Amperometric titrations applications

Diffusion Currents. Half-wave Potentials. Characteristics of the DME. Quantitative Analysis. Modes of Operation Used in Polarography. The Dissolved Oxygen Electrode and Biochemical Enzyme Sensors. Amperometric Titrations. Applications of Polarography and Amperometric Titrations. 

Diffusion currents. Half-wave potentials. Characteristics of the DME. Quantitative analysis. Modes of operation used in polarography. The dissolved oxygen electrode and biochemical enzyme sensors. Amperometric titrations. Applications of polarography and ampero-metric titrations. 

In cases where it proves impossible to find a suitable indicator (and this will occur when dealing with strongly coloured solutions) then titration may be possible by an electrometric method such as conductimetric, potentiometric or amperometric titration see Chapters 13-16. In some instances, spectrophotometric titration (Chapter 17) may be feasible. It should also be noted that ifit is possible to work in a non-aqueous solution rather than in water, then acidic and basic properties may be altered according to the solvent chosen, and titrations which are difficult in aqueous solution may then become easy to perform. This procedure is widely used for the analysis of organic materials but is of very limited application with inorganic substances and is discussed in Sections 10.19-10.21. 

Both lead ion and dichromate ion yield a diffusion current at an applied potential to a dropping mercury electrode of —1.0 volt against the saturated calomel electrode (S.C.E.). Amperometric titration gives a V-shaped curve [Fig. 16.14 (C)]. The exercise described refers to the determination of lead in lead nitrate the application to the determination of lead in dilute aqueous solutions (10-3 — 10-4lVf) is self-evident. 

Polarisation titrations are often referred to as amper-ometric or biamperometric titrations. It is necessary that one of the substances involved in the titration reaction be oxidisable or reducible at the working electrode surface. In general, the polarisation titration method is applicable to oxidation-reduction, precipitation and complex-ation titrations. Relatively few applications involving acid/base titration are found. Amperometric titrations can be applied in the determination of analyte solutions as low as ICE5 M to 10-6 M in concentration. 

Amperometric titrations have an even wider range of application than polarography. Although the titrant may be added from a burette, in many applications it is electrically generated in a coulometric cell (p. 261). Such an arrangement lends itself to complete automation and is particularly valuable for the titration of very small quantities. For examples of coulometric titrations with amperometric equivalence point detection see Table 6.5. 

Amperometric titrations are inherently more precise than polarography and are more generally applicable because the analyte need not itself be electroactive. Titrations involving the DME are not affected by changes in capillary characteristics as are conventional polarographic determinations, whilst working at a predetermined temperature is unnecessary provided that it remains reasonably constant throughout the titration. 

Dead-stop-end-point method was first introduced by Foulk and Bawden in 1926. Evidently, this particular technique is a modification of the classical amperometric titration. This technique is specifically applicable to only such systems where the phenomenon of oxidation-reduction exists both before as well as after the equivalence point has been duly accomplished. 

Phenylarsine oxide, C6H5As = O, is as effective as sodium thiosulfate in reducing iodine. It is more stable than thiosulfate. An advantage is that it is stable even in dilute solution. This substance is, however, highly toxic and is a suspected carcinogen. Because of its toxicity, its application is limited. One such application is in the amperometric titration of residual chlorine. The oxidation-reduction reaction of PAO is similar to thiosulfate. Its equivalent weight in iodine reaction is 168. 

Figure 4.6 Dual-polarized electrode amperometric titration curves. Both curves result from the application of a 0.25-V potential across two identical platinum electrodes that are immersed in the titration solution.

As with amperometric titrations, to have straight-line portions of the titration curve dilution corrections must be made because the response is directly dependent on the concentration of the ionic species. Also, the important data are taken before and after the equivalence point rather than precisely at the equivalence point. The general conditions for effective conductometric measurements of solutions are discussed in Chapter 5 and are directly applicable when the system is used as the endpoint detection method. A particularly complete review of the subject has been presented.8... 

In this chapter, after describing the useful technique of amperometric titrations, recent developments in amperometric and voltammetric sensors are summarized. Their application as biosensors to the study of biological compounds and in vivo, is described in Chapter 17. 

Rotating wire electrode — When a metal wire is rotating in a solution and used as an - electrode for - current measurements, the current will be enhanced due to the enhanced - mass transport of the - depolarizer to the electrode surface. However, since the hydrodynamic conditions are usually rather difficult to control at a rotating wire, especially when the wire is not a highly symmetric cylinder rotating around its own axis, the rotating wire electrode found only some application as an - indicator electrode in - amperometric titrations. 

Table 5 Applications of amperometric titrations with precipitation products...

Both potentiometric and amperometric methods for chlorine analysis actually measure iodine. The equilibrium concentration of iodine can be affected by the presence of halogens or halides with which complexes, such as Ig, l2 > l2Br , can be formed under certain conditions. Amperometric titration is a dynamic technique in which iodine is consumed, thereby disturbing the measurment equilibrium conditions. Reequilibration will occur until all iodine, including that which is complexed, has been titrated. The time required for this can he significant, especially where on-line applications are considered. Potentiometric measurements are made at equilibrium, so the complexed species would not be expected to contribute to the values determined. 

Spreadsheet Summary Amperometric titrations are the subject of the final exercise in Chapter 11 of Applications of Microsoft Excel in Analytical Chemistry. An amperometric titration to determine gold in an ore sample is used as an example. Titration curves consisting of two linear segments are extrapolated to find the end point. 

Advantages and Limitations of Radiometric Titrations. Radiometric detection of the equivalence point is a general method that does not depend on the chemical reaction employed. This contrasts with other methods of detection, which depend on specific chemical or physical transitions at the equivalence point. Amperometric titrations are applicable only to electrochemically active systems conductometric titrations apply only to ionic solutions, and so on. In principle, any titration system in which a phase separation can be effected is amenable to radiometric detection, provided there exist suitable radioactive labels. The major limitation of the method is the requirement for phase separation. In precipitation titrations, the phase separation is automatic and the method is well suited to this class of titrations. For other classes of titrations, special phase-separation methods, such as solvent extraction, need to be applied. At the present time, the method suffers from a lack of phase-separation techniques suitable for continuous monitoring of the titration curves. 

The various modes in which amperometry can be employed have resulted in its extensive utilization in analytical chemistry - in amperometric titration, amperometric sensing, and amperometric detection in flowing systems. The various applications of amperometry are discussed below. 

The most frequently used titrating agents in polarometric and amperometric titrations are silver and mercury compounds and some heteropoly acids. It should be stressed once more that little selectivity, especially for precipitation reactions, can be expected and thus in the analysis of mixtures the application of polarometric titrations must be preceded by separation techniques. 

Nearly all chemical sensors useful for liquid samples can be utiUzed to indicate titrations. Besides the preferred potentiometric, other electrochemical probes are also used, mainly amperometric and conductometric sensors. The so-called biamperometric titration works with simple wire pairs. Photometric and thermometric indication techniques are less common than electrochemical methods. Miniaturization does not play an important role for titration probes. Classical arrangements predominate to this day. Commercial titration instruments are only slowly starting to make use of the achievements of modern sensor technology. As an example, optodes have achieved a certain popularity in recent years for titration applications. 

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