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“Mechanism of Electrode Potential Formation and Their Classification”

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“Mechanism of Electrode Potential Formation and Their Classification” MINISTRY of PUBLIC HEALTH of UKRAINE ODESSA NATIONAL MEDICAL UNIVERSITY Department of Clinical Chemistry and Laboratory Diagnostics Medical chemistry Information block for first-year students of medical faculty Part 3. Thermodynamic and kinetic features of processes flow and electro kinetic phenomena in biological systems. “Mechanism of electrode potential formation and their classification” Control questions: 1) Electrode potential and mechanism of their occurrence. Nernst equation. 2) Normal hydrogen electrode. 3) Galvanic cell. Measurement of electrode potentials .Normal (standard) electrode potential. 4) Classification of electrodes: a) Electrodes of the first kind. b) Electrodes of the second kind. c) Ion selective electrodes. Glass electrode. d) Inert redox electrodes. 5) Potentials of biological systems. Diffusion potential (damage potential), membrane potential, phase potential. 6) The biological role of diffusion and membrane potentials. 1 Electrode potentials If a plate of any metal, for example zinc, is dipped into water, the ions of zinc being part of the metal crystal lattice, under the influence of polar molecules of water are hydrated, their bond with the lattice becomes weak and some amount of them coming out from the metal surface passes into water. An equivalent number of electrons stays on the metal: Between the metal cations, which have passed into water, and the negatively charged plate an electrostatic attraction arises. As a result, a reverse process - transition of metal ions into the plate - takes place. Metal ions will gain electrons from the metal plate and change into metal atoms. So a dynamic equilibrium is established in the system: It means that zinc ions pass from the plate into the solution and settle from the solution on the plate with identical rate. A double electrical layer is formed on the interface between the metal and the solution and a potential difference arises. If the liquid is water, then any metal is charged negatively and the layer of liq- uid adjoining to it is charged positively. A different picture is observed if a metal plate is immersed not into pure water, but into a solution of this metal’s salt. If an active metal is in contact with solution of its own salt, then its surface is charged negatively, and the value of this negative charge decreases with increasing of active concentration (activity) of its ions in the solution. If the metal is inactive, then the process of solution ion reduction will prevail and a plate of such metal will develop a positive charge. So, at immersing of a metal plate into a solution of its own salt, a potential dif- ference develops in the place of contact between the metal and solution. Its value and sign depend on the chemical nature of the metal and activity of its own ions in the solution. A conductor (metal) immersed in a solution of electrolyte is called an 2 electrode. Schematically an electrode may be written down as Cu|Cu2+, Ag|Ag+. Potential difference arising on the electrode-solution interface is called electrode potential. The value of electrode potential can be calculated according to the equation of W. Nernst (a German scientist) (1889): where E is electrode potential under given concentration of Men+ ions; R is the universal gas constant, 8.314 J /mol ∙ K; T is temperature, K; Fis the Faraday con- stant, 96500 C/mol; n is the number of electrons involved in an electrode reaction; a(Men+) is the activity of metal ions in a solution; E° is normal (standard) electrode potential. Standard electrode potential, E°, is the potential measured under standard conditions: the activity of metal ions in solution is equal to 1 mol/1, temperature is 298 K. At a temperature of 298 K the Nernst equation may be represented as: The science does not have any methods allowing to measure the absolute value of potentials of individual electrodes, it is possible to measure only the potential difference between electrodes. For this purpose it is necessary to use some electrodes as reference electrodes. A commonly accepted reference electrode is the normal (standard) hydrogen electrode. The normal hydrogen electrode represents platinized platinum (a platinum plate covered with black platinum) immersed in an acid solution where the activity of H+ ions is equal to 1 mol/1. Carefully cleared hydrogen gas at a pressure of 101325 Pa (1 atm) is run over this solution. The platinum surface becomes covered with a layer of gaseous hydrogen. An the interface between gaseous hydrogen and hydrogen ions equilibrium is established: Apparently, platinum does not react, but provides a surface for electrode pro- 3 cess. Potential of the standard hydrogen electrode is conditionally taken as zero. Hydrogen electrode may be attributed to gas-ion electrodes, because they con- sist of gas in contact with its cation (or anion) in the solution. So, for determining the electrode