Ionode IJ pH V5.1 - Manual (Page 13)

Ionode IJ Instruction Manual 13
Offset
Each pH electrode has a small offset which
varies from electrode to electrode. It is due to
an “asymmetry potential” across the membrane.
It is usually within ±0.2pH for a new electrode,
and requires correction by calibration in a buffer
at or near pH 7. The offset increases as the
electrode ages and is also an indication of the
condition
of the electrode, especially
cleanliness. A clean IJ44 should read within
±30mV (±0.5 pH) in a pH 7 buffer.
Slope
The SLOPE is a function of temperature (T) and
contains the conversion of the natural logarithm
to the base ten logarithm. It is defined as the
number of mV per unit of pH. It is manually or
automatically (ATC) compensated in pH
meters.
Table 1: Theoretical (Nernstian) Slope
T
(
o
C)
SLOPE(T)
(mV)
T
(
o
C)
SLOPE(T)
(mV)
0 54.197 30 60.149
5 55.189 35 61.141
10 56.181 38 61.737
15 57.173 40 62.133
20 58.165 45 63.126
25 59.157 50 64.118
A new electrode should have a slope between
95-102% of theoretical. Sub-Nernstian slope is
corrected by calibration with a second buffer.
As the pH membrane ages, the slope
decreases. This affects the accuracy. For best
performance, the slope should be >95%. Some
meters calculate and display % slope after