Jump to content

A note on pH meters

Recommended Posts

I have posted this before on another forum and was asked to re-post here. I hope it will help people to get better value out of their 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:

pH meter - Wikipedia, the free encyclopedia

Glass electrode - Wikipedia, the free encyclopedia

Reference electrode - Wikipedia, the free encyclopedia

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 <1, conductivity <100uS) it can sometimes be tricky to get the reading to stabilize. The addition of a bit of salt helps here as it raises the conductivity, making the measurement easier for the electrode, without affecting the pH (not any of the cichlid buffers – rock salt, aquarium salt or pool salt, anything that is pure NaCl without any additives).

After a measurement rinse with a bit of distilled or deionized water and store for next use.

Are pH meters more accurate than indicator liquids?

The short answer is: Yes, if maintained and used correctly they are more accurate than the commercial test kits for aquariums.

Let me explain why that is the case. Although individual indicators are relatively precise (they change colour over a very narrow pH range; for our purposes we can assume they do that at one specific pH) the drawback with all commercial pH test kits is that they have to cover a pH range. They do this by mixing a number of different indicators that all change colours at different specific pH values – and that is where the problem lies: they change colour only at certain pH values. To illustrate why that is a problem, let’s make up our own little pH test kit using our own (imaginary) indicators. To keep things simple, we use only three different indicators:

indicator A is blue at low pH and changes to colourless at pH7.4

indicator B is colourless at low pH and changes to yellow at pH 6.4 and to colourless at pH 8.6

indicator C is colourless at low pH and changes to red at pH7.8

If we make up a mix of the above indicators and test over the pH range, our mix will have the following colour chart:

pH 1-6.39 = blue (from A), pH 6.4-7.39 = green (colours of A and B), pH 7.4-7.79 = yellow (from B), pH 7.8-8.59 = orange (colours of B and C), pH 8.4-14 = red (from C).

So, what we are actually measuring with our pH test kit is a pH range, the colour only tells you that the pH is somewhere in that range. The same is true for all commercial pH tests, the intervals may be smaller, but every single colour covers a range of usually 0.2 pH units or more. You can’t tell exactly what the pH is, you can only say ‘the pH is between X and Y’. Even a cheap pH meter is usually accurate down to 0.1 pH units and that is the actual pH, it doesn’t give you a range. – Well, if the electrode is cared for properly, that is.

That's it in a nutshell. I hope the above helps a bit with understanding how a pH meter actually works and how to get the most out of it. Like any piece of equipment it just needs the appropriate level of care and proper handling to work accurately and last the distance. After all, you don’t park your car under water and expect it to run smoothly after that :D

Link to comment
Share on other sites

  • Create New...