When testing a pH Sensor (pH Sensor (PH-BTA), Tris-Compatible Flat pH Sensor (FPH-BTA), Go Direct® pH Sensor (GDX-PH)), it is best to place it into a known buffer solution. This allows you to see if the sensor is reading correctly (e.g., in a buffer pH 7, is the sensor reading close to pH 7). Do not place your sensor into distilled water to check for readings–distilled water can have a pH reading anywhere between 5.5 and 7.0, due to variable amounts of carbon dioxide dissolved from the atmosphere. Furthermore, due to a lack of ions, the pH values reported with the sensor in distilled water will be erratic.

If your pH Sensor is reading slightly off of the known buffer pH (e.g., reads 6.7 in a buffer 7), you may simply need to calibrate the sensor. You can calibrate the sensor in two buffer solutions for two calibration points. If you do not remember or know how to perform a calibration, refer to How do I calibrate my sensor?

If your readings are off by several pH values, the pH readings do not change when moved from one buffer solution to another different buffer, or the sensors response seems extremely slow, the problem may be more serious. Sometimes a method called “shocking” is used to revive pH electrodes. To shock your pH Sensor, perform the following steps:

1) Let the pH Electrode soak for 4-8 hours in an HCl solution between 0.1 and 0.5 M.
2) While the probe is soaking, prepare a batch of new storage solution. The recipe is: 10 grams of solid KCl dissolved in 100 mL of pH 4 buffer. Any pH 4 buffer mixture works well, but use only KCl for the salt – other salts do not work. (Note: Use this solution to store your pH probe when it is not in use.)
3) After the HCl soak, rinse off the probe and place it in 50-75 mL of the storage solution you made in Step 2. Soak the probe tip in this storage solution for 30-60 minutes.
4) Rinse the electrode and give it another try.

We have found that many old electrodes can be revived in this manner. Generally, storing them in a storage solution (buffer pH 4 + KCl) will keep them in good condition.

If you are using a pH electrode and none of this helps, you may need to replace it.

See also:
pH Sensor Troubleshooting and FAQs
What is the best way to clean the glass bulb of the pH electrode?
What do I do about mold growth in my bottle of pH electrode storage solution?