ANALYTICAL CHEMISTRY PRACTICE II ELECTROCHEMISTRY

Electrochemical cell: Electrochemistry: The interaction of matter with electrical energy and the resultant chemical transformations, physical changes, and the conversion of chemical energy into electrical energy. Electrochemistry examines the passage of electrons from one substance to another, and this electron transition determines the current that will give information about the substance being formed. Electrochemical reactions are oxidation-reduction or redox-type reactions. The electrochemical processes are carried out in an apparatus which takes the name of the electrochemical cell. Electrochemical cell: A solution containing the insoluble material was dissolved in the salt Electrodes for chemical conversion of the material It consists of an external circuit that connects the electrodes together.

Electrochemical Cells It is a system consisting of electrodes immersed in a suitable electrolyte solution. In order for an electric current to form in the electrochemical cell, an electron transfer reaction must take place in each of the electrodes, so that the electrons are brought into contact with one another by a metal conductor and between the solutions through one another.

VOLTAMMETRY AND POLAROGRAPHY Polarography was first discovered by Czech chemist Jaroslav Heyrovsky in 1922. Heyrovsky first found polarography, a special type of voltammetry, and won the Nobel Prize for Chemistry in 1959 for his invention. In the polarography, which is an important branch of the electrochemistry, the mercury electrode which is dripping as the working electrode is used instead of the voltammetry. Voltammetry; The electrochemical method is based on measuring the current as a function of the potential applied to the electrode. In voltammetry, a potential difference is applied between a small surface electrode and a large surface electrode. In this case, the potential difference calculated by the Nernst equation changes as the current in the cell does not pass, and the concentrations of reactant substances change to match the Nernst equation. This change is possible only when a reaction takes place in the cell and the current forms at the end of this reaction.

An electrolysis cell is filled with a suitable solution to be analyzed, the increasing potential differences are applied and the current values ​​passing through the cell are read. A curve is obtained when the potential difference is plotted against flow. This current-potential curve is called polarogram or voltamogram, depending on the type of electrode used. The analysis method that utilizes the current-potential curves in this way is called polarography or voltammetry. This method is called voltammetry, while the electrode used is the solid electrode (membrane, metallic, carbon electrode), used in the method of polarography, while the mercury electrode (dripping mercury electrode, suspended mercury electrode, stationary mercury drop electrode and mercury thin film electrode) are used in the experiment.

After the analyte begins to react with the electrode, the increase in current against the smallest potential change can be rapid. The magnitude of the current is limited by the rate at which the electroactive material reaches the electrode surface, and therefore does not increase after a certain potential value. In this region where the increase is not seen, the current magnitude is called the limit current. A small current is observed before the electroactive material reacts with the electrode. This current flow due to reasons such as the loading of the electrical double layer and the impurities in the solution is called residual current. In a small increase in the potential in the B-C region, the current increases greatly. The magnitude of the current in this region is called the diffusion current (id). The potential corresponding to the half of the diffusing current is called half-wave potential (E1/2).

Polarograms allow us to do both qualitative and quantitative analysis of the material: For each item there is a certain half-wave potential under certain conditions. It can be used for qualitative analysis due to the fact that the half-wave potentials are characteristic for electroactive substances. The quantitative analysis also utilizes the fact that the diffusion current is proportional to the concentration. This dependence is given by the first-order equation: id= 605 n D1/2 C m2/3 t 1/6 Id: diffusion current (in micro ampere) 1 uA = 10-6A (read from the device) N: the number of electrons in the electrode reaction D: diffusion coefficient of the solute C: concentration of solute in millimol / L (found) M: mercury mass in mg / sec T: drip time in sec 605: a number containing the geometry of the drop and the Faraday constant

The other parameters for the electrode and substance used in the ilkovic equation are constant, while there is a linear relationship between the id value and the concentration (id = kxC). By taking advantage of this, the amount of the substance in the unknown sample can be calculated from the calibration equation to be prepared with the aid of the standard substance. Also in quantitative analysis; Quantification can also be done by proportioning the read id values ​​for solubilizers known concentrations known to the unknown: Id standard / id sample = C standard / C sample

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