Nernst response curve

The emf data collected for each concentration of a metal ion solution (approximately 13 replicates) before the addition of hydroxide or silicate solution were averaged and the mean value used to calculate a Nernst response curve. Metal concentrations in ppm were converted to mols/dm and the Davies equation (19) was used to estimate the values of the mean molar activity coefficients of the metal ions in... 

For a reversible couple, the surface concentrations of the electroactive species for a simple reduction can be substituted into the Nernst equation to give an expression describing the potential-time response curve. At x = 0, CyC can be related to time by (xI/2 - t1/2)/t1/2, which gives... 

Response Curve of the Electrode. From Nernst s formula, the membrane potential of the glass electrode is proportional to the logarithm of the proton concentration. Protons are liberated from the cephalosporinase reaction. Therefore, hydrogen ion concentration in a sample solution measured by the pH electrode is proportional to the logarithm of the cephalosporin concentration... 

It is critical that pH electrodes are calibrated often as the slope of their response curve will change with temperature, as defined by the Nernst equation [2], Calibration should be performed with at least two certified pH calibration buffers that bracket the expected pH of the sample. Calibration is performed as described in the manufacturer s instructions provided with the pH meter. Calibration should be checked by measuring the pH of the first calibration buffer. The result should be no more than 0.02 pH units different from the certified pH of the buffer after adjusted for temperature. pH measurements are best performed when calibration buffers and samples are held at a constant temperature. 

Because each enzyme sensor has its own unique response, it is necessary to construct the calibration curve for each sensor separately. In other words, there is no general theoretical response relationship, in the same sense as the Nernst equation is. As always, the best way to reduce interferences is to use two sensors and measure them differentially. Thus, it is possible to prepare two identical enzyme sensors and either omit or deactivate the enzyme in one of them. This sensor then acts as a reference. If the calibration curve is constructed by plotting the difference of the two outputs as the function of concentration of the substrate, the effects of variations in the composition of the sample as well as temperature and light variations can be substantially reduced. Examples of potentiometric enzyme electrodes are listed in Table 6.5. 

A quite different response is found for couples O/R where Iq is large (in fact, where Iq > 10" /l or k > 10" cm s ). Then the electron transfer reaction at the surface is rapid enough that under most mass transport conditions obtainable experimentally, the electron transfer couple at the surface appears to be in equilibrium. Then the surface concentrations may, at each potential, be calculated from the Nernst equation, a purely thermodynamic equation, and the current may be calculated, for example, from equation (1.57). The I-E curve has the form shown in Fig. 1.16 the I-E curve crosses the zero current axis steeply and there is no overpotential for oxidation or reduction. Systems with these characteristics are often termed reversible . On the other hand, the limiting current densities do not depend on the kinetics of electron transfer closer to E. Hence the limiting current densities for reversible and irreversible reactions are the same. 

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