POTENTIOMETRIC TITRATIONS

Electrodes used in potentiometric titrations

            The experimental setup for potentiometric measurement comprises a set of an indicating and reference electrodes or two identical indicating electrodes, which should be treated carefully. Do not place the electrodes anywhere except attached to the electrode holder. At the end of the experiment rinse the electrodes and place each one in its housing as required.

            Glass electrode. Combined glass reference electrode consists of indicator and reference electrodes in the same body. Great care should be taken of it: never touch the glass part of the electrode with anything except soft tissue paper. While in use, the bulb of the glass electrode and the diaphragm of the reference electrode should be immersed in solution. For short-term storage the combined glass electrode should be immersed in solution of 2 M KCl. Buffer solutions of known pH are used for the pH calibration. The pH values of some buffers are temperature dependent. For high accuracy, calibration and measurements are to be performed at the same temperature.

            Ion-selective electrodes are used for detection of specific ions in a mixture of ions. The sensor element, ion-selective membrane, has a construction similar to that of glass electrode. For calibration of ion-selective electrodes a standard addition method is often employed.

            Silver indicating electrodes are silver wires with 1-2 mm diameter. When used in precipitation titration, the silver-salt precipitate should be occasionally removed from the electrode surface (mechanically with fine grade emery paper, or chemically immersing the electrode in NH₃ solution). It is simpler, however, to prevent the coating of the electrodes by addition of a surfactant as polyvinyl alcohol (1 drop 0.3% PVA to every 5 ml of solution).

            Mercury-coated indicating electrodes are reported to be prepared by lightly amalgamating a gold wire. The disadvantage in use of gold is that it is consumed with time by the amalgam formation. Silver wire used instead of gold, however, can serve many years. The preparation of mercury-coated silver electrode is done by the instructor in a hood (mercury vapors are poisonous!). The silver wire (~1.5 mm diameter) is (a) rubbed with emery paper, rinsed with distilled water and dried with tissue; (b) dipped into mercury to form an amalgame; (c) the mercury is gently spread on the wire with soft tissue. This electrode may be used during several runs of titrations without any renewal. For renewal, step (a) may be omitted.

            Platinum redox electrodes are used in redox potentiometric titrations. In excess of oxidant oxide films are formed on the platinum electrodes. The potential response of the electrode is distorted, and the film must be removed. Efficient pretreatment is achieved by cathodically polarizing the platinum electrode in 0.5 M H₂SO₄ at current density of 0.5 mA/cm² for 5 - 15 min. Platinum wire is recommended to use as an auxiliary electrode.

            Gold redox electrodes are seldom used in potentiometric titrations. According to our recent experience, the gold electrodes are better behaved than platinum electrodes in view of rate of response and stability toward formation of oxides. These features are of high importance in continuous mode of titration. A good example is the use of gold electrodes in the titration of ascorbic acid with bromine in continuous mode, where the response of the platinum electrode is unsatisfactory.

            Reference electrodes. Calomel and silver/silver-chloride electrodes are commonly used in potentiometric titration. In the case of possible interferences of chlorides (as in determination of halides), a mercurous sulfate electrode may be used. In the following series of experiments a home made Ag/AgCl/1 M KCl reference electrode is used. Its potential is -19 mV vs SCE, at 25⁰C.

Exp. 1.  Compexometric titration of Ca²⁺ and Mg²⁺ in drinking water

Chelating agents

            A chelating agent (ligand) is a multidendate compexing molecule, forming usually a 1:1 complex (chelate) with a metal ion M^z+, regardless of the value of the charge z of the metal ion.

            Complexometric titration using chelating agent Y (comprising n complexing groups) is far more advantageous than titration with monodendate ligand L (of the same type), for two reasons:

(1) The chelate MY is more stable than the ML_n complex due to thermodynamic considerations. The free energy change, ,

is characterized by similar  values, but very different  values, were  is the enthalpy and  is the entropy. More disorder is created by the dissociation of ML_n with the formation of n+1 species, compared to two species for the chelate.

(2) Stepwise complexation with monodendate ligands is usually characterized with successive formation constants relatively close to each other. Thus, there are several types of complexes in the vicinity of the end point, compared to a chelate, where a single 1:1 complex is usually formed.

            One of the most widely used chelating agents is EDTA (ethylenediaminetetraacetic acid) with six complexing groups (two nitrogen and four carboxylic groups). EDTA titrations have been applied to the determination of most metal cations with the exception of the alkali metal ions. Selectivity is obtained by controlling the pH. Ca²⁺ and Mg²⁺ have however close formation constants (K_f = 5.0·10¹⁰ and 4.9·10⁸ respectively), unsuitable for separate detection. Titrating with EDTA, the total concentration of Ca²⁺ and Mg²⁺ is determined. 

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