Fluorides concentration

Health and Safety Factors. The low solubiUty of calcium fluoride reduces the potential problem of fluoride-related toxicity. Water saturated with calcium fluoride has a fluoride concentration of 8.1 ppm as compared to the recommended water fluoridation level of 1 ppm fluoride ion. However, because the solubiUty of calcium fluoride ia stomach acid is higher, continued oral ingestion of calcium fluoride could produce symptoms of fluorosis. The adopted TWA limit for fluorides as F is 2.5 mg/m (68,69). 

Fluoride. A fluoride concentration of ca 1 mg/L is helpful in preventing dental caries. Eluoride is deterrnined potentiometrically with an ion-selective electrode. A buffer solution of high total ionic strength is added to the solution to eliminate variations in sample ionic strength and to maintain the sample at pH 5—8, the optimum range for measurement. (Cyclohexylenedinitrilo)tetraacetic acid (CDTA) is usually added to the buffer solution to complex aluminum and thereby prevent its interference. If fluoroborate ion is present, the sample should be distilled from a concentrated sulfuric acid solution to hydrolyze the fluoroborate to free fluoride prior to the electrode measurement (26,27). 

The total yield of products from alkanecarboxyhc acids increases, in most cases, by addition of anhydrous hydrogen fluoride. The optimum hydrogen fluoride concentration is much higher than catalytic and is related to the basicity of a carbonyl group. A mechanism for the formation of both 1,1,1-trifluoroalkanes and bis(l,l-difluoroalkyl) ethers has been proposed [206] (equation 102)... 

Based on a comprehensive investigation, Welham [453] recommends the use of a strong acid or high fluoride concentration in order to achieve optimal leaching. Specifically, the use of hydrofluoric acid with a concentration > 18% or fluoride concentration > 0.5 mol/1 was indicated. It is also noted that leaching at elevated temperatures leads to the separation of titanium by hydrolytic precipitation from the solution. 

Treat the fluoride sample solution in the same manner as described for the calibration curve after removing interfering ions and adjusting the pH to about 5 with dilute nitric acid or sodium hydroxide solution. Read off the fluoride concentration from the calibration curve and the observed value of the absorbance. 

The effect of fluoride was further demonstrated by the increase in plutonium solubility in deionized water from about 11 percent to essentially 100 percent by addition of sufficient NaF to raise the fluoride concentration to that of basalt ground water. It is likely that the enhanced solubility of plutonium in waters containing high fluoride concentrations is the result of stabilization of Pu(IV) in solution by formation of fluoride complexes. Normally Pu(IV) is the least soluble of the four... 

Figure 2. Solubility of maninite as a function of pH at 25°C for different fluoride concentrations. Formation of uranous fluoride complexes greatly enhances maninite solubility below pH 3-4. The increase of maninite solubility at higher pH results from the formation of uranous hydroxyl complexes. [Used with permission of Elsevier Science, fromLangmuir (1978) Geochim Cosmochim Acta, Vol. 42, Fig. 4, p. 555].

The first result in Table 2.2 is the value found directly. The next set of three results shows that recovery of fluoride after deliberate addition to the spiked samples is excellent. The fifth result is that obtained from an acidified sample. The sixth, seventh, eighth, and ninth sets of data show that the expected fluoride concentration is still obtained after deliberate addition of aluminium or iron in the form of their alums. Aluminium (III) and iron (III) form very strong fluoride complexes [70] and, provided that sufficient time is allowed for equilibration (as noted by Baumann [64] for very low fluoride concentrations), total flu-... 

As a final example, we consider a fluid of known fluoride concentration whose calcium content is set by equilibrium with fluorite (CaF2). The speciation of fluorine provides for two solutions to this problem. In dilute solutions, in which the free ion I dominates, the reaction,... 

Fig. 7. The 13C NMR spectra for [WO(X)(13CN)4]m illustrating the variation in chemical shift and MO W- C) coupling constants. For [WO(13CN)5]3 the total complex concentration [W(IV)] = 0.2 m, the total cyanide concentration [13CN] = 0.3 m, pH = 8.8 for [WO(F)(13CN)4]3 the total complex concentration [W(IV)] = 0.2 m, the total cyanide concentration [13CN] = 0.3 m, the total fluoride concentration [KF] = 0.33 m, pH = 5.2 and for [W0(H20)(13CN)4]2 the total complex concentration [W(IV)] = 0.2 m, the total cyanide concentration [13CN] = 0.3 m, pH = 3.1 (8). (Adapted with permission from Abou-Hamdan, A. Roodt, A. Merbach, A. E. Inorg. Chem. 1998, 37, 1278-1288. Copyright 1998 American Chemical Society).

