Scanning electrochemical microscopy: theory and experiment for the positive feedback mode with unequal diffusion coefficients of the redox mediator couple1

Theory for the chronoamperometric positive feedback mode of the scanning electrochemical microscope (SECM) is extended to include the situation where the oxidised and reduced forms of the redox mediator couple have arbitrary diffusion coefficients. Under typical positive feedback conditions, the solution initially contains a redox-active species, R, along with excess supporting electrolyte. The potential of the tip ultramicroelectrode (UME), positioned close to an interface of interest, is adjusted to a value where R is electrolysed to produce species O at a diffusion-controlled rate. O diffuses away from the tip towards the interface, where the reverse redox reaction occurs leading to the production, and diffusional feedback of R for electrolysis at the tip electrode. When positive feedback measurements are carried out under diffusion-controlled chronoamperometric conditions, the form of the normalised current–time behaviour, at a particular tip to interface distance, is found to be sensitive to the ratio of the diffusion coefficients of the O/R couple. As a steady-state is established, the normalised current becomes independent of the diffusion coefficient ratio and depends only on the tip to interface distance. Experimental measurements on the chronoamperometric oxidation of ferrocene (Fc) in acetonitrile solution at a Pt tip UME positioned close to a Pt substrate electrode, provide support for the theoretical predictions and demonstrate that the diffusion coefficient ratio can readily be determined from SECM chronoamperometry. Although Fc and Fc⁺ are often assumed to have the same diffusion coefficients, steady-state and chronoamperometric measurements at a conventional UME yield a value of 2.0×10^?5 cm² s^?1 for the diffusion coefficient of Fc, while SECM chronoamperometry indicates that the diffusion coefficient of Fc⁺ is 1.6×10^?5 cm² s^?1.     

Linear and non-linear analysis using the Oldham–Zoski steady-state equation for determining heterogeneous electrode kinetics at microdisk electrodes and digital simulation of the microdisk geometry with the fast quasi-explicit finite difference method1

A procedure which utilizes a linearized version of the Oldham–Zoski current-potential equation has been developed for the evaluation of electrode kinetic parameters from near-steady state voltammograms obtained at microdisk electrodes. The inherent advantage of the method is its mathematical simplicity relative to the other procedures already available in the literature. The procedure is applied to synthetic data obtained under near-steady state conditions from the fast quasi-explicit finite difference simulation of quasi-reversible electron transfer at a microdisk electrode. A procedure using non-linear optimization based on the Marquardt algorithm is also applied to the simulated data. Near-steady state voltammograms showing the effect of a potential dependent charge transfer coefficient are presented and analyzed. The robustness of the analysis is tested with voltammograms containing a random noise component and the near-steady state terms inherently present in experimental data. Application of the Oldham–Zoski equation to the simulated voltammograms enables the fidelity of both methods to be established over a wide range of heterogeneous kinetic conditions.     

A simple model for the measurement of charge transfer resistances of reactions with slow kinetics1

The response of an electrode, initially at equilibrium, to an applied potential step, is modelled with an equivalent circuit. The model is valid for reactions that are slow in comparison with the time constant of the decay of charging currents. In the measurement, an external resistor is added in series with the working electrode to match approximately the charge transfer resistance, R_CT, of the electron transfer. Extraction of R_CT is then particularly simple. In addition to R_CT, the uncompensated resistance, R_u, and the double layer capacitance, C_dl, can be determined. An advantage of the model is that even if the potentiostat rise time is far from ideal, R_CT can be obtained reliably if R_u is known. Criteria for applicability of the equivalent circuit model are derived.     

Microheterogeneous dispersion electrolysis with nanoscale electrode-modified zeolites1

This study demonstrates the utility of supported nanoscale Pt or RuO₂ particles as ensembles of ultramicroelectrodes for controlled potential electrolyses, including implications for their use in practical electrosyntheses. Microheterogeneous dispersions of nanoelectrode-modified aluminosilicate zeolites (M-Z) increase the effective electrode area of conventional large surface electrodes in electrolytes of customary ionic strength (μ≥0.1 M). For [Fe(CN)₆]^3? solutions (in pH 6.86 phosphate buffer), the time for total reduction of the iron complex using a dilute 2.5 mg/ml suspension density of 1 wt.% Pt–NaY was 67% less than that for the same solution with no electrode-modified zeolite added. The use of Pt–NaY dispersed in solutions of low electrolyte content (650 μM) exploits the nanoelectrode nature of the Pt particles and enables effective electron transfer to be conducted at applied potentials similar to those applied in high ionic strength electrolyte. In the absence of dispersed nanoelectrodes, no electrolysis can be sustained under these conditions of high resistance. Divorcing the electrocatalyst from the reactor electrodes allows inexpensive large area feeder electrodes to be used while the dispersed nanoscale electrodes can be a small mass of a high cost electrocatalyst. These studies demonstrate that heterogeneous dispersion electrolyses using electrode-modified zeolites can overcome the difficulties normally associated with industrial applications of electrosynthetic schemes: slow reaction times, difficult electrolyte separations, and high electrode costs.     

