I found some in a drawer. Photo below. I wouldn't try to keep these things for years. They do change over time. Hach puts expiration dates on theirs (couple of years). Understanding that you can sell more buffer capsules (or anything else) by stating shorter expiry dates and being the cynic that I am I tend to keep them beyond the expiry dates but I wouldn't go into 'years' and I certainly wouldn't use the solutions for more than a single brew or lab experiment. The pHydiron caps are a bit less than a buck apiece from Cole Parmer (and a bit more from Amazon - I think this may be the first time I've seen Cole Parmer the less expensive source of anything but there's a first time for everything). The Hach ones are $0.36 apiece. That's the whole idea behind the caps - that you can have a fresh buffer each time you go to work. Time isn't the only factor - in fact it may not be the main factor. Each time you put the probe into the buffer you are carrying some of whatever was on the probe with it. If you follow the recommendations of the Sticky that will be a very small quantity of DI water but even so you are diluting the buffer with that.
Remember that with a decent meter it will be the buffers that determine the accuracy of your meter. It seems 'penny wise and pound foolish' not to invest a $0.72 a brew (or $1.44 a month - i.e. make new buffers every 2 weeks - if you brew frequently) to be assured of the quality of your pH readings.
It is recognized that if you have a cheapo meter with stability of worse than 0.05 pH over 5 minutes that it isn't necessary to try to keep the buffer pH's within the ±0.02 promised on the label.
Now a question for you readers. How many were eagle eyed enough to note that the label in the photo says that these buffers are 'Directly Traceable to N.B.S. Standards'? That will give you some idea as to how long these have been lying in that drawer. I don't think I'll throw them out. I think I'll keep them around and compare the pH to a fresher buffer.
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