Boxes junction potential

Box 15-1 Systematic Error in Rainwater pH Measurement The Effect of Junction Potential... 

Junction potential. A junction potential exists at the porous plug near the bottom of the electrode in Figure 15-9. If the ionic composition of the analyte solution is different from that of the standard buffer, the junction potential will change even if the pH of the two solutions is the same (Box 15-1). This effect gives an uncertainty of at least —0.01 pH unit. 

A thermocouple is made by welding together at each end two wires made from different metals (Fig. 4.15). If one welded joint (called the hot junction) becomes hotter than the other joint (the cold junction), a small electrical potential develops between the joints. In IR spectroscopy, the cold junction is carefully screened in a protective box and kept at a constant temperature. The hot junction is exposed to the IR radiation, which increases the temperature of the junction. The potential difference generated in the wires is a function of the temperature difference between the junctions and, therefore, of the intensity of IR radiation falling on the hot junction. The response time of the thermocouple detector is slow thermocouples cannot be used as detectors for FTIR due to their slow response. 

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