Diffusion-layer titration

Fig. 14. Diffusion layer titration curve at a double electrode, when...

Fig. 8.11. Diffusion layer titration curve at a double hydrodynamic electrode (second order homogeneous reaction), (a) /det begins to rise when excess of B that did not react homogeneously reaches the detector electrode and A// /det = G(l/ar). (b) Assuming fast kinetics this is where linearity commences. From here...

Another application of double, generator/collector, electrodes is what is called diffusion layer titration. This can be used for a quantitative analysis of some species in solution. The technique was first suggested by Bruckenstein and Johnson [157], and has been followed up since then, with theory [33,458] and simulations [81,458,541,554] (naming just a selection of works). 

Tomcik P, Krajcikova M, Bustin D (2001) Determination of pharmaceutical dosage forms via diffusion layer titration at an interdigitated microelectrode array. Talanta 55(6) 1065-1070... 

We have verified that theory and experiment for the wall-jet ring-disc titration curves are in reasonable agreement and we have shown that successful diffusion layer titrations of some 6 different proteins can be carried out using the wall-jet cell. 

Albery WJ, Brackenstein S, Johnson DC (1966) Ring-disk electrodes. Part 4 - diffusion layer titration curves. Trans Faraday Soc 62 1938-1945... 

Bruckenstein S, Johnson DC (1964) Coulometric diffusion layer titrations using the ring-disk electrode with amperometric end point detection. Anal Chem 36 2186-2187... 

Basak J, Penar J, Sykut K (1987/1988) Digital simulation for determining rate constants in diffusion layer titration on the rotating ring disc electrode. Part II. Second order reactions. Ann Univ Mariae Curie - SModowska, Sectio AA XLII/XLIII 43-49... 

Svir IB, Oleinick AI, Compton RG (2003) Dual microband electrodes current distributions and diffusion layer titrations . Implications for electroanalytical measurements. J Electroanal Chem 560 117-126... 

Most successful is a rotating Pt wire microelectrode as illustrated in Fig. 3.75 as a consequence of the rotation, which should be of a constant speed, the steady state is quickly attained and the diffusion layer thickness appreciably reduced, thus raising the limiting current (proportional to the rotation speed to the 1/3 power above 200 rpm140 and 15-20-fold in comparison with a dme) and as a result considerably improving the sensitivity of the amperometric- titration. 

Macroscopic experiments allow determination of the capacitances, potentials, and binding constants by fitting titration data to a particular model of the surface complexation reaction [105,106,110-121] however, this approach does not allow direct microscopic determination of the inter-layer spacing or the dielectric constant in the inter-layer region. While discrimination between inner-sphere and outer-sphere sorption complexes may be presumed from macroscopic experiments [122,123], direct determination of the structure and nature of surface complexes and the structure of the diffuse layer is not possible by these methods alone [40,124]. Nor is it clear that ideas from the chemistry of isolated species in solution (e.g., outer-vs. inner-sphere complexes) are directly transferable to the surface layer or if additional short- to mid-range structural ordering is important. Instead, in situ (in the presence of bulk water) molecular-scale probes such as X-ray absorption fine structure spectroscopy (XAFS) and X-ray standing wave (XSW) methods are needed to provide this information (see Section 3.4). To date, however, there have been very few molecular-scale experimental studies of the EDL at the metal oxide-aqueous solution interface (see, e.g., [125,126]). 

Titrations of non-electroactive compounds can be carried out by homogeneous reaction with titrants electrogenerated in situ. This can be done using double hydrodynamic electrodes in the diffusion layer microtitration technique described in Section 8.7. 

Fig. 14.3 Diffusion-layer microtitration curves at the wall-jet ring disc electrode for titration of As(III) (X) with bromine (B) generated at the disc electrode from bromide. Solution 10-2 m KBr + 0.5 m H2S04. Analysis of the curve leads to [X] (from Ref. 4 with permission).

In the CD-MUSIC model, C l is obtained from titration data and C2 is chosen to provide a good fit to the salt dependency of specifically adsorbing ions (Hiemstra and van Riemsdijk, 1996). It should be noted that the diffuse layer model does not contain any capacitance parameters. 

Attard, P., Antelmi, D., and Larson, I., Comparison of the zeta potential with the diffuse layer potential from charge titration, Langmuir, 16, 1542. 2000. 

Among the six interfacial variables discussed in this section, the surface charge density oo, the surface potential (fo, and the potential at the OHP fd (usually called the diffuse layer potential), are most important in characterizing interfacial properties. The three remaining variables (i.e., ap, /p, and Od) can be estimated using Eqs. (5), (7), and (8) if oo, and /rf are known exactly. ao can be determined experimentally by the potentiometric titration method, and detailed explanation of the potentiometric titration is given, for example, by Yates [10]. The estimate of fo for the ceramic powder/aqueous solution interface is discussed in the next section, yd is perhaps the most important interfacial electrochemical parameter since it is closely correlated with the kinetic stability of a given colloidal suspension and it can be conveniently determined (approximately) experimentally. 

Both the surface potential ij/o and the diffuse layer potential ij/ defy a direct experimental determination. What can be measured are the surface charge density (To (via titration) and the electrokinetic potential or zeta-potential f, which corresponds to the concepmal hydrodynamic slip plane (or shear plane, SP in Fig. 3.3) between the Stem layer and the diffuse layer. Any measurement of the zeta-potential is, therefore, based on relative motion between the particle surface and the diffuse layer (cf. Sect. 2.3.7, Delgado et al. 2007). 

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