Contrary to popular belief, not all pH calibration fluids are the same. When purchasing pH calibration fluid, try to stick to major brand names such as Pinpoint, Hanna, and Milwaukee. A comparison done by Randy Holmes-Farley showed a huge discrepancy in several different brands. In one case a pH4 calibration fluid measured pH6! Bad calibration fluid could skew a normal reading of pH8.2 to something along the lines of pH7.6 to pH8.8.
While it is possible to reuse the calibration fluid, there’s an inherent risk of CO2 lowering the pH level in the calibration fluid. This is more prevalent in pH7 and 10 calibration fluids. Make sure to rinse the pH probe with RO/DI water after using each solution, and try to keep the solution sealed up as well as possible. While there are larger bottles of calibration fluid available, I find it easier just to stock up on several single use solutions. Remember to stock up on pH4.0 solution as well if you’re running a calcium reactor.
For more information, click here to read Randy’s full comparison on pH calibration fluids.