Does this sound like a pH probe problem?

I purchased a MW 102 pH meter a while back. I have only had the chance to use it a few times. I followed the manufacturer recommendations, as well as advice from aj and others on this site, when setting it up. I have calibrated it every time before I used it with 7.01 and 4.01 solution, which I purchased from my LHBS. The solutions are from Hanna, the red and green colored ones. I just purchased them in two small plastic bottles. There are no expiration dates on them.

When I have calibrated my meter, I typically have found that it reads high on the 7.01 solution (typically about 7.10-7.12). Once I leave it in the solution for about 10 min it settles out, and I perform a single point calibration there. I then rinse it off with distilled H2O, and move on to the 4.01 solution. It also seems to be off there, usually high by the same amount. I leave it in the solution for about 10 min, once it settles perform another calibration there. After each of these calibrations, the meter stays very stable at 7.01 or 4.01 where it was calibrated, without much drift at all over time.

However, if after completing the 4.01 calibration, I rinse it off and move it back to the 7.01 solution, it will read 7.12 or so again, even though I just calibrated it. So I will repeat the calibration at 7.01. Rinse it off, and move it into the 4.01 solution, and it will be off there as well.

I've repeated this process several times, and even though I have just calibrated it, it will be off. If I calibrate it at 7.01, then take it out and rinse it off and put it back into the 7.01, it will be right on.

Does this sound like a probe problem, or could it be a calibrating solution problem? I say that because I keep reading people talking about "mixing up a new buffer" when they calibrate, but I just use the same solution that I have had for a couple of months now. (note, I have never re-used any of the solutions, and always rinse the probe well between each measurement).

I would rather just purchase satchels to make up new buffers if that could be the problem, but don't want to waste time with that if it is clearly a problem with the probe.

If it is the probe, have people had success getting a replacement from Milwaukee? I purchased it from Amazon, not sure if that matters.

Thanks for any help you can provide.

JG

If the probe is not stored in Storage Solution, your probe is probably getting dehydrated. That will screw up the readings for quite a while.

Sorry, should have mentioned that the probe is being stored in pH probe storage solution - again purchased through the LBSH with limited labeling information. I assume it is a KOH solution, but it does not specify on the bottle. The solution is added to the cap, and the probe has been kept submerged in the solution since purchase (arrived in solution as well when I ordered it).

JG

Milwaukee recommends NOT rinsing in distilled water, use tap water. Distilled water removes
ions from the probe membrane.

Are you sure they recommend not using distilled for rinsing? There isn't much opportunity to leach ionic content in the short time a rinse is conducted. I do agree that you shouldn't store a probe in distilled water.

Calibration solutions should be replaced on a regular basis. In a critical application, the replacement is more frequent. From my experience, replacing on an annual basis seems good enough for our brewing applications.