Electrolysis stripping analysis

Several related bulk electrolysis techniques should be mentioned. In thin-layer electrochemical methods (Section 11.7) large AIV ratios are attained by trapping only a very small volume of solution in a thin (20-100 fxm) layer against the working electrode. The current level and time scale in these techniques are similar to those in voltammetric methods. Flow electrolysis (Section 11.6), in which a solution is exhaustively electrolyzed as it flows through a cell, can also be classified as a bulk electrolysis method. Finally there is stripping analysis (Section 11.8), where bulk electrolysis is used to preconcentrate a material in a small volume or on the surface of an electrode, before a voltammetric analysis. We also deal in this chapter with detector cells for liquid chromatography and other flow techniques. While these cells do not usually operate in a bulk electrolysis mode, they are often thin-layer flow cells that are related to the other cells described. 

Anodic stripping analysis The accumulation is usually carried out by the electrolysis at controlled potential for a definite time, t cc under reproducible mass-transport conditions. The deposition potential, E, imposed to the working electrode should be adjusted at the potential adequate to the plateau of the reduction voltammetric wave characteristic for the test metal (see Figs. 62A and 62B for a mixture of two tested species). Before adjusting the experiment, it is recommended to know the solubility data and diffusion coefficients of metals in mercury and their half-wave potentials (see Table 4). 

Potentiometric stripping analysis is carried out in several stages. After electrochemical generation of the mercury film on a graphite substrate, the elements to be determined are accumulated by electrolysis at constant potential. The next stage is the oxidation of the deposited elements by the oxidant present in the. solution. For this, the current circuit is disconnected. The deposited analytes are stripped in the order of their electrochemical jxitentials. Anodically deposited precipitates can similarly be stripped by chemical reduction. In all cases, potential - time curves with transition times proportional to concentration result [39]-[41]. 

Analyses for "copper, cadmium, and lead were carried out continually by DPASV. Zinc determinations were excluded to permit use of a lower electrolysis potential. The samples were analyzed at pH 4.9 by sparging with carbon dioxide. An 8-min. electrolysis at —1.0 V vs. silver/ silver chloride and a 25-mV pulse were used during the Seattle-Saanich portion of the trip (Leg 1) while a 10-min. electrolysis and a 50-mV pulse were used from Saanich to Seattle (Leg 2). Application of the DPASV technique resulted in greater sensitivity and thus shorter plating times for the low levels encountered. It also afforded better resolution for "copper than linear-sweep ASV. It should be pointed out, however, that DPASV does not result in shorter analyses times because the stripping portion of the analysis is very slow. Nevertheless, it is worthwhile to limit the time of electrolysis because this also reduces the concentrations of interfering metals accumulated in the mercury fllm. Under the... 

To carry out the determination of a metal ion by means of anodic stripping, a fresh hanging drop is formed, gentle stirring is begun, and a potential is applied that is a few tenths of a volt more negative than the half-wave potential for the ion of interest. Deposition is allowed to occur for a carefully measured period, which can vary from a minute or less for 10 M solutions to 30 min or longer for 10 M solutions. These times seldom result in complete removal of the analyte. The electrolysis period is determined by the sensitivity of the method ultimately employed for completion of the analysis. 

Hanging mercury drop electrode (HMDE) A microelectrode that can concentrate traces of metals by electrolysis into a small volume the analysis is completed by voltammetric stripping of the metal from the mercury drop. 

A three compartment electrolytic cell with two fritted disk separators was used. An auxiliary platinum electrode was placed in each of the two end compartments while an aluminium strip, to which a surface treatment based on chromic acid etching had been applied, was placed in the middle compartment. Ihe monomer solution contained methacrylic acid (0.436 M) and -methylenebisacrylamide (0.145 M) in water. The pH of the above solution was adjusted to desired values by adding either sulfuric acid or aqueous sodium hydroxide. The monomer solution (400 mi) was placed in the middle compartment while two end compartments contained water adjusted to the same pH as the monomer solution. While bubbling nitrogen through the monomers solution, the electrolysis was conducted at 10 vdts for 6 hours such that the cathodic reaction would occur in the center compartment. The polymer coating obtained on the aluminium cathode was washed with fresh water, dried in vacuum oven at room temperature, and weighed. The copolymer was then scraped off from aluminium and its composition determined by elemental analysis. 

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