(Posted with www.reefscapes.net and Breefcase's permission from works written in reefs.org)
Apparently, the pH calibration fluids sold in the United States are all derived from National Institute of Standards and Technology (NIST) standard pH solutions. (I believe this explains why we Yanks use calibration solutions which have pH values of 4.0, 7.0 and 10.0, instead of the 5.0, 7.0 and 9.0 solutions commonly used is Europe, where the use of pH meters for aquariums originated.)
Assuming that bit about NIST solutions to be true, the big enemy of old calibration fluids would be CO2 absorbed from the air, which acts like an acid in the calibration fluid just as it does in out tanks.
The pH 4.0 solution should hold its pH value very well over time, as it should be almost totally immune to absorbing CO2. Its pH is already close to the pH point at which all dissolved CO2 in the water is already either CO2 or H2CO3. So, absorbing more CO2 won't materially affect it.
The pH 7.0 standard is somewhat more susceptible to absorbing CO2 from the surrounding air, since it really wants to be at pH 4.0, too.
The real CO2-absorbance casualty among calibration fluids is the pH 10.0 standard solution. It's made from sodium carbonate and sodium bicarbonate, and it really wants to absorb CO2 and make acid out of it. Once it's been opened, its pH starts to fall immediately and it falls rapidly. (Don't breathe on it too much, either.) An open bottle that was right at 10.0 when opened will be 9.4 or even less in very short order -- a few days or weeks.
Using old or previously opened pH 10.0 fluid to calibrate your pH electrodes will bias your meter high, by an amount proportional to how far below 10.0 the fluid was. (This probably explains the apparent jump up in the pH of your water, matthew.)
If you want the best value for your buck and accurately-calibrated pH meters, buy the little one-time-use foil packs -- especially for the 10.0 fluid. Buy only what you will use in a year or so, and don't accept any that will be more than 2 years old for the 4.0 and 7.0, or more than 1 year old for the 10.0, at the time when you will get around to using them.
Remember, the shelf-life starts when the stuff is made, not when you buy it.
By the way, the best way to calibrate a meter is at two points on either side of the pH you plan to measure. That means 4.0 and 7.0 for you freshwater tank keepers, and 7.0 and 10.0 for us reefers and saltwater fish keepers. Some makes of meter reportedly require one or the other sets of fluids for calibration, regardless of what you plan to use it on.
It figures that we poor reefers would get saddled with the short-shelf-life, mean-tempered pH 10.0 calibration fluid....
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