Coulometric titration
Coulometric titration is an electrochemical method of investigation which can be used to determine thermodynamic data of various material systems. With the help of the Nernst equation mainly thermodynamic activities, partial and integral molar Gibbs energies (free enthalpy), enthalpies and entropies can be obtained from the measured voltages. The material to be investigated is introduced into a special cell as a cathode and loaded by various current or voltage profiles based on the desired data. Figure 1 shows the newly developed design of the CT [1].
During titration, the composition of the cathode material is varied in very small steps according to Faraday's law and the cell voltage is simultaneously recorded. The name of the method is explained by the analogy to classical titration techniques of chemistry.
To determine thermodynamic data, the equilibrium voltage of the cell is required, i.e. the voltage without current load. In order to simultaneously change the composition (current flow) and determine the equilibrium voltage (no current flow), intermittent methods such as GITT (Galvanostatic Intermittent Titration Technique) are used. This technique alternates between current pulses and subsequent relaxation periods to achieve equilibrium.
Due to the increased temperature level during the measurements (usually between 360 °C and 500 °C) diffusion processes are favored and the equilibrium is quickly reached. Due to the special design up to three cells per furnace can be measured, which allows a variation of the parameters.
With the experimental setup it is also possible to perform classical EMF measurements (electromotive force). Here, too, basically the equilibrium voltage of the cell is measured, but in contrast to coulometric titration, the cell temperature is varied instead of the electrode composition. Furthermore, a combination of the two individual methods is possible, which allows the most important thermodynamic data to be recorded in a shorter time.
A further advantage of CT is the fact that no samples with the target composition have to be prepared in advance (based on the active species). Manual preparation involves the risk of errors, which are reduced to a minimum by in situ sample preparation.
In principle, the possible temperature range is only limited by the melting points of the materials used. The lower limit of the range is determined by the electrolyte (typically molten salts) and the upper limit by the stability of the electrode and housing materials used. The technique can be applied by varying the electrolyte, even at lower temperatures, down to room temperature.