pH Buffer solutions are required to calibrate a pH controller with a pH sensor (probe). The 3 most common pH buffer solutions are pH4, pH7 and pH10, and are usually a different colour to clearly distinguish between them. pH 4 is often red, pH7 green and pH10 blue. It is recommended to write the pH value on the cap of each bottle to prevent them getting mixed up.
pH buffer solutions should always be stored in a cool dry place and should be discarded responsibly if any remains after the expiry date, which is printed on the bottle.