AMPEROMETRIC TITRATION (EN: amperometric titration; DE: amperometrisches Titrieren; FR: titrage amperometrique; ES: titulacion amperimetrica; RU: амперометрическое титрование) is one of the titrimetric methods of the analysis of substances.
The moment during the titration, when the quantity of the reagent, which is added, becomes strictly equivalent to the quantity of the substance, which is determined, is called the point of equivalence, and within the experiment, it is called the end point of the titration. During the amperometric titration, they determine the latter according to the change of the strength of the limiting diffusive current (see the "Polarography" article), which is flowing through the solution. The method has been proposed by the Czech scientist J.Heyrovsky in 1927. They apply onto the indicative electrode (Figure 1) such voltage, at which the limiting diffusion current flows through the solution of the analysed substance, of the titrant, and of the product of their reaction (indicative substances). Because the value of the latter is proportional to the concentration of the electro-active substance, as it changes during the titration, the magnitude of the limiting diffusion current also changes. During the amperometric titration, they plot the graph of the dependence of the strength of the current on the volume of the titrant, which is consumed. The intersection of the two branches of the curve of the titration corresponds to the end point.
Depending on the type of the reaction during the amperometric titration, and on the substance, which participates within the electrode reaction, they obtain various shapes of the curves of the amperometric titration. At the Figure 2, the "a" curve corresponds to the titration of the electrochemically active substance by the solution of the inactive reagent (for example, the titration of Zn2+, Cd2+, Ni2+, and others by the chelator III); the "b" curve is the opposite case: the titration of Mg2+ by the 8-hydroxyquinoline.
The "v" curve is plotted for the case, when the explored substance and titrant are reduced at the electrode (for example, the determination of Fe3+ by the a-nitroso-b-naphthol). If, at the electrode, one substance is reduced, and the second one is oxidized, then the shape of the curve corresponds to the "g" case; if there interact the electrochemically inactive substance and titrant with the formation of the electrochemically active substance, then they obtain the "d" curve (for example, the titration of arsenates by the potassium iodide, during the interaction of which there is formed the electrochemically active I2). The "e" curve corresponds to the case of the titration with indicators (for example, the titration of alkali by the acid with the indicator, which is made of the mixture of I- and IO3- ions, which also causes the formation of the electrochemically active I2).
They distinguish the amperometric titration with one and two indicative electrodes. In the first case, they use the mercury drop electrode or the rotating platinum electrode as the indicative one. There may serve as the reference electrode the hydrogen, mercuric-iodine, calomel, mercuric-sulfate, or chlorine-silver electrode. During the amperometric titration with two indicative electrodes, they use instead of the reference electrode the electrode, which they place into the solution, which is titrated. Amperometric titration permits to identify 10^-6 grams of the substance within the titrated volume. They determine by the method of the amperometric titration more than 60 elements within the various industrial and natural materials, including ores, rocks and minerals, and also many organic substances, for example alkaloids, aldehydes, amines, ketones, organic acids, colourants, sugars, phenols, and others.
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