How long can a pH probe be stored before first use?

Document ID

Document ID TE6790

Published Date

Published Date 07/12/2021
How long can a pH probe be stored before first use?
Concerns and risks with long term storage of pH electrodes.
It is not recommended that a probe be stored for more than 30 days before first use. It is not recommended that spare probes be purchased ahead of time and stored long term until they are needed. This is for several reasons, and is more important for some probes than others.

The first reason is that some types of probe failures are only covered under warranty for the first 30 days. Meaning that if the probe is received in that condition it would be replaced, but if a claim is received of a probe in such a condition outside of that 30-day period it would likely not be replaced under warranty. This is because these types of failures are rare, and if it's reported more than 30 days after it was during its most likely that the problem was caused during the storage time and it was not received in this condition. Hach recommends that all products are inspected once received so that any out of box issues can be reported to tech support immediately.

The larger concern is that the probes are shipped in an amount of storage solution that should be sufficient to store the probe without affecting its response long enough to account for shipping and cross over time of swapping out older electrodes with their replacements, which for some customer locations can take up to a few weeks.

Over time some of the water in the storage solution will evaporate; this impacts the solution in two ways. One is that the volume level decreases, the other is that the concentration of electrolyte in the remaining solution increases.

This increase of electrolyte concentration will come to equilibrium with the internal electrolyte solution and cause an offset in the probe since the electrolyte concentration is different than that inside of the reference. This happens naturally as the probe ages, the electrolyte concentration can decrease slightly over time, which causes the offsets that are corrected when the probe is calibrated. But in this case the concentration is increased and likely at a faster rate than the rate at which it decreases in normal use and storage. This initial offset can cause the probe to not recognize buffers during an initial calibration. This is somewhat less of a concern for refillable probes since the electrolyte solution can just be replaced (see What is the procedure for replacing the filling solution in a refillable pH probe?), but this is not the case for non-refillable probes. In some cases, this can be corrected with additional conditioning by soaking the probe for 1-2 hours in 4 pH buffer but not in all cases.

The decrease in the volume level can be extremely detrimental to probes with open junctions like the PHC101 or 5051T. These probes have open junctions on the sides of the probe, unlike the porous pin junctions that are located near the glass bulb for other probes. As the volume level decreases and the probe is stored on its side, if the open junction is exposed to the air, the internal gel electrolyte will dry up as well. This will cause the gel to contract and can usually be seen if the gel is pulled off the inside walls of the electrode or air bubbles forming in the gel. If this happens the probe is unrecoverable.

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