Electrochemical processes

Electrochemistryisthe study of chemical properties and reactions involving ions in solution, including electrolysis and electric cells.

Main concepts of electrochemistry.

Electrical double layer isa model of the interface between an electrode and the solution close to it. In this model a sheet of one type of electrical charge surrounds the surface of the electrode and a sheet of the opposite charge surrounds the first sheet in the solution. In the Helmholtz model the double layer is regarded as consisting of two planes of charge, with the inner plane of ions from the solution being caused by the charge on the electrode and the outer plane being caused by oppositely charged ions in the solution responding to the first layer of ions. In the Gouy - Chapman model (diffuse double layer) thermal motion of ions is taken into account. Neither model is completely successful since the Helmholtz model exaggerates the rigidity of the structure of the charges and the Gouy-Chapman model underestimates the rigidity of the structure. The Stern model improves on both models by assuming that the ions next to the electrode have a rigid structure, while taking the second layer to be as described by the Gouy-Chapman model.

Electrochemical equivalentis the mass of a given element liberated from a solution of its ions in electrolysis by one coulomb of charge.

Electrode isa conductor that emits or collects electrons in a cell, thermionic valve, semiconductor device, etc. The anode is the positive electrode and the cathode is the negative electrode.

Electrode potential is the potential difference produced between the electrode and the solution in a half cell. It is not possible to measure this directly since any measurement involves completing the circuit with the electrolyte, thereby introducing another half cell. Standard electrode potentials are defined by measuring the potential relative to a standard hydrogen half cell using 1.0 molar solution at 25°C. The convention is to designate the cell so that the oxidized form is written first. For example,

Pt(s) | H₂(g)H⁺ (aq) | Zn²⁺(aq) | Zn(s) .The e.m.f. of this cell is -0.76 volt (i.e. the zinc electrode is negative). Thus the standard electrode potential of the Zn²+| Zn half cell is -0.76 V.

Electrolyte is a liquid that conducts electricity as a result of the presence of positive or negative ions. Electrolytes are molten ionic compounds or solutions containing ions. i.e. solutions of ionic salts or of compounds that ionize in solution. Liquid metals, in which the conduction is done by free electrons, are not usually regarded as electrolytes. Solid conductors of ions, as in the sodium-sulphur cell, are also known as electrolytes.

Electrolysis is the production of a chemical reaction by passing an electric-current through an electrolyte. In electrolysis, positive ions migrate to the cathode and negative ions to the anode. The reactions occurring depend on electron transfer at the electrodes and are therefore redox reactions. At the anode, negative ions in solution may lose electrons to form neutral species. Alternatively, atoms of the electrode can lose electrons and go into solution as positive ions. In either case the reaction is an oxidation. At the cathode, positive ions in solution can gain electrons to form neutral species. Thus cathode reactions are reductions.

Electrolysis –law-based methods

Electrodeposition is the process of depositing one metal on another by electrolysis, as in electroforming and electroplating.

Electrolytic refining is the purification of metals by electrolysis. It is commonly applied to copper. A large piece of impure copper is used as the anode with a thin strip of pure copper as the cathode. Copper (II) sulphate solution is the electrolyte. Copper dissolves at the anode: Cu ® Cu²⁺ + 2e and is deposited at the cathode. The net result is transfer of pure copper from anode to cathode. Gold and silver in the impure copper form a so-called (anode sludge at the bottom of the cell, which is recovered.

Electrolytic separation is a method of separating isotopes by exploiting the different rates at which they are released in electrolysis. It was formerly used for separating deuterium and hydrogen. On electrolysis of water, hydrogen is formed at the cathode more readily than deuterium, thus the water becomes enriched with deuterium oxide.

Electroplating is a method of plating one metal with another by electrodeposition. The articles to be plated are made the cathode of an electrolytic cell and a rod or bar of the plating metal is made the anode. Electroplating is used for covering metal with a decorative, more expensive, or corrosion-resistant layer of another metal.

Methods based on the movement of charged particles in electric field (electromigration)

Electrodialysis is a method of obtaining pure water from water containing a salt, as in desalination. The water to be purified is fed into a cell containing two electrodes. Between the electrodes there is placed an array of semipermeable membranes alternately semipermeable to positive ions and negative ions. The ions tend to segregate between alternate pairs of membranes, leaving pure water in the other gaps between membranes. In this way, the water being fed is separated into two streams: one of pure water and the other of more concentrated solution.

Electrophoresis (cataphoresis) is a technique for the analysis and separation of colloids, based on the movement of charged colloidal particles in an electric field. There are various experimental methods. In one the sample is placed in a U-tube and a buffer solution added to each arm, so that there are sharp boundaries between buffer and sample. An electrode is placed in each arm, a voltage applied and the motion of the boundaries under the influence of the field is observed. The rate of migration of the particles depends on the field, the charge on the particles and on other factors, such as the size and shape of the particles. More simply, electrophoresis can be carried out using an adsorbent, such as a strip of filter paper, soaked in a buffer with two electrodes making contact. The sample is placed between the electrodes and a voltage applied. Different components of the mixture migrate at different rates, so the sample separates into zones. The components can be identified by the rate at which they move. In gel electrophoresis the medium is a gel, typically made of polyacrylamide, agarose or starch. Electrophoresis, which has also been called electrochromatography, is used extensively in studying mixtures of proteins, nucleic acids, carbohydrates, enzymes, etc. In clinical medicine it is used for determining the protein content of body fluids.

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