Chapter 25: Q.24.4 (page 686)
Why is the reference electrode placed near the working electrode in a three-electrode cell?
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Distinguish between (a) voltammetry and amperometry, (b) linear-scan voltammetry and pulse voltammetry, (c) differential-pulse voltammetry and square-wave voltammetry, (d) an RDE and a ringdisk electrode, (e) faradaic impedance and double-layer capacitance, (f) a limiting current and a diffusion current, (g) laminar flow and turbulent flow, (h) the standard electrode potential and the half-wave potential for a reversible reaction at a working electrode, (i) normal stripping methods and adsorptive stripping methods.
What is the purpose of the electrodeposition step in stripping analysis?
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.
Concentration
Dopamine, mM Peak Current, nA
0.093 0.66
0.194 1.31
0.400 2.64
0.596 4.51
0.991 5.97
(a) Use Excel to perform a least-squares analysis of the data to determine the slope, intercept, and regres-
sion 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 cur-
rent of 2.92 nA. This value is the average of duplicate experiments.
(c) Calculate the standard deviation of the unknown concentration and its 95% confidence interval
assuming that each of the data in the table was obtained in a single experiment.
Why are stripping methods more sensitive than other voltammetric procedures?
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 Faraday’s 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 Simpson’s 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.
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