Fluoride solution

The i-V curves, as shown in Fig. 5.2, are different for p-Si and n-Si in the dark due to the difference in the concentrations of holes, which are required for the anodic reactions, in the two types of materials. Large currents can be obtained on p-Si by anodic polarization which gives a forward bias to increase the concentration of holes at the surface. On the other hand, for nondegenerated n-Si the anodic current is limited by the availability of holes. The i-V curve for n-Si becomes identical to that for p-Si when n-Si is illuminated at a sufficiently high light intensity. 

The anodic polarization curves of p-Si or strongly illuminated n-Si in fluoride solutions are typically characterized hy two peak currents, Ji and J2, and two plateau currents, J3 and A as shown in Fig. 5.2. At anodic potentials up to that at J], the electrode behavior is characterized hy an exponential dependence of current on potential and by the uneven dissolution of silicon surface leading to the formation of porous 

FIGURE 5.2. Current-potential curves for dark and illuminated p- and n-Si electrodes in a 2.5 wt % HF electrolyte. After Lehmann and Foil. (Reproduced by permission of The Electrochemical Society, Inc.) 

The potentials of the two current peaks, Ji and J2, extend to more positive values for lowly dopedp-Si A difference of as much as 1 V for the first current peak and about 1.5 V for the second is found between lowly and highly dopedp-type materials as shown in Fig. 5.4. This is attributed to a larger potential drop in the space charge layer of the lowly doped sample. 

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Hydrochloric acid should not be used for acidifying the alkaline solution since the yellow colour, due to the ferric chloride formed, causes the Prussian blue to appear greenish. For the same reason, ferric chloride should not be added—as is frequently recommended a sufficient concentration of ferric ions is produced by atmospheric oxidation of the hot alkaline solution. The addition of a little dfiute potassium fluoride solution may be advantageous in assisting the formation of Prussian blue in a readily filterable form. 

Ammonia. Ammonia (qv) reacts with excess fluorine ia the vapor phase to produce N2, NF, N2F2, HF, and NH F. This reaction is difficult to control ia the vapor phase because of the iatense heat of reaction, and ia some cases only N2 and HF are produced. Nitrogen trifluoride was obtained ia 6% yields ia a gas-phase reaction over copper (42). Yields of ca 60% are achieved by the reaction of fluorine and ammonia ia a molten ammonium acid fluoride solution (43,44). 

In addition, there are other methods of manufacture of cryoHte from low fluorine value sources, eg, the effluent gases from phosphate plants or from low grade fluorspar. In the former case, making use of the fluorosiHcic acid, the siHca is separated by precipitation with ammonia, and the ammonium fluoride solution is added to a solution of sodium sulfate and aluminum sulfate at 60—90°C to precipitate cryoHte (26,27) ... 

Manufacture. Anhydrous ammonium bifluoride containing 0.1% H2O and 93% NH4HF2 can be made by dehydrating ammonium fluoride solutions and by thermally decomposing the dry crystals (7). Commercial ammonium bifluoride, which usually contains 1% NH F, is made by gas-phase reaction of one mole of anhydrous ammonia and two moles of anhydrous hydrogen fluoride (8) the melt that forms is flaked on a cooled dmm. The cost of the material in 1992 was 1.48/kg. 

Silver difluoride [7783-95-1], AgF2, is a black crystalline powder. It has been classified as a hard fluorinating agent (3) which Hberates iodine from KI solutions and o2one from dilute aqueous acid solutions on heating. It spontaneously oxidizes xenon gas to Xe(II) in anhydrous hydrogen fluoride solutions (20). 

Nitrosyl chloride (178), nitrosyl chloride—hydrogen fluoride (NOF -3HF, NOF -6HF) (179), nitrous acid—hydrogen fluoride solutions (180,181), or nitrogen trioxide (prepared in situ from nitric oxide and oxygen) (27) can be used in place of sodium nitrite in the dia2oti2ation step. 

Niobic Acid. Niobic acid, Nb20 XH2O, includes all hydrated forms of niobium pentoxide, where the degree of hydration depends on the method of preparation, age, etc. It is a white insoluble precipitate formed by acid hydrolysis of niobates that are prepared by alkaH pyrosulfate, carbonate, or hydroxide fusion base hydrolysis of niobium fluoride solutions or aqueous hydrolysis of chlorides or bromides. When it is formed in the presence of tannin, a volurninous red complex forms. Freshly precipitated niobic acid usually is coUoidal and is peptized by water washing, thus it is difficult to free from traces of electrolyte. Its properties vary with age and reactivity is noticeably diminished on standing for even a few days. It is soluble in concentrated hydrochloric and sulfuric acids but is reprecipitated on dilution and boiling and can be complexed when it is freshly made with oxaHc or tartaric acid. It is soluble in hydrofluoric acid of any concentration. 

