Potentiometry working electrode

In potentiometry, a two-electrode setup is used and is given schematically in Fig. 1.3. This setup consists of a working and a reference electrode, and it is the aim of this method to measure equilibrium conditions at the surface of the working electrode. Under equilibrium, the Nernst equation is valid ... 

Open-circuit potential (OCP) — This is the - potential of the - working electrode relative to the - reference electrode when no potential or - current is being applied to the - cell [i]. In case of a reversible electrode system (- reversibility) the OCP is also referred to as the - equilibrium potential. Otherwise it is called the - rest potential, or the - corrosion potential, depending on the studied system. The OCP is measured using high-input - impedance voltmeters, or potentiometers, as in - potentiometry. OCP s of - electrodes of the first, the second, and the third kind, of - redox electrodes and of - ion-selective membrane electrodes are defined by the - Nernst equation. The - corrosion po-... 

In electrochemical detection, the potential of a working electrode can be measured versus a reference electrode, usually while no net current is flowing between the electrodes. This type of detection is referred to as potentiometry. Alternatively, a potential is applied to the working electrode with respect to the reference electrode while the generated oxidation or reduction current is measured. This technique is referred to as amperometry. When applying a negative po-... 

Polarisation titrations are often referred to as amperometric or biamperometric titrations. It is necessary that one of the substances involved in the titration reaction be oxidizable or reducible at the working electrode surface. These titrations are based on generally on the principles of polarography, voltammetry and potentiometry. 

Electrochemical analytical techniques are some of the oldest in chemistry and can be divided into potentiometry, voltammetry and conductimetry. They are most important as detectors after chromatographic separations and as chemical and biological sensors. They generally involve the use of electrodes that are housed in electrochemical cells. All electrochemical cells contain two electrodes but some have three. The first electrode is the actual working electrode (also called a sensing or indicator electrode) and the second is a combined reference electrode and auxiliary (counter) electrode. If there are three electrodes, the reference and counter electrodes are separate. 

Owing to its simplicity and flexibility, potentiometry is probably the most widely used analytical technique. It is most commonly used for measuring pH and for the selective determination of analyte concentrations in a wide variety of sample solutions. Potentiometry is based on the measurement of the potential difference between the reference and working electrodes in a voltaic cell. In this type of cell, as mentioned above, a spontaneous redox chemical reaction occurs due to one reagent being oxidised (losing electrons) at the anode... 

Potentiometry requires a reference electrode, a working electrode and a potentialmeasuring instrument, e.g. voltmeter, otherwise known as a potentiometer. The test solution must be in direct contact with the working electrode, which is sometimes referred to as the chemical sensor as it is sensing the output of a chemical reaction. The reference electrode can also be placed in the test solution or can be brought into contact with the test solution via a salt bridge. The measured potential can be related to the concentration of the species being measured and this approach is called direct potentiometry. 

Figure 15.6 An electrochemical measuring system for potentiometry. The indicator (sensing, working) electrode responds to the activity of the analyte of interest. The potential difference developed between the reference electrode and the indicator electrode is read out on the potentiometer (voltmeter). [Courtesy of Thermo Orion, Beverly, MA (www.thermoorion.com).]...

Electrochemical transducers are commonly used in the sensor field. The main forms of electrochemistry used are potentiometry (zero-current cell voltage [potential difference measurements]), amperometry (current measurement at constant applied voltage at the working electrode), and ac conductivity of a cell. 

With no current through the electrolytic cell, it does not matter whether the electrodes are large or small the equilibrium potentials are the same. But with current flow, the current density and therefore the voltage drop and the polarization, will he much higher at the small electrode. An increased potential drop will occur in the constrictional current path near the small electrode, and in general the properties of the small electrode will dominate the results. The small electrode will be the electrode studied, often called the working electrode. It is a monopolar system, meaning that the effect is determined hy one electrode. The other electrode becomes the indifferent or neutral electrode. Note that this division is not true in potentiometry, electrode area is unimportant under no-current conditions. 

Armalis S, Kubiliene E (2000) Stripping potentiometry signal enhancement using mercury film-coated reticulated vitreous carbon working electrode. Anal Chim Acta 423 287—291... 

In potentiometry, all ions present in the solution principally contribute to the potential of the working electrode. As the ratio between the analyte concentration and that of other species in the solution generally is rather low, the analyte contribution to the detector signal is often low, which results in relatively poor detection limits. To circumvent this problem, ion-selective membranes (ISM), which permit only some ions to pass through the membranes, are commonly employed. In this way, detection limits down to 10 mol/L can be achieved. The ISM also reduces the influence from matrix components, which allows measurements in complex matrices such as blood or seram without interferences. The long-term stability of these electrode may, however, be a problem, as the electrodes might have to be replaced after a few hours or days. Common analytes are inorganic anions and cations, especially alkali and alkaline earth metals ions. A further application is the indirect detection of amino acids, where the... 

Three broad classifications of electrochemical methods are used in this chapter. Po-tentiometric methods include zero-current potentiometry and methods in which current of controlled magnitude is apphed to the working electrode, such as in potentiometric stripping analysis (PSA). Amperometric methods consider all techniques in which current is measured these include constant-potential amper-ometry and amperometric measurements made in response to a variety of applied potential waveforms in voltammetric methods. Impedimetric methods comprise a final classification in these methods, faradaic currents are generally absent, and impedance, conductance, or capacitance is the measured property. 

The functional grouping is as follows. The electrode which is under study is called working electrode in voltammetry or indicator electrode in potentiometry. The electrode the potential of which is practically constant and used to make comparison of electrode potentials, i.e., to define the value of the potential of the electrode on the scale based on standard hydrogen electrode, is called a reference electrode. The electrode that serves to maintain the current in the circuit formed with the working electrode in voltammetric experiments in three-compartment cells is the auxiliary (or counter) electrode. 

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