Fig. 1.2 The calculated fraction of each component in an aqueous HF solution as a function of pH for a fixed total fluoride concentration of 7.5 mol I-1. Redrawn from [Vel].

The characteristic shape of the anodic voltammogram of a Si electrode in aqueous fluoride media, as shown for example in Fig. 3. Id, is surprisingly stable against changes in fluoride concentration (cF) or pH. When the potential of a p-type Si electrode is swept anodic of OCP a steep current rise near 0 V is observed, followed by a sharp peak (Jj) and a narrow plateau (J2). Then a second broad maximum (Ji) is found around a positive bias of 1.5-2.5 V, followed by a broad plateau (J4) extending over several volts, as shown in Fig. 4.7. When electrode rotation is used, these curves are pen-reproducible for a given solution. The hysteresis of the curves approaches zero for slow sweeps [Ch3]. 

Equation (19) is valid for a large number of aromatic substances investigated in the system HE + NaF. A higher alkali fluoride concentration was necessary for aromatic substances of high basicity. For solubility... 

Boyd et al. have electroplated technetiiun under a variety of conditions. Optimum results are obtained at pH 5.5 in the presence of about 10 M fluoride concentration. Yields are higher when copper instead of platimun cathodes are used. At a current density of 100 mA/cm 98.5% of technetiiun is deposited in 2 h. However, yields of 98-99% are obtained at similar current densities even with platinum cathodes, at pH 2 to 5 and fluoride concentrations of 5 x 10 M with plating times of up to 20 h... 

Examination of 13 workers with present or past occupational exposure found 8 with abnormalities of pulmonary function chest X rays were negative, and preshift urinary fluoride concentrations did not exceed 4mg/l. Air sampling showed concentrations ranging from 0.1 to 1.8 ppm. Dryness of the nasal mucosa and epistaxis were attributed to boron trifluoride exposure in workers exposed to high concentrations for 10-15 years. Exposures of 50 ppm for 3 0-60 min are expected to be lethal to humans. ... 

Workers exposed to an airborne fluoride concentration of 5mg/m complained of eye and respiratory tract irritation and nausea. The lethal oral dose of sodium fluoride for humans has been estimated to be 32-65 mg F/kg of body weight. Effects from ingestion are diffuse abdominal pain, diarrhea, and vomiting excessive salivation, thirst, and perspiration painful spasms of the limbs and sometimes albuminuria." Gastrointestinal effects produced after the acute ingestion of toxic amounts of fluoride likely arise from the corrosive action of hydrofluoric acid, which is produced within the acidic environment of the stomach. Cardiac arrest after accidental exposure to high levels of fluoride has been attributed to the development of hypocalcemia and/or hyperkalemia. ... 

Fluoride concentration in the urine has been used as a biological indicator of fluoride." Most absorbed fluoride is excreted rapidly in the urine. A portion is stored in bone, but a nearly equal amount is mobilized from bone and excreted. Some storage of fluoride occurs from the ingestion of as little as 3 mg/ day. Evidence from several sources indicates that urinary fluoride concentrations not exceeding 5 mg/1 in preshift samples taken after 2 days off work are not associated with detectable osteosclerosis and that such changes are unlikely at urinary levels of 5-8 mg/1." Preshift urinary fluoride concentration is considered to be a measure of the worker s body (skeletal) burden of fluoride, whereas the postshift sample is taken to be representative of exposure conditions during that work shift. 

Biological monitoring of urinary fluoride concentration provides an indication of total fluoride intake. Data indicate that a postshift urinary fluoride level of less than 8 mg/1, averaged over an extended period of time, will not lead to osteosclerosis, although a minimal or questionable increase in bone density might develop after many years of occupational exposure. ... 

The effect of fluoride ions on the electrochemical behaviour of a metal zirconium electrode was studied by Pihlar and Cencic in order to develop a sensor for the determination of zirconium ion. Because elemental zirconium is always covered by an oxide layer, the anodic characteristics of a Zr/Zr02 electrode are closely related to the composition of the electrolyte in contact with it. These authors found the fluoride concentration and anodic current density to be proportional in hydrochloric and perchloric acid solutions only. In other electrolytes, the fluoride ion-induced dissolution of elemental zirconium led to an increase in the ZrOj film thickness and hindered mass transport of fluoride through the oxide layer as a result. The... 

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