Chemical analysis using galvanic cells

The measurement of the potential of a galvanic cell can be used to determine the concentration of the ions in a solution via the Nemst equation. For example, the cell reaction of the DanieU cell is  

The most widespread use of this analytical technique is the measurement of the concentration of hydrogen ions (pH). To illustrate this, consider how the combination of a standard electrode with a (nonstandard) hydrogen electrode can be used to measure the concentration of hydrogen ions present at the hydrogen electrode. The standard electrode is often a particularly stable electrode, caUed a calomel electrode, which uses the redox couple Hg2Cl2/ Hg, Cl . The cell is  

Where E is the cell constant, which depends on the Cl concentration. The pH can be read directly on a 

Apart from the calomel electrode, one of the most common standard electrodes used in pH meters is a silver/silver chloride electrode. This consists of a silver wire coated with silver chloride immersed in a four-molar solution of potassium chloride (KCl), saturated with silver chloride (AgCl). The half-reaction is  

In practice, the hydrogen ion selective electrode and the standard electrode are packaged together in a 

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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. 

The combination of chemistry and electricity is best known in the form of electrochemistry, in which chemical reactions take place in a solution in contact with electrodes that together constitute an electrical circuit. Electrochemistry involves the transfer of electrons between an electrode and the electrolyte or species in solution. It has been in use for the storage of electrical energy (in a galvanic cell or battery), the generation of electrical energy (in fuel cells), the analysis of species in solution (in pH glass electrodes or in ion-selective electrodes), or the synthesis of species from solution (in electrolysis cells). 

In principle the activity coefficients yb of solute substances B in a solution can be directly determined from the results of measurements at ven temperature of the pressure and the compositions of the liquid (or solid) solution and of the coexisting gas phase. In practice, this method fails unless the solutes have volatilities comparable with that of the solvent. The method therefore usually fails for electrolyte solutions, for which measurements of ye in practice, much more important than for nonelectrolyte solutions. Three practical methods are available. If the osmotic coefficient of the solvent has been measured over a sufficient range of molalities, the activity coefficients /b can be calculated the method is outlined below under the sub-heading Solvent. The ratio yj/ys of the activity coefficients of a solute B in two solutions, each saturated with respect to solid B in the same solvent but with different molalities of other solutes, is equal to the ratio m lm of the molalities (solubilities expressed as molalities) of B in the saturated solutions. If a justifiable extrapolation to Ssms 0 can be made, then the separate ys s can be found. The method is especially useful when B is a sparingly soluble salt and the solubility is measured in the presence of varying molalities of other more soluble salts. Finally, the activity coefficient of an electrolyte can sometimes be obtained from e.m.f. measurements on galvanic cells. The measurement of activity coefficients and analysis of the results both for solutions of a single electrolyte and for solutions of two or more electrolytes will be dealt with in a subsequent volume. Unfortunately, few activity coefficients have been measured in the usually multi-solute solutions relevant to chemical reactions in solution. 

Obviously, in everyday applications galvanic cells are operated in irreversible fashion during discharge and charge thus, the voltages and operating conditions are not subject to the analysis provided in this chapter. Rather, the near steady state conditions considered here can only be used in a thermodynamic analysis of chemical processes detailed below. 

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