Instability usually occurs for a couple of reasons. Every combination glass pH electrode has two wires. One that goes to to the tip of the bulb (indicating pH probe) and the other that is inside the electrode body (reference probe). The reference wire is separated from the sample solution through a porous frit known as a junction.
A circuit is formed with a meter from the wire that is inside the indicating pH probe, through the glass bulb, into the solution, then through a junction and then to the internal reference. If there is build up on the glass pH indicating bulb then it will impede the flow of current from the internal wire and the sample solution. The same with the junction. If the porous frit gets clogged then it will insulate the internal reference wire and prevents the circuit from being complete.
Since the meter is reading correctly in the buffer then it is safe to say that the probe is not dirty unless the previous calibration was done with a dirty probe then the build up would automatically be compensated for by the calibration process. BTW when I say dirty I am referring to a build on the glass that can be hard to see with the naked eye. That is organic growth (i.e. algae, etc. ) can be on the bulb. The surface of the bulb would be slightly hazy in appearance.
Most likely the junction is not clogged as well. Without asking my guess is that any storage solution that was used from the last time dried out. When a pH probe is not stored in storage solution then two things will occur. The first is the layer of water on the glass bulb (known as the hydrated layer) dries up. The hydrated layer is part of the measuring circuit. A probe will read one pH when the probe is dry and a different pH when it is hydrated. It takes about 4 hours for the hydration layer to completely form.
The second issue when a probe is stored dry is the effect on the porous junction. The junction is a barrier from the inside of the probe and the outside solution. This junction is also known as a liquid junction. Basically it is wet and solution/ions migrate from inside the probe through the junction to the outside. If the junction is dry the diffusion of ions is impeded. A dry junction will also show instability in readings.
So what to do. Before your next calibration let the probe sit inside storage solution or buffer for 3-4 hours. This will make sure that the bulb is hydrated and the junction is free flowing. This will help increase the response time.
If you use IPA (Isopropyl Alcohol, not beer) to clean then it will be important to store in solution for 3-4 hours. The IPA will dehydrate the glass bulb. It might be better to use an enzymatic cleaner. Since the wort has a lot of protein an enzymatic cleaner will be the best to break them down. There are protein cleaning solutions for pH probes. Some contact cleaning solutions are enzymatic cleaners but it would be hard to say how effective they would be. Ideally solutions for pH probes are made for pH probes so they are always the first choice.
If you provide the make and model of the pH meter there can be some additional information that can be provided.
If your meter read within 0.02 pH in the buffer then that is very good. That means there has not been much change to the probe since the last calibration. If the probe was stored dry then I would expect that number to drift over time as the hydration layer forms.
Lastly, compare the pH value on the meter to the value on the buffer packet. A pH meter can be calibrated to a buffer but show a different value after calibration. Some manufactures program the meter to display the solution pH (value on the bottle/packet) that is dependent on temperature. This is not the same as temperature compensation provided by a temperature sensor. That is for a different phenomenon related to membrane potentials, which is what a pH electrode is based on.
Topic: pH probe calibration (Read 4050 times)
