Cleaning Procedures - Orp Sensors; Checking The Reference Electrode - Emerson Rosemount 398R Reference Manual

Maintenance
February 2019
6.4

Cleaning Procedures - ORP Sensors

Clean platinum ORP electrodes by using a tissue to rub the metal surface with a paste of baking
soda (sodium bicarbonate). A clean platinum electrode is bright and shiny.
6.5

Checking the Reference Electrode

Some processes contain substances, for example, sulfides, that poison the reference electrode.
Poisoning alters the electrode potential. For example, sulfide poisoning converts the reference
electrode from a silver/silver chloride electrode into a silver/silver sulfide electrode, causing a
shift in potential of several hundred millivolts.
A good way to check for poisoning is to compare the voltage of the reference electrode with a
silver/silver chloride electrode that is known to be good. The reference electrode from a new
sensor is the best choice. To check the suspect electrode, place both sensors in a beaker
containing buffer or a solution of potassium chloride. Connect the reference leads to a volt
meter and measure the potential difference. If the suspect electrode is good, the difference
should be no more than about 20 mV.
Refer to Figure 6-3. A poisoned reference electrode usually requires replacement. A laboratory
silver/silver chloride reference electrode can be used in place of the second sensor. All Rosemount
pH sensors have a silver/silver chloride reference, and most sensors use gelled saturated
potassium chloride for the fill. The potentials of a good sensor reference electrode and a saturated
silver/ silver chloride laboratory electrode will agree within about 20 mV.
Figure 6-3: Checking the Potential of the Reference Electrode
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Reference Manual
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Maintenance