Saturated calomel electrode errors with

Most common reference electrodes are silver-silver chloride (SSC), and saturated calomel electrode (SSC, which contains mercury). The reference electrode should be placed near the working electrode so that the W-potential is accurately referred to the reference electrode. These reference electrodes contain concentrated NaCl or KC1 solution as the inner electrolyte to maintain a constant composition. Errors in electrode potentials are due to the loss of electrolytes or the plugging of the porous junction at the tip of the reference electrode. Most problems in practical voltammetry arise from poor reference electrodes. To work with non-aqueous solvents such as acetonitrile, dimethylsulfoxide, propylene carbonate, etc., the half-cell, Ag (s)/AgC104 (0.1M) in solvent//, is used. There are situations where a conventional reference electrode is not usable, then a silver wire can be used as a pseudo-reference electrode. 

The end-point of the titration is found visually with thymol blue as indicator, or potentiometrically with glass and saturated calomel electrodes. The error of the determination does not exceed 1.4%. Alkoxy groups do not interfere in the procedure. 

V, (recalculated with the CODATA value for the saturated calomel electrode and corrected for an obvious misprint in the error limits) has been rejected. 

For electromotive force measurements there are many suitable electrodes, and only the most novel features can be summarised here. The glass electrode is generally usable in these solvents. It has been found that in solutions of hydrochloric acid in anhydrous acetic acid, the glass electrode is subject to acid errors of as much as 70 mV relative to the chloranil electrode, owing to the incorporation of chloride ions in the surface gel layer of the glass. ° Since ethylenediamine reacts with calomel but not with mercury (II) chloride, a suitable reference electrode for this solvent can be constructed of a mercury pool in contact with ethylenediamine saturated with respect to both HgClg and LiCl. °... 

The saturated copper-copper sulfate reference electrode consists of metallic copper immersed in saturated copper sulfate, as shown in Fig. 3.6. It is used primarily in field measurements where the electrode must be resistant to shock and where its usual large size minimizes polarization errors. The accuracy of this electrode is adequate for most corrosion investigations, even though it falls somewhat below the precision obtainable with the calomel or silver chloride electrodes. The half-cell reaction is... 

The internal element appears as a grayish-white cylindrical pack with shiny mercury at the top of the element, if it is a calomel internal. This mercury-mercurous chloride half cell provides a potential of 244 mV versus the normal hydrogen electrode at 25°C if it is surrounded by saturated potassium chloride filling solution. It is important that this element be kept wet and uncontaminated in order to provide a stable and reproducible potential. With use, the pack may show some separation within the element tube, but this usually does not cause error or deviation of its potential. 

A normal calomel reference electrode may be used in solution containing up to 68% methanol with a large but stable junction potential. A means of reducing this potential is to change the filling solution as discussed in Chapter 3. For example, a methanol solvent saturated with potassium chloride may provide a more stable reference electrode. This error is also discussed in Section 6.2.I.E. 

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