Good pH measurement technique is a bit of a grey area for some of our users. Questions such as "How often should I calibrate?" and "How often should I replace my electrode?" are common for anyone starting a pH measurement process.
Inappropriate electrode storage is something we see crop up from time to time. We come across an electrode stored dry. Usually, the incorrect reason behind this is that dry storage will keep the electrode inert and viable for longer.
What actually happens is that the electrode dries out permanently if stored dry long enough. The sensing glass of a pH electrode is actually composed of three discrete glass layers: a hydrated outer glass gel layer, a dry middle layer and a hydrated inner layer. The hydrated layer is responsible for providing the electrode with sensitivity to pH changes.
This leads to pH drift, slow response times and incorrect values. Fortunately, you can "revive" an electrode that has been stored dry in most cases by immersing the bulb and junction in a pH storage solution for at least an hour. After that, you can calibrate the electrode and be back in business.
We get it. You want to make sure your pH sensing glass is nice and clean for the next measurement, so you carefully wipe your electrode with a paper towel. However, this can cause more problems than a little residual buffer in your sample.
The electrode sends a voltage to the meter that is based on the pH of the sample in which it is immersed. Wiping the pH glass can produce a static charge (think of it as rubbing a balloon and observing the static charge that builds up). Static charge interferes with the electrode voltage reading. When the voltage reading is incorrect, the pH value at which the voltage is interpreted is also negated. What's more, the hydrated glass layer you've worked to develop through proper storage is interrupted by wiping the sensing glass with a paper towel.
If necessary, you can blot the electrode with a lint-free paper towel to remove excess moisture, taking care not to rub the glass surface.
storage in pure water (such as deionized, distilled or reverse osmosis water) is also a major mistake in pH electrode storage. This most often occurs when a user runs out of storage solution but wishes to keep the electrode hydrated. This poses another, far worse problem.
Deionized water contains virtually no ions. The pH electrode is full of ions, both in the filling solution and interacting in the hydrated part of the pH sensing glass. So, when an electrode is immersed in a solution containing no ions, the ions in the electrode will want to move into the solution with the intention of eventually establishing an equilibrium. With most of the ions gone from the electrode due to repeated storage of deionized water, the electrode will be rendered unusable. Glass also degrades much more rapidly, reducing electrode life.
If you ever come across an electrode stored in deionized or distilled water, remove it immediately. If the electrode is rechargeable, replace the filling solution with a fresh one. Once the filling solution has been changed, store the electrode in the storage solution and calibrate the electrode.
We've seen a lot of interesting applications since we published the pHep, pH measurement was opened up to applications beyond the laboratory as a variety of samples and substances were measured, from soil to wine and everything in between.
With so many different samples, it makes sense to have cleaning solutions developed specifically for these applications.
This is because the deposits that form on the electrode cover the detection glass. For example, grease from oils or food can coat the electrode. As a result, you will be measuring the deposits and the sample, rather than just the sample. A slow response time can also be seen from dirty electrodes. You may even record the value when it seems stable, but in reality it drifts very slowly towards the "real" value. This can happen even if the electrode "looks" clean; a very thin layer of oil or scale may still be on the electrode.
The best way to clean the electrode is to use a cleaning solution specially formulated for pH electrodes. Even better would be to use one that has been developed for the application for which you are using the electrode. For example, there are cleaning solutions ideal for removing stains from electrodes. This way, you can be sure that residues are completely removed from the electrode.
Calibration is certainly one of the most common processes we're asked about. The frequency of calibration is a major concern. We also get a lot of questions about which buffers to use for their application. Sometimes, the frustration is so real that users stop calibrating altogether. Fortunately, all questions can be answered by understanding how calibration works.
All pH electrodes are based on a principle known as theNernst equation. The Nernst equation takes a voltage reading (mV) and correlates it with the ion concentration (or pH). This correlation forms a straight line. For pH electrodes, the theoretical mV value at pH 7 is 0 mV (neutral) and the slope of the line is 59.16 mV. This means that, in theory, the electrode will change its output by 59.16 mV for each pH unit you use (for example, pH 6 to pH 7 would be 59.16 mV / pH unit). This is all in theory, as electrodes will change slope and shift with age.
In reality, the electrode may behave slightly differently from the theoretical behavior (for example, a slope of 58.2 mV and a shift of 8 mV). Calibration compensates for this by determining the actual slope and offset of your electrode using known buffers, and updating the algorithm in the meter accordingly.
A pH 7 buffer should always be included to achieve the offset (neutral) point. This means that if your sample has a pH of 8.6, pH 7 and pH 10 buffers should be used.
The frequency of calibration ultimately depends on the accuracy of your figures. Daily calibration is ideal; however, we understand that calibration takes a little longer than what might be a crazy schedule.
Not all pH electrodes are created equal. Even with the best technique, you may still not get the best measurements. In fact, some electrodes are better suited to certain applications than others. Less-than-ideal electrode use can result in longer response times and shorter electrode life.
Consider the "standard" pH electrode. It's usually made of glass, with a large spherical bulb at the end that forms the sensing glass part. There is usually a small ceramic junction that allows electrolyte flow from the reference part of the electrode. This electrode is functional for a wide variety of applications, but not ideal for all samples.
Problems arise when pH is measured in semi-solid/solid samples or samples containing solids suspended in solution. Such samples include wastewater and foodstuffs. Ion-poor samples can also pose problems in terms of response time and stability (such as drinking water).
In such cases, it may be ideal to use an electrode specially adapted to these different types of sample. Conical sensing tips with open junctions enable direct measurement of solid and semi-solid samples, eliminating the need to manufacture a slurry. Electrodes with multiple ceramic junctions allow the electrolyte to diffuse more rapidly into the sample, enabling greater stability in pH measurements of low-conductivity samples.
