Hanna Instruments HI 8711 - Manual (Page 36)

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Oxidizing pre-treatment: immerse the electrode for some minutes in
HI 7092 solution.
If no pre-treatment is performed, the electrode will have long response
times.
If working with refillable electrodes, always check the internal electrolyte
level and refillwith HI 7071 solution, if necessary (the level must be at
least 2.5 cm below the filling hole).
If measurements are taken in solutions containing sulfides or proteins,
the cleaning of the electrode junction must be performed (see "Cleaning
Procedure" section for details).
To check the correct functioning of the ORP electrode, immerse it into
HI 7020 test solution and verify that the reading is within 200 and
275 mV.
After the test, rinse the electrode thoroughly with water and proceed
with the oxidizing or reducing pre-treatment before taking any
measurement.
When not in use, the electrode tip should be kept moist and far from
any type of mechanical stress which might cause damage. For this
reason, it is reccommended to store the electrode with a few drops of
HI 70300 storage solution in the supplied protective cap.