Hi – I’m Sherri Renberg from the global liquid analysis marketing group, and I’d like to thank the many liquid analysis experts who have contributed to this blog series. We hope you will enjoy these useful answers to some of the most frequently asked questions we get from users about pH measurement.
While some of the questions are basic, that’s why they’re valuable. pH is a measurement where it never hurts to go back to the fundamentals. We’ll cover a few questions in this blog, and more in future.
Q) What is the shelf life of a pH sensor?
A) pH glass electrodes must remain hydrated which is why all manufacturers ship pH sensors with a cap saturated in a liquid solution. After being on the shelf for some time, the liquid solution inside the sensor cap can go dry, which is the primary reason sensors go bad on the shelf. It’s a good idea to re-saturate the pH sensor cap with a 4-buffer about every (6) months that the sensor remains on the shelf to extend the shelf life of the probe. The best way to determine if the sensor is functioning accurately is to see if it calibrates properly using the two-point calibration method.
Q) What is the proper way to install a pH sensor?
A) Most manufacturers insert an air bubble inside their glass electrodes to allow for temperature and pressure changes. Without this, pH sensors could crack with large temperature or pressure swings. If a sensor is mounted horizontally, the air bubble inside the sensor can move to the tip of the sensor, which can cause poor readings because it can impede the transfer of hydrogen ions. Therefore, pH sensors should be mounted at least 10 degrees above horizontal to ensure correct measurement. Sensors can also be installed vertically.
Q) I have a pH loop and I’m getting a “low slope” error message. What does this mean?
A) If you are getting a “low slope” error message, there are a few possible causes:
• The sensor may be coated or dirty. Try cleaning the sensor and repeating the calibration.
• The glass is dry and needs to be rehydrated before calibration. To rehydrate the sensor, soak it in pH 4 buffer solution overnight. Theoretically, a brand new sensor’s slope should be 59.16mV when the sensor is set to auto-temperature compensate to 25oC, however, a new sensor could potentially have a slope as low as 55mV/pH without causing any problems. Note that the calibration is only as good as the chemicals are fresh. Make sure there are no air bubbles on the glass and that the sensor is left in the solution long enough to stabilize the reading.
• The glass is old and may need replacing.
Q) What affects the accuracy of a pH calibration?
A) The first thing to consider when trying to get an accurate pH measurement is the proper calibration of your equipment. Make sure that you take the appropriate time to calibrate your pH meter or analyzer with a quality standard buffer solution.
Room temperature, buffer temperature, and sample temperature all impact the calibration process. Try to simulate the actual environment the sensor will be operating in for the best calibration results.
As the pH sensor depends on its glass tip to make readings, the cleanliness and the quality of the glass can also impact your accuracy. Time, heat, and harsh chemicals gradually eat away at the glass surface, changing its properties and degrading the quality of the reading.
Q) What buffer calibration errors can occur when calibrating my pH sensor?
A) Buffer solutions have a stated pH value at 25°C (77°F), but when that value is 7 pH or above, the actual pH of the buffer will change with temperature. The values of the buffer solution at temperatures other than 25°C (77°F) are usually listed on the bottle. The pH value at the calibration temperature should be used or else errors in the slope and zero values, calculated by the calibration, will result. An alternative is to use the “buffer recognition” feature on modern pH analyzers, which automatically corrects the buffer value used by the analyzer for the temperature.
Another type of calibration error can result from not allowing enough time for the buffer calibration to complete. If the pH sensor is not given enough time to fully respond to the buffer solution, it can cause errors, especially in the case of a warm pH sensor not being given enough time to cool down to the temperature of the buffer solution. Current pH analyzers have a “buffer stabilization” feature, which prevents the analyzer from accepting a buffer pH reading that has not reached a prescribed level of stabilization.
This is just a start of some of the great questions users have sent us. We’ll share some more in a future blog. What kind of questions do you have about pH measurement?