Controlled-current techniques coulometric methods

Coulometric Titrations Controlled-current coulometric methods commonly are called coulometric titrations because of their similarity to conventional titrations. We already have noted, in discussing the controlled-current coulometric determination of Fe +, that the oxidation of Fe + by Ce + is identical to the reaction used in a redox titration. Other similarities between the two techniques also exist. Combining equations 11.23 and 11.24 and solving for the moles of analyte gives... 

Two distinctly different coulometric techniques are available (1) coulometric analysis with controlled potential of the working electrode, and (2) coulometric analysis with constant current. In the former method the substance being determined reacts with 100 per cent current efficiency at a working electrode, the potential of which is controlled. The completion of the reaction is indicated by the current decreasing to practically zero, and the quantity of the substance reacted is obtained from the reading of a coulometer in series with the cell or by means of a current-time integrating device. In method (2) a solution of the substance to be determined is electrolysed with constant current until the reaction is completed (as detected by a visual indicator in the solution or by amperometric, potentiometric, or spectrophotometric methods) and the circuit is then opened. The total quantity of electricity passed is derived from the product current (amperes) x time (seconds) the present practice is to include an electronic integrator in the circuit. 

Other techniques for studying protein molecnles in solntion are less infln-enced by these microscopic effects. Square-wave voltammetry is widely used due to its great sensitivity, and even a low density of productive sites on the electrode may give rise to a sharp and analyzable response [28]. The electrode may also be rotated to achieve forced convection and hydrodynamic control of solution redox species, while amperometric (and coulometric) measurements—where the current (or charge) is recorded following a potential step—enable the time and potential domains to be deconvoluted [28,29]. These options complement each other to provide a detailed picture of the thermodynamics and kinetics of redox processes. Finally, bulk electrolytic methods enable samples of a particular redox state to be prepared quantitatively for spectroscopic examination, at precise electrode potentials that may lie outside the range of conventional chemical titrants. 

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