pH Calibration Solution 4.0 - 4 fluid ounces
To obtain reliable and consistent measurements it's important to calibrate your pH meter because of how the electrode changes over time. Regularly calibrating your pH meter will adjust your electrode based off any changes that may have occurred and ensures that your readings are accurate and repeatable.
Most modern meters use 4.0 and 7.0 solutions to calibrate both the low and high end of the spectrum.
Follow the calibrating instructions for your meter. Use only enough of the solution to submerge the tip of the meter, don't reuse or pour the solution back into the bottle. Rinse with purified water in between calibration solutions to avoid inaccurate readings.