How to take a pH reading and recommended electrode maintenance

Taking a pH reading with your meter can have hurdles that may cause issues especially when it comes to correctly maintaining and caring for your electrode. Below, we summarize our hints and tips with taking readings, our most heard questions to help you understand how to properly care for your electrode and our troubleshooting guide should you be experiencing difficulty. 

 

Taking a pH reading : hints & tips

  • A pH meter will usually need to be calibrated before use and often this is a 2 or 3 point calibration. 
  • Rinse off the electrode using deionized water between samples.
  • With refillable electrodes, the filling cap should be opened when taking a reading.
  • Correct electrode care and maintenance should help prolong the lifespan of your electrode (see our Q & A below). 
  • After use, store your electrode wet - in pH electrode storage solution. 
Using a ph meter Using a ph meter

Electrode care and maintenance : Q & A

Instructions may change from electrode to electrode, however here we provide some general tips and pointers to follow. Correct maintenance and care of your electrode and meter will ensure it gives accurate results and will enhance its longevity.

What should I do when I receive a new electrode?

When you receive a new electrode, it should arrive in a soaker bottle containing electrode storage solution. It is normal to see deposits of KCl crystals on the outside of the electrode, simply rinse these off with water. Leave the electrode in water or weak HCl for around 15 minutes to help reactivate the reference junction.
You may notice bubbles in the glass bulb, simply shake the electrode in a downwards motion to remove them.

How do I store my pH electrode?

pH electrodes should always be stored wet. They are usually stored in electrode storage solution, otherwise known as 3.0M KCl, in an electrode soaker bottle. This ensures the glass membrane is kept hydrated. If you do not have 3.0M KCl to hand, it is possible to store in pH 4 or pH 7 buffer, but never in deionized water which will cause ions to leach out of the electrode. 

Why are there crystals on the outside of my electrode?

After storage, it is normal to notice crystals forming on the outside of the electrode. They are caused by the electrode storage solution drying to the outside of the electrode. These will not affect it, simply rinse off with water.

How should I clean my electrode?

Between taking readings in different samples or between buffers, the electrode should be rinsed off with deionised water. Do not rub the electrode with a tissue or cloth as this could damage the glass membrane. From time to time, your electrode may need more that simply rinsing off. Here, we detail some of the more specialist cleaning methods.  

Before any special cleaning procedure, you should drain the reference electrode and refill with fresh solution after if it is refillable. Rinse the electrode with water following cleaning and leave to soak in storage solution for at least 10 minutes before use.

  • General cleaning: use a mild detergent or weak HCl (0.01M). Place your electrode in this for around 15 minutes. 
  • Protein and/or food buildup: a cleaner with pepsin (1%) is recommended. You would usually soak for around 15 minutes (or longer if required). Contact lens enzymatic solution will also have the same effect.  More specific cleaning agents are also available for example for foodstuff samples.
  • Oils and grease: wash the bulb in a mild detergent to help break up the oily residues
  • Sulphides: if your electrode has silver ion-containing electrolyte, sulphides may react causing a blockage. This can be cleaned by soaking in a thiourea solution until the discoloration disappears. 
  • Salt deposits: soak electrode in HCl (0.1M) for around 5 mins followed by soaking in NaOH (0.1M) for around 5 mins.

If you would like to read more on cleaning electrodes, click here

How do I unblock the reference junction?

If your reference junction has become blocked, you may be able to clean this by heating the reference portion of the electrode in diluted KCl solution to between 60°C and 80°C for approximately 10 minutes.

Should I wipe my electrode when cleaning it?

No, you should never wipe the electrode bulb as this may scratch or damage the membrane.

How do I refill my electrode?

Not all electrodes are refillable; the gel type ones have non-refillable electrolyte. For refillable electrodes, open the filling cap and remove any old fill solution with a syringe. Replace the cap and leave the electrode to stand upright for about an hour.

How do I calibrate my pH electrode?

