Oxidation-reduction reactions current generation

Electrochemistry is best defined as the study of the interchange of chemical and electrical energy. It is primarily concerned with two processes that involve oxidation-reduction reactions the generation of an electric current from a spontaneous chemical reaction and the opposite process, the use of a current to produce chemical change. 

Electrochemistry works using the principles of oxidation-reduction reactions which generate electric currents or, more simply, the conversion of chemical information into an electrical signal. Electrochemical cells or sensors usually contain a working electrode, to which a potential is applied, and a reference electrode. The oxidation-reduction reaction that ensues is then recorded as an electric current which is a measurement of the analyte from the reaction. Electrochemical methods can be further subdivided into amperometric (measures current), potentiometric (measures potential), conductometric (measures the conductive properties of the medium), impedimetric (measures resistance and reactance) or field effect (measures current through charge accumulation at a gate electrode). ... 

Electron-transfer reactions occur all around us. Objects made of iron become coated with mst when they are exposed to moist air. Animals obtain energy from the reaction of carbohydrates with oxygen to form carbon dioxide and water. Turning on a flashlight generates a current of electricity from a chemical reaction in the batteries. In an aluminum refinery, huge quantities of electricity drive the conversion of aluminum oxide into aluminum metal. These different chemical processes share one common feature Each is an oxidation-reduction reaction, commonly called a redox reaction, in which electrons are transferred from one chemical species to another. 

Batteries are a practical application of the galvanic cell in that an oxidation-reduction reaction generates an electric current. A battery that has an enormous impact on our lives is the automobile battery, shown in Figure 10.3. 

What feature of an oxidation-reduction reaction allows it to be used to generate an electric current (21.1)... 

Many oxidation-reduction reactions may be carried out in such a way as to generate electricity. Such an arrangement for the production of an electric current is called a galvanic or electrochemical cell. In principle, this may always be done for spontaneous, aqueous, oxidation-reduction reactions for which the following experimental requirements are met. 

We have seen that an oxidation-reduction reaction can be used to generate an electric current, in fact, this type of reaction is used to produce electric currents in many space vehicles. An oxidation-reduction reaction that can be used for this purpose is hydrogen and oxygen reacting to form water. 

For an electrical current to flow and light up the bulb shown in Figure 8.5, there must be a voltage difference (difference in electrical potential) between the iron and copper bar. These bars are used to transfer electrons between a wire and solution—and in this case, they actually participate in the oxidation-reduction reaction that occurs. The metal bars are called electrodes. Because this cell is used to generate a voltage and to extract electricity from a chemical reaction, it is called a voltaic cell. 

The electric current generated by this oxidation-reduction reaction is measured for determining the CO concentration in the sample. 

EC sensors are relatively sensitive, as they react to chemical vapor concentrations at the low parts-per-million level. However, EC sensors are not as selective as colorimetric detectors (see Chapter 10). They may respond to various chemicals simultaneously without differentiation capability. This is because the oxidation-reduction reaction between the chemicals in the sample and the electrolyte controls the detection. Any chemicals contained in the sample that will react with the electrolyte on the working electrode surface will generate electrical current and are detected together with those from targeted chemicals. Using a chemical filter may reduce or eliminate some of the chemical interference potential. The nse of a second working electrode that responds to different sets of chemicals from the first working electrode within the same sensor may also lead to better selectivity. 

Coulometry measures the amount of current flowing through a solution in an electrochemical oxidation or reduction reaction and is capable of measuring at ppm or even ppb levels of reactive gases. Thus a sample of ambient air is drawn through an electrolyte in a cell and the required amount of reactant is generated at the electrode. This technique tends to be non-specific, but selectivity can be enhanced by adjustment of pH and electrolyte composition, and by incorporation of filters to remove interfering species. 

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