A how-to-guide to calibrating and maintaining your pH sensor
Kajal Jaiswal, courtesy of Mettler Toledo | November 10, 2023The measurement of pH is an important aspect of many scientific, medical and industrial applications. The pH sensor is a sensitive device that can drift from its original calibration, leading to inaccurate pH measurements. Thus, regular calibration is essential to ensure that the pH measurements obtained are accurate and reliable. By performing regular calibration, the sensor can be adjusted to the correct reference value.
Comprehensive guide to pH sensor calibration
Preparation
Before starting the calibration process, ensure that the pH meter is turned on and functioning correctly.
The pH meter has different buffer groups, which should be selected based on the range of pH values being measured. The buffer solution should have a pH value within the range of the sample to be measured.
If the pH meter does not have automatic temperature capture, ensure that the temperature of the buffer solutions is entered correctly.
Next, clean the beaker thoroughly to prevent contamination from previous solutions. Ensure that enough of each buffer solution is poured into the beaker to cover the electrode completely during calibration.
Before calibration, rinse the electrode with distilled or deionized water to remove any contaminants. For refillable electrodes, ensure that the electrolyte filling hole is open.
Calibration
Immerse the electrode into the first buffer solution and start the calibration on the pH meter.
Start with the lowest pH value. Wait until the pH meter reading stabilizes to determine the endpoint. This usually takes around 40 seconds.
Once the measurement has reached the endpoint, take the electrode out of the buffer solution, and rinse it with distilled or deionized water. This ensures that no buffer solution remains on the electrode surface.
To add calibration points, repeat the previous three steps with the next buffer solution. Once all the required calibration points have been added, end the calibration process on the pH meter.
Source: Mettler Toledo
Evaluation
After calibrating a pH sensor, it is crucial to evaluate the calibration results. Check if the pH values match the known pH values of the buffer solutions. If the pH values do not match the known values, the calibration may be inaccurate. If the calibration results are acceptable, save the calibration on the pH meter. This ensures that the pH meter uses the correct calibration when measuring pH values in the future.
By following this methodical approach, the pH sensor is calibrated correctly to obtain accurate and reliable results. However, accurate and reliable pH measurements can only be guaranteed if pH sensors are properly stored and maintained.
Best practices for properly cleaning pH sensors
Maintenance
After every measurement, thoroughly clean the sensor with deionized water to ensure that any residual chemicals or substances do not affect the accuracy of future readings.
Rinse the sensor probe and junction thoroughly with deionized water to remove any leftover samples. Use solvents such as ethanol or acetone to clean the sensor surface from any accumulated organic matter.
Finally, rinse the sensor again with deionized water to remove any remaining traces of solvent and facilitate the rehydration of the sensor membrane.
It is essential to avoid rubbing the sensor surface as this can damage the sensor membrane and affect the accuracy of future measurements. Instead, gently blot dry the excess water using tissue paper.
Periodically recondition the sensor in 0.1 M hydrochloric acid based on the sensor performance to remove any buildup of contaminants on the electrode surface. The frequency of reconditioning depends on the number of samples analyzed per day and the age of the sensor. Aging sensors may require more frequent conditioning. The sensor must be re-calibrated after reconditioning.
Ensure that the electrolyte level is above the sample level to prevent the influx of samples into the electrode compartment. Care must be taken to refill the correct electrolyte, i.e., 3M KCl.
Storage
Store the sensor in the wetting cap filled with InLab storage solution when not in use to keep it moist and prevent drying. Never store the sensor dry or in distilled water, as this may cause the membrane to dry out or become damaged affecting the pH-sensitive glass membrane. The glass membranes are delicate and should be handled with care.
By following the steps mentioned above, the performance of the pH sensor is maintained, and the lifetime of the sensor can be prolonged.
In conclusion, calibration helps to prevent costly errors and downtime and ensures compliance with regulatory standards. Accurate calibration of pH measurements is crucial for ensuring their reliability in critical applications. As pH measurements are critical in various scientific, medical and industrial fields, it is imperative to give priority to proper calibration and maintenance of pH sensors to guarantee precise and trustworthy pH measurements.