Using a digital pH meter for pH measurement is the ideal solution for reliable testing in a range of industries and environments. Calibration is one form of maintenance that helps to keep pH electrodes continually accurate, which is particularly important in laboratory environments where high precision is key.
Why is calibration important?
Calibration is a fundamental part of pH meter and electrode maintenance. Drift can occur in pH readings over time and through use due to electrode ageing and chemical exposure.
Calibrating takes account of these changes in the pH electrode and realigns the electrode and the pH meter using known reference standards which are referred to as pH buffer solution. Regular calibration is important for continued accuracy and repeatability.
How often should you calibrate?
How often you will need to calibrate depends on how often you use your meter, what substance you are testing and how well the electrode is cleaned and stored. For example, if an electrode was being used to test clean water once a day then monthly calibration may be suitable.
An electrode used to test battery acid 20 times a day would require more frequent calibration to remain accurate. Another factor in determining calibration frequency is the level of accuracy required.
Many operators only need to measure pH to 0.1 of a unit and will not need to calibrate as frequently. An operator who requires 3 decimal point accuracy may need to calibrate daily or before each sample is tested.
Using calibration solution
pH calibration solution, also referred to as buffer solution, is supplied ready to use in a range of pH values. The most commonly used solutions are pH 7 and pH 4, as 7 is the halfway point on the pH scale.
The majority of pH measurements are performed in the acid range, making pH 4 the most suitable second buffer in many cases. Solutions in other pH values are available for more specialist requirements or when measuring samples in the alkaline range, where it is common to choose a pH 10 buffer in place of pH 4.
Calibrating at pH 7 and pH 4 or pH 7 and pH 10 is known as a two point calibration and is sufficient for the majority of pH applications and accuracy requirements.
Certain professional meters such as the HI-6221 Advanced pH Meter may offer up to 5 point calibration to achieve maximum accuracy over a particular required pH range. It is important that you use fresh solutions each time, and that you check to ensure your solutions are uncontaminated and in date before use.
The calibration process
Preparing for calibration
If fitted, remove the protective cover from the electrode bulb and rinse using deionised (DI) water to remove any salt crystals.
After rinsing, shake gently to remove excess water. It is important to avoid wiping the sensing tip/bulb as this can cause static electricity build-up which will affect the results of testing. If your electrode is refillable, check the level of the reference solution and refill if required.
Tip: The formation of salt crystals on a pH electrode is normal, and is typically caused when storage solution evaporates during transport or if the electrode is not used for a long period of time.
How to calibrate your pH electrode with your pH meter
Modern pH meters, such as the Hanna HI-6221 advanced pH and ORP benchtop meter, will automatically recognise the value of the pH buffer solution the electrode has been placed in and will provide on-screen instructions once they are in calibration mode.
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Select calibration mode on your pH meter
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Pour out an amount of pH solution into a container sufficient to immerse the measurement bulb and reference junction of your electrode. If using a sachet it may be easier to simply open the sachet and insert the electrode.
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Place the electrode into the first pH buffer as indicated on the pH meter screen.
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Wait for the reading to stabilise, then place the electrode within the second pH buffer solution when prompted.
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Wait for a stable reading and finalise calibration.
- Rinse the electrode with DI water to remove any residual buffer solution and it is ready for use.
For Hanna meters with GLP data
Many Hanna pH meters include a Good Laboratory Practice (GLP) screen as a method to verify that the electrode’s calibration data is suitable for use in measurement. This data includes the slope and offset of the electrode, using the Nernst equation.
The slope of your electrode should be between 85-105% (51-62 mV/pH unit) with an offset of ±30mV. If your electrode is reading as outside of this range, cleaning, reconditioning and recalibrating is the best place to start. If the problem persists, the electrode may need replacing.
How to use storage solution
If your electrode is dry, you can rehydrate it by soaking the bulb and junction in electrode storage solution for an hour or longer. Storing the electrode in storage solution helps to improve lifespan and performance.
Storing electrodes in other solutions, such as water, can lead to microbial growth. Storage solution prevents the reference electrolyte from leaching out and keeps the junction clear.
If you don’t have any storage solution, pH 4 or 7 buffers can be a suitable temporary option.
How to clean and store your pH electrode
Soak your electrode in a suitable cleaning solution for at least 15 minutes pre-calibration to dissolve any contaminants.
Use a cleaning solution designed for what you are testing. For example, if you are testing soil, choose a solution that is formulated to remove organic dirt and stains.
After using cleaning solution, rinse your electrode thoroughly in distilled or deionised water, then soak in storage solution for at least 2-3 hours before calibration. It is recommended that you store your electrode overnight where possible.
Buy pH calibration solution from Hanna Instruments UK
Hanna Instruments stocks a wide selection of pH calibration solutions to suit different industries and specialisations. Browse our full range of pH calibration buffers online here.
Get in contact with us via email or at 01525 850 855 to speak to our experts about the different meters, testers and solutions available to suit your testing requirements.
