Isopotential Value - Thermo Scientific Orion Versa Star Pro User Manual

Thermo Scientific

Isopotential Value

Versa Star Pro meters with pH, pH/ISE and pH/LogR modules offer the option for operators to
enter an isopotential value for pH and ISE measurements. The slope of all electrodes changes
with temperature, based on the Nernst equation. The slopes at the different temperatures
intersect at the isopotential point, which is the point at which the electrode potential does not
vary with temperature. For most pH electrodes, this point is 7.000. For ion selective electrodes
(ISE), if the isopotential point is known or can be determined experimentally, entering this value
makes temperature compensation for an ISE possible when a one or two point calibration is
performed.
Determining an Isopotential Value for an Ion Selective Electrode
1. Prepare several standards with concentrations spanning the measuring range of the ion
selective electrode.
2. Measure the mV value of the standards at room temperature, about 20 °C to 25 °C.
3. Measure the mV value of each standard at 75 °C.
4. Measure the mV value of each standard at 10 °C.
5. On semi-logarithmic graph paper, plot the concentration values on the log axis versus the
mV values on the linear axis.
6. The lines will intersect at the isopotential value. Read the concentration from the graph for
this point from the log axis, this is the isopotential value.
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Appendix A Channel Specific Settings and Features
Orion Versa Star Pro Meter User Manual
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