Electrochemical measurement of diffusible

The electrode is a critical component in the electrochemical measurement of diffusion coefficients. The general type of electrode to be used (i.e., UME or conventional electrode, see Chapters 5,6, and 11 for more detail) has a fundamental impact on the type of measuranent to be made. As shovra in Figure 19.1a, at a conventional electrode diffusion normally is planar (i.e., comes from essentially one direction toward the electrode surface). With planar diffusion there is a depletion of the redox species close to the surface, resulting in a current that decays with time. As such, under planar diffusion control the current is measured as a function of time. With an RDE, forced convection causes efficient, steady-state transport of species to the electrode surface (Figure 19.1b) and time-independent current results. With... 

K. Iwanaga, M. Eguchi, M. Watanabe, and T. Hibiya, 2000, Electrochemical measurement of diffusion coefficient of oxygen in molten silicon ,... 

It must be noted that the effective diffusion coefficient (Di)eff is obtained by electrochemical measurements of air gas-diffusion electrodes with sufficiently thick gas layer so that the limiting process is the gas... 

Chronoamperometry has proven useful for the measurement of diffusion coefficients of electroactive species. An average value of it1/2 over a range of time is determined at an electrode, the area of which is accurately known, and with a solution of known concentration. The diffusion coefficient can then be calculated from it1/2 by the Cottrell equation. Although the electrode area can be physically measured, a common practice is to measure it electrochemically by performing the chronoamperometric experiment on a redox species whose diffusion coefficient is known [6]. The value of A is then calculated from it1/2. Such an electrochemically measured surface area takes into account any unusual surface geometry that may be difficult to measure geometrically. 

B. Robertson, B. Tribollet, and C. Deslouis, "Measurement of Diffusion Coefficients by DC and EHD Electrochemical Methods," Journal of The Electrochemical Society, 135 (1988) 2279-2283. 

In general, two methods are used to determine trace amounts of fluorine in different natural materials, fodder, and foodstuffs. These are photometric determination based on the reaction of fluoride ions with the lanthanum-alizarin complexing agent and electrochemical measurements of fluoride concentrations by means of F-selective electrodes (Brill 1995). When measuring fluorine in the fluids and tissues of mammals, it is pre-separated by distillation or diffusion. As applied to fluorine determination in plants, good results are demonstrated using alkaline hydrolysis with subsequent fluoride concentration measurements, or in an acid medium by means of ion-selective potentiometry (Galloway et al. 1975). 

Table 2 lists some measurements of diffusion coefficients in microemulsions of different surfactants [51]. Some data on SDS micellar solutions are included for comparison. The electrochemical measurements in all SDS systems were performed using ferrocene as probe to obtain the apparent diffusion coefficients. These values are compared to QELS... 

The principle of the electrochemical measurement of oxygen diffusion in a metal consists in bringing the metal from a well-defined state into another well-defined state and following the diffusion-controlled relaxation process electrochemically. For example, the metal sample is placed on one side of the solid electrolyte ZrOa and functions as one electrode of a galvanic cell. On the other side of the electrolyte there is a practically unpolarizable electrode such as porous platinum in contact with air, or an Fe/FeO electrode, which has a fixed oxygen partial pressure of about 10 atm at 800°C. The following cell may be used ... 

As described above, useful information is available from electrochemical measurements of CD systems. Since CD is a large macrocyclic compound, incorporation of small guest molecule in the bulky CD makes the apparent diffusivity of guest lower. Taking adsvantage of this phenomenon the formation constant of inclusion complex can be easily determined by quantitative treatment of the variation of peak current (CV) or diffusion current (polarography) on the CD concentration in the electrolyte solution. 

Linear scan voltammetry (LSV) and cyclic voltammetry (CV) (see Chapter 11) are among the most common electrochemical techniques employed in the laboratory. Despite their utility, however, they are not particularly well suited to careful measurements of diffusion coefficients when using electrodes of conventional size. We will briefly discuss techniques for measuring D with LSV and CV, but the reader should be cautioned that these measurements under conditions of planar diffusion (i.e., at conventional electrodes) are probably useful to only one significant digit, and then only for nemstian systems with no coupled homogeneous reactions and with no adsorption. For more reliable results with LSV and CV, UMEs should be used. 

Much more accurate measurements of diffusion coefficients can be obtained with LSV or CV using UMEs. These measurements are much less dependent on the electrochemical reversibility of the redox couple. Measurement of the diffusion-limited current from a voltammogram recorded at a microelectrode is demonstrated in Figure 19.3c. The concept is identical to that already discussed for chronoamperometry at UMEs at slow scan rates (i.e., long times) the current becomes steady state as long as the potential is well past Ey2-The dependence of D on the steady-state current is given by equation (19.3) for a hemispherical UME and equation (19.4) for a disk UME. The time considerations for CV are the same as those discussed above for chronoamperometry, except that the time is estimated from the scan rate and the difference between the final potential and Ey . 

Chatenet, M., Aurousseau, M. and Durand, R. (2000). Electrochemical Measurement of the Oxygen Diffusivity and Solubility In Concentrated AUcahne Media on Rotating Ring-disk and Disk Electrodes Application to Industrial Chlorine-soda Electrolyte, Electrochim. Acta, 45, pp. 2823-2827. 

Robertson, B., Tribollet, B. Deslouis, C. (1988). Measurement of diffusion-coefficients by DC and EHD electrochemical methods. Journal of the Electrochemical Society 135(9) 2279-2284. 

Zistler M, Wachter P, Schreiner C, Gores HJ (2010) Electrochemical measurement of tiiiodide diffusion coefficients in blends of ionic liquids Results for improving a critical parameter of dye-sensitized solar cells. J Mol Liq 156(l) 52-57. doi 10.1016/j.molliq.2010.04.021... 

The diffusion coefficients of the constituent ions in ionic liquids have most commonly been measured either by electrochemical or by NMR methods. These two methods in fact measure slightly different diffusional properties. The electrochemical methods measure the diffusion coefficient of an ion in the presence of a concentration gradient (Pick diffusion) [59], while the NMR methods measure the diffusion coefficient of an ion in the absence of any concentration gradients (self-diffusion) [60]. Fortunately, under most circumstances these two types of diffusion coefficients are roughly equivalent. 

The measurement of transport numbers by the above electrochemical methods entails a significant amount of experimental effort to generate high-quality data. In addition, the methods do not appear applicable to many of the newer non-haloalu-minate ionic liquid systems. An interesting alternative to the above method utilizes the NMR-generated self-diffusion coefficient data discussed above. If both the cation (Dr+) and anion (Dx ) self-diffusion coefficients are measured, then both the cation (tR+) and anion (tx ) transport numbers can be determined by using the following Equations (3.6-6) and (3.6-7) [41, 44] ... 

Transport numbers for several non-haloaluminate ionic liquids generated from ionic liquid self-diffusion coefficients are listed in Table 3.6-7. The interesting, and still open, question is whether the NMR-generated transport numbers provide the same measure of the fraction of current carried by an ion as the electrochemically... 

With electrochemical methods such as chronoamperometry, cyclovoltammetry (CV), or conductivity measurements, the diffusion coefficients of charged chemical species can be estimated in highly dilute solutions [16, 17]. 

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