Coulometric techniques

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. 

As described in Chapter 8, current-potential curves in polarography and voltammetry are useful for obtaining mechanistic information on electrode reactions. However, for complicated electrode processes, the information obtained from the current-potential curves is not conclusive enough. In order to get more conclusive information, it is desirable to confirm the reaction products and/or intermediates by some other technique. In this chapter, we focus our discussion on such techniques. We deal with electrolytic and coulometric techniques in Section 9.1 and the combinations of electrochemical and non-electrochemical techniques in Section 9.2. 

Coulometry comprises a set of techniques in which the total charge required (not the current, as in potentiometry) to oxidize or reduce the chemical species of interest is measured. The prime virtue of coulometric techniques is that they link the quantity of substance determined directly to the quantity of electrical charge, and thus expensive and often difficult procedures for standardization or calibration can be minimized or eliminated. 

Coulometric techniques involve the determination of the quantity of material electrolyzed from the amount of charge passed through an electrochemical cell during electrolysis. Faraday s law relates the measured charge to the amount of material electrolyzed,... 

An exhaustive coulometric technique can be used both as an analytical tool and as a preparative tool. These two applications often require different cell designs. The present discussion is restricted to analytical coulometric cells. The design of electrochemical cells for preparative electrolysis has been treated in Chapter 22 and elsewhere [10]. 

In coulometry the stoichiometry of the electrode process should be known and should proceed with 100% current efficiency, and the product of reaction at any other electrode must not interfere with the reaction at the electrode of interest. If there are intermediate reactions, they too must proceed with the desired accuracy. In practice the electrolytic cell is designed to include isolation chambers. Losses of solute through diffusion, through ionic or electrical migration, and simply through bulk transfer must be minimal. Finally, the end point has to be determined by one of the many techniques used in titrations generally, whether coulometric or not. Both indeterminate and determinate end-point errors limit the overall accuracy achieved. Cooper and Quayle critically examined errors in coulometry, and Lewis reviewed coulometric techniques. 

The coulometric technique is based on the same chemistry but its fundamental difference is the way by which the iodine is introduced into the titration cell (Figure 20.14). 

Water Analysis. The water gained by a sample was measured on a duPont 26-321A moisture analyzer. This Instrument uses a coulometric technique to measure the total amount of water in a sample. Samples were heated for 15 minutes above their Tg to drive off the water. 

All in all, with their simplicity, accuracy, relative low cost, and wide applicability, coulometric techniques deserve serious consideration by the analyst. 

Coulometry. Faraday s laws of electrolysis, enunciated in 183 form the basis of coulometric techniques. By the beginning of the present century the silver coulometer had been shown to provide an accurate means for the measurement of quantities of electricity. An excellent survey of various chemical and other coulometers is available ( ). The electronic digital coulometer, first described in 1962 ( ), was a major practical advance. 

KF titration is available in coulometric or volumetric techniques volumetric titration is normally used for a broad range in moisture concentration (0-100%), while the coulometric techniques is normally used for small water concentrations (ppm levels to low percent levels). 

This review session discusses the development of a couple of coulometric techniques... 

An analytical technique in which the amount of electricity passing between two electrodes in an electrochemical cell is measured. The chloride meter is an example of a coulometric technique. 

Alfonsi (62, 92-95) has applied controlled-potential coulometric techniques to the determination of copper in a wide variety of brasses, bronzes, solders, and other alloys. Yamada (96) has constructed a novel capillary type micro-coulometer to estimate traces of copper in eluates from paper chromatograms. In an effort to characterize quantities of trace elements in living bodies, Suzuki and Yamamoto (97) used controlled-potential coulometry to determine 100-600 jxg of copper in plant materials. Farrar and co-workers (98) used similar techniques to recover copper from other fission elements in partially spent reactor fuels. Controlled-potential coulometry can be of considerable value here for remote-control monitoring of high radiation-level facilities (99). 

Manning, Ball, and Menis (162) have carried out polarographic and coulometric reductions of molybdenum (VI) in a nitrilotriacetic acid medium and have applied their findings to the analysis of thorium-uranium oxide mixtures. The determination of molybdenum in steel using coulometric techniques has been reported by Ibrahim and Nair (163) who reduced molybdenum at —0.40 V vs. SCE in a sodium acetate buffered chloride medium. Chromium interference can be removed by pre-reduction with alcohol. The catalytic effect of lower oxidation states of molybdenum in the reduction of perchlorate has been used as an indirect electro-analytical method for the determination of perchlorate (159, 164). 

VS. SCE in 0.1 M hydrochloric acid. The polarographic reduction waves of thallium (I) and lead (II) in molar hydrochloric acid overlap so closely that direct coulometric separation is not convenient, but Meites (227) ingeniously combined polarographic and controlled-potential coulometric techniques to yield two sets of data which can be resolved to the individual concentrations of the two components through simultaneous equations. 

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