For use with Hanna™ GroCheck TDS (HI983301), GroCheck Combo (HI981404N) meters and Trimeter. TDS solution are produced with high-quality potassium chloride in various concentrations. They are standardized using a conductivity meter calibrated with NIST potassium chloride. Hanna™ is one of the few producers to offer calibration solutions for lab and field applications with packages from 20 to 500 mL. Our packaging has been designed to keep air and light from damaging the calibration solutions.