A functional pH sensor works because of the hydrated gel layer on the glass membrane of its bulb. For glass pH electrodes, possibly the most important need for long-term sensor health is that this bulb stays hydrated. If you’re dealing with a dehydrated bulb, don’t be so quick to throw it out. It is possible to rehydrate a pH sensor, and get it back in working condition.

Imagine this: A sensor is removed from storage, and for whatever reason, that sensor never finds its way into your process. It’s removed from its wetting cap and left to sit on the shelf. Now let’s say a week later, a process sensor needs replaced, and you recall seeing a spare on the shelf. You go to put it in your process, only to find the measurements of this seemingly brand new sensor are all off. It’s highly likely that the sensor’s bulb has become dehydrated

So what do you do? Toss the sensor out and buy a new one? Well, you could. But actually, your sensor may still have some life left. In this article, we’ll examine why the bulb requires hydration, what happens when it dries out, and a few tricks you can do to get it back in working condition.

What Hydration Does and What Happens Without It

The glass membrane of a pH sensor is constructed of a unique, hydrogen ion-sensitive glass with a thin gel layer covering the membrane. The ion exchange from the process sample and the membrane creates a charge, which is then communicated to the rest of the sensor and eventually translated into a numerical pH reading. Without a healthy gel layer, the ion exchange cannot properly take place, and readings will be largely inaccurate.

The hydrated layer also acts as a conductor between the sample and the internal reference solution of the sensor. Without water molecules present, this ionic pathway breaks down and the circuit is incomplete or worse, erratic. This will cause readings to drift and result in inaccuracy.

How to Rehydrate a pH Sensor

The process of rehydrating a dry sensor is relatively simple, but must be done precisely for a chance at regaining your sensor’s accuracy.

• The first suggestion to hydrate your sensor is to soak the bulb (not the entire sensor) in a typical storage grade KCL solution. This is the type of solution that is housed in the wetting cap that typically arrives with the sensor in packaging. Leave the sensor in the KCL solution overnight for the best chance at recovering your sensor’s hydration.

• Avoid soaking the sensor in de-ionized water. This will further disturb the ion concentration in the layer and further complicate re-hydration.

• Following an overnight soak in the KCL solution, gently blot dry (do not wipe) the sensor bulb.

• Next, perform a few 2 or 3 point calibrations to test the sensor’s accuracy. Be sure to verify your sensor slope falls within 90-100%. If everything seems to read efficiently, your sensor should be back in working condition.

Prevent Sensor Failure Before It Starts

Of course, it’s possible that your dehydrated sensor is beyond saving if left out to dry longer than 48 hours. To ensure your sensor stays hydrated, make sure to always store your sensor in a storage solution when not in use. Use the manufacturer’s cap to keep it sealed and upright and avoid long dry spells during any kind of transport.

A dried-out sensor might seem like a small oversight, but in a high-precision environment, the ripple effect is real. From lost calibration time, questionable readings, to full batch rejections, the problems can add up. Taking a few extra minutes to try and  re-hydrate (or better yet, store properly) can save on replacement costs and keep your measurement loop reliable.

For more helpful tips on sensor storage, general maintenance, and best practices for keeping your loop working efficiently, consider scheduling a training with M4 Knick to further optimize your liquid analytics needs.