Most pH probes should be stored in a 1M potassium hydroxide solution when not in use. This is not critical though. I just leave mine in buffer all the time when not in use and the probes can last a couple years this way. Storing it in buffer "can" decrease the life of the probe somewhat as well as slow the response time. (I do it anyway though with no ill effects) if your slope gets below 55, then its time to get a new probe. The gel on the interior of the probe is potassium hydroxide which can be leached out over time if not "recharged". For long term storage, I would definitely go with the Storage solution though. Hope this helps.