Mobility of ions

Phases of heterogeneous system will come in movement owing to interaction with an electric field if to system to put a constant potential difference. Movement of particles of a disperse phase in an electric field on a direction to opposite charged electrode is called electrophoresis. Electrophoresis has been opened F. Rejssom in 1807.

Electrophoresis is movement of the weighed particles (phials of gas, colloidal particles and macromolecules) in a liquid under action of an electric field. The method combining influence on an organism of a constant current and introduction of medicinal substances, is called medical electrophoresis or iontophoresis.

Professor V. Vilensky has applied a constant current in a combination to medicinal substances for the first time in Russia in 1859, theoretical substantiations have found acknowledgement in researches of the physicist, the neuropathologist, psychiatrist A.B. Scherbak.

Electrophoresis has received wide application in clinical researches of whey of blood, gastric juice, urine, spinal fluid in modern medicine. Electrophoresis is applied in physiotherapy. Usually two basic methods are applied - macroscopic and microscopic electrophoresis.

Macroscopic electrophoresis is used for division of the substances which are being a mix, and their subsequent allocation.

Microscopic electrophoresis is used for determination of mobility of ions, cells, particles in an electric field, e electrokinetic potential, and also electrochemical properties of a surface of investigated substances.

Speed of movement of particles of a disperse phase can be found from Smoluchovsky equation: Where υ - speed of movement of particles; ε - dielectric permeability of the dispersive environment; Е - a gradient of potential of an electric field; ζ -electrokinetic potential; η -factor of viscosity of the dispersive environment.

We can apply the equation (1) for erythrocyte, leukocytes, microorganisms and other cells. Electrophoresis mobility of protein molecules and colloidal particles depends on their size and the form. The factor, depending on the size and the form of particles, is entered for calculations into the equation (1). The equation (1) is applied to calculation of size of electrokinetic potential. For this purpose it is necessary to know intensity of an external field, dielectric permeability and factor of viscosity of environment, and also speed of movement of a disperse phase. One of methods of electrophoresis consists in the following: Investigated disperse system is placed on bottom of V-figurative tube and buffer solution is poured lateral knees of a tube. The distinct border has undressed should be between an investigated liquid and a buffer solution. The electrodes, connected to a source of a direct current, are plunged into each knee of V- figurative tube. The created electric field causes moving a disperse phase of an investigated solution, and the border between disperse system and a buffer solution moves. Moving of border is registered by means of long-focus optics. If the investigated mixture contains some components, each component moves with the speed proportional to size of potential ζ. As a result the mixture is divided into a number of functions. The curve having a number of peaks turns out at registration of a signal. The height of peaks serves as a quantity indicator of the given functions. Separate fractions of proteins of blood plasma are allocated and are investigated, by means of this method. Given method has extended after development of techniques of this method by Tizelius. On device Tizelius it is possible to receive results of high accuracy, but it is the complex and bulky device.

Fig. 1. The diagram of electrophoresis analysis on Tizelius: I - normal whey; II - plasma of blood at myelom; III - whey at nephrosis.

Method of electrophoresis on a paper less exact, but more simple. This method offered Viland and Fisher, is applied now. It allows to divide fibers, nucleinic acids, and other biologically important substances.

The certain quantity of an investigated solution is put on the special filtering paper moistened by a buffer solution. The ends of this strip of a paper are connected through the trays filled by a buffer solution. Electrodes are connected to a source of a constant current and fall to trays with a buffer solution. Components of an investigated mixture move at inclusion of a current. Mobility of separate components depends on size of potential ζ, according to the equation (1). After the termination of experience investigated substances settle down on various distance from a line of start. The tape of a paper is necessary for drying up and painting the dye showing investigated substances. In the further the divided components are exposed to the quantitative analysis. Macroscopic methods of electrophoresis are applied for division and research of electrochemical properties of colloidal solutions. Microscopic methods of electrophoresis are used for studying electrochemical properties of suspensions of various cells: erythrocytes, leukocytes, bacteria, sexual cells. Suspensions of cells in a small amount are located in the special chamber, filled by a buffer solution. The electrodes connected to a source of a constant current are entered into this chamber also. Under action of an electric field a cell starts to move to opposite charged electrode. Speed of moving of cells is defined by means of a microscope supplied by eyepiece a micrometer.

