Fluoride electrode potential

Ion-selective electrodes are a relatively cheap approach to analysis of many ions in solution. The emf of the selective electrode is measured relative to a reference electrode. The electrode potential varies with the logarithm of the activity of the ion. The electrodes are calibrated using standards of the ion under investigation. Application is limited to those ions not subject to the same interference as ion chromatography (the preferred technique), e.g. fluoride, hydrogen chloride (see Table 10.3). 

There are two types of fluoride lon-selective electrodes available [27] Onon model 96-09-00, a combination fluoride electrode, and model 94-09-00, which requires a reference electrode The author prefers to use Onon model 94-09-00 because it has a longer operational life and is less expensive When an electrode fails, the reference electrode is usually less expensive to replace The Fisher Accumet pH meter, model 825 MP, automatically computes and corrects the electrode slope It gives a direct reading for pH, electrode potential, and concentra tion in parts per million The fluoride lon-specific electrode can be used for direct measurement [2S, 29] or for potenPometric titration with Th" or nitrate solutions, with the electrode as an end point indicator... 

The pressed disc (or pellet) type of crystalline membrane electrode is illustrated by silver sulphide, in which substance silver ions can migrate. The pellet is sealed into the base of a plastic container as in the case of the lanthanum fluoride electrode, and contact is made by means of a silver wire with its lower end embedded in the pellet this wire establishes equilibrium with silver ions in the pellet and thus functions as an internal reference electrode. Placed in a solution containing silver ions the electrode acquires a potential which is dictated by the activity of the silver ions in the test solution. Placed in a solution containing sulphide ions, the electrode acquires a potential which is governed by the silver ion activity in the solution, and this is itself dictated by the activity of the sulphide ions in the test solution and the solubility product of silver sulphide — i.e. it is an electrode of the second kind (Section 15.1). 

Mercury cyanide, 5, 1062 Mercury electrodes potential range aqueous solution, 1, 480 Mercury fluoride, 5. 1059 Mercury fulminate, 2, 7, 12 5, 1063 Mercury halides, 5, 1049 Mercury iodate, 5,1068 Mercury iodide, 5. 1059 Mercury ions Hgf... 

Fig. 5-17. Interfacial tension y of a mercury electrode observed in aqueous solutions of various anions as a function of electrode potential pu = potential of zero diarge in sodium fluoride solution in which no contact adsorption occurs. [From Grahame, 1947.]...

Ion hopping is a familiar concept in the chemistry of solid-state conductors, e.g. in the semiconductor industry. In the fluoride electrode, fluoride vacancies in.side the solid LaF lattice allow for conduction of charge (see Figure 3.11), in turn registered by the electrode as a potential. The emf is zero if the internal and external solutions are the same because the same numbers of fluoride ion enter the crystal from either face. 

Fig. 6.3. The effect of pH on the potential of the Orion Research lanthanum fluoride electrode in NaF solutions of various concentrations. The potential change with pH in the acidic region is caused by the formation of HFJ. (After Butler [53].)...

As to what, exactly, might be the fluorinating agent in the proposed Radical Mechanism we have seen how some workers favour that of fluorine absorbed on or in the nickel fluoride layer, whereas others prefer that of high valence fluorides. In this respect, it is interesting to note how different workers interpret what are apparently very similar phenomena upon opening the electrical circuit of a previously conditioned ECF anode, e.g., Watanabe [169], and the decay of electrode potential, as an example of the first, and Sartori et al. [186], and the persistence of chemical activity, as an example of the second. Perhaps, in reality, the dynamic equilibrium which relates fluoride ion, nickel fluorides, and atomic... 

When a fluoride electrode was immersed in standard solutions (maintained at a constant ionic strength of 0.1 M with NaN03), the following potentials (versus S.C.E.) were observed ... 

Using an expanded scale pH meter, such as the Orion 801, pipet 10 ml of the stock solution into a small beaker and add 10 ml of Tisab (Orion No. 94-09-09). Determine the electrode potential using a fluoride electrode Orion 94-09. Comparison is made by bracketing with fluoride standards prepared similarly. 

Electrode Potential.—This can be determined only in solutions of fluorides, since tantalum shows valve action in all other electrolytes, and even in the case of fluorides there is some reason to believe that oxide formation takes place, vitiating the results. The potential at the electrode... 

An example is described here for the measurement of fluoride ions in solution. The fluoride electrode uses a LaF3 single crystal membrane and an internal reference, bonded into an epoxy body. The crystal is an ionic conductor in which only fluoride ions are mobile. When the membrane is in contact with a fluoride solution, an electrode potential develops across the membrane. This potential, which depends on the level of free fluoride ions in solution, is measured against an external constant reference potential with a digital pH/mv meter or specific ion meter. The measured potential corresponding to the level of fluoride ions in solution is described by the Nernst equation ... 

