High-frequency titrations oscillometry

As already indicated conductimetric measurements are normally made with alternating current of frequency 103Hz, and this leads to the existence of capacitance as well as resistance in the conductivity cell. If the frequency of the current is increased further to 106 — 107 Hz, the capacitance effect becomes even more marked, and the normal conductivity meter is no longer suitable for measuring the conductance. 

The cell employed must be re-designed, and in a typical form the metal electrodes (which need not be platinum) encircle the outside of the glass container and are situated about 2.5 cm apart. Thus they are not in contact with the liquid which can be advantageous for dealing with corrosive materials. Various forms of apparatus suitable for use with such cells have been devised.18,19 

At these high frequencies, the retarding effect of the ion-atmosphere on the movement of a central ion is greatly decreased and conductance tends to be increased. The capacitance effect is related to the absorption of energy due to induced polarisation and the continuous re-alignment of electrically unsymmetrical molecules in the oscillating field. With electrolyte solutions of low dielectric constant, it is the conductance which is mainly affected, whilst in solutions of low conductance and high dielectric constant, the effect is mostly in relation to capacitance. 

Clearly for titration purposes, it is low-dielectric constant conducting solutions which will be important, and addition of a suitable reagent to such a solution permits the plotting of a titration curve from which the end point can be deduced as described in Section 13.7. It should be noted that in view of the enhanced conductance in the high-frequency field, the maximum concentration of reagents is much smaller than with normal conductimetric titrations, and the maximum concentration will depend on the frequency chosen. It is found that 

Coulometric analysis is an application of Faraday s First Law of Electrolysis which may be expressed in the form that the extent of chemical reaction at an electrode is directly proportional to the quantity of electricity passing through the electrode. For each mole of chemical change at an electrode (96487 x n) coulombs are required i.e. the Faraday constant multiplied by the number of electrons involved in the electrode reaction. The weight of substance produced or consumed in an electrolysis involving Q coulombs is therefore given by the expression 

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Oszillometrie/ oszillometrische Titration/ Hochfrequenztitration oscillometry, high-frequency titration... 

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