The important data, describing electrochemical properties of biological surfaces, are received by means of methods electrophoresis. Alive the protoplasmatic surface is always charged negatively, all biological surfaces possess negative electrokinetic potential. It is established on the basis of numerous experiments. It is not known any example of positive potential of a surface of alive object.

The size ζ - potential can have various values for different cells. At the person it makes approximately 16,3 mV. Potential of erythrocyte is very stable size. For example, distinctions are not present in size ζ - potential erythrocyte at people of various races and a floor. Distinctions are not observed also between representatives of different groups of blood. Electrophoresis mobility of erythrocyte does not change at any diseases of blood, including at many forms of anemia. Electrochemical properties of a surface of erythrocyte have the big stability and constancy.

Scientists have come to conclusion, that the electrokinetic potential of erythrocyte is caused dissociation of acid groups of molecules of phospholipids (cephalin) on a surface of erythrocyte and not connected with processes of adsorption of fibers and ions. The size of electrokinetic potential of erythrocyte changes if there is a change of physical and chemical structure of the surface of a cell. It is observed at some diseases, for example hemoblastosis, lymphosarcoma. For other uniform elements of blood ζ - the potential is studied much more poorly, than for erythrocyte. Leukocytes move to the anode at electrophoresis, as well as erythrocytes, but their mobility approximately to 2 times below mobility of erythrocyte. Electrophoresis mobility of leukocytes is rather close to mobility of quartz particles. The phenomenon of electrophoresis is observed at migration of leukocytes in the inflammatory centers. The electrokinetic phenomena can promote migration of leukocytes. Processes of destruction of structures and accumulation of free molecules occur in the inflamed sites, mainly organic acids; it leads to shift pH in the sour party. As a result of these physical and chemical changes the boundary site between the inflamed and not inflamed fabric gets superfluous positive potential in size up to 100-150 мВ. Leukocytes possess negative electrokinetic potential, they move through a wall of a capillary to a fabric in a direction to positively charged inflamed site.

Bacterial cells possess negative potential, which can change in very wide limits: from zero up to tens millivolt. Owing to this studying the majority of bacteria managed to be divided on two groups. Bacteria which surface has the protein nature belong to the first group. Dissociation ofionogenic groups of protein molecules causes a charge and ζ - potential of such cells. ζ - the potential of these cells varies at change pH environments, as the degree of dissociation of ionogenic groups depends from pH. Bacteria which surface consists from polysacharide concern to the second group. The charge of cells in this case is caused by adsorption of ions from dispersive environment polysacharide of a surface.

Such division is conditional because properties of a surface of bacterial cells can change at change of external conditions. So, for example, the potential ζ of golden staphylococcus under usual conditions remains to constants at the big change pH of environments. If bacteria are cultivated in the environment rich with glucose, then dependence ζ - potential from size pH is observed. This dependence appears in consequence of accumulation on a surface of cells of groups of the protein nature. Thus, the knowledge of mobility of ions, application of a method electrophoresis is good means of studying of electrochemical properties of biological surfaces: abilities to ionization and ability to adsorption of molecules and ions.

Conductivity of electrolytes is carried out due to the ions, arising at dissolution and splitting of molecules of substances. Molecules break up to positively charged ions - cation and negatively charged ions - anion. The phenomenon of splitting of soluble substance on ions is called electrolytic dissociation.

If two electrodes to ship in electrolyte and to bring to them a voltage, then under action of an electric field ions with negative charges (anion) will move to the anode, and ions with positive charges (cation) - to the cathode. If the potential difference on the electrodes, located on distance L from each other, is equal φ₁-φ_2, then intensity of an electric field of electrolyte is defined under the formula E= (φ₁-φ₂)/L

The electric field acts on the charged particles with constant force, forcing them to move to electrodes with some constant speed. If more intensity, then ions will move more quickly. Speed of moving of ions is directly proportional to intensity of an electric field υ= υ₀E, where υ₀- the coefficient of proportionality named by mobility of ions: υ₀ = υ/E.

Intensity of an electric field is measured in V/m, speed of movement of ions - in m/sec. Mobility of ions of the certain kind is expressed by their speed of moving in solvent under action of an electric field and measured in m²/V∙ sec

Mobility of various ions under identical conditions of moving depends on the sizes of ions, valency. Mobility is size characteristic for the certain kind of ions. Probably on size of mobility of ions to define a kind of an ion or to divide a mixture of ions by electrolytic method.

Поиск по сайту: