Flow-coulometry

The second method is the flow-coulometry and can be applied to orthophosphates with concentrations of 5x10 1x10 M contained in the interstitial water in sediments. The advantage of this method is that only small amounts of sample ( 100 p.1) are needed and the analytical results are entirely free from interference due to silicate ions. 

Flow coulometry experiments were performed to study the reduction of U02 in nitric, perchloric, and sulfuric acid solutions [56]. The results of these studies show a single two-electron reduction wave attributed to the U02 /U + couple. The direct two-electron process is observed without evidence for the intermediate U02" " species because of the relatively long residence time of the uranium ion solution at the electrode surface in comparison to the residence time typically experienced at a dropping mercury working electrode. The implication here is that as the UO2 is produced at the electrode surface, it is immediately reduced to the ion. As the authors note a simplified equation for this process can be written, Eq. (7), but the process is more complicated. Once the U02" species is produced it experiences homogeneous reactions comprising Eqns (8) and (9) or (8) and (10) followed by chemical decomposition of UOOH+ or UO + to [49]. 

Kihara et al. employed flow coulometry to study the electrode reactions for Np ions in various acidic media [49]. Flow coulometry has an inherent advantage over the conventional hulk coulometry methods in that the electrolysis can be achieved rapidly to aid in the characterization of unstable electrode products. The resulting coulopo-tentiograms for the Np02 /Np02 and Np /Np " " couples indicate reversible processes in nitric, perchloric, and sulfuric acids. The differences in potentials between the various acids are attributed to the associated stability constants of the electrode products with the anion of the acid in each case. Table 2 contains the half-wave potentials for each couple in the various acids. 

Tab. 2 Half-wave potentials (V versus Ag/AgCl) in acidic solution from flow coulometry studies [49]...

As with U and Np ions, flow coulometry experiments were conducted to further study the electrode reactions of Pu ions in acidic aqueous solutions [49]. The results from these studies confirm the reversibility of the one-electron couples Pu02 " /Pu02 and Pu" /Pu " " in nitric, perchloric, and sulfuric acid solutions. The further reduction of Pu02 resulted in an irreversible two-electron transfer yielding Pu +. The flow coulometry results in the mixed phosphoric-nitric acid solutions confirm the overall conclusions that have been reached from the stationary and rotated working electrode experiments described previously, in which PuO + is the primary Pu(IV) product from Pu02 reduction. 

Recently flow coulometry, which uses a column electrode for rapid electrolysis, has become popular [21]. In this method, as shown in Fig. 5.34, the cell has a columnar working electrode that is filled with a carbon fiber or carbon powder and the solution of the supporting electrolyte flows through it. If an analyte is injected from the sample inlet, it enters the column and is quantitatively electrolyzed during its stay in the column. From the peak that appears in the current-time curve, the quantity of electricity is measured to determine the analyte. Because the electrolysis in the column electrode is complete in less than 1 s, this method is convenient for repeated measurements and is often used in coulometric detection in liquid chromatography and flow injection analyses. Besides its use in flow coulometry, the column electrode is very versatile. This versatility can be expanded even more by connecting two (or more) of the column electrodes in series or in parallel. The column electrodes are used in a variety of ways in non-aqueous solutions, as described in Chapter 9. 

Aoyagi H, Yoshida Z, Kihara S. 1987. Plutonium and uranium ion determination and differentiation based on twin electrode flow coulometry. Anal Chem 59 400-405. 

Flow coulometry experiments were performed to study the reduction of U02 in nitric, perchloric, and sulfuric acid solutions [56]. The results of these studies show a single two-electron reduction wave attributed to the cou-... 

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