Most electrodes require either two or three point calibration. This is carried out with pH buffer solution. There is no set calibration schedule in most cases, as this depends on the samples and conditions you are working in. Many people will choose to calibrate at the start of their work – say once a day. Click here to see our step by step calibration guide.

What's the difference between 20ºC buffer and 25ºC buffer?

pH is temperature dependent; this relationship is given in the Nernst equation.  The buffers will be their designated pH value at that set temperature. Most buffer bottles come printed with a temperature compensation table. Read more here.

How long will my electrode last?

pH electrodes are considered to be a consumable product so you should be expecting to have to change your electrode at intervals. The exact length of time it would last will differ greatly and depend on factors such as your sample type. Even in the best case, you can expect your electrode to last no longer than 1-3 years. 

How do I know when my electrode needs replacing?

Try our troubleshooting guide below if you are experiencing issues. If you have tried cleaning, unclogging and reviving the electrode and it is still not working correctly, it may be time to replace. You can also check the mV value in pH 7 buffer, if the reading is greater than +/- 30mV it may be getting towards the end of its life. Read more about what mV mode is here

Troubleshooting

Here we list some of the more common issues reported to us and techniques you can try to get your electrode and meter back to working correctly.

Dried out electrode

If your electrode is dry, you will want to soak it in KCl or pH 4 buffer to rehydrate the membrane for an hour or longer. It may not however be possible to revive your dried out electrode. Others ways of reconditioning electrodes are by cleaning with weak HCl or other cleaners then soaking in KCl. If cleaning and reconditioning does not revive your electrode response, it may be time to replace the electrode.

Air bubbles in the electrode

Shake the electrode downwards to remove any air bubbles.

Long response times
  • Low ionic strength samples such as pure water will generally have a longer response time. It is therefore recommended that you select an electrode designed specifically for low ionic strength samples.
  • Gel filled electrodes will have a slower response than liquid-filled. It may be worth considering switching if you are experiencing long response times.
  • As electrodes age, they will respond more slowly. If the junction has become clogged this could result in a longer response time. If you are using a single junction electrode, you could consider getting a double junction which would be more resistant to clogging. You can try unclogging the junction using the following method: 

Heat the reference portion of the electrode in diluted KCl solution (storage solution) to between 60°C and 80°C for approximately 10 minutes.

  • The electrode may have dried out causing the longer response time. Try soaking in storage solution before taking your reading.
Difficulty calibrating
  • Ensure buffer is within expiry date and not contaminated. If you do not calibrate often, sachets can be a great way to ensure this.
  • Check the mV readings, they should be at ± 25mV for pH7 and -+177mV±10% for pH4 and 10. If your mV readings are out, this indicates and issue with the probe, buffer or user error. If the readings are acceptable, this indicates an issue with the meter itself. 
  • Try a factory reset on the meter and re-calibration to see if this helps. 
Meter not calibrating at pH 10

The pH 10 buffer is the quickest to go off, so check the expiry date. You should not be putting used buffer back in the bottle as this will compromise it. If there is no issue with the buffer, check you mV readings on the meter. 

pH readings are lower than expected

Is your sample of a low ionic strength, such as steam condensate? If so, this could account for readings being lower than expected. Low ionic strength samples will have a lower stabilisation time due to the lack of ions present that the pH meter is reading. If you are not using a specially designed low ionic pH electrode, then readings may be lower than expected due slow stabilisation and the meter 'accepting' a value lower than the true value. Try swtiching electrode to see if this helps. 

mV reading are within the acceptable range but the meter is not accepting calibration

If your mV readings in buffers are within the acceptable range: pH 7 0 +/-25mV and pH 4/10 177mV +/- 10% then this indicates a problem with the meter as opposed to issues with the probe, buffers or user error. Try a factory reset of the meter and re-calibration. If this doesn't work, its probably time to contact your supplier/the manufacturer as your meter may have a software issue. 


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