Effective use and care of pH meters and probes

Tom Barr

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Reposted from Bernd from Australia:



A note on pH meters

There seems to be a lot confusion out there about the use and usefulness of pH meters. You often read comments that they are inaccurate or don’t work at all – I’d venture a guess that >95% of those ‘wrong’ measurements are due to incorrect handling of the electrode, mostly incorrect storage.
Although all of them do wear out over time and need the electrode replaced (this may not be possible in cheap versions) they should last for quite a while (1year +) and are usually more accurate than required for keeping any type of fish.

To help with this situation I thought I’d write a brief summary about pH electrodes, how they work and how to care for them so that you get accurate measurements and a decent lifetime out of them.
There are different types of pH meters around (pH pens, pH controllers etc.) but they all work according to the same principles and like all sensitive equipment they need to be treated correctly to get good use and correct measurements out of them.

First: how do they actually work?
I’ll give simplified explanation in this post, for anyone interested in more details, have a look here:
http://en.wikipedia.org/wiki/PH_electrode
http://en.wikipedia.org/wiki/Glass_electrode
http://en.wikipedia.org/wiki/Reference_electrode
A pH electrode is made from three basic parts: a glass electrode, a reference electrode, and a voltage meter. The most important part is the glass electrode, this is the part that is exposed to the water and senses the concentration of H+ in the water (pH is an expression of that concentration). The glass electrode contains buffer of pH 7.00 and is connected to the water it measure through a fragile porous membrane. If there is a difference in pH between the inside and outside an electric potential is generated that can be measured by the voltage meter. Inside the glass electrode sit a reference electrode (usually a silver chloride electrode, sometimes a calomel electrode) that provides a constant known electrical potential and provides the reference point for the voltage meter. As the change in pH is proportional to the change in electrical potential over almost the complete pH range the measured voltage can be directly translated into a pH reading that you will see on the display.

Why is correct storage and conditioning important?
As mentioned above, the electrode is connected to the water through a fragile porous membrane. As most solids apart from metals don’t conduct electricity and the membrane is not made from metal (can be special glass, asbestos or other materials) the one thing that allows the pH electrode to work is the fact that water sits in the membrane (together with some salts), which makes this membrane a conductor and allows the electrical potential to be measured. And this is the reason why storage and conditioning of the pH electrode is so important. If the electrode is stored dry over some period of time, this membrane dries out and a drop in conductivity inside the membrane occurs that leads to vastly incorrect measurements.

How do I correctly store and condition a pH electrode?
To ensure accurate measurements the membrane in the electrode must be fully hydrated (contain enough water), otherwise the readout will be incorrect.
They can be stored dry – they usually come out of the box that way – but that really is only a good option for long-term storage. If stored dry they need to be conditioned before use, i.e. the membrane needs to be hydrated. The easiest way to achieve this is by submerging the electrode in tap water for about a day – do not use distilled or deionised or RO water as this will kill the electrode. Once the electrode is conditioned it needs to be calibrated using the appropriate buffers for calibration – if you don’t have any calibration solutions, get some or you will never ever get a correct reading from your pH meter.
In between measurements the electrode needs to be stored correctly to prevent the membrane from drying out again. There are a couple of different ways to do that. For shorter periods of time (a few days) they can just be stored in tap (or tank) water; this will keep the membrane wet. It will however slowly leach some of the components from the solutions inside the electrode, so will reduce the life-span of your pH meter. To get truly accurate measurements it is necessary to calibrate again if they have been stored in water for more than half a day or so.
The best way to store the electrodes is in a storage solution. What exactly that is depends on what type of electrode you have and can usually be obtained from the manufacturer. A common storage solution for a silver chloride electrode is e.g. 3M KCl with some AgCl in it. The correct storage solution will maintain the correct hydration state of the membrane and prevent leaching, thus prolonging the life of your electrode. Before using an electrode stored this way, remove from the storage solution and briefly rinse in distilled or deionized water, then calibrate and do the measurements.
After prolonged usage the electrodes can accumulate some dirt in or on the membrane which affect the accuracy of the measurements. This can be taken care of with cleaning solutions available from the supplier or alternatively with dilute HCl (3.7% is common). Doing the occasional cleaning will again prolong the life of your pH meter.
One sign of the electrode coming to the end of its life is that it is further and further off when measuring the pH of the calibration between calibrations. Doing calibrations before each series of measurements will still give accurate readings for some time, but eventually the electrode will give only erratic readings. Then it is time to get a new one,but that should only happen after quite some time…

How do I correctly measure the pH of a sample?
After storing the electrode in any type of storage solution, rinse with some distilled or deionized water to remove any trace of storage solution, as this may affect the pH in your sample. As pointed out previously, to get the most accurate measurement it is important to calibrate the electrode once it has been in storage for more than a few hours. Single point calibrations are ok, two-point calibrations provide more accurate readings – follow the instructions by the manufacturer. After calibrating again rinse with some distilled or deionized water to remove any trace of calibration buffer, as this will affect the pH in your sample. To measure the pH of a tank, I find it easiest to take a solution in clean (!) small plastic container or a water glass and fill it with enough water to cover the electrode, submerge the electrode, swirl it around a bit and then just let it sit for a few minutes to let the reading stabilize while I do something else. If you don’t get a stable reading after a few minutes it is likely a sign that the electrode is not properly conditioned (or already too damaged/old).
With very soft water (GH and kH
 

Calcuttan

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Mar 3, 2016
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This is a useful write up for novices like me. I have a question - you mention not to use distilled water to condition the electrode the first time saying, "it will kill the electrode". Thereafter you suggest using distilled water. I am a little confused by this. Is distilled water bad only the first time? My manufacturer (Hanna) suggests keeping the electrode dipped in pH buffer for a long time before the first use. After that to rinse with clear water between calibrations and measurements etc. I was thinking of using "distilled water" instead of "clear water". But seeing your words, "it will kill the electrode", I was wondering if it will be safe to use distilled water to rinse. Could you please clarify when to use or not use distilled water?
 

Tom Barr

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The initial conditioning/storage of the probe is what that was specific too,


Not rinsing say between measures. In that case, you can use DI water etc.
 

Calcuttan

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Mar 3, 2016
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Thanks a lot. In fact after reading your article I searched the Hanna site where they have given some technical explanation as to why distilled water is not good in certain situations. It went a little over my head but I got the message :) Thanks once again for clarifying.
 

dmartinx

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Oct 25, 2008
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Pacifica, Ca
Appreciate the summary Tom. What is an estimate of the life expectancy of a pH probe that is continually submerged such as the American Marine system that uses pH to regulate CO2 inflow rates?
 

Tom Barr

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Not sure, I replace yearly as rule and use a permanent sharpie to write on the probe the date of service.
 
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Justlikeapill

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Jul 26, 2015
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Question: I have read to calibrate monthly for regular grade probes and every other month is fine for lab grade. Is this good advice?


Also, I have read that if the probe dries out it is ruined. Is this in terms of drying out because your sump level got too low and you weren't on top of things for a day, or does this really only apply storing it dry?


I have been doing a two-point calibration monthly with 7.0 and 10.0 calibration solutions. This is just what came with my probes. I assume these would be best for rift lake cichlids and SW fish, but for our purposes would it be better to use 4.0 and 7.0 solutions? If it is justified, does it matter enough for me to toss all of my 10.0 solution and go buy 4.0 solution?
 

Tom Barr

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When in doubt, generally replace the pH probe. 4 and 7 calibration works best for our purposes.


I never let them dry out. You can check them against a new probe and see the performance and then you have a decent back up should the older one fail or you have questions about them.