Cathodic reductive stripping

Adsorptive stripping voltammetry (AdSV) is of increasing importance in trace determinations of pharmaceutical compounds. In this method, the preconcentration step is adsorptive rather than electrolytic, resulting in an adsorbed film of the analate on the electrode surface. The stripping step typically uses LSV or the differential pulse mode in either the cathodic or anodic direction, as required. The HMDE is typically used for cathodic reductive stripping, whereas carbon or noble metal electrodes are used in the adsorptive mode. 

Oxide films can be stripped off iron by using bromine in methanol followed by heating at 300 °C in N2 to remove FeBr2 (Mayne and Ridgeway, 1971). The thickness of such films can be measured by weighing, by cathodic reduction and from the interference colours of the films the latter technique can also be applied to measurement of film thickness in situ. The first order interference colours of hematite films on iron are yellow/brown, mauve, blue and silver grey and the second order colours are pinky-blue, blue and greenish-blue (Evans, 1963). 

Lead and mercury are deposited as micron-sized clusters, predominantly at intercrystallite boundaries [105] so does lithium from the polyethylene oxide solid electrolyte. What is more, Li intercalates into the sp2-carbon [22, 138], Thus, observations on the Li intercalation and deintercalation enable one to detect non-diamond carbon on the diamond film surface. Copper is difficult to plate on diamond [139], There is indirect evidence that Cu electrodeposition, whose early stages proceed as underpotential deposition, also involves the intercrystallite boundaries [140], We note that diamond electrodes seem to be an appropriate tool for use in the well-known electroanalytical method of detection of traces of metal ions in solutions by their cathodic accumulation followed by anodic stripping. The same holds for anodic deposition, e.g. of, Pb as PbCh with subsequent cathodic reduction [141, 142], Figure 30 shows the voltammograms of anodic dissolution of Cd and Pb cathodically predeposited from their salt mixtures on diamond and glassy carbon electrodes. We see that the dissolution peaks are clearly resolved. The detection limit for Zn, Cd, and Pb is as low as a few ppb [143]. 

With ferrous ion or cathodic reduction, conversion of plutonium from Pu to Pu is so rapid that back extraction of plutonium to the aqueous phase and reduction there to Pu can be carried out simultaneously in a single multistage contactor. With tetravalent uranium, reduction of plutonium is slower, so that additional contactor volume is desirable to complete back extraction. With hydroxylamine, reduction of plutonium is so much slower that it is preferable first to return both uranium and plutonium to the aqueous phase by stripping with dilute nitric acid and then to reduce the plutonium in equipment providing sufficient residence time for reduction to proceed to completion. Finally, the uranium is reextracted by TBP. 

Olson, C. and Adams, R.N. (1963) Carbon paste electrodes application to cathodic reductions and anodic stripping voltammetry. Anal Chim. Acta,... 

The process is one of electrolytic reduction and the apparatus is similar to that shown in Fig. 77, p. 144. It consists of a small porous cell (8 cm. x 2 cm. diam.) surrounded by a narrow beaher (ii cm. X 6 cm. diam.). The oxalic acid, mixed w lth too c.f. 10 per cent sulphuric acid (titrated against standard baryl.a solution] forms the cathode liquid and is placed in Iht beakei. The porous cell is filled with the same strength of siilphuiic acid and foims the anode liquid. The electrodes ara made from 01 dinary clean sheet lead. The anode consists of i thiu strip projecting about two inches from the cell and tliu... 

Van den Berg [131] used this technique to determine nanomolar levels of nitrate in seawater. Samples of seawater from the Menai Straits were filtered and nitrite present reacted with sulfanilamide and naphthyl-amine at pH 2.5. The pH was then adjusted to 8.4 with borate buffer, the solution de-aerated, and then subjected to absorptive cathodic stripping voltammetry. The concentration of dye was linearly related to the height of the reduction peak in the range 0.3-200 nM nitrate. The optimal concentrations of sulfanilamide and naphthyl-amine were 2 mM and 0.1 mM, respectively, at pH 2.5. The standard deviation of a determination of 4 nM nitrite was 2%. The detection was 0.3 nM for an adsorption time of 60 sec. The sensitivity of the method in seawater was the same as in fresh water. 

Absorptive cathodic stripping voltammetry has been used [151,152] to determine nanomolar levels of nitrite in seawater. The nitrite is derivatised by diazotisation with sulfanilamide and coupled with 1-naphthylamine to form an azo dye. The dye adsorbs onto a mercury drop electrode and its reduction is fully reversible. The concentration of dye is linearly related to concentration of nitrite in the range 0.3-200 nM. Down to 0.3 nM nitrite can be determined in seawater for an adsorption time of 60 seconds. 

Donat and Bruland [217] determined low levels of nickel and cobalt in seawater by a voltammetric technique, and the nioxime complexes of the two elements were concentrated on a hanging mercury drop electrode. The current resulting from the reduction of Co (II) and Ni (II) was measured by differential pulse cathodic stripping voltammetry. Detection limits are 6 pM (cobalt) and 0.45 nM (nickel). 

Certain trace substances such as selenium (IV) can be determined by differential cathodic stripping voltammetry (DPCSV). For selenium a rather positive preconcentration potential of-0.2 V is adjusted. Selenium (IV) is reduced to Se2", and Hg from the electrode is oxidised to Hg2+ at this potential. It forms, with Se2" on the electrode, a layer of insoluble HgSe, and in this manner the preconcentration is achieved. Subsequently the potential is altered in the cathodic direction in the differential pulse mode. The resulting mercury (II) peak produced by the Hg11 reduction is proportional to the bulk concentration of SeIV in the analyte. 

Van den Berg [620] also reported a direct determination of sub-nanomolar levels of zinc in seawater by cathodic stripping voltammetry. The ability of ammonium pyrrolidine dithiocarbamate to produce a significant reduction peak in the presence of low concentrations of zinc was used to develop a method capable of achieving levels two orders of magnitude below those achieved with anodic stripping voltammetry. Interference from nickel and cobalt ions could be overcome by using a collection potential of 1.3 V, and interference from... 

Cyclohexane-1,2-dione dioxime (nioxime) complexes of cobalt (II) and nickel (II) were concentrated from 10 ml seawater samples onto a hanging mercury drop electrode by controlled adsorption. Cobalt (II) and nickel (II) reduction currents were measured by differential pulse cathodic stripping voltammetry. Detection limits for cobalt and nickel were 6 pM and 0.45 mM, respectively. The results of detailed studies for optimising the analytical parameters, namely nioxime and buffer concentrations, pH, and adsorption potential are discussed. 

Cathodic stripping voltammetry has been used [807] to determine lead, cadmium, copper, zinc, uranium, vanadium, molybdenum, nickel, and cobalt in water, with great sensitivity and specificity, allowing study of metal specia-tion directly in the unaltered sample. The technique used preconcentration of the metal at a higher oxidation state by adsorption of certain surface-active complexes, after which its concentration was determined by reduction. The reaction mechanisms, effect of variation of the adsorption potential, maximal adsorption capacity of the hanging mercury drop electrode, and possible interferences are discussed. 

The situation is different on GC where only one irreversible cathodic wave is observed at —1.6 V, associated with disintegration of the compound, and one anodic wave on the reverse scan, caused by stripping of the lead released by the reduction ... 

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