pH is a useful parameter which is used across all industries for quality control, environmental monitoring, lab work and much more.
There are 10 key steps which should be followed to ensure pH measurements using a meter are accurate.
Why do I need to calibrate?
Before measurements are taken the electrode or measuring sensor should be calibrated with pH buffers.
pH buffers are liquids which have a set and constant pH. They can be bought as ready to use liquids in bottles or as single use sachets, or in powder form which must be made up with deionised water in a volumetric flask.
Exposing the electrode to pH buffers is effectively like showing the electrode what pH 4 “looks like”, or what pH 7 “looks like”. It will then use this as a reference point later when measuring unknown samples.
What are the main steps in pH measurement?
The steps below outline the general principles, but you should check the manual of your meter also for specific requirements.
1. Put the meter into calibration mode.
This will ensure the readings are stored and used as a reference point when you are measuring samples.
2. Decide on the calibration type and collect the buffers.
Some meters can perform a 2 point calibration, 3 point calibration or more. The more points in a calibration the more reference points the meter has to look back to later. A 2 or 3 point calibration is usually recommended, though check what your meter is capable of – some will only have the option of a 1 or 2 point. The manual will also tell you which buffers it will automatically recognize.
3. Decant the buffers into suitable containers.
If you’re using a bottle of buffer you should pour a small amount (around 10ml) into a beaker or other container. This makes sure the whole bottle is not contaminated by any traces on the electrode. Even when using sachets it is good practice to decant them into a beaker so there is room to move the electrode back and forth in the buffer.
4. Perform the first point of the calibration.
Check the manual of your meter to see how to do this, but for most meters the electrode should be placed into the buffer solution so the sensor end is immersed. Gently wave the electrode back and forth in the buffer, being careful not to strike the sides or base of the beaker.
Most modern meters automatically recognize some buffers, but for some you may need to tell the meter what pH value it is – by a turn screw or entering the value. Check that the value has been recognised correctly and saved.
5. Rinse the electrode
The electrode or sensor should be rinsed with water to remove any traces of the first buffer. This will make sure the second calibration point is from fresh.
6. Perform the next point of the calibration and rinse again.
Repeat steps 4 and 5 until you’ve completed all points of the calibration. Rinse the electrode or sensor thoroughly to remove all traces of buffer after the final point.
7. Put the meter into measurement mode
Check the manual to see how to do this as it will vary on each model.
8. Measure your sample
Put the electrode into a beaker of your sample and gently wave back and forth until a stable reading is given.
9. Rinse the electrode
Rinse thoroughly in water to remove any traces of the sample. If you have more samples repeat steps 8 and 9.
10. Store the electrode properly.
The membrane on the electrode is very delicate and will easily break if it is stored incorrectly. Put the clean electrode into storage solution – usually this will be 3 molar KCl (Potassium Chloride) but consult the manual of your pH meter. See more information on storage here.
Why does temperature matter?
Temperature affects pH readings due to the relationship given in the Nernst equation, this means when measuring pH you should always measure temperature also. Most meters will then automatically compensate for the temperature and give an accurate reading (this might be listed as ATC – automatic temperature compensation).
Electrodes may have temperature measurement built in to the electrode itself (called a 3-in-1 electrode) or the meter might have a separate temperature probe. If there is a separate temperature probe then both the electrode and the temperature probe should be held together in the buffers and samples when you are calibrating and measuring.
If you have any questions about pH measurement or calibration, just contact us