Isopotential point, electrodes

When electrodes are manufactured, every practical attempt is made to minimize this area. Electrodes that do not meet an asymmetry potential specification are eliminated. The asymmetry potential, in this case, is any difference in potential between glass and SCE reference electrodes, when immersed in pH 7 buffer (see Section 3.1.4 for an exact definition). Since it is not possible to obtain an isopotential point with electrodes, the pH meter isopotential point is set at the most likely point, pH 7. Since this point is only an estimation of the electrodes isopotential point, a slight error is observed if the measuring temperature is different than the buffering temperature. This is because a change in the slope made by changing the temperature compensator of the meter may not revolve around the same point as the slope of the electrode pair. 

Therefore, in most cases, pH values measured at high temperatures in dilute solution should be considered approximate values only. In cases where the investigators address this problem and are careful to select a suitable electrode (namely, one that manufacturers claim to have almost hysteresis-free pH measurement and a stable isopotential point over the temperature range), the error associated with electrode performance will be small, and differences in reported pH values will correspond to differences in actual pH. In cases where pH is measured in concentrated sucrose solutions, the reported pH value should be considered as a nominal value only, and the differences in nominal pH values might not correspond to actual differences in hydrogen ion activity. 

E° [equation (15.4)] is also referred to as the offset, the zero potential point, or the isopotential point, since theoretically it is defined as the pH that has no temperature dependence. Most pH electrode manufacturers design their isopotential point to be 0 mV at pH 7 to correspond with the temperature software in most pH meters. The offset potential is often displayed after calibration as an indication of electrode performance. Typical readings should be about 0 30 mV in a pH 7 buffer. In reality, E° is composed of several single potentials, each of which has a slight temperature coefficient. These potentials are sources of error in temperature compensation algorithms. 

Isopotential point — In potentiometric measurements with the use of a -> ion-selective electrode (ISE) cell, the isopotential point is the potential difference between the internal and external reference electrodes which is independent of temperature. The isopotential point is governed by a particular activity of the ion being determined. Both ISE and the outer reference electrode must be specified. When an isothermal cell is used with identical reference electrodes, the isopotential point is defined by the activity of the sensed ions that gives zero net - membrane potential, e.g., sensed activity is the same in the inner and outer (test) solution. Calibration lines for different cell temperatures have different slopes, but they intersect at a common activity point. Cells with temperature gradients are not recommended. 

In multiscan cyclic voltammetry measurements, e.g., of a redox film-coated electrode, the isopotential point is the potential of the same current value for voltam-mograms of different cycles. That is, by analogy to, e.g., the isosbestic point in absorption spectroscopy of two species remaining in equilibrium, this is the potential at which voltammograms of two redox species being in equilibrium in the film cross each other. A pair of such redox species can have several isopotential points in their multicyclic voltammograms. 

Fig. 7.3 The relationship between electrical potential and pH. The solid line shows the response of a calibrated electrode while the other plots are for instruments requiring calibration 1 has the correct slope but incorrect isopotential point (calibration control adjustment is needed) 2 has the correct isopotential point but incorrect slope (slope control adjustment is needed).

At pH 7, where H = OH, the voltage from the electrode is zero. This is called the isopotential point (Fig. 4). In theory, this point is temperature-independent. The International Union for Physical and Applied Chemistry (lUPAC) (1) operational pH scale is defined as fhe pH relative to a standard buffer measured using a hydrogen electrode. In practice, a pH electrode is calibrated with standard buffers of pH 7.00 and pH 4 or 9 fo defermine the isoelectric point and slope, respectively. Conventional pH meters will read accurately over a range of 2.5-11 and beyond these ranges, accuracy cannot be assured. However, recently, instruments have become available that carry out calibration to allow correction for nonideal electrode behavior allowing accurate measurements between ranges of pH 1 and 13. 

The temperature compensator is keyed to the zero-millivolt isopotential point of the meter. Since this point may be varied with a zero control (see Section 2.4), the compensation will vary with this control setting. The electrodes, on the other hand, are fixed as to their isopotential point and cannot be varied. It is only when the isopotential point of electrodes and the meter are identical, or nearly so, that temperature compensation can be applied. In other words, if slope correction is being applied with the zero control at other than pH 7, the standard buffer solution and sample should be at the same temperature since the temperature compensator does not apply the proper correction. (See Figure 2.4.)... 

The purpose of the slope and zero (isopotential) controls is to provide a pH readout that closely follows the electrode response, thus increasing the accuracy of the measurement. If a high degree of accuracy is not required, typically greater than 0.05 pH unit or more, the slope control should be turned off or set at 100% and the zero control set to display the typical isopotential point at pH 7 and no second slope/span adjustment made. 

Also remember that the temperature compensator setting becomes more critical as the pH value deviates from 7. As discussed in Chapter 2, a meter is designed with zero millivolts at pH 7 and the temperature compensator control has no effect at this point. Since this point is the isopotential point of the electrode, any change in slope (mV/pH) due to temperature change has no effect. 

Isopotential point A potential which is not affected by temperature changes. It is the pH value at which dEldt for a given electrode pair is zero. Normally, for a glass electrode and SCE reference, this potential is obtained approximately when immersed in pH 7 buffer. 

Rgure 4-1 b. Changes In the Glass Measuremsnt Electrode cause a Horizontal Shift of the Isopotential Point... 

Some of the resistances in Equation 4-lj are large. Fortunately, the input leakage current (Ij) that flows through these resistances and the amplifier input is extremely small (about 1 picoamp or one trillionth of a milliamp). The current flows from the positive measurement electrode terminal to the negative reference electrode terminal so that the sign of the potential drop is negative compared to the measurement electrode s outer potential and consequently causes the isopotential point to shift down and the pH measurement to go upscale. 

Changes in ionic strength will cause changes in the liquid junction potential of the reference electrode. If the concentration of positive ions increases, the liquid junction potential increases and shifts the isopotential point up and the pH downscale. If the concentration of negative ions increases, the liquid junction potential decreases or becomes more negative and shifts the isopotential point down and the pH upscale. 

Gas bubbles can result from gas entrainment from excessive agitation, gas evolution from a reaction, or a gas reagent. The gas bubbles cause a high resistance and reduce hydrogen activity at the measurement electrode upon impact. The result is an intermittent shift of the isopotential point to the left and down, which creates upscale pH noise. 

Note from the definition is follows that the pH of the isotherm intersection point in Fig. 2.13 represents the isopotential pHf of the Metrohm EA121 combined electrode.)... 

Anodes and cathodes need not be separate electrodes but can be areas on the same piece of metal. O Halloran et al. [4] have developed a technique in which isopotential contours on the corroding electrode may be mapped (see Fig. 1). As the technique involves gathering a large number of data points, a microprocessor is used. A small reference electrode is passed across a corroding specimen close to its surface and the potential differences relative to another fixed reference electrode are recorded. The potential profile reflects the ion current density in the vicinity of the corroding surface and... 

The zero (isopotential) control provides the flexibility to standardize at a point other than pH 7 and then make a slope adjustment without affecting the standardization point. It provides greater accuracy by allowing the two-point calibration to take place over a narrower pH range. The zero control provides a potential to offset the ideal standardization potential and thus provide zero millivolts at a point other than pH 7. The zero control is first adjusted to pH 10.0 when in the standby mode which separates the electrodes from the meter. Then the electrodes are standardized in a pH 10.0... 

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