slow involves slow decomposition of zirconium fluoro complexes [14], These are first prepared by adding an appropriate amount of hydrofluoric acid (HF) to the zirconyl salt and these complexes are decomposed in the presence of phosphoric acid, with a slow stream of nitrogen or water vapor passing through the system. The rate of precipitation of zirconium phosphate is controlled by the rate of removal of HF from the system, and when this is very slow, a highly crystalline a-ZrP is obtained. The gamma form of the metal phosphate differs significantly from the alpha and current discussion will be concerned with the latter phase. 

Porous silicon was discovered over 35 years ago by Uhlir.28 The porous material is created by electrochemical dissolution in HF-based electrolytes. Hydrofluoric acid, on its own, etches single-crystal Si extremely slowly, at a rate of only nanometers per hour. However, passing an electric current between the acid electrolyte and the Si sample speeds up the process considerably, leaving an array of deep narrow pores that generally run perpendicular to the Si surface. Pores measuring only nanometers across, but micrometers deep, have been achieved under specific etching conditions. 

Current efficiency may be increased by adding a substance such as hydrofluoric acid which raises the anode potential, and also by adding sulphurous acid or hydrogen sulphide which destroys Caro s acid but does not affect persulphuric acid. The addition of sulphurous acid to the point of saturation in sulphuric acid of density 1 38 raises the current efficiency to 92 per cent. The addition of hydrochloric acid to the bath has a beneficial effect because it raises the anode potential and also destroys Caro s add, and so removes the harmful depolarising effect of this substance. It has been shown that the concentration of persulphuric acid increases with rise in current density, but the final concentration of Caro s acid is independent of the current density. 

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