Galvanic cell measurements, standard

Galvanic cell measurements, standard free energy of formation... 

A problem with compiling a list of standard potentials is that we know only the overall emf of the cell, not the contribution of a single electrode. A voltmeter placed between the two electrodes of a galvanic cell measures the difference of their potentials, not the individual values. To provide numerical values for individual standard potentials, we arbitrarily set the standard potential of one particular electrode, the hydrogen electrode, equal to zero at all temperatures ... 

Table 2. Measured voltages and equilibrium constants for some galvanic cells using standard electrodes at 25 °C (all ions and soluble species at 1 M and all gases at 1 atm).

A second source of standard free energies comes from the measurement of the electromotive force of a galvanic cell. Electrochemistry is the subject of other articles (A2.4 and B1.28). so only the basics of a reversible chemical cell will be presented here. For example, consider the cell conventionally written as... 

The potentials of the metals in their 1 mol U salt solution are all related to the standard or normal hydrogen electrode (NHE). For the measurement, the hydrogen half-cell is combined with another half-cell to form a galvanic cell. The measured voltage is called the normal potential or standard electrode potential, E° of the metal. If the metals are ranked according to their normal potentials, the resulting order is called the electrochemi-... 

The zinc-copper galvanic cell is under standard conditions when the concentration of each ion is 1.00 M, as shown in Figure 19-13. The cell potential under these conditions can be determined by connecting the electrodes to a voltmeter. The measured potential is 1.10 V, with the Zn electrode at the higher (more negative) potential, so Zn gives up electrons and E eii = 1.10 V ... 

Equation expresses an important link between two standard quantities. The equation lets us calculate standard electrical potentials from tabulated values for standard free energies. Equally important, accurate potential measurements on galvanic cells yield experimental values for standard potentials that can be used to calculate standard free energy changes for reactions. 

Potentiometry is used in the determination of various physicochemical quantities and for quantitative analysis based on measurements of the EMF of galvanic cells. By means of the potentiometric method it is possible to determine activity coefficients, pH values, dissociation constants and solubility products, the standard affinities of chemical reactions, in simple cases transport numbers, etc. In analytical chemistry, potentiometry is used for titrations or for direct determination of ion activities. 

It has been emphasized repeatedly that the individual activity coefficients cannot be measured experimentally. However, these values are required for a number of purposes, e.g. for calibration of ion-selective electrodes. Thus, a conventional scale of ionic activities must be defined on the basis of suitably selected standards. In addition, this definition must be consistent with the definition of the conventional activity scale for the oxonium ion, i.e. the definition of the practical pH scale. Similarly, the individual scales for the various ions must be mutually consistent, i.e. they must satisfy the relationship between the experimentally measurable mean activity of the electrolyte and the defined activities of the cation and anion in view of Eq. (1.1.11). Thus, by using galvanic cells without transport, e.g. a sodium-ion-selective glass electrode and a Cl -selective electrode in a NaCl solution, a series of (NaCl) is obtained from which the individual ion activity aNa+ is determined on the basis of the Bates-Guggenheim convention for acr (page 37). Table 6.1 lists three such standard solutions, where pNa = -logflNa+, etc. 

Potentiometry is the most widely used electroanalytical technique. It involves the measurement of the potential of a galvanic cell, usually under conditions of zero current, for which purpose potentiometers are used. Measurements may be direct whereby the response of samples and standards are compared, or the change in cell potential during a titration can be monitored. 

The voltage we measure is characteristic of the metals we use. As an additional example, unit activity solutions of CuCE and AgCl with copper and silver electrodes, respectively, give a potential difference of about 0.45 V. We could continue with this type of measurement for aU the different anode-cathode combinations, but the number of galvanic cells needed would be very large. Fortunately, the half-reactions for most metals have been calculated relative to a standard reference electrode, which is arbitrarily selected as the reduction of hydrogen ... 

Rg. 7.14. A galvanic cell composed of a copper electrode in cupric ion solution and a standard hydrogen electrode (a) gives a measurable potential difference Ee (b). 

We start with a simple reversible redox reaction for which we can directly measure the free energy of reaction, Ar<7, with a galvanic cell. This example helps us introduce the concept of using (standard) reduction potentials for evaluating the energetics (i.e., the free energies) of redox processes. Let us consider the reversible interconversion of 1,4-benzoquinone (BQ) and hydroquinone (HQ) (reaction 14-5 in Table 14.1). We perform this reaction at the surface of an inert electrode (e.g.,... 

If we now make a whole series of galvanic cells with solutions at unit activity and use the standard hydrogen electrode as one-half of every cell, then the measured cell voltage (Fceii) for each cell will be given, according to Equation 17-1, by either... 

Measurements of the potentials of galvanic cells at open circuit give information about the thermodynamics of cells and cell reactions. For example, the potential of the cell in Figure 1, when the solution concentrations are 1 molar (1 M) at 25°C, is 1.10 V. This is called the standard potential of the cell and is represented by E°. The available energy (the Gibb s free energy AG°) of the cell reaction given in equation (3) is related to E° by... 

In voltaic cells, it is possible to carry out the oxidation and reduction halfreactions in different places when suitable provision is made for transporting the electrons over a wire from one half-reaction to the other and to transport ions from each half-reaction to the other in order to preserve electrical neutrality. The chemical reaction produces an electric current in the process. Voltaic cells, also called galvanic cells, are introduced in Section 17.1. The tendency for oxidizing agents and reducing agents to react with each other is measured by their standard cell potentials, presented in Section 17.2. In Section 17.3, the Nernst equation is introduced to allow calculation of potentials of cells that are not in their standard states. 

Instruments and Methods of Measurements. A Leeds and Northrup Type K-3 universal potentiometer, in conjunction with a General Electric Model 29 galvanometer, was used to measure electromotive force. The potentiometer was calibrated by means of a Weston Standard Cell which had been calibrated against a National Bureau of Standards (NBS) certified standard cell. Galvanic cells which were maintained at constant temperatures of 25°, 35°, and 45°C d= 0.01° by being immersed in a water bath at the desired temperature. The temperatures of the baths were set using a Fisher Scientific calibrated standard thermometer, with calibration traceable to the NBS. An adaptation of the cell sketched by Ives and Janz (II) was used. The modification of the cell was that described by Mclntrye and Amis (10). 

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