WPI EVOM3, Instruction Manual

Page: 32 / 52

28 World Precision Instruments
A 10–15 minute immersion of the tips is recommended before each daily use. (These
treatments may be reduced to once per week as experience guides). DMEM deposits
cause resistance measurements to become unstable and the coating appears as
a dark crust on the electrode surfaces. See "Appendix A: KCL Testing Mixtures" on
page 36. A properly chlorided silver surface will appear to be a dark silvery grey
coloration.
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CAUTION : Too much chloriding can also lead to instabilities. A short 5 minute
soak in ammonia water (3%) will convert the AgCl back to Ag. After a DI water
rinse, repeat the chloriding. Follow this with an electrode test.
Storing the Electrodes
After a daily use, wash and store the electrodes dry.
Testing the STX2-PLUS Electrodes
If another electrode is being used, refer to that electrode's manual.
To make sure the electrode is operating properly, a resistance test in KCl is used. This
test demonstrates electrode functionality.
40 mM KCl 80 mM KCl 160 mM KCl
38-66 Ω 27–44 Ω 12–20 Ω
At the lower molarities (10–20 mM), a DMEM coated electrode will begin to show
instabilities at the higher concentrations. If the resistance readings in the KCl mixtures
are over 10% out of range, then the electrode may need cleaning. See "Cleaning/
Maintaining the STX2-PLUS Electrodes" on page 26. An Excel scatter plot of the
molarity vs. resistance (as a power) should show an R
2
value > 0.99x.
NOTE : These resistance readings can vary due to fluid volume and electrode depth.
These values are a reference. An Excel scatter plot of the molarity vs. resistance
(as a power) should show an R
2
value > 0.99x. If the R
2
value is acceptable, then the
electrode can be used even if it is slightly out of range. If the resistance readings in the
KCl mixtures are out of range, then the electrode may need cleaning. See "Cleaning/
Maintaining the STX2-PLUS Electrodes" on page 26. Electrodes which are not being
properly maintained may begin to show instabilities indicating the need for cleaning.
Regular maintenance of the electrode is strongly recommended for longevity.