Application of the mean spherical approximation and ion association to describe the activity coefficients of aqueous 1:1 electrolytes1

The unrestricted version of the mean spherical approximation has been applied to describe the concentration dependence of the mean ionic activity coefficients for 12 aqueous electrolyte solutions for which extensive permittivity data are also available. By using the actual solvent permittivity rather than that of the pure solvent, it is shown that this primitive model fits the experimental data in almost all cases to the maximum concentrations for which the permittivity can be described by the use of three adjustable parameters, namely, an effective ionic diameter, σ₊ or σ_?, the effective ion pair diameter σ_ip, and the association constant K_as for ion pair formation. The values of K_as obtained are discussed with respect to results from experiments reported in the literature.     

A two-dimensional Born–Oppenheimer treatment of intramolecular electron transfer reactions

The Born–Oppenheimer (BO) formulation of polar solvation is developed and implemented at the semiemperical (PM3) configuration interaction (CI) level, yielding estimates of electron transfer (ET) coupling elements (V₀) for intramolecular ET in several families of radical ion systems. In contrast to the traditional treatment based on a single solvent coordinate and a fixed gas-phase coupling element, the present treatment yields a self-consistent characterization of kinetic parameters in a 2-dimensional solvent framework which includes an exchange coordinate. The dependence of V₀ on inertial solvent contributions and on donor/acceptor separation (r_DA) is discussed.     

Remounting multiple casting prior to final cementation

A technique has been presented for mounting and refining multiple restorations (castings) to a high degree of accuracy. This technique offers a solution to many problems encountered when making final occlusal adjustments in the mouth. The exact technical adjustment procedure to be followed after remounting will be described in a following article.     

Normal handling of phosphate in the salivary glands of X-linked hypophosphataemic mice

In X-linked hypophosphataemia (Hyp gene) there is reduced renal tubule re-absorption of phosphate and an osteomalacic bone disorder. To determine whether altered phosphate transport also occurs in the salivary gland, saliva was analysed from normal and Hyp mice at 10 weeks of age. The effect of plasma inorganic phosphate on the composition of saliva was controlled by placing both genotypes on either a control diet or a low-P diet for three days. Inorganic phosphate, total phosphate, calcium, sodium, potassium, chloride, amylase, sialic acid, protein, osmotic pressure, percentage water, percentage inorganic solids and sample volume were measured. The Hyp mice fed the control diet showed a significant decrease in salivary-inorganic phosphate, total phosphate, osmotic pressure and sialic acid. These four variables were similarly lower in the normal mice fed the low-phosphate diet. The changes seem to stem from the low plasma inorganic phosphate and not from expression of the Hyp gene in the salivary gland. The total evidence from various organ systems suggests that the altered phosphate transport in X-linked hypophosphataemia may be restricted to the kidney and is not a generalized phosphate-transport deficiency.     

Cleft lip and palate in nigerian children and adults: A comparative study

A survey of cleft lip and palate in 160 Nigerians is presented. In clefts of the lip alone, there was no sex predilection whereas cleft lip and palate cases showed a slight male preponderance. 119 (74·4%) cases presented below the age of 12 years and 41 (25·6%) cases were first seen above 12 years of age. Only a few cleft lip and palate cases survived to adulthood possibly because of malnutrition combined with the lethal effects of associated congenital abnormalities. This study confirms the view that late primary repair of clefts is less likely to produce impaired growth of the maxilla due to contraction of scar tissue commonly seen in early surgery. The importance of early primary repair is emphasised.     

Effects of bone grafting on maxillary bone healing in the growing pig

The healing of defects of the lateral wall of the maxilla was studied in the growing pig. In Series I the effect on bone healing of AAA-bone and Kiel bone was compared with that of autografts, and in Series II a comparison was made between autografts and AAA-bone implants. It was found that both autografts and AAA-bone implants improved the rate of healing, but the Kiel bone had no effect. Comparison between autografts and AAA-bone implants in Series II did not show any significant differences. Therefore, it was concluded that AAA-bone can be a useful substitute for autografts in grafting procedures in the maxilla.     

The pathogenesis and microbiology of idiopathic cervicofacial abscesses

Sixty-three cases of idiopathic cervicofacial abscesses were surveyed. Several debilitating conditions, probably responsible for diminished resistance to infection, were found in association with the abscesses. The condition mainly affected children under two years of age, and the submandibular and submental regions were the most common sites of occurrence. The predominant infecting organism was Staphylococcus aureus. The abscess cavity was a statistically significant source of S. aureus, but the nose, ear, mouth, and skin did not appear to be significant sources of the pathogen. Antibiotics such as Dalacin C and gentamicin were found to be active against all the strains of S. aureus. Although incision and drainage was the most effective method of treatment for the abscesses, appropriate antibiotics may be prescribed for those cases where suppuration has not yet taken place.     

Subluxation of the atlanto-axial joint

A case of post operative atlanto-axial dislocation is presented which was diagnosed on CT scan although the clinical signs and symptoms should have suggested the diagnosis. It is recommended that all patients who present with torticollis in the early post operative period should be considered to have C1-C2 dislocation and an orthopaedic opinion should be sought.