Orion Benchtop pH/ISE Meter Instruction Manual
Automatic Blank Correction
120
This blank may be a true reagent blank, traces of analyte ion in the
reagents, or it may be the “mud” value of the electrode. It could also be an
interferent in the reagents that becomes apparent at low levels of analyte
ion, or it could be any combination of these effects.
The expanded version of the Nernst equation traditionally used for blank
correction is as follows:
E = Eo + S x log(C + b) where b is the blank.
In a multi-point calibration a set of equations is generated and the
relationship between them evaluated.
For example the equations generated in a three point calibration would be
as follows:
E1 = Eo + S x log (C1 + b )
E2 = Eo + S x log (C2 + b )
E3 = Eo + S x log (C3 + b )
The meter evaluates the relationships between the three potentials E1, E2,
and E3 and the three concentrations C1, C2, and C3. If the relationships
indicate that blank correction is desired, a blank will automatically be
calculated, and the non-linearity will be corrected for in a Nernstian
manner. If the appropriate conditions are not met, the blank is set at zero
and each segment of the multipoint calibration is treated independently.
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.
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