The sensitivity of liquid sensor, ion-selective microelectrodes to changes in temperature and solution level

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Abstract
Current procedures for the manufacture of ionselective microelectrodes using liquid ion sensors result in two kinds of electrode, one with a short (10–100 μm) column of sensor at the tip and one with a long column (> 1 mm). We report here that the signal from a long-column microelectrode is very sensitive to small fluctuations in bath temperature and solution level when a recording is made above room temperature. This sensitivity is explained by the effects of temperature on the potentials developed at the interfaces between the ion sensor and the bordering aqueous solutions (the internal filling solution of the electrode and the test solution). If the temperature at the two interfaces is changed by different amounts, these interfacial potentials will also change by different amounts resulting in a change in the output of the electrode. Such an effect occurs for longcolumn electrodes because the column of sensor is usually not completely immersed in the heated test solution. Hence fluctuations in solution temperature and level will induce fluctuations in temperature along the column of sensor producing variations in the electrical signal from the electrode. In contrast, sensitivity to temperature and solution level is virtually absent in short-column microelectrodes since their column of sensor is fully immersed in the test solution and thereby at uniform temperature. In conclusion, in order to produce noise-free recordings of ion activities, the use of short-column microelectrodes is recommended for experiments performed above (or below) room temperature.