Standardization of Electrodes

FIGURE 4.8 Laboratory platinum electrode and reference calomel electrode. (1) Platinum wire of 1.024 mm diameter, (2) waterproof epoxy, (3) triple-distilled mercury, (4) glass tubing, (5) silicon sealant covered with heat-shrink tubing, (6) test tube, (7) saturated KCl, (8) calomel-mercury-KCl paste, (9) mercury, (10) saturated KCl, (11) salt bridge, and (12) copper wire. 

When quinone is added to the buffer solution, it is reduced to hydroquinone as shown below Quinone + 2H+ + 2e = Hydroquinone, E° = 0.699 V According to Equation 4.68 

In a saturated solution, if the ratio of quinone/Hydroquinone = 1, then Equation 4.80 reduces to 

Expected electrode potential with SCE can be calculated using the following equation  

Newly constructed platinum electrodes placed in quinone/hydroquinone solutions at pH 7.0 should give an electrode potential of 0.042 V or 42 mV. When placed in a pH 4.0 buffer solution, they should give a potential of 0.219 V or 219 mV. The amount of quinhydrone added to the pH buffer solution should be enough to obtain a final concentration of at least 1 g L . Quinhydrone solutions should be made fresh every time electrodes are calibrated. 

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Ammonium acetate solutions formed by neutralizing acetic acid using ammonium hydroxide are essentially neutral. Thus, these solutions are suitable for standardization of electrodes, and for use as titration standards. Solutions must be used while fresh, however, as they become acidic on standing. 

Ammonium acetate is used for preserving meats as a mordant in the dyeing of wool in analytical chemistry for standardization of electrodes, and in titra-... 

Figure C2.8.3. A tliree-electrode electrochemical set-up used for the measurement of polarization curves. A potentiostat is used to control the potential between the working electrode and a standard reference electrode. The current is measured and adjusted between an inert counter-electrode (typically Pt) and the working electrode.

The last example presented in this section deals with the pitting corrosion of Fe in CIO solutions. Perchlorate is less known as an aggressive ion but reveals some unique and remarkable characteristics with regard to pitting corrosion. For example, the critical pitting potential (1.46 V against a standard hydrogen electrode (SHE) for Fe/1 M NaClO ) can be measured with an accuracy of less than 4 mV [61] which is very unexpected if compared to... 

Figure 3-1 Voltage Measurements on a Silver-Silver Chloride, Hydrogen Cell at 298.15 K. The contribution of the Standard Hydrogen Electrode is taken as zero by convention.

Standard Hydrogen Electrode The standard hydrogen electrode (SHE) is rarely used for routine analytical work, but is important because it is the reference electrode used to establish standard-state potentials for other half-reactions. The SHE consists of a Pt electrode immersed in a solution in which the hydrogen ion activity is 1.00 and in which H2 gas is bubbled at a pressure of 1 atm (Figure 11.7). A conventional salt bridge connects the SHE to the indicator half-cell. The shorthand notation for the standard hydrogen electrode is... 

Quantitative Analysis Using External Standards To determine the concentration of analyte in a sample, it is necessary to standardize the electrode. If the electrode s response obeys the Nernst equation. 

The immersion of glass electrodes in strongly dehydrating media should be avoided. If the electrode is used in solvents of low water activity, frequent conditioning in water is advisable, as dehydration of the gel layer of the surface causes a progressive alteration in the electrode potential with a consequent drift of the measured pH. Slow dissolution of the pH-sensitive membrane is unavoidable, and it eventually leads to mechanical failure. Standardization of the electrode with two buffer solutions is the best means of early detection of incipient electrode failure. 

Corrective action should be initiated when value is > — 0.23 V against the standard hydrogen electrode (SHE). Plant-specific values should be estabUshed for protection of stainless steels and nickel-based critical components. 

