Microelectrode polarography

More positive potentials can be attained by employing solid microelectrodes, but then negative potentials often cannot be employed. In 1941 Laitinen and Kolthoff (LI) coined the general term voltammetry for current-voltage curves obtained at all microelectrodes, including mercury and solid microelectrodes. Polarography is a special type of voltammetry in which the electrode is the DME. Either oxidation or reduction of a test substance can take place at microelectrodes, depending on the applied potential and the test substance. 

See also DNA Sequencing. Enzymes Enzyme-Based Electrodes. Forensic Sciences Blood Analysis. Immunoassays, Techniques Enzyme Immunoassays. Microelectrodes. Polarography Techniques Organic Applications. Purines, Pyrimidines, and Nucleotides. Sensors Chemically Modified Electrodes. Voltammetry Organic Compounds. 

Indicator Electrode and Working Electrode In polarography and voltammetry, both terms are used for the microelectrode at which the process under study occurs. 

Stripping is the most sensitive form of voltammetry. In anodic stripping polarography, analyte is concentrated into a single drop of mercury by reduction at a fixed voltage for a fixed time. The potential is then made more positive, and current is measured as analyte is reoxidized. In cyclic voltammetry, a triangular waveform is applied, and cathodic and anodic processes are observed in succession. Microelectrodes fit into small places and their low current allows them to be used in resistive, nonaqueous media. Their low capacitance... 

The slope of this straight line is 16.91 x n V-1 at 25 °C. However, it is more common to use the inverse function E = Ei/2 + 2.303 x (RT/nF) log [(fi, - I) /I], with the slope 0.059/nV. Both functions are called the logarithmic analysis of DC polarogram. They both cross the potential axis at the half-wave potential, which corresponds to I = Ii/2. The main characteristic of fast and reversible electrode reactions is that the half-wave potential is independent of the drop life-time in DC polarography, or the rotation rate of the rotating disk electrode, or the radius of microelectrode. If this condition is satisfied, the slope of the logarithmic analysis indicates the number of electrons in the electrode reaction. 

The current-potential relationship of the totally - irreversible electrode reaction Ox + ne - Red in the techniques mentioned above is I = IiKexp(-af)/ (1+ Kexp(-asteady-state voltammetry, a. is a - transfer coefficient, ks is -> standard rate constant, t is a drop life-time, S is a -> diffusion layer thickness, and

logarithmic analysis of this wave is also a straight line E = Eff + 2.303 x (RT/anF) logzc + 2.303 x (RT/anF) log [(fi, - I) /I -The slope of this line is 0.059/a V. It can be used for the determination of transfer coefficients, if the number of electrons is known. The half-wave potential depends on the drop life-time, or the rotation rate, or the microelectrode radius, and this relationship can be used for the determination of the standard rate constant, if the formal potential is known. 

Polarography depends on the current-voltage changes arising at a microelectrode when diffusion is the rate-limiting step in the discharge of ions. Both qualitative and quantitative analyses are possible if the substance is capable of undergoing cathodic reduction or anodic oxidation. 

The microelectrodes of voltammetry/polarography Again in FIA and in high-performance liquid chromatography (HPLC) some of these electrodes may function as detectors. 

Classical DC polarography uses a linear potential ramp (i.e., a linearly increasing voltage). It is, in fact, one subdivision of a broader class of electrochemical methods called voltammetry. Voltanunetric methods measure current as a function of applied potential where the WE is polarized. This polarization is usually accomplished by using microelectrodes as WEs electrode... 

We mentioned that polarography is a form of voltammetry in which the electrode area does not remain constant during electrolysis. It is possible, however, to use electrode materials other than mercury for electroanalysis, provided that the potential window available is suitable for the analyte in question. Table 15.7 summarizes the accessible potential ranges for liquid mercury and for solid platinum electrodes. The precious metals and various forms of carbon are the most common electrodes in use, although a great many materials, both metallic and saniconducting, find use as analytical electrode substrates. Voltammetry is conducted using a microelectrode as the WE under conditions where polarization at the WE is enhanced. This is in sharp contrast to both poten-tiometry and coulometry where polarization is absent or minimized by experimental conditions. 

What are the advantages of mercury electrodes for electrochemical measurements What are the advantages of the DME versus a Pt microelectrode for polarography What are the disadvantages of the DME Describe the method of anodic stripping voltammetry. What analytes can it be used to determine Why is stripping voltammetry more sensitive than other voltammetric methods ... 

This class of titration is based on the measurement of the diffusion current at a dropping-mercury electrode or at a rotating platinum microelectrode of the substance being titrated or of the reagent being used. It is known that in polarography the diffusion current obtained from a given ion is proportional to the concentration of that ion at a suitable potential and this fact is used in amperometric titrations. 

---