Standard potential from cells with liquid junctions

The Standard Potentials of the Alkali Metals from Cells with Liquid Junctions. The determination of the standard potentials of sodium and potassium, using cells without liquid junctions, has already been described in Chapter 10. It is of interest to compare the value obtained in that way for potassium with the result of measurements on cells with liquid junctions, especially as the available data for computing the standard potentials of lithium, rubidium and cesium are of the latter type. Lewis and Keyes2 have found the potentials of the cells ... 

Standard Potentials in Acetonitrile at 25°C from Cells with Liquid Junctions... 

An operational definition endorsed by the International Union of Pure and Applied Chemistry (lUPAC) and based on the work of Bates determines pH relative to that of a standard buffer (where pH has been estimated in terms of p"H) from measurements on cells with liquid junctions the NBS (National Bureau of Standards) pH scale. This operational pH is not rigorously identical to p H defined in equation 30 because liquid junction potentials and single ion activities cannot be evaluated without nonthermodynamic assumptions. In dilute solutions of simple electrolytes (ionic strength, I < 0.1) the measured pH corresponds to within 0.02 to p H. Measurement of pH by emf methods is discussed in Chapter 8. 

The Standard Potentials of the Halogens. The standard potential of chlorine, obtained from cells without liquid junctions, has been considered on page 198. The standard potential of iodine has been determined by Maitland and more recently by Jones and Schumb6 and Jones and Kaplan.7 It is most convenient to take the solid as the standard state of iodine, instead of the gas at one atmosphere pressure as was done with chlorine, Jones and Schumb used cells of the type... 

In Eqs. (122) and (123), M(Hg) is an alkali metal amalgam electrode, MX the solvated halide of the alkali metal M at concentration c in a solvent S, and AgX(s)/Ag(s) a silver halide-silver electrode. Equation (124) is the general expression for the electromotive force " of a galvanic cell without liquid junction in which an arbitrary cell reaction 0)1 Yi + 0)2Y2 + coiYi + , takes place between k components in v phases. In Eq. (124) n is the number of moles of electrons transported during this process from the anode to the cathode through the outer circuit, F the Faraday number, and the chemical potential of component Yi in phase p. Cells with liquid junctions require the electromotive force E in Eq. (124) to be replaced by the quantity E — Ej), where Ey> is the diffusion potential due to the liquid junction. The standard potential E° for the cell investigated by Eq. (122) is given by the relationship... 

The standard potential of am electrode is defined as the standard potential of a cell in which the other (reference) electrode is the arbitrary zero of potential (equation (2)) as described above. In this chapter the methods for obtaining standard potentials from emf measurements of cells without liquid junctions will be discussed, and the available data will be used for computing such potentials. The order adopted will be, more or less, that of the increasing complexity of the methods employed, Later chapters will deal with liquid junctions and the less accurate standard potentials that can be obtained from emf values of cells containing such junctions. 

The Variation of the Standard Potentials of Some Electrodes with the Temperature. In a number of cases the standard potentials of galvanic cells without liquid junctions have been determined over a range of temperatures. From these determinations it has been possible to prepare Table V, which gives the standard potentials of a number of electrodes at intervals of 12.5° from 0° to 50°. Some slight adjustments, of the order of 0.2 millivolt, of the original data have been necessary to bring the figures into accord with the Ho values at 25° adopted in this book. A more complete table of standard potentials of the elements at 25° will be found at the end of Chapter 14. 

In Chapter 10 standard potentials were obtained from measurements on galvanic cells involving only one electrolyte. These cells without transference thus do not involve surfaces between solutions of electrolytes, more commonly called liquid junctions. Although the measurements on cells without liquid junctions can be much more readily interpreted than the results from cells containing such boundaries most of the earlier work was carried out with the latter type of cell. Thus for instance instead of measuring the potential of the cell ... 

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is... 

As a liquid junction potential is avoided, the cell potential consists merely of the electrode potentials of the hydrogen and the silver/silver chloride reference electrode. Chloride at known concentrations, mcl, must be added to the (chloride-free) buffer solution to use the silver-silver chloride electrode in cells without transference as a reference. This is different from silver/silver chloride reference systems with fixed potentials used for example as standard references in single-rod glass electrodes. 

The Silver-Silver Chloride Electrode.—In recent years the silver-silver chloride electrode has been frequently employed as a reference electrode for accurate work, especially in connection with the determination of standard potentials by the use of cells containing chloride which are thus free from liquid junction potentials. The standard potential of the Ag, AgCl(s), Cl electrode is obtained as follows the e.m.f. of the cell... 

Most pH determinations are made by electrometric methods, the pH of the unkown solution (X) being calculated from that of a known standard (S) and the emf ( x and s) of a cell composed of a hydrogen ion-responsive electrode (for example, a glass electrode or a hydrogen gas electrode) coupled with a reference electrode (calomel, silver-silver chloride). This cell is filled successively with the standard solution S and with the unknown solution X. A liquid junction potential j exists where these solutions make contact with the concentrated KCl solution of the reference electrode. From the Nernst equation for the cell reactions and assuming an ideal hydrogen ion response ... 

If pH measurements are made at a temperature other than that at which the standardization is made, other factors being equal, the liquid-junction potential will change with temperature. For example, in a rise from 25° to 38°C, a change of -1-0.76 mV has been reported for blood and —0.55 mV for buffer solutions. Thus, for very accurate work, the cell should be standardized at the same temperature as the test solution. 

The required attributes listed above effectively limit the range of primary buffers available to between pH 3 and 10 (at 25 °C). Calcium hydroxide and potassium tetraoxalate tire excluded because the contribution of hydroxide or hydrogen ions to the ionic strength is significant. Also excluded are the nitrogen bases of the type BH+ (such as tris(hydroxymethyl)aminomethane and piperazine phosphate) and the zwitterionic buffers (e.g. HEPES and MOPS (10)). These do not comply because either the Bates-Gu enheim convention is not applicable, or the liquid junction potentials are high. This means the choice of primary standards is restricted to buffers derived from oxy-carbon, -phosphorus, -boron and mono, di- and tri-protic carboxylic acids. The uncertainties (11) associated with Harned cell measurements are calculated (1) to be 0.004 in pH at NMIs, with typical variation between batches of primary standard buffers of 0.003. 

The pH meter cell must be standardised with buffers of known pH. Aqueous buffers are sometimes used when the solvent under investigation is a methanol-water mixture. One must then assume that liquid junction potentials thereby introduced are approximately constant from one test solution to another. 2 There is also a risk that transfer of a glass electrode from aqueous to methanolic solution may cause ir-reproducible changes in its asymmetry potential. A better procedure is to prepare standard buffers in methanol. Three such buffers were assigned pH values by the same procedure used for standard aqueous buffers. The assigned values, designated pH (5), should closely approximate pC %). If an unknown solution X is now measured in the standardised cell, the operational definition of its pH is... 

The potentials of these reference electrodes can be checked against a standard half-cell, but another method must be used when the reference electrode is not separated from the electrolysed solution, when the liquid junction potential is unknown or when the potential of the reference electrode is not very reproducible— as with the mercurous sulphate electrode. In this method the unknown half-wave potential is measured against the half-wave potential of a standard substance, whose half-wave potential against S.C.E. or N.C.E. in the used supporting electrolyte is accurately known. 

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