Is there anything more tedious than pH-ing a solution? Standing there adjusting the pH of your buffer, adding acid or alkali drop by drop until you get to the right pH… With pH-ing being so boring, it’s in our best interests to keep the equipment in good working order so that we needn’t fuss over the pH meter, and can just get on with what really needs doing. So in this article, we’ll talk about how to look after your pH meter.
How a pH Meter Works
Let’s start by thinking a little bit about how a pH meter works. Without going into all the gritty details, your pH meter probably consists of two main parts: the electrode and the screen. It’s fairly obvious that the latter part is there to tell you what the pH of your solution is, but the electrode is a little more complicated. Most pH meters will only have one probe (some older versions may have two) that contains two different electrodes: the glass electrode and the reference electrode. The glass electrode is the most important in reading pH, and consists of an electrical wire made of silver submerged in potassium chloride, surrounded by a special silica and metallic salt-coated conductive tube. The reference electrode is usually made of the same material as the glass electrode, but its tube lacks any special conductive coating.
When submerged in a solution, hydrogen ions will migrate towards the conductive glass electrode and replace some of the metal ions in the coating. This creates a small voltage that the silver electrode is able to pick up, which is then passed on to the voltmeter, which converts the signal into a pH reading. The reference electrode acts as a baseline measurement, and completes the electrical circuit.
Storage of the Electrodes
Now that we know a little about how the pH meter works, we can better understand how to take care of it. Obviously this is a very sensitive instrument, and needs to be taken care of properly. The simplest thing that can be done is to keep the probe submerged in liquid. The special conductive glass coating shouldn’t be left to dry out… always keep it hydrated, or it may stop working! You can buy special storage solutions for this purpose. Either keep the probe submerged in a falcon tube, or keep the probe covered with the special cap it came with. The exact type of solution used will depend on your meter, as different probes have different requirements. Combined electrodes (those consisting of both the glass and reference electrodes) are typically stored in a concentrated solution of whatever is inside the probe – the concentration should be higher to prevent diffusion out of the probe. pH meters with two separate electrodes can have their glass electrodes stored in an acidic solution of roughly pH 3. In either case, never use distilled or deionized water, as prolonged submersion in water can encourage diffusion out of the electrode, which will affect its sensitivity.
Cleaning the pH Meter
If your meter has been left to dry out and is now covered in dust and the crystallised remnants of whatever it was submerged in – you may want to consider getting it professionally serviced. It may still be ok, but could need a special treatment to get it working properly. Otherwise, if it’s in pretty good condition, there are things that can be done to keep it running smoothly. Special washes can be bought to treat the probes and can be used regularly (perhaps once a month or so, depending on how often the pH meter is used) to maintain the special conductive surface of the probe. Junctions in the meter may become clogged sometimes too – follow the manufacturer’s instructions for cleaning in these instances.
It’s also important to clean the electrode between uses – don’t just take it out of your solution you’ve been working with and put it back in the storage solution. Rinse the probe with distilled or deionized water (which is ok for washing and rinsing – just not for submersion!) and then dry it quickly by blotting (NOT wiping!) before submerging it in your next solution or returning it to the storage solution.
Something people may occasionally be a bit lax with is calibration. As part of the care for your pH meter, always be sure to calibrate it regularly. This makes your readings more accurate, and may also give you an indication of whether the electrode is damaged or not. Calibration is a pretty simple task, and you should follow the manufacturer’s instructions for your specific machine, but basically you simply need to submerge the probe in a solution of known pH (these can be bought specifically for this purpose) and tell the machine what it should be reading.
Even if you take the best care of your pH meter, it’s vulnerable to the effects of aging. The electrodes aren’t going to work perfectly forever, and there may come a stage where it simply needs replacing. A good sign of an old electrode is a delayed response time while you’re taking your measurements. An older electrode may also need more frequent calibrations, and you’ll need to pay attention to the conditions in which the pH meter is kept: keep it in a cool place, keep the electrode hydrated and avoid any situations in which the electrode could be dropped or damaged.
What are your tips for keeping a pH meter in good working order?