3 addition method, Addition method – Metrohm 781 pH/Ion Meter User Manual
Page 153
9 Annex
780/781 pH/Ion Meter, Manual
143
3- and more point calibration
With at least 3 calibration solutions the calibration function is deter-
mined according to the principle of the smallest mean-square error. The
required calibration parameters Slope, E(0) and c(blank) are calculated
iteratively. If c(blank) is sufficiently small to have no measurable influ-
ence, what means that c(blank) would be smaller than approx. 0.2 % of
the smallest measured value, then it is set as zero and not shown. This
limit represents the highest possible resolution of ISE measured values
for a resolution of 0.05 mV for the measured potential. In such a case a
linear relationship is obtained between Ui and log(ci).
Variance: This will always be calculated and outputted when at least 4
calibration standards are used. If c(blank) = 0 then the var-
iance will also be calculated for 3 standards:
=
(
)
F
n
U
U
i
2
calculated
,i
i
−
−
∑
n: no. of meas. points
F: degree of freedom:
2 for c(blank) = 0
3 for c(blank) ≠ 0
9.2.3
Addition method
In addition measurements (standard or sample addition or subtraction)
a linear relationship between U and log(c) is generally assumed. This
means that c(blank) is not calculated but is set as zero and the com-
pensation curve (linear regression) is determined according to the prin-
ciple of the smallest mean-square error. The concentration of the ion to
be determined in the sample solution is then calculated from the initially
measured potential UA and the regression data as follows:
Slope
)
0
(
E
U
A
A
10
)
Ion
(
c
−
=
Please note that this result represents the primary result. The final result
is calculated by taking into account the calculation parameters (Sample
size, Vtotal and Factor; see also Section 6.5.4) and shown directly.
size
.
Smpl
V
Factor
)
Ion
(
c
)
Ion
(
c
total
A
⋅
⋅
=
If the parameter Smpl.size is set to '
OFF
' then cA(Ion) will simply be mul-
tiplied by Factor.