Thermo Scientific Orion DUAL STAR User Manual page 105

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 interference 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 = E + S * log (C + b)
o
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:
E = E + S * log (C
1 o
E = E + S x log (C
2 o
E = E + S x log (C
3 o
The meter evaluates the relationships between the three potentials E
and the three concentrations C
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,
E - E > 10 mV
3 1
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 C
Thermo Scientific Orion DUAL STAR
where b is the blank
+ b)
1
+ b)
2
+ b)
3
, C and C
1 2
pH/ISE Meter User Guide
™
. If the relationships indicate that blank
3
.
3
Advanced Meter Features
, E and E
1 2 3