What is a high frequency alternating current
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High frequency titration
In contrast to conductivity titration, in which the change in resistance is measured with electrodes immersed in the sample solution, high-frequency titration works with electrodes that are attached to the outside of the measuring vessel. This arrangement has the advantage that a change in the electrodes due to chemical reactions and adsorption is excluded. Of course, the electrodes cannot be polarized either. A disadvantage of the method is the much greater measurement effort.
The measuring cell used is absolutely impermeable to direct current because the glass wall between the electrodes and the solution has a very high ohmic resistance. In relation to a high-frequency alternating current, however, it behaves like a capacitor, the capacity of which depends on the composition of the sample solution with which the vessel is filled. Since the composition of the filled solution changes during the titration, the resulting change in the alternating current resistance can be followed.
A basic measuring arrangement consists in connecting a capacitor, an induction coil and an ohmic resistor in series in an alternating current circuit. As a result of the phase shift between current and voltage, so-called reactive resistances result on the coil as on the capacitor, the size of which depends on the alternating current frequency and on the parameters of the coil or the capacitor. For each resonant circuit there is a frequency at which the reactances of the coil and capacitor compensate each other. Because of the only effective ohmic resistance there, the current intensity reaches a maximum value here. This is called the resonance case.
If the above-described measuring cell is switched into the AC circuit instead of the capacitor, the change in the solution composition caused by the titration will affect its capacity. The current strength flowing in the resonant circuit can now be used as a measured variable, but also, for example, that on the ohmic resistance R. omitted voltage drop. Overall, different variants of the measurement are used, but their aim is always to determine the alternating current resistance or its reciprocal value, the alternating current conductance of the titration point. Because of the electrolyte inside the cell, the AC conductance is made up of the aforementioned reactive component and an ohmic part, which is determined by the conductivity of the solution. This part, also known as the active component, changes in a characteristic way during the titration.
If the AC conductance of the measuring cell is determined and plotted against the logarithm of the specific conductivity of the electrolyte solution filled in the cell, a bell-shaped curve is obtained, which is also known as the HF characteristic curve. For this purpose, the frequency of the excitation voltage must be selected so that the conductance of the active component rx is large compared to the susceptibility cxso that the measured change in the AC conductance is practically caused by the change in the ohmic conductivity (active component method).
For the sake of completeness, it should be mentioned that it is also possible to change the reactive component cx depending on the course of the titration, which under certain conditions also allows an indication of the equivalence point. Instead of the capacitance measuring cells, inductance measuring cells are also used. In these, the inductance of a coil is influenced by an electrolyte solution arranged in its interior.
High-frequency titrations are used whenever the composition of an electrolyte solution interferes with the resistance measurement between immersed electrodes. Another important area of application is titrations in non-aqueous solvents.
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