The Three-Electrode Measurement

It should be noted that during measurement in a three-electrode cell, the potential of the counter electrode might change substantially. This, however, does not in any way influence the measured potential of the working electrode with respect to the reference. 

The three-electrode arrangement can be used equally well if the potential between the working and reference electrodes is controlled and the current flowing through the working and counter electrodes is measured. Details of this mode of measurement are discussed below. 

Residual JRs Potential Drop in a Three-Electrode Cell 

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Before testing, the whole test cell is placed into a beaker filled with electrolyte solution for electrolyte intake, after which the cell is placed in a vacuum oven at 60°C for at least 30 min to remove trapped air inside the cell. Note that some test cells include a reference electrode to measure the individual electrode potentials, similar to the three-electrode measurement discussed above. However, locating this reference electrode inside the thin layer electrolyte (separator) is a challenge. 

The three-electrode system serves two important purposes. Because the reference electrode carries no current, but merely measures a potential relative to the working electrode, its stabiUty is not unduly influenced by the electrolysis. Furthermore, because it is placed close to the working electrode the measured potential difference is more nearly representative of the tme potential difference between the working electrode and the sample solution. This latter is the significant quantity in electro analysis. 

Accurate pH measurement is surprisingly complex. 3 A recent review(4) states that if three electrodes measuring pH at one point in an industrial process agree to within 0.1 pH unit, then the electrodes are probably broken, coated, or still covered by their protective caps ... 

Fig. 7.36. The three-electrode system required to measure electrode overpotentials, i.e., dtf - The potential between the working electrode and the reference electrode when both J<]> and 3e correspond to the same reaction is equal to the overpotential, T). The tube joining the reference electrode and the working electrode is a Luggin capillary. It helps diminish the inclusion of illicit IR drop in the measurement...

Positive feedback iR compensation in three-electrode measurements As described in Section 5.3, the influence of iR-drop is serious in two-electrode polarography or voltammetry. The influence is eliminated considerably with three-electrode instruments, if the tip of the reference electrode is placed near the surface of the indicator electrode. However, there still remains some iR-drop, which occurs by the residual resistance at... 

Figure 19.8—A selective electrode designed from a MOSFET (metal oxide semiconductor field effect transistor). A specific reaction can be monitored by putting an enzyme in contact with the electrodes. This schematic shows the three electrodes used for amperometric measurement.

The 2 x SSC buffer solution contained 300 mM of NaCl and 30 mM C3H5Na307 (pH 7.4). The acetate buffer solution (ABS) was prepared at the final concentration of 0.25 M, with 10 mM of NaCl (pH 4.7). All oligonucleotides, probes and complementary strands were purchased from Sigma Genosys (UK). All measurements were performed using screen-printed electrodes (SPEs). SPEs were used as disposable. The three-electrode system used was formed by a graphite working electrode, a counter electrode and a silver-based reference electrode. 

Immerse the three electrodes in a cell renewing the 25 mL of bidistilled water and measure the potential once it remains stable for each ISE. 

Potential control or potential measurements are fundamental to electroanalytical studies, so the cells used are usually of the three-electrode type. A typical cell for electroanalytical work, such as linear sweep and cyclic voltammetry, is shown in Fig. 6.2. 

Returning to the three-electrode setup, it could seem that no ohmic drop would affect the measurement of the potential difference between the working and reference electrodes, since there is practically no current flow between both electrodes. However, this is not totally true. The reference electrode is located at a given distance from the working electrode surface, and, as a result of this separation, the potential difference measured contains a part of the ohmic drop in the solution which is called residual ohmic drop, IRU (with I being the current and Ru the uncompensated resistance). For more details concerning the minimization of the ohmic distortion of the current-potential response, see Sects. 1.8 and 5.4. 

To minimize errors in voltammetric work, the three-electrode configuration (Figure 5.2b) is commonly used. The cell current flows between the working electrode and the counter or auxiliary electrode, while the potential of the working electrode is measured with respect to the reference electrode using a high-impedance measuring device. This avoids internal polarization of the reference electrode and compensates for the major portion of the iR drop in the cell. 

Three-electrode system — A measurement system with a - potentiostat that uses three electrodes - working, -r counter, and - reference. The systems work in such a way that a desired potential is imposed to the working electrode vs. the reference electrode. The current in the cell flows only between the working and counter electrodes. The reference electrode is not loaded with the current, therefore it preserves its potential even under conditions of high current flowing in the cell. The application of the three-electrode system allows also the elimination of -> salt-bridge resistance and consequently the ohmic potential drop which influences the recorded -> voltammograms. Three-electrode systems do not compensate the entire resistance in the cell. See also - electrochemical cell, -> IRU potential drop. 

Undivided cell — Electrochemical cells where all electrodes (two or three) are placed in the same compartment. Undivided cells are typically used for analytical experiments at small or -> microelectrodes in aqueous solutions when a -> two-electrode system is applied, or in a -> three-electrode measurements in nonaqueous media with a (platinum) -> quasireference electrode. A requirement in the use of an undivided cell is that the reaction products produced at the counter electrode do not reach or perturb the behavior of the working electrode. 

The following procedure was followed for all copper titrations at 25°C and constant pH. The three electrodes were first preconditioned for 30 min in a solution at pH 8 containing 0.1 M Tris base, 0.05 M HCl and sufficient CUSO4 to achieve a pCu of 13.0 to 13.5. The electrodes were then rinsed several times with distilled water and placed for 30 min in a portion of the solution to be titrated. The electrodes were then placed in a fresh 70 mX-portion of the same solution contained in a 100 mJl borosilicate glass beaker and titrated with CUSO4. Sufficient time was allowed for the electrodes to reach steady state potentials after each copper addition. At no copper addition, 60 min was allowed. For copper concentrations 10 M and > 10"7 m, measurements were made 30-60 min and 20-30 min, respectively, after each qopper addition. After reaching steady state, pCu and pH values were... 

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