The sensitivity of a pH electrode diminishes gradually up to a point that it needs to be replaced. The following procedure allows you to test your electrode to see if it has passed the threshold for producing acceptable results. In general, if the asymmetry potential (AP) of an electrode is smaller than ±40 mV, and/or its efficiency is above 85%, it is still able to produce acceptable readings.

For this test procedure, 4.01 pH and 7.00 pH buffer solutions are required. It is assumed that the pH meter used in this process has recently been calibrated.

1. First, rinse the electrodes with DI water. This rinsing process cleanses the electrode from any impurity solutions on the electrode. These impurities can be solutions from the prior uses, the solution in which the electrode has been stored in, or the process medium.
2. Dry the electrode from any rinse water by gentle touch onto a soft tissue (such as a KimWipe).
   Important Note: Rubbing the electrode on the tissue creates static charge build ups in the electrode, which can reduce the accuracy of the electrode.
3. Set the pH/mV switch on the pH meter to the mV position
4. The pH meter has to be set to zero. For this purpose, connect a precision mV generator with 0 mV input to the pH meter, and adjust the standardize/zero control on the pH meter such that 0.0 mV reading is attained.
5. Remove the precision mV generator and connect the pH electrode.
6. Place the electrode along with its automatic temperature compensator (ATC) instantaneously in a 7.00 pH buffer solution. A thermometer can be used instead of the ATC, if the electrode doesn’t include one.
7. Wait for 30 seconds for the electrode – ATC/thermometer pair to reach thermal equilibrium with the buffer solution. If a thermometer is used, adjust the dial on the meter to match with the reading.
8. Record the mV reading and its polarity. The mV reading is the AP of the electrode. The smaller this number is, the more accurate the electrode will function; i.e. the voltage leakage of the electrode will be smaller.
9. Repeat steps 1 and 2.
10. Repeat steps 3 through 5 with a 4.01 buffer solution.
11. Calculate the difference between the reading for 7.00 and 4.01 pH and buffer solutions. The slope of the electrode is this number divided by 1.769 mV.

Notes:

• In order to increase the precision of the calibration, fresh pH buffer solutions should be used.
• If the pH meter/electrode pair is to be used to measure pH values above 7.00, instead of the 4.01 pH standard solution, a 10.00 pH buffer standard solution can be employed. However, due to the reduced stability of high pH solutions,
  the 4.01 pH buffer solution is more preferred.
• It is assumed that the ATC has already been calibrated and has an accurate precision.
• In order to get the most accurate characterization, it is recommended that this procedure be performed at room temperature, 25ºC. Otherwise, the pH should be adjusted based on the temperature.

For a list of pH meters in Mazian Lab Equip‘s inventory, please check http://mazianlab.com/cats/PH-Meter-Conductivity-Meters.htm