potential of any electrode, a cell comprising the electrode under investigation as one of the electrodes and a standard hydrogen electrode as the reference electrode must be assembled. Electrodes in this electro- chemical cell must be joined with a wire, and solutions must be joined with a salt bridge that conducts electricity, but prevents the ions from mixing among themselves in the solution. Electrons flow from the electrode where oxidation takes place (anode), to the electrode where reduction occurs (cathode). Thus the electrochemical cell produces electricity as a result of spontaneous chemical process. Such system is called the galvanic cell. Potential difference between electrodes is measured by a potentiometer. Dif- ference between potentials of electrodes is known as electromotive force (EMF) of the cell. (It should be more exactly noted that EMF is the maximum voltage obtain- able from the cell). EMF is always calculated in the following way: If the normal hydrogen electrode is used as the reference electrode, whose elec- trode potential is equal to zero, EMF of the cell will immediately give the electrode potential of the other electrode. Electrode potential is called oxidation potential if oxidation process occurs on the electrode relative to the standard hydrogen electrode, and on the contrary, it is called reduction potential if reduction takes place on the electrode relative to the standard hydrogen electrode. It means that the electrode where reduction takes place relative to the standard hydrogen electrode has a positive reduction potential. The electrode where oxidation occurs relative to the standard hydrogen electrode has a negative reduction potential. Reduction potential is equal to oxidation potential, but 4 an opposite sign. In practice electrode potentials are expressed as reductions potentials. As it has been mentioned, if a metal is immersed in a solution of its own ions in concentration of 1 mol/1 at 298 K, then the potential arising under these conditions is named the standard electrode potential of this metal. When all the metals are arranged in the order of their standard reduction potentials increase, an elec- trochemical series of metals is obtained: The electrochemical series of metals coincides with the activity series empiri- cally established by Russian scientist N. Beketov by investigating metal displacement from their compounds by other metals. Relative activity of metals can be estimated by comparison of their oxidation potentials, because metallic properties are characterized by the ability of metals to lose electrons. So, a metal with a greater oxidation potential will be more active than a metal with a lower oxidation potential. That is why any metal of the electro- chemical series can displace metals that are on its right side from the salt solution. Classification of electrodes All electrodes may be divided into 4 basic types: 1. Electrodes of the first kind, reversible relative to cation or anion. 2. Electrodes of the second kind, reversible relative to both cation and anion. 3. Ion-selective electrodes. 4. Inert oxidation-reduction electrodes. Electrodes of the first kind. Metal immersed in a solution of its own salt can be an example of an electrode of the first kind, reversible relative to cation: Cu|Cu2+; Zn|Zn2+; Ag|Ag+. Gas-ion + - electrodes such as: (Pt) 1/2H2|H ; (Pt) C1 |1/2C12 maybe also an example of the first- kind electrodes. 5 Their potential is equal to: where a(cat.) and a(an.) are the activities of cation and anion, mol/1, respectively. Electrodes of the first kind are often used as indicator electrodes. Since such electrodes rapidly respond to the concentration of the analyte ion in a solution, it is possible to calculate activity of its ions in a solution. For example, the concentration of H+ ions may be determined using the hydrogen electrode. The Nernst equation for the hydrogen electrode can be written as: E = 0.059 a(H+) or E = -0.059pH. Electrodes of the second kind.It is metal covered with a layer of its insoluble salt being in contact with a solution containing an anion of the salt. For example, the so-called silver/silver chloride electrode: Ag|AgCl, KC1. It is a silver wire covered with silver chloride and immersed in a solution of potassium chloride. During its work reactions occur: The potential arises at the Ag|Ag+ interface. The potential of silver/silver chlo- ride electrode has a sign «+» in relation to the normal hydrogen electrode. If a solution of KC1 is saturated, in other words, a system consists of the silver electrode immersed in a solution saturated with both silver chloride and potassium chloride, the activity of Ag+ ions is determined by the solubility product constant KspAgCl of AgCl: 6 Consequently, Then: The saturated silver/silver chloride electrode is often used as a reference elec- trode (EAg+/AgCl = 0.201 V) instead of the standard hydrogen electrode, which is dif- ficult to operate. The saturated calomel electrode Hg|Hg2Cl2, KClsatd is also referred to electrodes of the second kind, and is also used as a reference electrode.
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