The HI4004-45 solution is for the conditioning and storage for the HI4004 and HI4104 calcium liquid membrane ion selective electrodes (ISEs). Measurement of calcium in solutions is determined by calcium ion exchange through the organic polymer membrane. The membrane of the half-cell or combination calcium ISE should be presoaked in HI4004-45 solution for at least one half-hour before calibration to optimize the sensor response. The HI4004-45 is a 500 mL bottle of conditioning and storage solution that ensures optimum measurement performance from the HI4004 and HI4104 calcium ISEs.