HI7061-050 is the GroLine general purpose cleaning solution that can be used for a variety of applications. Electrodes can become dirty from use and will produce inaccurate results even as they read correctly in a pH buffer. Hanna's cleaning solutions eliminate impurities and residues that are left on electrode surfaces when immersed in samples during measurement or when stored incorrectly. Hanna suggests cleaning the bulb and junction of your electrode on a regular basis to ensure that the probe is always clean and to prevent any clogging of the junction. The HI7061-050 is for 500 mL of general purpose cleaning solution.