Hot oleum (>50°C), strong alkalis, fluoride solutions, sulphur trioxide Strong alkalis, especially >54°C, distilled water >82°C, hydrofluoric acid, acid fluorides, hot concentrated phosphoric acid, lithium compounds >1 77°C, severe shock or impact applications Strong oxidizers, very strong solvents... 

One change of the aqueous potassium fluoride solution 1-Chloroheptene... 

Hydroxybenzoic acids are easily eonverted into hydroxybenzotrifluorides by treatment with sulfur tetrafluoride in a hydrogen fluoride solution under mild conditions [2/5] An exception is salicylic acid, which becomes a resin, however, a 70-75% yield of 2-hydroxybenzotrifluoride can be obtained by carrying out the fluonnation in a mixture of three parts of hydrogen fluoride and one part of benzene [216] (equation 110)... 

Cyclic ethylene carbonate and its halogeno derivatives are converted into 2 2 difluoro 1,3 dioxolanes, which are useful as inhalation anaesthetics by treatment with sulfur tetrafluonde m an anhydrous hydrogen fluoride solution at 100-150 °C [239] (equation 126)... 

Chlorine monofluoride m a hydrogen fluoride solution reacts rapidly at low temperature with haloacetate and halopropanoate esters to give 61-80% yields of a a difluoroalkyl ethers [242] (equation 128)... 

Nitrosyl chloride [55], nitrosyl fluoride-hydrogen fluoride liquid complexes (NOF3HF, NOF 6HF) [56], nitrous acid-hydrogen fluoride solutions [57, 5 ] nitrogen trioxide (prepared in situ from nitric oxide and oxygen) [59] and rert-butyl nitrite-hydrogen fluoride-pyndine [60] have been substituted for sodium nitrite in the diazotization step... 

Sodium silicate and silico fluoride solutions as concrete surface hardeners 9/104... 

Both sodium silicate and silico fluoride solutions are applied to clean, dry, sound concrete floors as dilute aqueous solutions (10-15 per cent solids) in two to three applications, taking care to ensure that all material penetrates and is absorbed into the concrete surface. The silicate or silico fluoride reacts with the small amount of free lime in the cement to form glassy inert materials in the surface, and the successful application of both materials depends upon filling the micropores in the surface of good-quality concrete, leaving its surface appearance and non-skid characteristics virtually unchanged. 

Tantalum and niobium oxides dissolve very slowly in HF solutions. Thus, it is recommended to use a high concentration of HF or a mixture of HF and H2SO4 at a temperature of about 70-90°C. The best precursors for the preparation of fluoride solutions are hydroxides. Both tantalum hydroxide, Ta205 nH20, and niobium hydroxide, M Os-nHjO, dissolve well, even in diluted HF solutions. 

No available data was found on the precipitation from fluoride solutions of niobium and tantalum fluoride compounds containing tri- and tetravalent metals. 

The fact that tantalum and niobium complexes form in fluoride solutions not only supplements fundamental data on the coordination chemistry of fluoride compounds, but also has a broad practical importance. This type of solution is widely used in the technology of tantalum and niobium compounds in raw material digestion, liquid-liquid extraction, precipitation and re-pulping of hydroxides, and in the crystallization and re-crystallization of K-salts and other complex fluoride compounds. 

NMR, Raman and IR spectroscopy are most frequently used to investigate the complex structures of fluoride solutions containing tantalum and niobium. Most investigations of such solutions were performed on the liquid-liquid extraction of tantalum and niobium, with the objective of describing the mechanism of the process. These publications will be discussed separately. 

Raman spectra of fluoride solutions containing niobium were investigated by Keller [171]. Solutions were prepared by dissolving niobium fluoride compounds in solutions of hydrofluoride acid, HF, of different concentrations. 

Complexes in fluoride solutions - niobium-containing solutions... 

Table 44. Composition of niobium-containing complexes in fluoride solutions of different concentrations (after Keller [171]).

Only two types of complex ions are present in fluoride solutions NbOF52 and NbF6 the equilibrium of which is described by Equation (47). Higher concentrations of HF lead to the formation of NbF6 ions. 

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