Equivalence point spectrophotometric titration

Photometric and spectrophotometric measurements are useful for locating the equivalence points of titrations. This application of absorption measurements requires that one or more of the reactants or products absorb radiation or that an absorbing indicator be added to the analyte solution. 

In acid-base titrations the end point is generally detected by a pH-sensitive indicator. In the EDTA titration a metal ion-sensitive indicator (abbreviated, to metal indicator or metal-ion indicator) is often employed to detect changes of pM. Such indicators (which contain types of chelate groupings and generally possess resonance systems typical of dyestuffs) form complexes with specific metal ions, which differ in colour from the free indicator and produce a sudden colour change at the equivalence point. The end point of the titration can also be evaluated by other methods including potentiometric, amperometric, and spectrophotometric techniques. 

A. Direct titration. The solution containing the metal ion to be determined is buffered to the desired pH (e.g. to PH = 10 with NH4-aq. NH3) and titrated directly with the standard EDTA solution. It may be necessary to prevent precipitation of the hydroxide of the metal (or a basic salt) by the addition of some auxiliary complexing agent, such as tartrate or citrate or triethanolamine. At the equivalence point the magnitude of the concentration of the metal ion being determined decreases abruptly. This is generally determined by the change in colour of a metal indicator or by amperometric, spectrophotometric, or potentiometric methods. 

In a spectrophotometric titration, absorbance of light is monitored as titrant is added. For many reactions, there is an abrupt change in absorbance when the equivalence point is reached. The Fajans titration is based on the adsorption of a charged indicator onto the charged surface of the precipitate after the equivalence point. The Volhard titration, used to measure Ag+, is based on the reaction of Fe3+ with SCN- after the precipitation of AgSCN is complete. 

Campbell et al. [70] developed a BASIC program, LESTEQ, for calculation of the exact equivalence point volume in spectrophotometric titrations by use of ancillary indicators. The program is based on the non-llnear least-squares fit of the titration data to a model equation by means of a modification of the Gauss-Newton multi-parameter optimization method, and calculates the indicator and stability constants. The form of the model equation used by LESTEQ depends on the stoichiometry of the titration reaction and on the nature of the indicator reaction. The equation... 

Photometric or spectrophotometric measurements are useful for locating the equivalence point of a titration, provided the analyte, the reagent, or the titration product absorbs radiation." Alternatively, an absorbing indicator can provide the absorbance change necessary for location of the equivalence point. 

Among them, volumetric methods are presumably the most widely used for water analysis. They are titrimetric techniques which involve a chemical reaction between a precise concentration of a reagent or titrant and an accurately known volume of sample. The most common types of reactions as used within this method are acid-base neutralization, oxidation-reduction, precipitation, and complexation. The use of an indicator which identifies the equivalence point is required to develop this kind of method. The modem laboratories usually employ automated endpoint titrators, which largely improve the efficiency and reliability of the determination. Moreover, spectrophotometric, potentiometric, or amperometric methods to determine the endpoint of the reaction can... 

Other titration designations have also been given in the literature. Some are named after the instrumental method used to detect the equivalence point. We can list, for example, conductometric, potentiometric, amperometric, spectrophotometric, and thermometric titrations. Others are named after the titrant nature. We speak then of iodometric, complexometric, and acidimetric titrations. 

We must also keep in mind that equivalence points of EDTA titrations can be detected by using several instrumental methods. A first method, potentiometry, was just mentioned. There are also other potentiometric methods, based on other principles than the previous one, that may be used. Amperometric and conductometric methods have been proposed equally (see electrochemical methods of analysis). Finally, we ll mention photometric and spectrophotometric indications. 

Spectrophotometric measurements can also be used to determine the equivalent point in a titration when the analyte, the reagent or the product of the titration absorb in the UV-vis region. 

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