Reference Electrode Calibration

A precision pH-meter (accuracy of 0.005 pH units) and a glass and calomel reference electrode calibrated against pH standards were used to measure pH values (activity units) which were converted into - logio [H ] using the Debye-Hiickel equation. However, the absolute accuracy that can be achieved with the applied electrode calibration is at best +0.02 pH-units, which is not sufficient for the determination of accurate results from potentiometric titration data. This problem becomes evident from the results shown in Table 1 of [1971K1C/STE]. The value of = 0.5 was reached at pH = (3.00 + 0.05) in titrations starting at initial acid concentrations below 0.01 M (/ = 0.013-0.016 M) and at pH = (3.40 + 0.05) in titrations starting at initial acid concentrations of about 0.03 M (/ = 0.037 M). 

The question is, how can we show that this has happened In cathodic protection of steel in soil or water it is usual to do this by achieving a potential of -770 mV or -850 mV against a copper/copper sulphate half cell on the surface as the system is switched off (the instant off potential). However, these criteria are not appropriate for steel in atmospherically exposed concrete for a nnmber of theoretical and practical reasons. Two of the practical reasons are the difficulty in accurately measuring an absolute potential over a nnmber of years when reference electrodes calibration may drift, and the fact that if an absolnte minimum (or maximum negative) potential is achieved then some parts of the structure will be overprotected as the corrosion environment varies so rapidly and severely across a high resistance electrolyte like concrete. 

Ion-selective electrodes are a relatively cheap approach to analysis of many ions in solution. The emf of the selective electrode is measured relative to a reference electrode. The electrode potential varies with the logarithm of the activity of the ion. The electrodes are calibrated using standards of the ion under investigation. Application is limited to those ions not subject to the same interference as ion chromatography (the preferred technique), e.g. fluoride, hydrogen chloride (see Table 10.3). 

Finally, calomel electrodes (and more especially hydrogen electrodes) are not suitable for field measurements because they are not sufficiently robust. The calomel electrodes are however essential for calibrating the field reference electrodes. Saturated KCI calomel electrodes are the most suitable because there is then no doubt about the reference potential of the calibrating electrode. Lack of adequate calibration is a common cause of cathodic protection system mismanagement. 

As an alternative to plotting a calibration curve, the method of standard addition may be used. The appropriate ion-selective electrode is first set up, together with a suitable reference electrode in a known volume (Ft) of the test solution, and then the resultant e.m.f. ( t) is measured. Applying the usual Nernst equation, we can say... 

Changes in the reference electrode junction potential result from differences in the composition of die sample and standard solutions (e.g., upon switching from whole blood samples to aqueous calibrants). One approach to alleviate this problem is to use an intermediate salt bridge, with a solution (in the bridge) of ions of nearly equal mobility (e.g., concentrated KC1). Standard solutions with an electrolyte composition similar to that of the sample are also desirable. These precautions, however, will not eliminate the problem completely. Other approaches to address this and other changes in the cell constant have been reviewed (13). 

The Vacuum Reference The first reference in the double-reference method enables the surface potential of the metal slab to be related to the vacuum scale. This relationship is determined by calculating the workfunction of the model metal/water/adsorbate interface, including a few layers of water molecules. The workfunction, — < ermi. is then used to calibrate the system Fermi level to an electrochemical reference electrode. It is convenient to choose the normal hydrogen electrode (NHE), as it has been experimentally and theoretically determined that the NHE potential is —4.8 V with respect to the free electron in a vacuum [Wagner, 1993]. We therefore apply the relationship... 

It has become fairly common to adopt the manufacture of combinations of internal reference electrode and its inner electrolyte such that the (inner) potential at the glass electrode lead matches the (outer) potential at the external reference electrode if the glass electrode has been placed in an aqueous solution of pH 7. In fact, each pH glass electrode (single or combined) has its own iso-pH value or isotherm intersection point ideally it equals 0 mV at pH 7 0.5 according to a DIN standard, as is shown in Fig. 2.11 the asymmetry potential can be easily eliminated by calibration with a pH 7.00 0.02 (at 25° C) buffer solution. 

