Electrode response curve

A nonlinear type of electrode response curve ABC, Figure 2.5) is more likely. This curve is accentuated for illustrative purposes. When the slope adjustment is made, the nonlinearity remains with only the two points being on the correct output. If the initial standardization were made at pH 10 and the slope adjustment at pH 13.0, the reading displayed on placing the electrodes again in pH 10 buffer would not be 10. As shown in Figure 2.5, the reading would correspond to that of point B. To avoid this interaction between standardization and slope adjustment controls, it is necessary to set the standardization point to zero potential. 

A tracing of the electrode signal during a cycle of turning aeration off and on is shown in Fig. 24-15. The rate of supply is zero (after bubbles have escaped) in the first portion of the response curve thus, the slope equals the uptake rate by the organisms. When aeration is resumed, both the supply rate and uptake rate terms apply. The values for C — C can be calculated from the data, the slope of the response curve at a given point is measured to get dC/dt, and the equation can be solved for K a because all the other values are known. 

As shown below, the influence of three quite distinct dynamic processes play a role in the overall measured oxygen concentration response curve. These are the processes of the dilution of nitrogen gas with air, the gas-liquid transfer, and the electrode response characteristic, respectively. Whether all of these processes need to be taken into account when calculating KLa can be seen by examining the mathematical model and making simulations. 

Figure 9 summarizes the electrode responses toward a variety of DNA-binding substrates [14c]. For intercalators (quinacrine, acridine orange, and safranin) and groove binders (spermine and spermidine), a steep rise followed by a saturation of the concentration response curve is commonly observed. If one compares the specific concentration which gives a 50% response in the increment of the cathodic peak current (A/p ) for each substrate, a selectivity order of quinacrine acridine orange > spermine > spermidine > safranin can be estimated. The binding constants measured in aqueous media for the affinity reaction with ds DNA are as follows quinacrine, 1.5 x 10 (38 mM NaCl)... 

Potentiometry Electrochemical reaction Exchange of charge Electrode potentials E=m Response curve ... 

It is clear from the Nemst equation that the temperature of the solution affects the response slope (2.303A7//0 of the calibration curve. The electrode voltage changes linearly in relationship to changes in temperature at a given pH therefore, the pH of any solution is a function of its temperature. For example, the electrode response slope increases from 59.2mV/pH at 25°C to 61.5 mV/pH at a body temperature of 37°C. For modem pH sensing systems, a temperature probe is normally combined with the pH electrode. The pH meter with an automatic temperature compensation (ATC) function automatically corrects the pH value based on the temperature of the solution detected with the temperature probe. 

In this method, an entire calibration curve is measured for the primary ion in a constant background of interfering ion. aj(BG) is the activity of the constant interfering ion in the background. afiDL) is the low detection limit (LDL) of the Nernstian response curve of the electrode as a function of the primary-ion activity. In the mixed interference method the selectivity is calculated from the following equation ... 

Examination of the behaviour of a dilute solution of the substrate at a small electrode is a preliminary step towards electrochemical transformation of an organic compound. The electrode potential is swept in a linear fashion and the current recorded. This experiment shows the potential range where the substrate is electroactive and information about the mechanism of the electrochemical process can be deduced from the shape of the voltammetric response curve [44]. Substrate concentrations of the order of 10 molar are used with electrodes of area 0.2 cm or less and a supporting electrolyte concentration around 0.1 molar. As the electrode potential is swept through the electroactive region, a current response of the order of microamperes is seen. The response rises and eventually reaches a maximum value. At such low substrate concentration, the rate of the surface electron transfer process eventually becomes limited by the rate of diffusion of substrate towards the electrode. The counter electrode is placed in the same reaction vessel. At these low concentrations, products formed at the counter electrode do not interfere with the working electrode process. The potential of the working electrode is controlled relative to a reference electrode. For most work, even in aprotic solvents, the reference electrode is the aqueous saturated calomel electrode. Quoted reaction potentials then include the liquid junction potential. A reference electrode, which uses the same solvent as the main electrochemical cell, is used when mechanistic conclusions are to be drawn from the experimental results. 

Fig. 8 The dependence of the electromotive force (EMF) ofCd(ll) ion-selective electrode on logarithm of Cd(ll) concentration in M NaNOs at pH 7. Solid lines, calculated response curves on the basis of Eq. (7) in Ref 399.

Figure 15. (top,) Spectral response curves for a reduced SrTiOs electrode before and after aging in F solution, (bottom) 1-V data for the same aging conditions. The I-V and spectral response curves were obtained in 0.1 M NaOH, and the electrode was aged in 1M KF for 2 days at +5 V vs. SCE. 

