Thermo Scientific VSTAR-ISE, Справочное руководство

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Thermo Scientific Orion VERSA STAR pH/ISE/Temperature Module
When all three of the following conditions are met, blank correction is invoked.
1. The concentration of the first standard is zero, or the slope of the electrode between the first and second standards
is less than the slope between the second and third.
2. Potential differences between points are significant. For example, E3 - E1 > 10 mV
3. The blank correction algorithm converges at reasonable blank and slope values. Conditions in steps 1 and 2 prevent
failure to converge in most situations. However, the slope will attain any value necessary for convergence to a
calculated blank value of 3 x C3.
When blank correction is implemented, the slope value for the electrode may be outside the range of values normally
considered acceptable during an ordinary calibration. For best results, calibration standards should be close in range to the
expected sample concentrations and should bracket the expected sample concentration. When conditions 1, 2 and 3 are not
met, the calibration data is handled by the multipoint calibration method described earlier.
In calibrations with more than three points, a combination of methods is utilized. Automatic blank correction is used if the
lowest three points satisfy the criteria and multipoint calibration is used for the other points. The slope displayed after
calibration is an average of the slope calculated in the automatic blank correction algorithm and the slope(s) for each
additional segment.
Determining an Isopotential Value
1. Prepare several standards with concentration ranges over the measuring range of the electrode.
2. Measure the millivolt value of the standards at room temperature, about 20 °C to 25 °C.
3. Measure the millivolt value of each standard at 75 °C.
4. Measure the millivolt value of each standard at 10 °C.
5. On semi-logarithmic graph paper, plot the concentration values on the log axis versus the millivolt 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.