Controlled-current techniques coulometry

Two general techniques are used for coulometric analysis controUed-potential (potentiostatic) coulome-try and controlled-current (amperostatic) coulometry. In controUed-potential coulometry, the potential of the working electrode (the electrode at which the analytical reaction occurs) is maintained at a constant... 

Coulometric titration. For this technique, often designated controlled-current or amperostatic coulometry, it is useful to distinguish between redox, complex-formation and precipitation titrations on the one hand and acid-base titrations on the other and to discuss each group separately. 

Controlled-current electrolysis in flowing solution has been extremely useful for analytical purposes. The prevalent techniques are constant-current coulometry and coulometric titrations, which are discussed in Chapter 25. 

Several coulometric and pulse techniques are used in electroanalytical chemistry. Rather low detection limits can be achieved, and kinetic and transport parameters can be deduced with the help of these fast and reliable techniques. Since nowadays the pulse sequences are controlled and the data are collected and analyzed using computers, different pulse programs can easily be realized. Details of a wide variety of coulometric and pulse techniques, instrumentation and applications can be found in the following literature controlled current coulometry [6], techniques, apparatus and analytical applications of controlled potential coulometry [7], coulostatic pulse techniques [8], normal pulse voltammetry [9], differential pulse voltammetry [9], and square-wave voltammetry [10]. 

Galus Z (1994) Eundamentals of electrochemical antilysis, 2nd edn. Harwood, Chichester Delahay P (1954) New instrumental methods in electrochemistry. Wiley, New York Macdonald DD (1977) Transient techniques in electrochemistry. Plenum, New York Janata J, Mark HB Jr (1969) Application of controlled-current coulometry to reaction kinetics. In Bard AJ (ed) Electroanalytical chemistry, vol 3. Mtircel Dekker, New York, pp 1-56 Harrar JE (1975) Techniques, apparatus, and aneilytical appUcations of controlled-potentitil coulometry. In Bard AJ (ed) Electroanalytical chemistry, vol 8. Marcel Dekker, New York, pp 2-167... 

Different electrochemical techniques depend on whether they are bulk methods as in conductometry or interfacial methods. The latter may be static as in potentiometry or dynamic. Dynamic methods are classified on the basis of the current used. Conductometry uses a constant current but controlled current methods include voltametry, amperometry and coulometry. 

Among electrochemical techniques,cyclic voltammetry (CV) utilizes a small stationary electrode, typically platinum, in an unstirred solution. The oxidation products are formed near the anode the bulk of the electrolyte solution remains unchanged. The cyclic voltammogram, showing current as a function of applied potential, differentiates between one- and two-electron redox reactions. For reversible redox reactions, the peak potential reveals the half-wave potential peak potentials of nonreversible redox reactions provide qualitative comparisons. Controlled-potential electrolysis or coulometry can generate radical ions for smdy by optical or ESR spectroscopy. 

Controlled-potential coulometry is quite often used as an ancillary technique to polarography to calculate the number of electrons involved in a polarographic reduction. The determination of an n value for some antibacterial aminoacridines using this technique demonstrated that with the exception of 5-aminoacridine, all other substituted aminoacridines undergo a one-electron reduction that corresponds to the first step of the current-voltage curve [17]. This fact substantiated the formation of an intermediate stable semiquinone radical, which is of... 

Analytical Applications of Coulometry. The major advantage of coulometry is its high accuracy, because the reagent is electrical current, which can be well controlled and accurately measured. Coulometry is used for analysis, for generation of both unstable and stable titrants on demand , and for studies of redox reactions and evaluation of fundamental constants. With careful experimental technique, it is possible to evaluate the Faraday constant to seven significant figures, for example. 

A complete system providing both a sensor and an actuator would be ideal in the field of process control, but because of a lack of truly reliable chemical sensors on the market the concept has not been widely implemented. One exception relates to the analytical method of coulometry, a technique that offers great potential for delivering chemical compounds to a controlled reaction. Especially attractive in this context is the method of constant- current coulometry, which can be carried out with an end-point sensor and a coulomet-ric actuator for maintaining a generator current until the end-point has been reached. In this case both of the required devices can be miniaturized and constructed with the same technology. 

A large ohmic potential drop is often present in OTTLE cells owing to the nonuniform current distribution within the thin-layer cavity caused by the large distance between the working electrode and counterelectrode. This is not a problem, as experiments generally involve exhaustive electrolyses, where any ohmic drop can be out-waited . The OTTLE cell design enables the techniques of thin-layer electrochemistry, cyclic voltammetry, controlled potential coulometry and UV-visible spectros-... 

---