Buffer Solution pH 4.0 (± 0.02) is used as a standard buffer in Molecular Biology as a calibration buffer for the pH meters. A buffer solution is one which resists changes in pH when small quantities of an acid or an alkali are added to it. It is an acidic buffer solution which has a pH less than 7. This pH buffer solution calibrates the pH meter to a pH of 4.0 at 25°C.