Chelometric titrations

Types of Chelometric Titrations. Chelometric titrations may be classified according to their manner of performance direct titrations, back titrations, substitution titrations, redox titrations, or indirect methods. 

In resolving complex metal-ion mixtures, more than one masking or demasking process may be utilized with various aliquots of the sample solution, or applied simultaneously or stepwise with a single aliquot. In favorable cases, even four or five metals can be determined in a mixture by the application of direct and indirect masking processes. Of course, not all components of the mixture need be determined by chelometric titrations. For example, redox titrimetry may be applied to the determination of one or more of the metals present. 

An interesting indirect end-point detection method is based on the work of Siggia et al [143], and Reilley and Schmid [119], who originally developed the technique for chelometric titrations of electro-inactive metals with a mercury indicator electrode. Assume that metal ion M is titrated by titrant Y (ionic charges are omitted for the sake of simplicity),... 

Acid-base, redox, precipitation and chelometric titrations are usually dealt with in textbooks on analytical chemistry. The titration curves in these titrations can be obtained potentiometrically by use of appropriate indicator electrodes, i.e. a pH-glass electrode or pH-ISFET for acid-base titrations, a platinum electrode for redox titrations, a silver electrode or ISEs for precipitation titrations, and ISEs for... 

Conductimetry is used to detect and determine ionic species in solution. It plays important roles in environmental analyses and in the detection of ion chromatography. It is also used in acid-base, precipitation and chelometric titrations to detect end-points. However, the biggest advantage of conductimetry is displayed in the fundamental studies of solution chemistry. Its applications to chemical studies in non-aqueous solutions will he discussed in Chapter 7. 

Figure 12-9 Titration of Ca2+ with EDTA as a function of pH. As the pH is lowered, the end point becomes less distinct. The potential was measured with mercury and calomel electrodes, as described in Exercise 15-6 in Chapter 15. [C. N. Reilley and R. W. Schmid, "Chelometric Titration with Potentiometric End Point Detection Mercury as a pM Indicator Electrode Anal. Chem. 1958, 30. 947.]...

Sadek, F. S., and C. N. Reilley Ultramicro Chelometric Titrations with Potentiometric End Point Detection. Microchem. J. 1, 183 (1957). 

Reilley, C. N. and R. W. Schmid. 1958. Chelometric titrations with potentiometric end point detection, mercury as pM indicator electrode. Anal. Chem. 30 947-953. 

We can measure the pM potentiometrically if a suitable electrode is available, for example, an ion-selective electrode (see Chapter 13), but it is simpler if an indicator can be used. Indicators used for chelometric titrations are themselves chelating agents. They are usually dyes of the o,o -dihydroxy azo type. 

With the exception of the alkah metals, nearly every metal can be determined with high precision and accuracy by chelometric titration. These methods are much more rapid and convp ient than gravimetric procedures and have largely replaced them, except in those few instances when greater accuracy may be offered and required. 

Procedures especially suited to silicate rocks, minerals, and refractory silicates and aluminosilicates have been described by Bennett and Reed (290). A history of analytical methods was published by Andersson (291), and three new spectro-photometric procedures were developed. Available chemical methods listed by Meites (292), in addition to the conventional gravimetric and colorimetric methods, also include precipitation of the silicate ion as the cobalt salt, which is then determined by chelometric titration, and as a nitrogen base salt which is titrated with perchloric acid. 

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