Oxine, Titration

Full details are given for the determination of aluminium by this method. Many other metals may be determined by this same procedure, but in many cases complexometric titration offers a simpler method of determination. In cases where the oxine method offers advantages, the experimental procedure may be readily adapted from the details given for aluminium. 

Among the most important indirect methods of analysis which employ redox reactions are the bromination procedures for the determination of aromatic amines, phenols, and other compounds which undergo stoichiometric bromine substitution or addition. Bromine may be liberated quantitatively by the acidification of a bromate-bromide solution mixed with the sample. The excess, unreacted bromine can then be determined by reaction with iodide ions to liberate iodine, followed by titration of the iodine with sodium thiosulphate. An interesting extension of the bromination method employs 8-hydroxyquinoline (oxine) to effect a separation of a metal by solvent extraction or precipitation. The metal-oxine complex can then be determined by bromine substitution. 

The term radiometric analysis is often used in a broad sense to include all methods of determination of concentrations using radioactive tracers. In a more restricted sense it refers to a specific analytical method which is based on a two-phase titration in the presence of a radioactive isotope. The endpoint of the titration is indicated by the disappearance of the radioisotope from one of the phases. Figure 9.4 illustrates two cases, (a) the determination of Ag in a solution by titration with Nal solution containing ( y t 1.57x10 y),and (b) the determination of Fe in an aqueous solution, to which trace amounts of radioactive Fe (EC 2.73 y) has been added. In case (a) the Agl precipitate is radioactive but the solution has little radioactivity until all the Ag has been precipitated. The activity of the solution is measured by a liquid flow GM-detector (Ch. 8). In the latter case (b) a two-phase liquid-liquid analytical technique is used ( 9.2.6) the titrant contains a substance (oxine) which extracts Fe(II) from the aqueous to the chloroform phase. The radioactivity of the organic phase is followed by liquid scintillation (sampling) to determine the end point of the titration. 

From another standpoint, oxine forms insoluble precipitates with several metallic ions called oxinates (see Part V). The precipitation reaction may be quantitative if the pH conditions are judiciously chosen. They depend on the nature of the metallic ion. The formula of the precipitates are M(C9H60N) where n is the valency of the metallic ion. Exceptions to this rule are rare. From these considerations, it results a methodology of determination of metallic ions. The metallic ion is quantitatively precipitated by oxine at the judicious pH value. After filtration of the precipitate, this one is treated by a strong acid that displaces oxine from it. The liberated oxine is titrated by an excess of a bromate/bromide mixture in acidic medium. The bromine in excess is titrated by indirect iodometry. 

The quantitative determination of metallic ions, which follows the precipitation as oxinates, may be carried out, depending on the case, by gravimetry (see Chap. 38) or by dissolution of the precipitate in hydrochloric acid and subsequent titration of oxine by bromometry (see Chap. 19). Moreover, in certain conditions, oxine is electroactive. As a result, it can be determined by amperometry. 

The easiest method for the determination of zinc ions in aqueous solution is by complexometric titration. A suitable instrumental method is by atomic absorption. Gravimetrically, zinc can be precipitated with 8 hydroxyquinoline (or oxine) and weighed as the oxinate. 

Like zinc ions, cadmium ions can be titrated in buffered solutions with EDTA but the end-point is more easily located by high frequency titration than by an indicator. Among the instrumental methods suitable for the determination of cadmium in dilute solutions, atomic absorption is ideal and polarography is also suitable. Gravimetrically, cadmium can be precipitated as the oxinate similar to zinc. In addition, it can be precipitated as molybdate. 

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