AccuraSea Seawater Reference Solution is the only calibration solution of its kind made from real seawater. It's pure natural seawater micron filtered and UV sterilized then adjusted with reverse osmosis water to match the standard S 35 at 25 C (77 F). Use it to accurately calibrate seawater refractometers and to check the accuracy of swing-arm hydrometers. You may also want to use it as a reference for test kits used to measure the concentrations of various seawater components, since it is natural seawater with the correct and well-known concentrations of all dissolved salts. Be sure.