Laboratory experiments have shown that IGSCC can be mitigated if the electrochemical potential (ECP) could be decreased to —0.230 V on the standard hydrogen electrode (SHE) scale in water with a conductivity of 0.3 ]lS/cm (22). This has also been demonstrated in operating plants. Equipment has been developed to monitor ECP in the recirculation line and in strategic places such as the core top and core bottom, in the reactor vessel during power operation. 

Fig. 6. Band edge positions of several semiconductors ia contact with an aqueous electrolyte at pH 1 ia relation to the redox (electrode) potential regions (vs the standard hydrogen electrode) for the oxidation of organic functional groups (26,27).

Other Coordination Complexes. Because carbonate and bicarbonate are commonly found under environmental conditions in water, and because carbonate complexes Pu readily in most oxidation states, Pu carbonato complexes have been studied extensively. The reduction potentials vs the standard hydrogen electrode of Pu(VI)/(V) shifts from 0.916 to 0.33 V and the Pu(IV)/(III) potential shifts from 1.48 to -0.50 V in 1 Tf carbonate. These shifts indicate strong carbonate complexation. Electrochemistry, reaction kinetics, and spectroscopy of plutonium carbonates in solution have been reviewed (113). The solubiUty of Pu(IV) in aqueous carbonate solutions has been measured, and the stabiUty constants of hydroxycarbonato complexes have been calculated (Fig. 6b) (90). 

An electrolyte starter is almost a standard product like a motor and the manufacturer, depending upon the number of starts and the speed control requirement, can adjust the quantity of electrolyte, depth of electrodes etc. 

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). 

The electrolysis is carried out at a reference potential of -2.4 volts vs a standard calomel electrode. An initial current density of 0.0403 amp/cm is obtained which drops to 0.0195 amp/cm at the end of the reduction, which is carried on over a period of 1,682 minutes at 15° to 20°C. The catholyte is filtered, the solid material is washed with water and dried. 430 g of the 2,3-bis-(3-pyridyl)-butane-2,3-diol is recrystallized from water, MP 244° to 245°C. 

Ideally, one would prefer to compare anodic and cathodic potential limits instead of the overall ionic liquid electrochemical window, because difference sets of anodic and cathodic limits can give rise to the same value of electrochemical window (see Figure 3.6-1). However, the lack of a standard reference electrode system within and between ionic liquid systems precludes this possibility. Gonsequently, significant care must be taken when evaluating the impact of changes in the cation or anion on the overall ionic liquid electrochemical window. 

The thermodynamic driving force behind the corrosion process can be related to the corrosion potential adopted by the metal while it is corroding. The corrosion potential is measured against a standard reference electrode. For seawater, the corrosion potentials of a number of constructional materials are shown in Table 53.1. The listing ranks metals in their thermodynamic ability to corrode. Corrosion rates are governed by additional factors as described above. 

As A will be a function of current density, T will be a function of electrode area, and comparisons should therefore be made with cells of standard size. Equation 12.12 shows that high throwing indices will result when polarisation rises steeply with current (AE, AEj) and cathode efficiency falls steeply (cj >> f i)- The primary current ratio, P = affects the result because... 

Since the single potential of a metal cannot be measured it is necessary to use a suitable reference elecrode such as the Hg/Hg2Cl2/KCl electrode or the Ag/AgCl/KCl electrode, and although potentials are frequently expressed with reference to the standard hydrogen electrode (S.H.E.) the use of this electrode in practice is confined to fundamental studies rather than testing. 

It is apparent that since the electrode potential of a metal/solution interface can only be evaluated from the e.m.f. of a cell, the reference electrode used for that purpose must be specified precisely, e.g. the criterion for the cathodic protection of steel is —0-85 V (vs. Cu/CuSOg, sat.), but this can be expressed as a potential with respect to the standard hydrogen electrode (S.H.E.), i.e. -0-55 V (vs. S.H.E.) or with respect to any other reference electrode. Potentials of reference electrodes are given in Table 21.7. 

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