The pH value of samples is one of the most commonly measured parameters in the laboratory — but precisely because it is so ubiquitous, pH measurement is often a bit of an afterthought. And as we focus more on the new and fancy techniques, the nitty-gritty details of accurate pH measurement may get forgotten.

So, how do you tell when your pH measurement may be in trouble? And what do you do about it?

How can I tell if my pH measurement is done?

You place your sensor, start the measurement, and wait until the value on the screen stops changing. But how long is long enough? As stabilization times can depend on sample and sensor age, you cannot just measure for the same duration every time. Modern pH meters offer help by indicating whether a measurement result is stable or not. On some, you can even adjust the criteria to fit your needs.

My pH measurement takes a long time!

This type of issue generally comes in two flavors, with different ways to fix them. Option one: your sensor used to measure faster, but it has now slowed down considerably. In this case, the first option would be cleaning with an appropriate cleaning solution. Option two: your sensor takes a long time to stabilize in a specific sample but generally measures faster in calibration buffers. In that case, the sensor is probably not optimal for the sample of your choice. Typical candidate samples for such behavior are pure water samples, samples containing organic solvents, and many solid and semisolid foods.

Honey, I broke the pH sensor!

Most practically used pH sensors are made partially or wholly out of glass. That means they remain susceptible to damage from bumps and drops. However, you can minimize that risk by using a suitable electrode holder. The best ones feature exactly vertical movement and an adjustable stopper that prevents you from hitting the table. If it works for your sample, you should always consider switching to a sensor with a polymer shaft.

Those sensors don’t last as long as they used to

Aging is a fact of life for pH sensors. But that doesn’t mean that you are helpless. Good storage and maintenance can increase your sensor’s lifetime significantly. Most importantly, never store your sensor dry or in ion-poor solutions. If measuring demanding samples, use appropriate cleaning solutions when the sensor performance drops.

How long can I use my pH buffer once opened?

That is mostly a function of handling and how much buffer you use per calibration. The biggest risk is buffer contamination. The risk of such contamination can be reduced by handling bottles carefully, e.g., by closing the bottle immediately after pouring the required quantity into the calibration beaker. Never should you calibrate directly in the buffer bottle. But why not take the guesswork out of the equation? Many pH buffers are now available in single-use sachets, eliminating the need for guesswork.

So, these are just a few pointers to address the greatest pains in pH measurement. What are your greatest issues with pH measurements? Let us know in the comments!

You can learn about sensors for various pH applications here. Our webinar on Fundamentals of pH can also help you understand all about pH measurements and its best practices.

To contact the author of this article, email pHmatters@mt.com