First, try calibrating your pH Sensor. Please see the Specifications and User Guide for calibration instructions.

The accuracy and response times of pH Sensors change as they age. While a well-maintained pH Sensor will work well for many years, it will need to be calibrated more frequently as it gets older.

If your pH Sensor is very old (5+ years old), it may need to be calibrated before each use. At some point the probe will no longer read accurately, whether or not it has been calibrated.

Here are some points to consider:
1. An important indicator of the viability of a pH Sensor is the difference in potential (delta V) between two buffer solutions when calibrating the probe. For example, we use pH 4 and pH 7 buffer solutions to calibrate pH Sensors. The potential reading in a pH 4 buffer solution should be very close to 2.50 V. The potential reading in a pH 7 buffer solution should be very close to 1.75 V. More importantly, the delta V between pH 4 and pH 7 should be 0.75 V.

2. As a pH Sensor ages, the delta V during calibration lessens. A general (but not hard and fast) rule is that when a calibration using pH 4 and pH 7 solutions produces a delta V of less than 0.5 V, the accuracy of the probe will be poor and probably only suitable for strong acid-strong base titrations.

How do I save a custom calibration to my pH sensor?
The pH electrode I have been using no longer seems to work or is slow to respond. Is there anything I can do to fix it?
pH Sensor Troubleshooting and FAQs