Have you ever wondered what is going on when you calibrate a pH system, or what to do when something doesn’t work as it should? Many labs often resort to external calibration of their pH meters to gain confidence in their measurements, but is this necessary? In this article, we will break down some of the myths surrounding calibration, validation, and verification. Removing this confusion should help you to make informed choices in your lab and enable you to have greater confidence in your measurements.
"Verification is the measurement of a standard that does not form part of the calibration set in order to ensure that the calibration is accurate and the standard reads within a given tolerance of its certified value."
How Does a pH System Work?
A pH measuring system consists of a meter, an electrode, a temperature sensor, and various calibration buffers. For high confidence in the accuracy of a pH measurement, the entire system must be working well together. The electrode picks up a signal (in mV) from the sample based on the activity of the hydrogen ion. This signal is passed down the cable of the electrode and is interpreted as a pH value based on the last calibration that was performed on the meter. The meter itself is calibrated using high purity, known pH value buffers which allow the meter to assign a mV value to specific buffer values. The measurements are heavily temperature dependant and as such a temperature measurement is always taken with a pH measurement.
Calibration, Validation, and Verification - What Do They Mean?
Calibration involves an adjustment to the pH value that is assigned to a particular mV value. Accuracy is assured by using fresh, high-quality standard pH buffers that are certified traceable to the regional or national metrology institute of choice. Examples include the National Institute of Standards and Technology (NIST) in the United States or the Physikalisch-Technische Bundesanstalt (PTB) in Germany, as well as many others around the globe.
Validation is the process of determining if a system is capable of performing as per specified criteria. It ensures all functional parts of the meter are working within specification although no adjustments are made. A validation usually follows a defined protocol provided by the manufacturer, which covers aspects such as the products used, the application that they will be used for, and the environment an instrument will operate in. Other checks include whether temperature measurements are within a given tolerance, as well as voltage checks and meter performance at calibration.
It is a way of verifying and providing confidence that all is as it should be. A reading is taken for a buffer that is near the pH level of interest, or which falls within the calibration range, but is not one of the calibration buffers. If the verification reading is within the expected range (e.g., +/- 0.1 pH or 0.05 pH), we can have confidence in the entire measuring system.
Can I Perform These Processes Myself in My Lab?
All of the processes described in this article can be performed by the user. It makes sense to do them in-house since you can take action quickly and it will improve your understanding of your pH system and how it is used.
For support with carrying out procedures in your lab, or to access full documentation, please contact your local Thermo Fisher Scientific representative or our European Technical Support team at: techsupport.labequipment.eu@thermofisher.com
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