Polarography

Polarography - method of qualitative and quantitative chemical analysis, based on obtaining the curves of dependence of size of a current on voltage in the circuit consisting of the investigated solution and the immersed electrodes, one of which is highly polarized, and the other almost depolarizes. The receipt of such curves - paragram - produced with the assistance of polarographs.

Fig. 1. Scheme of polarography. Fig. 2. Polarograph.


The scheme is simple polarograph given in Fig. 1. From the potentiometer 2 connected to a battery 1, serves the gradually increasing tension on polarized, usually drip mercury electrode 3 and depolarizes electrode 4 - layer mercury with relatively large surface. The amount of current flowing through the analyzed solution 5, measured by the galvanometer 6. When determining the recoverable substances polarizable electrode connected to negative terminal of an external power source, the definition of oxidizing substances from the positive pole. On the basis of data obtained draw polarograph. In avtoregistracia polarograph curve plotted automatically. If the solution of the same analyte polarograph has the form shown in Fig. 2. The curve ab called polarographic wave. Height h of this wave is proportional to the concentration of the analyte in solution. Voltage E1/2, corresponding to half the height of waves and called potential of the half-wave, depends on the nature of the analyte. If the solution contains several substances, capable to recover or to oxidize in polarizable electrode, polarograph consists of several waves. Thus the nature of each substance is determined by the values of E1/2, and their concentration in solution is the height h of the corresponding waves.
Polarographic method is characterized by high sensitivity. For the analysis is usually enough 3-5 ml of the analyzed solution. An analysis by autoregistry.exe of polarography only lasts about 10 minutes. Polarography used for determining the objects of biological origin of the content of poisonous substances (e.g. mercury, lead, thallium and other), to determine the degree of saturation of blood with oxygen, research composition of the exhaled air of harmful substances in the air of industrial enterprises. For a number of diseases (malignant tumor, radiation sickness and other) polarography serum blood and protein-free filtrate significantly changed, so you can use them when making a diagnosis.

Polarography - physical-chemical method of analysis based on obtaining current-voltage curves (subprograms, polarization curves), which expresses the dependence of the current on voltage in the circuit consisting of the investigated solution and two immersed electrodes, one of which must be strongly polarized.
As a polarizable electrode usually use drip mercury electrode, which can serve as the cathode (when defining elektrovosstanovlenie substances)and an anode (if defined substances capable of elektrooniline). The second auxiliary electrode is almost not polarized mercury electrode with a large surface. You can also use the solid electrodes, such as platinum, with the surface of polarizable electrode must be thousands of times smaller than the surface of the auxiliary electrode.
Getting current-voltage curves produced with the assistance of polarograph, simple, with a visual reference values of current and voltage (Fig. 1) and more complex, with automatic registration of polarization curves. Via battery 6 and potentiometer 5 drip on the mercury electrode 2 and auxiliary electrode 3 polarograph you can submit any certain tension. Through the electrolytic cell 1, containing the test solution, the current flows, the value of which is measured by sensitive galvanometer 4. The test solution, in addition to the analyte, contains an excess indifferent electrolyte, Nona which in the conditions of the experiment is not discharged at the electrodes. Adding this electrolyte (the so-called background) provides high electrical conductivity of the solution and the creation of conditions in which the change is applied external voltage is equal to the change of the potential of drip mercury electrode. The latter (Fig. 1, 2) is the cathode and is used to determine elektrovosstanovlenie substances. With the gradual increase in the external voltage is first of all the current goes to the loading of the electrode, and the current in the circuit remains negligible (the so-called residual current - Fig. 2, plot OA curve), which indicates the absence of electrochemical reaction. When reaching the voltage corresponding to the first point, there is a sharp increase in the current, indicating the beginning of a recovery defined ions on drip mercury cathode. With the further growth of the stress concentration recovering ions from the surface of the cathode falls to zero, and the diffusion rate them in the near-cathode space becomes maximum and constant, as a result, the current in the circuit reaches a limit values (point b), which does not change with further increasing the voltage. Such current call limit diffusive shock (section bc curve). The resulting curve Oabc called polarographic wave. Height h of this wave determined by the ordinate of point b, is proportional to the concentration of recovering ions in solution. This allows you to use polarography for the quantitative determination of substances in solution. Potential e½ , when the current reaches half of the limiting diffusion current, called the potential half. The potential value of the half-wave is determined by the nature of the discharge on a polarizable electrode ions. Thus, determination of potentials on the half-wave gives an opportunity to judge the quality of the analyzed solution. If the solution is somewhat ion that is able to recover (or to oxidize) on polarizable electrode, polarograph consists of several waves. Recovery (or oxidation) of each type of ions corresponds to a certain wave (Fig. 3). This allows you to define multiple substances in a sample. Thus the nature ions is defined by the values of the potentials of half-waves e½, and their concentration on the heights of the corresponding waves.
Polarographic analysis method has high sensitivity and gives the opportunity to identify substances with a very insignificant (to 0.0001%) of their concentration in solution. For analysis enough 3-5 ml of solution; the number of the solution can be reduced to 0,1-0,5 ml analysis on avtoregistracia polarograph takes about 10 minutes
Polarography widely used in biomedical research for qualitative and quantitative determination in biological objects and medicinal products of inorganic and organic electrolytes, protein, hormones, vitamins and other substances, to determine the degree of saturation of blood with oxygen, composition of the exhaled air, for definition of harmful substances in the air of industrial enterprises. For a number of diseases (malignant tumor, radiation sickness and other) height polarographic wave serum and protein-free filtrate is undergoing significant changes, which can be used to develop new methods of diagnosis and to determine the effectiveness of treatment.

Fig. 1. The scheme is simple polarograph with visual reference.
Fig. 2. Polarization curve (polarograph).