Dilute Fluoride Solutions Research Articles

The influence of intrapore cation on the fluorination of zeolite Y

Abstract The influence of the intrapore cation on the fluorination of zeolite Y from dilute fluoride solutions has been studied, revealing fluoride reacts with the zeolite framework in the presence of a Bronsted acid to form [SiO3F] and [AlO3F] moieties. 29Si{1H} Cross-polarised MAS NMR indicates the reaction proceeds by the substitution of surface hydroxide moieties for fluoride. The fluorination reaction is strongly influenced by the nature of the intrapore cation. Intrapore Bronsted acids facilitate fluorination of the framework by in situ ion-exchange, releasing the acidic ions to the zeolite surface. The fluorination reaction may be further promoted by the presence of intrapore alkaline earth cations (viz. Mg2+, Ca2+, Sr2+ and Ba2+). The conclusions of this work are significant to the preparation of fluorinated zeolite catalysts, the application of zeolites in defluoridation and the labelling of zeolite-based tracers with 18F for application in positron imaging techniques.

Role of the P–F Bond in Fluoride-Promoted Aqueous VX Hydrolysis: An Experimental and Theoretical Study

Following our ongoing studies on the reactivity of the fluoride ion toward organophosphorus compounds, we established that the extremely toxic and environmentally persistent chemical warfare agent VX (O-ethyl S-2-(diisopropylamino)ethyl methylphosphonothioate) is exclusively and rapidly degraded to the nontoxic product EMPA (ethyl methylphosphonic acid) even in dilute aqueous solutions of fluoride. The unique role of the P-F bond formation in the reaction mechanism was explored using both experimental and computational mechanistic studies. In most cases, the "G-analogue" (O-ethyl methylphosphonofluoridate, Et-G) was observed as an intermediate. Noteworthy and of practical importance is the fact that the toxic side product desethyl-VX, which is formed in substantial quantities during the slow degradation of VX in unbuffered water, is completely avoided in the presence of fluoride. A computational study on a VX-model, O,S-diethyl methylphosphonothioate (1), clarifies the distinctive tendency of aqueous fluoride ions to react with such organophosphorus compounds. The facility of the degradation process even in dilute fluoride solutions is due to the increased reactivity of fluoride, which is caused by the significant low activation barrier for the P-F bond formation. In addition, the unique nucleophilicity of fluoride versus hydroxide toward VX, in contrast to their relative basicity, is discussed. Although the reaction outcomes were similar, much slower reaction rates were observed experimentally for the VX-model (1) in comparison to VX.

Electrochemical Growth of Micrometer-Thick Oxide on SiC in Acidic Fluoride Solution

Anodic polarization of SiC at modest potential in dilute fluoride solution of pH 3 surprisingly gives rise to the growth of micrometer-thick surface layers, clearly revealed with scanning electron microscopy. The reaction occurs at p-type SiC in the dark and at n-type SiC under (supra)bandgap illumination. The surface layer was shown by Rutherford backscattering spectrometry (RBS) to consist of silicon dioxide and to contain excess oxygen. Elastic recoil detection (ERD) indicated only a low level of carbon and fluoride in the layer but a considerable content of hydrogen. The growth kinetics was characterized in situ by spectroscopic ellipsometry and electrical impedance spectroscopy. The results suggest the formation of a duplex layer: a thin inner dielectric oxide and a thick hydrated outer oxide. The latter must have a considerable degree of porosity to allow diffusion/migration of reactants and products during oxide growth.

Etch Rates of Anodic Silicon Oxides in Dilute Fluoride Solutions

The etch rates of anodic oxides in dilute fluoride solutions have been determined by using a new approach to the analysis of anodic current oscillations. The variation of the thickness of the anodic oxide with time has been measured by in situ Fourier transform infrared spectroscopy. The change in oxide thickness can be simulated by considering the time-dependent current and the etch rate of the oxide in the HF-containing solution. The pH-dependent etch rates can be well fitted for different concentrations of by applying Judge’s model [J. Electrochem. Soc., 118, 1772 (1971)]. The coefficients in Judge’s model are obtained for the etching process of the respective anodic oxides. The electronic properties of the Si/anodic oxide interface, which have been investigated by in situ photovoltage and photoluminescence measurements, correlate with the anodic current. © 2003 The Electrochemical Society. All rights reserved.

学校歯科保健活動における低濃度フッ化物洗口法導入によるう蝕予防効果の研究

The purpose of this study was to clarify the cariostatic effect when mouth rinsing with dilute fluoride solution was introduced into school dental health programs.The subjects were 300 children (160 boys and 140 girls), who used a 100ppm fluoride mouth rinse with for 6 years. These subjects were analysed by cohort observation. As controls, 943 schoolchildren (479 boys and 464 girls) in 1986 and 1987 who had no fluoride mouth rinsing were analysed by longitudinal observation and were compared with the fluoride group. A precise oral examination was carried out once a year. An evaluation of cariostatic effect was performed by DMFT index, DMF rate for each tooth type, and comparison between fluoride groups of DMF rate of upper and lower first molar for each school grade at time of eruption.The increment of DMFT index in the fluoride group was less than in the control group. It showed a statistically significant cariostatic effect in the higher grades for both boys and girls. The DMF rate for each tooth type of 1, 2, 4, 5, 6, 7, 5 and 7 was reduced in the fluoride group and showed a statistically significant difference for both boys and girls. The DMF rate of upper first molar erupted during the first or second school grade showed no difference, but there was cariostatic effect when the molar erupted during the third grade.As a result, we recognized that a high cariostatic effect was obtained by introducing mouth rinsing with a solution of 100ppm fluoride into school dental health programs. Observation of DMF rate for each tooth type classified by school grade at time of eruption was a useful method for evaluating the cariostatic effect of fluoride mouth rinsing.

Interaction of dilute fluoride solutions with hydrous iron oxides

The amount of F- retained by hydrous iron oxide suspensions was found to be a function of F- concentration (2-100 mg L-1), pH, and structural form of the iron oxide. Uptake was minimal at pH >7, and increased with decreasing pH (e.g, with 10 mg L-1 NaF in solution, the amount sorbed by freshly precipitated hydrous iron oxide varied between 7 mg/g at pH 6 and 35.5 mg/g at pH 4.5). With fixed pH (between 4.5 and 6) the amount sorbed varied with concentration in accordance with a Langmuir isotherm relationship. The pH effect was countered at lower values by solid dissolution, hence each F- concentration studied exhibited a pH (between 3.5 and 4.5) where uptake was a maximum. Solid dissolution was accompanied by the formation of soluble iron(III) fluoro complexes. In acid media, the amount of F sorbed by hydrous iron oxide >> goethite > limonite >> hematite. In environmental systems, F interaction and retention should thus be most pronounced in acid media, containing recently precipitated hydrous oxides.

Interaction of clays with dilute fluoride solutions

Interaction between dilute (mg L−1) NaF solutions and clay suspensions (0.08 % w/v) has been examined as a function of pH (range 3 to 8), clay type (Na+- or Ca2+-kaolinite, illite, montmorillonite) and NaF concentration. No F loss from solution was detected at pH > 6.5, while enhanced uptake was found on decreasing the pH, especially in the 4 to 3 region. Removal of F from 1 to 6 × 10−4 M NaF was only slightly dependent on weight of solid, but did increase with [F−]. It is proposed that F losses are due to the formation of sparingly soluble Al species (e.g. cryolite, Na fluoro silicate), occasionally augmented with CaF2 formation (Ca2+-clays). The Al is released by proton attack on the lattice, following conversion of the suspended solids into the unstable H+-form, either through acid addition (pH < 5) or through hydrolysis of the Na+-form material. The latter process was most pronounced with the illite and montmorillonite samples. The amount of F fixed by montmorillonite was roughly double that held by the other two clays, and had a maximum value (pH 3) of ∼ 4 mg g−1, using 11 mg L−1 NaF solutions. Soluble fluoro-complexes, similar in quantity to the retained F, were detected, in many of the studies. It was concluded that contact of the clay components of soils or sediments with mg L−1 levels of F in adjacent aqueous phases would result in only a minor proportion being retained.

Errors in the direct potentiometric electrode method of fluoride determination: adsorption, illumination and temperature effects

Pre-soaking of clean plastic bottles for 1 day in dilute hydrochloric acid reduces adsorption of fluoride from dilute fluoride solutions, stabilising storage. Changes in ambient illumination cause an electrode potential drift, implying an increase in the fluoride concentration on commencement of irradiation. The potential does not necessarily return to the original value on cessation of irradiation. The extent of temperature dependence of the electrode is directly proportional to the state of deterioration (“age”) of the electrode membrane. Older electrodes show greater potential drift per degree of temperature change than newer electrodes; this is often large in the 15–30 °C range. Although the extent of drift is variable, the direction of drift is uniform, a temperature rise producing an apparent fall in the fluoride concentration. A temperature hysteresis exists on sample heating and cooling.