Sensing performance for H-,. Sensing performance of the amperometric sensor was examined for the detection of H2 in air. Figure 3 shows the response curve for 2000 ppm H2 in air at room temperature. The response was studied by changing the atmosphere of the sensing electrode from an air flow to the sample gas flow. With air the short circuit current between two electrodes was zero. On contact with the sample gas flow, the current increased rapidly. The 90% response time was about 10 seconds and the stationary current value was 10yUA. When the air flow was resumed, the current returned to zero within about 20 seconds. 

In order to confirm such potential shift we observed the actual behavior of the sensing and counter electrode potential under both open and short circuit conditions. Each potential was measured against a silver reference electrode which was attached to the sensor element as shown in Figure 6. Figure 7 depicts the response curves against 500 ppm H2 in air under both conditions. When the circuit is open, the change in potential occurs only at the sensing... 

Sensors of this type were constructed as multi-electrode arrays bearing chohnesterases of differing sensitivity to organo-phosphates, and the assay extended to milk [35]. Both spiked milk and milk from shops inhibited the activity of the electrodes. In two instances, estimates of the level of organo-phosphates in spiked milk made using sensors were very close to those made using GC-MS. This appears to have been a fortunate co-incidence as the response curves used for calibration were not linear. Nine out of ten milk samples from shops inhibited at least some members of the array. In only one case did the GC-MS assay find insecticides in the samples, but the insecticides were not organo-phosphates. The inhibition shown by the enzyme-based arrays was reversible by pyridine-2-aldoxime... 

An estimate of the amount of pesticide the SPCE has been exposed to in molar diazinon equivalents can be found as follows a calibration curve can be obtained by plotting the degree of inhibition (ratios of electrode responses) against the concentration of pesticide standard. Fit a curve to the data and estimate the amount of pesticide in the wool extracts from the ratios for the electrodes exposed to wool extracts. As the standard used was diazinon, this will only give the pesticide concentration in diazinon equivalents. 

We will now discuss how the relationship between potential, mass transport and current manifests itself experimentally in some situations typically met in electroanalytical chemistry. In Sections 6.7.1-6.7.3, we discuss the response curves for two families of electroanalytical methods that are conducted under conditions where linear semi-infinite diffusion to/from planar working electrodes prevail. These are chronoamperometry and double... 

The discussions will be based on a reduction process, Equation 6.6 reproduced below, studied under the conditions where the substrate O is the only electroactive species initially present in the solution and we will initially assume that the product of the electrode process, R, is stable under the conditions of the experiment. (The transposition to oxidation processes is straightforward. Be aware, however, that the minus and plus signs in some of the equations given below will then have to be interchanged.) This is followed by a discussion of how follow-up reactions involving R affect the response curves. 

The use of dual-polarized electrodes was first suggested more than 70 years ago 2 the subject has been reviewed thoroughly by two more recent publications.3,4 Almost all modem commercial pH meters have provision for imposing a polarizing current of either 5 or 10 nA to make possible measurements by dual-polarized electrode potentiometry. Such a provision is included because dual-polarized potentiometry is by far the most popular endpoint detection method for the Karl Fischer determination of water. For this titration a combination of reagents is used, including iodine the response curve is similar to that of Figure 4.3b. In practice the response is many times more sensitive than... 

The linear response range of the glucose sensors can be estimated from a Michaelis-Menten analysis of the glucose calibration curves. The apparent Michaelis-Menten constant KMapp can be determined from the electrochemical Eadie-Hofstee form of the Michaelis-Menten equation, i = i - KMapp(i/C), where i is the steady-state current, i is the maximum current, and C is the glucose concentration. A plot of i versus i/C (an electrochemical Eadie-Hofstee plot) produces a straight line, and provides both KMapp (-slope) and i (y-intercept). The apparent Michaelis-Menten constant characterizes the enzyme electrode, not the enzyme itself. It provides a measure of the substrate concentration range over which the electrode response is approximately linear. A summary of the KMapp values obtained from this analysis is shown in Table I. 

A quantitative measure of the interaction of chemical waves and steady electric fields is the velocity response curve, i.e. a plot of the dependence of a plane wave velocity on the electric field applied along the direction of propagation. Note that in Figure 8 the curve measured for a modification of the BZ medium terminates at around 10 volts/cm (for waves propagating toward the negative electrode) since for larger fields the waves are annihilated. Further details on these measurements are found in Ref. 1 where the special chemical wave medium recipes developed to make the measurements are given. 

Fig. 2.17. Plots of the current at +0.1 V for a poly(aniline)/poly(vinylsulfonate)-coated glassy carbon electrode (deposition charge 150 mC, geometric area 0.38 cm2) rotated at 9 Hz in 0.1 mol dm- 1 citrate/phosphate buffer at pH 7 as a function of the NADH concentration showing the stability of the electrode response. Four replicate calibration curves recorded in succession over 4h using the same electrode are shown ( ) run 1 ( ) run 2 (A) run 3 and (O) run 4. The solid line is drawn as a guide for the eye.

Fig. 6. Typical response curve of a potentiometric enzyme electrode.

---