MODEL | SECTION 11.0 |
| MAINTENANCE |
11.3.3 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 elec- trode 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 sus- pect electrode, place both sensors in a beaker containing buffer or a solution of potassium chloride. Connect the ref- erence 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
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 chlo- ride 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.
45