Metrohm 781 pH/Ion Meter - Manual (Page 150)

9.2 Evaluation
140
780/781 pH/Ion Meter, Manual
The calculation of the calibration data depends on the number of pH
buffers used:
1-Point calibration
Slope:
The theoretical slope value of 100.0% is used.
pH(0) =
N
1
1 U Slope
U
pH
⋅ −
−
2-Point calibration
Slope =
( )
( )
N 1 2
1 2
U
1
pH pH
U U
−
⋅
−
−
pH(0) =
N
U Slope
U
pH
⋅ −
− ;pH , U = mean value
3- and more point calibration
With at least 3 calibration buffers a compensation curve according to
the principles of the smallest mean-square error (linear regression) will
be calculated.
Slope =
( ) ( )
( )
N
i
2
i
i
i i
U
1
pH pH
U U pH pH
−
⋅
−
− ⋅ −
∑
∑
pH(0) =
N
U Slope
U
pH
⋅ −
−
Variance =
( )
2 n
U U
i
2
calculated ,i i
−
−
∑
;n = no. of measuring points