For the drain current IA of the ISFET, again eqn. 2.101 is valid there is a freedom of choice as to the drain-bulk voltage Vjb> but once its value has been chosen one has to calibrate with buffers. In fact, a reference electrode is not essential, but it contributes to more stable results. 

It remains possible to check the correctness of the end-point detection by calibration on samples of known composition under the same measurement conditions a similar procedure consists in the differential titrations introduced by Pinkhof and Treadwell, who used a reference electrode, identical with the indicator electrode, but dipped it into a solution buffered to the end-point potential value67. 

For current practice, the described method of pH measurement is too tedious. Moreover, not hydrogen but glass electrodes are used for routine pH measurements (see Section 6.3). Then the expression for the EMF of the cell consisting of the glass and reference electrodes contains a constant term from Eq. (6.3.10), in addition to the terms present in Eq. (3.3.3) this term must be obtained by calibration. Further, a term describing the liquid junction potential between the reference electrode and the measured solution must also be included. 

The wireless pH capsule (Medtronic Inc.) is oblong in shape and contains an antimony pH electrode, a reference electrode at its distal tip, a battery, and a RF transmitter. The whole device is encapsulated in epoxy. The capsule is introduced into the esophagus on a catheter through the nose or mouth and is attached to the lining of the esophagus with a clip. The probe monitors the pH in the esophagus and transmits the information via RF telemetry at a rate of 6 per second (0.17 Hz) to a pager-sized receiver that is worn by the patient on a belt. Prior to implantation, the capsule is calibrated with its receiver in pH buffer solutions of pH 1.07 and pH 7.01 [168],... 

Potentiometric measurements with ISEs can be approached by direct potentiometry, standard addition and titrations. The determination of an ionic species by direct potentiometry is rapid and simple since it only requires pretreatment and electrode calibration. Here, the ion-selective and reference electrodes are placed in the sample solution and the change in the cell potential is plotted against the activity of the target ion. This method requires that the matrix of the calibration solutions and sample solutions be well matched so that the only changing parameter allowed is the activity of the target ion. 

The pH meter is standardized (calibrated) with the use of buffer solutions. Usually, two buffer solutions are used for maximum accuracy. The pH values for these solutions should bracket the pH value expected for the sample. For example, if the pH of a sample to be measured is expected to be 9.0, buffers of pH = 7.0 and pH = 10.0 should be used. Buffers with pH values of 4.0,7.0, and 10.0 are available commercially specifically for pH meter standardization. Alternatively, of course, homemade buffer solutions (see Chapter 5) may be used. In either case, when the pH electrode and reference electrode are immersed in the buffer solution being measured and the electrode leads are connected to the pH meter, the meter reading is electronically adjusted (refer to manufacturer s literature for specifics) to read the pH of this soluiton. The electrodes can then be immersed into the solution being tested and the pH directly determined. 

The SHE is experimentally cumbersome, and is hazardous to use owing to the involvement of elemental hydrogen gas, while the values of h+,H2 can fluctuate quite badly during operation because of the cyclic nature of bubble formation. For these reasons, the SHE is avoided experimentally unless a secondary reference electrode requires calibration. We will not consider the SHE any further because it is so unlikely that an analyst would in fact wish to calibrate a new reference electrode. 

The lUPAC Commission for Analytical Nomenclature defines the calibration curve [138] as the dependence of the electromotive force of the given ISE -reference electrode cell on the logarithm of the activity or concentration of the given substance. It is recommended that the potential be plotted on the ordinate (the vertical axis) and the logarithmic function of the activity or concentration on the abscissa (the horizontal axis), with the concentration increasing from the left to the right. 

In the bypass position, the carrier solution flows through the bypass loop and across the ISFET. The sample is injected into the sampling valve and is introduced into the carrier solution. The bypass loop has a high hydrodynamic resistance and thus the solution proceeds to the detector. The reference electrode is always immersed only in the carrier solution and is electrically connected with the ISFET through the solutioa The apparatus is regularly calibrated by K, Ca and pH standard solutions. 

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