Maintenance
Instruction Manual
February 2014
LIQ_MAN_6081-P
34
Maintenance
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 voltmeter and measure the potential difference. If the suspect electrode
is good, the difference should be no more than about 20 mV. Refer to Figure 11-1. A poisoned
reference electrode usually requires replacement.
Figure 11-1. Checking the Potential of the Reference Electrode
Refer to the wiring diagram(s) for the sensors to identify the reference leads.
A laboratory silver/silver chloride reference electrode can be used in place of the second sensor.
All Rosemount Analytical 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.
11.3.4 Rejuvenating Reference Electrodes
Occasionally, a poisoned or plugged reference electrode can be reconditioned. Although the
electrode seldom recovers completely, the procedure might extend the life of the sensor by a few
weeks.
1. Clean the sensor as thoroughly as possible.
2. Soak the sensor for several hours in a hot (NOT BOILING) 3% potassium chloride solution.
Prepare the solution by dissolving 3 g of potassium chloride in 100 mL of water.
3. Soak the sensor in pH 4 buffer at room temperature overnight.
4. Calibrate the sensor in buffers and retest it in the process liquid.
11.4
ORP Sensor Maintenance
11.4.1 Frequency of Cleaning
The frequency at which an ORP sensor should be inspected and cleaned can be determined only
by experience. If the process liquid coats or fouls the sensor, frequent cleaning may be necessary.
If the process does not contain a high level of suspended solids, the need for regular cleaning will
be less.
11.4.2 Cleaning Procedures
The platinum electrode is easily cleaned by using a tissue to rub the metal surface with a paste of
