Reference electrodes, aqueous solutions mercury

A reference electrode composed of mercury, mercurous chloride (calomel), and a saturated aqueous chloride solution. 

Both lead ion and dichromate ion yield a diffusion current at an applied potential to a dropping mercury electrode of —1.0 volt against the saturated calomel electrode (S.C.E.). Amperometric titration gives a V-shaped curve [Fig. 16.14 (C)]. The exercise described refers to the determination of lead in lead nitrate the application to the determination of lead in dilute aqueous solutions (10-3 — 10-4lVf) is self-evident. 

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. 

Undoubtedly, the mercury/aqueous solution interface, was in the past, the most intensively studied interface, which was reflected in a large number of original and review papers devoted to its description, for example. Ref. 1, and in the more recent work by Trasatti and Lust [2] on the potentials of zero charge. It is noteworthy that in view of numerous measurements of the double-layer capacitance at mercury brought in contact with NaF and Na2S04 solutions, the classical theory of Grahame [3] stiU holds [2]. According to Trasatti [4], the most reliable PZC value for Hg/H20 interface in the absence of specific adsorption equals to —0.433 0.001 V versus saturated calomel electrode, (SCE) residual uncertainty arises mainly from the unknown liquid junction potential at the electrolyte solution/SCE reference electrode boundary. 

The most common electrode of this type is the saturated calomel electrode (SCE) which consists of mercury in contact with a layer of insoluble Hg2Cl2 immersed in a saturated aqueous solution of KC1. The SCE is used as a secondary standard reference electrode. At 25°C it has an electrode potential of + 0.2415 V. 

Other careful electrochemical measurements of the oxidation potentials of 2,4,6-tri-t-butylphenol and 2,6-di-t-butyl-4-methylphenol in acetate buffered ethanol or acetonitrile have been measured by Mauser et al.184). They determined the static potentials using a boron carbide indicator and a mercury/mercury-acetate reference-electrode. Since in this case the oxidation of the phenols and not the phenolates to the phenoxyls has been determined the oxidation potentials cannot be compared with those in Table 12. For other electrochemical oxidations of phenols in buffered aqueous solutions using a graphite electrode see Ref. 185 186>. 

Electrical Properties of Interfaces. Compilation of Data on the Electrical Double layer on Mercury Electrodes. J. Lyklema, R. Parsons, Eds. Publ. U.S. Dept, of Commerce, Natl. Bur. Standards, Office of Standard Reference Data (1983). (Contains electrocapillary data for the mercury-aqueous solution interface.)... 

A zinc amalgam electrode has been employed as reference electrode in dimethyl sulfoxide (DMSO) [205] and ammonia [206]. The saturated zinc amalgam was prepared [207] by electrolytic deposition of zinc into a layer of pure mercury from an aqueous ZnS04 solution. The solution in equilibrium with the Zn(Hg) consisted of a saturated solution of Zn(C104)2, 4DMSO or ZnCF, 6NH3. 

The compound is an air-stable, white, crystalline solid, soluble in water. Its aqueous solution is stable between pH 4 and 7.5. Its polarogram in 0.5 M tris(hydroxymethyI)aminomethane + 0.5 M NaCl buffer, pH 7.5, obtained with a dropping mercury electrode, shows two waves with half-wave potentials —1.06 and —1.16 V versus SCE. There is a shoulder on the electronic spectrum in the UV region at240nm(c= 1.4-10 M" cm ). The 250-MHz WNMR spectrum in H2O-D2O (90 10) solution shows six resonance lines with relative intensities 2 1 2 2 2 1 at = —93.8, — 103.7, - 104.8, — 118.6, — 189.8, and — 196.8 ppm, respectively (reference external 2 M Na2WO4 in alkaline D2O). 

Fig. 1 0.1 Experimental data for the interfacial tension of the mercury aqueous electrolyte solution (1 M) interface as a function of the potential drop across the cell at 18°C. The reference electrode was a calomel electrode with 1 M KCl.

In the case of the study of the adsorption of molecular solutes at the polarizable interface, different tactics are often employed in data analysis. Suppose a study of the adsorption of pyridine is carried out at the mercury solution interface from an aqueous solution containing KCl. The Gibbs adsorption isotherm for this system using a reference electrode reversible to the CU anion is... 

Let us now consider a specific chemical system in which a mercury surface contacts an aqueous KCl solution. The potential of the mercury is controlled with respect to a reference electrode having no liquid junction with the test solution. Suppose also that the aqueous phase contains a neutral species, M, that might be interfacially active. For example, the cell could be... 

An example of a polarizable interface is that between a mereury electrode and liquid water, since the concentration of mercury ions in the aqueous phase is quite negligible. In this case, it is common to assume a a practical convention that equilibrium at the interface exists when the emf of the mercury electrode-reference electrode pair vanishes, since a Galvani potential difference between mercury and water cannot be measured. An example of a reversible interface is that between a hydrous oxide solid and liquid water. In this case, and OH ions can cross the interface freely and are potential-determining. Equilibrium at the interface is established when the net ion transport across the interface vanishes, i.e., when there is no change in the pH value of the aqueous phase. Note that the interface between a soil particle and the soil solution is in general reversible. Any charged species that is adsorbed by the particle and found in the soil solution is potential-determining. 

The object here will be to review in outline the current status of the electrical double layer in organic solvent systems. The emphasis will be on broad conclusions rather than experimental methods and numerical data, details of which can be found in the original papers. In contrast to the more general field of electrode processes in organic media on which a substantial literature exists, the double layer has been neglected by comparison. Less than 40 papers have been published, the majority in recent years. Consequently the field is still in a relatively exploratory stage and many of the conclusions are tentative. As is the case in aqueous solutions most of the work has been concerned with the mercury electrode to which the following discussion refers exclusively. 

In electrochemical cells without liquid junctions, the two electrodes are in contact with the same electrolyte of uniform concentration. For example, the cell shown in Figure 2.23 is made of a lead electrode and a lead amalgam electrode (lead dissolved in mercury), in contact with an aqueous solution of PbCl2. This cell corresponds to the schematic representation (2.123), where M and M" refer to the metal of the two conductors attached to the voltmeter. 

In 1890 Ostwald introduced the normal calomel electrode ( ) as a reference electrode of fixed potential in equilibrium with aqueous potassium chloride solution. Ostwald calibrated a normal calomel electrode against a dropping mercury electrode and obtained a mean value of 0.560 volt. He referred to this value as the absolute value of the electrode potential of the normal calomel electrode. Ostwald recommended the use of the normal calomel electrode as the null electrode, or the standard electrode, in the measurement of the potential difference at a metal-solution junction. He suggested that the electromotive potential measurement with the normal calomel... 

Reference electrodes of mercury have been used by several investigators in an attempt to measure single electrode potentials. Stastny and Strafelda (5 ) concluded that the zero charge potential of such an electrode in contact with an infinitely dilute aqueous solution is -0.1901V referred to the standard hydrogen electrode. Hall ( ) states that the potential drop across the double layer under these conditions is independent of solution composition when specific adsorption is absent. Daghetti and Trasatti (7, ) have used mercury reference electrodes to study the absolute potential of the fluoride ion-selective electrode and have compared their estimates of ion activities in NaF solutions with those provided by other methods. Their method is based on the assumption that the potential drop across the mercury I solution interface is independent of the electrolyte concentration once the diffuse layer effects are accounted for by the Gouy-Chapman theory. 

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