Metrohm 781 pH/Ion Meter - Manual (Page 140)

8.6 pH electrode test
130
780/781 pH/Ion Meter, Manual
The following table lists the preset tolerances for the three types of elec-
trode:
excellent
electrode
good
electrode
passing
electrode
Electrode type: standard
Streaming potential dU [mV] ≤ 2.5 ≤ 3.0 ≤ 4.0
Total drift [mV] ≤ 2.0 ≤ 2.5 ≤ 3.0
Slope s [%] 96.5 ≤ s ≤ 101 96 ≤ s ≤ 102 95 ≤ s ≤ 103
Response time [s] ≤ 45 ≤ 50 ≤ 60
Offset potential U
off
[mV] -15 ≤ U
off
≤ 15
Electrode type: gel => all (reference-) electrodes filled with Idrolyte
Streaming potential dU [mV] ≤ 3.0 ≤ 3.5 ≤ 4.5
Total drift [mV] ≤ 2.5 ≤ 3.0 ≤ 4.0
Slope s [%] 96.5 ≤ s ≤ 101 96 ≤ s ≤ 102 95 ≤ s ≤ 103
Response time [s] ≤ 60 ≤ 75 ≤ 90
Offset potential U
off
[mV] -15 ≤ U
off
≤ 15
Electr. type: non-aqueous => all (reference-) electrodes filled with TEABr or LiCl in ethanol
Streaming potential dU [mV] ≤ 3 ≤ 4.5 ≤ 6
Total drift [mV] ≤ 5 ≤ 7 ≤ 9
Slope s [%] 88 ≤ s ≤ 120 80 ≤ s ≤ 130 70 ≤ s ≤ 140
Response time [s] ≤ 60 ≤ 75 ≤ 90
Offset potential U
off
[mV] -10 ≤ U
off
≤ 70
The tested pH electrode is given that evaluation for which it fulfills all the
criteria. If one of the tolerances for a usable (passing) electrode is not
observed then the electrode test will be concluded with the message
' electrode out of limits. ' in the report. If further test criteria are
also not fulfilled then the electrode test evaluation will be ' electrode
test not passed '. In addition a more specific error message will be
shown on the display and will also be included in the report (see follow-
ing section).
TIP !
Instead of the stored tolerances for most Metrohm electrodes you can
set your own values (see Section 6.2.7). This may be helpful for spe-
cial applications if, for example, other quality requirements are placed
on the pH electrode or work is carried out with a reference electrode
which displaces the offset potential U
off
beyond the required –15 to
+15 mV (standard and gel electrodes).