Use, Care and Maintenance of DO Electrode (Part 2)

The last article introduced the basic types of DO electrodes, preparations before use, and two-point calibration methods. This article will continue to introduce the performance testing, maintenance, and maintenance of DO electrodes.

4.Performance Test of DO Electrode

Each electrode has its own zero point and slope, and with the prolongation of use time, the electrolyte is gradually consumed, and the slope and zero point of the electrode will also change accordingly. And through the change of slope and zero point, we can infer the performance of the electrode.

#Slope judgment method:

After calibrating with air as the calibration medium, refer to the standard of air current in the polarographic dissolved oxygen electrode current signal table (see the table below) to judge whether the slope of the DO electrode is normal.

If it is in the warning or alarm range, replace the electrolyte or membrane of the electrode and then re-calibrate. If it is still in the warning or alarm range after calibration, the electrode needs to be returned to the factory for repair.

#Zero judgment method:

After calibrating with nitrogen with a purity of 99.995% as the calibration medium, refer to the zero point current standard in the polarographic dissolved oxygen electrode current signal table (see the table below) to judge whether the zero point of the DO electrode is normal.

(Polarographic dissolved oxygen electrode current signal meter)

5.The current value of the electrode in the air is abnormal

The current value of the electrode in the air refers to the current value of the electricity exposed to the air, which is generally expressed as an absolute value. Different types of DO electrodes have different current value ranges in the air. See the electrode instruction manual for details.

#The current value in the air is low, possible reasons and solutions:

1)The surface of the platinum cathode is covered with oxides

In this case, when the head of the inner electrode faces the light source and observes the cathode, it can be seen that the surface of the cathode appears black. You can use sandpaper with a grade of 1000 or more to lightly sand the head of the platinum wire for several times until the surface of the platinum wire is shiny. Do not over-sand as this will damage the inner electrode head.

2)The platinum cathode fails to contact the dissolved oxygen film

Check whether the dissolved oxygen membrane is screwed in place, if not, you must screw the membrane in place, and tighten the membrane protective cover until the black sealing ring is no longer visible.

Check the DOF membrane, if there are excessive protrusions, as shown in the figure below, so that the cathode cannot touch the membrane, the DOF must be replaced.

#The current value in the air is too high, the possible reasons and solutions:

Reason ①: Insufficient electrode polarization

Treatment method: Confirm whether the polarization time is consistent.

Reason ②: The electrode is fouled

To clean the electrode, deionized water should be used instead of ethanol-containing cleaning solution.

Reason ③: electrolyte aging

Change the electrolyte

Reason ④: Membrane aging or damage

Replace the membrane

Reason ⑤: The cable is damaged

Replace the cable, the transmitter should display a very low and stable current value when the electrode is not connected.

Reason ⑥: The transmitter is damaged

Replace the transmitter

6.Maintenance of DO Electrodes

During use, inaccurate or unstable measurement is most likely to occur due to membrane blockage, which is often caused by the attachment of tiny ions to the membrane surface. Such blockages are generally not easy to detect with the naked eye. For such pollution, the electrode can be removed and soaked in 3%~5% dilute hydrochloric acid for several hours before use.

When the electrode is not used for a long time, the protective cap should be covered and stored in a protective box.