The pH glass sensor cannot measure basic buffer values above pH 10

The pH glass sensor cannot accurately measure basic buffer values above pH 10. What are the reasons?

Possible reasons:

The pH glass sensor might be incompatible with the sample solution. This depends whether the sample is dissolved in aqueous or non-aqueous media and the corresponding pH glass sensor with aqueous or non-aqueous electrolyte solution is used accordingly. The reference electrolyte might be contaminated and the pH sensitive glass membrane might be dehydrated. If the junction of the sensor is not completely covered with sample solution or the fill height of the reference electrolyte inside the sensor is lower than the sample solution level then exchange of ions between the sample and reference will occur that affects the pH measurement.

If calibrating with a pH 10 or higher buffer, it may have shifted down in pH due to CO₂ contamination.

Recommendations:

Calibrate the sensor with basic buffer pH-12 and check whether the sensor is working as expected. Use aqueous pH glass sensor for aqueous sample solutions (EG11) and the dedicated non- aqueous pH glass sensor for non- aqueous sample. If the reference electrolyte is contaminated, then drain it through the aperture and refill it with the new electrolyte approximately 1 cm below the aperture. Activate the glass membrane by placing the sensor in a tenfold diluted reference electrolyte solution. Calibration is imperative after reference electrolyte replacement. Dehydrated glass membranes of pH sensors dedicated for non-aqueous titrations can be reactivated by placing it for several hours in 0.1 mol/L HCl. Use a sufficiently high sample solution volume so that the diaphragm is completely covered and there is proper exchange of ions between the sample and the electrolyte. If the fill height of the reference electrolyte is below the sample solution level, replace the electrolyte with fresh one and make sure that sufficient volume is filled in.

Caustic pH buffers slowly lower in pH when exposed to the air. Your pH sensor may be working well, but the calibration buffer is no longer correct. Caustic buffers should be used immediately when poured into a beaker for testing and never reused. Collapsible bag carboy style buffer containers are also preferred, because as the buffer is drained from the bag, fresh air does not come in and introduce more CO₂ as buffer is removed like with a bottle.