What is calibration and why do you need to calibrate a pH electrode?
22 April 2016
22 April 2016
Calibration can sound like a daunting technical term, but when you look closer this is a simple principle to understand.
We’ve taken a look at what calibration is, and why it’s important to calibrate your pH electrode.Everything you need to calibrate – pH electrode, meter and pH buffers
Imagine there is a man standing near to you, and I’ve asked you to guess how tall he is.
You look at him, then make your guess – you might be close, but you will probably not get the correct answer.
Now we stand him next to a height chart, that shows the exact height every cm. Your guess this time will be much more accurate because you have something to compare it to.
Calibration works in just the same way – it gives the instrument a definite comparison point.
When you calibrate a pH electrode you expose it to a pH buffer that you know has a definite pH value (this is like the height chart that has a definite cm value)
Later when you put the electrode into an unknown solution it remembers this and can use it as a comparison.
The pH electrode includes delicate layers which are sensitive to ions, this is how the readings are taken.By calibrating the electrode you can make sure that the reaction it has to these ions is correct.
After a calibration you can carry on measuring unknown samples, but over time the electrode’s reaction will have deteriorated. This is why you need to re-calibrate regularly.
Most people will calibrate their pH electrode at the beginning of each day, or at the start of each session to keep their results accurate.
Some meters will have a prompt to tell you when you need to re-calibrate – the TRUEscience meter has a clear on-screen message to tell you, and you can even alter what frequency this is set to.
Depending on your meter, you may be able to do a calibration with 1, 2, 3 or even more points.
Generally a 3 point calibration is sufficient for good accuracy results.
The number of points refers to the number of pH buffers you are calibrating with.pH buffers are used to calibrate the pH electrode – these are liquids that have a known and fixed pH value
The meter should confirm when a calibration is successful – check the manual of your meter to see exactly how this looks for your meter. It may be through a message or icon on screen.
On a TRUEscience meter the pop up message at the end of a calibration shows you it has ended, and tells you how accurate it was. You can also see on the main meter screen a green tick and “calibrated” message, so you know that the electrode has a current, valid calibration.
If a calibration was not successful, try to find out why, and calibrate again. Until you get a successful calibration your results will not be accurate.
If you want to perform an extra check that calibration was successful, you can do a QC check.
This is where you measure another pH buffer (not one you calibrated with) as an additional check that the electrode is responding correctly. For example, if you calibrated with pH 4, 7 and 10 buffers, you may do a QC check with a pH 6 buffer.
This is a great way to validate the calibration and ensure the highest levels of accuracy.
Some meters, such as the TRUEscience meter, have this feature built in with the ability to save the QC check result in the meter for future reference.
Calibration sets up set comparison points
We’ve seen that calibration is just setting your electrode against known standards as a comparison point. When you then measure an unknown sample, the electrode has something to compare it to. For pH electrodes, our known standards are pH buffers.
As the electrode goes on testing, the calibration will expire, you then need to calibrate again, aka recalibrate. Usually this is done daily, but some meters will prompt you when you need to recalibrate.
Make sure calibrations are a success
Check after calibrating that the calibration was successful and saved. You should be able to tell through the display on your pH meter. For extra confirmation do a QC check with a different buffer.
Browse our other posts or contact us if you have any other questions about calibration or pH testing