Bluelab pH 4.0 Buffer 250ml is a pH meter calibration solution that improves the accuracy and lifespan of the instrument. Calibration is easy to perform and applies to all Bluelab pH meters and pens. After cleaning the pH probe, calibrate with calibration solutions at pH 4.0 and 7.0 to start measuring pH immediately. Repeat calibration after cleaning the probe or when the instrument requests calibration.