A study of the differential capacitance of indium in some aqueous solutions

Summaryo1.Of the systems studied, the indium/sodium sulphate system is the only one to show potential regions where the electrode-aqueous solution interphase is considered to be uncomplicated by adsorption or the intrusion of films. An estimate of the p.z.c. for this system is −0.95±0.02 V.2.It is not possible to determine unequivocally the p.z.c. for indium in fluoride electrolytes, although a well-defined capacitance minimum occurs at −0.93±0.02 V in dilute fluoride solutions.3.For unacidified chloride and perchlorate electrolytes a well-defined capacitance minimum is not observed as the ionic concentration is lowered.4.In unacidified solutions of KF and NaClO4 at high ionic concentrations, the anodic rise in electrode capacitance due to lattice oxidation is suppressed, due to the irreversible formation of highly insoluble films at the electrode surface.5.A pseudocapacitance peak at −0.90 V in KF solutions, −0.90 V in NaClO4 solutions and −0.95 V in KCl solutions is concluded as being due to OH− ion interaction with the indium electrode surface and subsequent dissolution of the metal lattice. Of the systems studied, the indium/sodium sulphate system is the only one to show potential regions where the electrode-aqueous solution interphase is considered to be uncomplicated by adsorption or the intrusion of films. An estimate of the p.z.c. for this system is −0.95±0.02 V. It is not possible to determine unequivocally the p.z.c. for indium in fluoride electrolytes, although a well-defined capacitance minimum occurs at −0.93±0.02 V in dilute fluoride solutions. For unacidified chloride and perchlorate electrolytes a well-defined capacitance minimum is not observed as the ionic concentration is lowered. In unacidified solutions of KF and NaClO4 at high ionic concentrations, the anodic rise in electrode capacitance due to lattice oxidation is suppressed, due to the irreversible formation of highly insoluble films at the electrode surface. A pseudocapacitance peak at −0.90 V in KF solutions, −0.90 V in NaClO4 solutions and −0.95 V in KCl solutions is concluded as being due to OH− ion interaction with the indium electrode surface and subsequent dissolution of the metal lattice. The concentration of solution soluble indium species in equilibrium with the electrode at the capacitance peak has been calculated for the fluoride and chloride cases (1 mol l−1, unacidified solutions) at these peak potentials.

A study of the differential capacitance of indium in some aqueous solutions

Summary o 1. Of the systems studied, the indium/sodium sulphate system is the only one to show potential regions where the electrode-aqueous solution interphase is considered to be uncomplicated by adsorption or the intrusion of films. An estimate of the p.z.c. for this system is −0.95±0.02 V. 2. It is not possible to determine unequivocally the p.z.c. for indium in fluoride electrolytes, although a well-defined capacitance minimum occurs at −0.93±0.02 V in dilute fluoride solutions. 3. For unacidified chloride and perchlorate electrolytes a well-defined capacitance minimum is not observed as the ionic concentration is lowered. 4. In unacidified solutions of KF and NaClO 4 at high ionic concentrations, the anodic rise in electrode capacitance due to lattice oxidation is suppressed, due to the irreversible formation of highly insoluble films at the electrode surface. 5. A pseudocapacitance peak at −0.90 V in KF solutions, −0.90 V in NaClO 4 solutions and −0.95 V in KCl solutions is concluded as being due to OH − ion interaction with the indium electrode surface and subsequent dissolution of the metal lattice. The concentration of solution soluble indium species in equilibrium with the electrode at the capacitance peak has been calculated for the fluoride and chloride cases (1 mol l −1 , unacidified solutions) at these peak potentials.

A Study of the Dissolution of SiO2 in Acidic Fluoride Solutions

Data are presented on the dissolution rates of films in a variety of acidic fluoride solutions. It is shown that the rate of dissolution is linearly dependent on the concentration of and HF in relatively dilute fluoride solutions. Both reactions have a similar temperature dependence characterized by an activation energy of 9.1 and 8.1 kcal/mole, respectively. The reaction with is about four to five times as fast as that with HF. Extrapolation to more concentrated solutions suggests that higher polymeric species of the type may also be active in the dissolution process. Raman data indicates that no single product species is formed although the product spectrum is consistent with the presence of some .

The role of iso-ionic exchange in the uptake of 18F labelled fluoride by hydroxyapatite and enamel

When 18F is used to measure the uptake of fluoride from dilute fluoride solutions by hydroxyapatite or enamel, it is probable that the observed uptake of 18F does not give a true estimate of the uptake of stable fluoride because some 18F in the solution phase exchanges iso-ionically with stable fluoride in the solid phase. To demonstrate the extent of this iso-ionic exchange radioactivity measurements and chemical analyses for 18F and stable fluoride were made upon the same samples. When the hydroxyapatite originally contained very little fluoride, the uptake of 18F was exactly comparable to the uptake of stable fluoride determined chemically. However, powdered hydroxyapatite samples, pretreated to raise their fluoride content, and powdered enamel samples containing 172 ppm F, showed little or no uptake of stable fluoride but a significant uptake of fluoride as 18F. Under the conditions of the experiment iso-ionic exchange of radiofluoride for stable fluoride must have been responsible for the observed uptake of 18F. It would appear therefore that 18F should not be used in the measurement of fluoride uptake by enamel from dilute fluoride solutions. The uptake of 18F is theoretically predictable in terms of the stable fluoride concentrations in the solutions, and the number of replaceable sites in the hydroxyapatite. This theory is in reasonable accord with experimental behaviour. Uptake by powdered enamel, which is sensibly at equilibrium, is completely different in character from uptake by intact enamel where kinetic considerations apply. Hence the behaviour of intact enamel is not in general predictable from data obtained on powdered enamel.