Chapter 25: Q 25.4 (page 686)
Why is the reference electrode placed near the working electrode in a three-electrode cell?
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Why is it necessary to buffer solutions in organic voltammetry?
A new method for determining ultrasmall () volumes by anodic stripping voltammetry has been proposed (W. R. Vandaveer and I. Fritsch, Anal. Chem.). In this method, a metal is exhaustively deposited from the small volume to be measured onto an electrode, from which it is later stripped. The solution volume is related to the total charge required to strip the metal by
where is the number of moles of electrons per mole of analyte, is the faraday, and is the molar concentration of the metal ion before electrolysis.
() Beginning with Faradayโs law (see Equation ), derive the above equation for .
() In one experiment, the metal deposited was from a solution that was in . The solution was electrolyzed for min at a potential of versus a gold top layer as a pseudo reference. A tubular nano band electrode was used. The silver was then anodically stripped off the electrode using a linear-sweep rate of . The following table represents idealized anodic stripping results. By integration, determine the total charge required to strip the silver from the tubular electrode. You can do a manual Simpsonโs rule integration or do the integration with Excel From the charge, determine the volume of the solution from which the silver was deposited.
() Suggest experiments to show whether all the was reduced to in the deposition step.
() Would it matter if the droplet were not a hemisphere? Why or why not?
() Describe an alternative method against which you might test the proposed method.
The working curve for the determination of dopamine at a nanoneedle electrode by differential-pulse voltammetry (Figure 25-38) was constructed from the following table of data.
(a) Use Excel to perform a least-squares analysis of the data to determine the slope, intercept, and regression statistics, including the standard deviation about regression.
(b) Use your results to find the concentration of dopamine in a sample solution that produced a peak current of 2.92 nA. This value is the average of duplicate experiments.
(c) Calculate the standard deviation of the unknown concentration and its confidence interval assuming that each of the data in the table was obtained in a single experiment.
A solution containing was analyzed voltammetrically using the standard addition method. Twenty-five milliliters of the deaerated solution, which was in , produced a net limiting current of } at a rotating mercury film working electrode at a potential of (versus ). Following addition of of a standard solution, the resulting solution produced a current of }. Calculate the concentration of the sample.
In experiment 1, a cyclic voltammogram at an HMDE was obtained from a solution of data-custom-editor="chemistry" at a scan rate of . In experiment 2, a second CV is to be obtained from a solution of data-custom-editor="chemistry" using the same HMDE. What must the scan rate be in experiment 2 to record the same peak current in both experiments if the diffusion coefficients of data-custom-editor="chemistry" are , respectively. Assume that the reductions of both cations are reversible at the HMDE.
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