Electrical Methods Of Analysis

Electrical methods of analysis make use of the relationships between electrical and chemical phenomena as outlined in Sec. 3.9. Such methods are particularly useful in water chemistry, as they lend themselves to continuous monitoring and recording. The pH meter is probably the most widely used electrical method of analysis. In this j case a glass electrode and a reference electrode are inserted in a solution, and the : electrical potential or voltage across these electrodes is a measure of the concentra- ; tion of hydrogen ions in the solution. Methods based upon this principle are said to i be potentiometric,

In other electrical methods, suitable electrodes are introduced into a solution ' and a small measured voltage is applied. The current that flows is dependent upon the composition of the solution and so may be used to make analytical measurements. Methods based upon this principle are said to be polarographic. There are : many modifications of the different electrical methods of analysis, and the distinction between them is not always readily apparent. Other electroanalytical methods that have been adequately discussed in Sec. 3.9 are conductimetry, which measures the ability of a solution to carry a current, and oculometry, which is a measure of the . equivalence relationship between the quantity of electricity required to effect a ; given quantity of chemical change.

Potentiometric Analysis

The principles of the electrochemical cell were discussed in Sec. 3.9. Equation (3.42) gives the relationship between the relative potential of an electrode and the concentration of a corresponding ionic species in solution. If use is made of this equation, the measurement of the potential of an electrode can permit the calculation of the activity or concentration of a component of the solution. A potentiometric analysis requires a special electrode designed for measurement of the specific ion of interest, a reference electrode, and a potential measuring device.

As an example, the chloride concentration in a solution can be measured using the cell assembly diagrammed in Fig. 12.14. A reference electrode shown here as a saturated calomel electrode is used in conjunction with the electrode used to mea-

Figure 12.14

Cell assembly for Potentiometrie analysis for chlorides.

Figure 12.14

Cell assembly for Potentiometrie analysis for chlorides.

sure the constituent of interest. This particular reference electrode with potential Esce is a metal contacting a slightly soluble salt described later in this section. The chloride measuring electrode is actually a silver-silver chloride electrode, at which the following reaction occurs:

The potential of this half-cell, on the basis of Eq. (3.42), is given by the following:

The measured potential of the complete cell thus becomes

This equation indicates that the potential of the whole cell varies with the log of the chloride activity or approximately with the log of the chloride concentration. Once this cell is calibrated with standard chloride solutions, it can be used to measure the chloride concentration in unknown solutions. This is the basis for all potentiometric methods of analysis. The major requirement is for an electrode that is specific for the ion of interest and that is also relatively free from interactions with other constituents of the solution. Various electrodes of interest are described in the following.

Gas Electrode A gas electrode consists of a strip of nonreactive metal, such as platinum or gold, in contact with both the solution and a gas stream. Both electrodes in Fig. 3.8 are of this type, and the reactions indicated by Eqs. (3.39) and (3.40) are typical of gas electrodes. One gas electrode of particular importance is the hydrogen electrode, the standard electrode to which the potentials of other electrodes are related. It consists of a strip of sheet platinum coated with platinum black, immersed in a solution that is 1.0 N with respect to hydrogen ions and bathed with a stream of hydrogen gas under 1 atm of pressure. The reaction occurring at this electrode is that given by Eq. (3.39). The hydrogen electrode is assigned a value of zero, and potentials related to it are designated by the prefix £H. The hydrogen electrode is cumbersome to use, and in practice other electrodes are commonly used for reference.

In shorthand notation the hydrogen electrode can be described as follows:

PtjH2(Patm)|HCl(CM)

The vertical lines separate the different phases of the electrode, and the pressure of the gas and concentration of the solution can be included as indicated.

Metal Electrode A metal electrode consists of a metal in contact with its ions in solution as indicated in Fig.12.15. An example would be an iron wire immersed in water. The notation for this electrode would be Fe|Fe2+(C), and the half-cell reaction at the electrode would be |Fe ^Fe2+ + e~. Generally, this type of electrode j might be used for analysis of metals such as silver, copper, mercury, lead, and cad- : mium that show reversible electron transfer between the metal and its ions in solu- • tion. Other metals cannot be used that do not develop reproducible potentials, generally due to strain or crystal deformations in their structures or because of oxide coatings on their surfaces. Metals in this category include iron, nickel, cobalt, and chromium.

Negatively charged

Figure 12.15

Development of a single-electrode potential on a metal electrode immersed in water.

Negatively charged

Figure 12.15

Development of a single-electrode potential on a metal electrode immersed in water.

Oxidation-Reduction Electrode This electrode consists of a nonreactive electrode immersed in a solution of ions in both reduced and oxidized form. An example would be a platinum wire immersed in a solution containing both ferrous and ferric chloride. An oxidation-reduction electrode used in conjunction with a reference electrode is illustrated in Fig. 12.16. An oxidation-reduction electrode could be designated as Pt|FeCl2(C), FeCl3(C). The reaction occurring at this electrode would be Fe2+ ^ Fe3+ + e~. Sometimes oxidation-reduction electrodes are used as internal indicators to show the stoichiometric end point during the oxidation-reduction type of titrations, as discussed in Sec. 11.4. They are also of interest as a means of showing the conditions that exist in biological systems. The ability to measure the relative oxidation-reduction (redox) potential of such a system would be highly desirable. Unfortunately, in complex wastewater systems that are generally not in equilibrium and that contain many oxidants and reductants, meaningful measurement of the redox potential is often not possible. However, oxidation-reduction measurement with the electrode system has sometimes been useful in an empirical way to indicate whether a system is aerobic and oxidizing or anaerobic and reducing.

Electrode with Metal Contacting Slightly Soluble Salt This type of electrode consists of a metal in contact with one of its slightly soluble salts, while the salt, in turn, is in contact with a solution containing a common anion. The silver-silver chloride electrode illustrated in Fig. 12.14 is of this type. The example of greatest interest is the calomel electrode, which is used extensively as a reference electrode. The elements of a calomel electrode are shown in Fig. 12.17, The electrode contains mercury in contact with the slightly soluble Hg2Cl2, which, in turn, is in contact with a solution of KC1; Hg|Hg2C],jKCI(C). The reaction that occurs at this electrode Is Hg + CI"

Figure 12.16

Electrode assembly for measurement of oxidation-reduction potentials (ORP).

Figure 12.16

Electrode assembly for measurement of oxidation-reduction potentials (ORP).

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