HI-50021 is a premium quality calibration check solution used exclusively for the HI9813-6 portable pH/EC/TDS/Temp. meter. Each sachet has the lot number and expiration date stamped on it and is made of light block foil ensuring freshness each time one is opened. Hanna’s line of solutions have been specially formulated to have an expiration of 5 years from the date of manufacture for an unopened sachet. Each sachet contains 20 mL of solution that is used to verify the calibration and functionality of the pH electrode.