In these clusters tantalum atoms are bound to other tantalum atoms and are also edge bridged via halide. As our deposit was completely amorphous without any XRD peak we concluded that it did not consist of crystalline tantalum but rather of such clusters. We varied the electrode potential for deposition and tried deposition with very low constant current densities, but in no case was crystalline tantalum obtained. Thus, the electrochemical window of our liquid was surely wide enough, but for some reason the electrodeposition stopped before Ta(0) was obtained. When we studied the literature dealing with metal clusters we found that the cluster chemistry with fluoride seems to be less comprehensive. Consequently... 

Electrode Calibration Pipet 50 mL of the Buffer Solution into a plastic beaker. Place the fluoride ion and reference electrodes (or a combination fluoride electrode) into the plastic beaker and stir. At 5-min intervals, add 100 pL and 1000 pi. of the 1000 mg/kg Fluoride Standard and read the potential, in millivolts, after each addition. The difference between the two readings is the slope of the fluoride electrode and should typically be in the range of 54 to 60 mV at 25°. If the difference in potential is not within this range, check, and, if necessary, replace the electrode, instmment, or solutions. 

Nonaqueous electrolyte solutions can be reduced at negative electrodes, because of an extremely low electrode potential of lithium intercalated carbon material. The reduction products have been identified with various kinds of analytical methods. Table 3 shows several products that detected by in situ or ex situ spectroscopic analyses [16-29]. Most of products are organic compounds derived from solvents used for nonaqueous electrolytes. In some cases, LiF is observed as a reduction product. It is produced from a direct reduction of anions or chemical reactions of HF on anode materials. Here, HF is sometimes present as a contaminant in nonaqueous solutions containing nonmetal fluorides. Such HF would be produced due to instability of anions. A direct reduction of anions with anode materials is a possible scheme for formation of LiF, but anode materials are usually covered with a surface film that prevents a direct contact of anode materials with nonaqueous electrolytes. Therefore, LiF formation is due to chemical reactions with HF [19]. Where does HF come from Originally, there is no HF in nonaqueous electrolyte solutions. HF can be produced by decomposition of fluorides. For example, HF can be formed in nonaqueous electrolyte solutions by decomposition of PF6 ions through the reactions with H20 [19,30]. 

A fluoride electrode, in which the membrane is a single crystal of lanthanum fluoride doped with europium to increase the conductivity, is one of the best ion-selective electrodes available. Conduction through the membrane is facilitated by the movement of F" ions between anionic lattice sites which in turn is influenced by the F ion activities on each side of the membrane. If the electrode is filled with a standard solution of sodium fluoride, the membrane potential is a function of the fluoride activity in the sample solution only. Thus,... 

In the electrode-potential data for the halides, it is noteworthy that the first oxidation is most facile for the most electronegative halide and becomes increasingly more difficult in the series F < Cl < Br < I, with a particularly large jump (greater than 0.2 V) from F to Cl. The fluoride is also easiest to oxidize to a di-cation. The... 

Note that from an electrode kinetic point of view, the Nemst equation does not give any information as to the actual species that establishes the electrode potential. In the case of a fluoride-containing solution, it is very possible that one of the fluoro-iron(III) complexes rather than the Fe " ion participates in the electron-exchange reaction at the electrode. Complexation usually stabilizes a system against reduction. In the example just considered, because complexation is stronger with Fe(III) than with Fe(II), the tendency for reduction of Fe(III) to Fe(II) is decreased. It is apparent that coordination with a donor group, in general, decreases the redox potential. In the relatively rare instances where the lower oxidation state is favored (e.g., complexation of aqueous iron with phenanthroline), the redox potential is increased as a result of coordination. 

However, the electric potential of the electrocatalyst at its interface with the electrolyte (and thus the facility for charge transfer) can be easily and extensively altered at will to control rate and selectivity. For instance, a decrease of electrode potential by about 0.15 V can change the product selectivity for vinyl fluoride and chloride reduction on palladium by as much as 80% (31). In contrast, gas phase parallel reductions, with 5 kcal/mol difference in activation energies, would require a temperature increase from 500 K to 730 K for a comparable selectivity change. We should note here that the electrocatalytic specificity of the above reductions is quite similar to that of conventional heterogeneous catalytic reactions, but differs from that of conventional electrolytic reduction on noncatalytic electrodes (32). 

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