Water purified by distillation so as to free it from dissolved salts and other compounds. Distilled water in equilibrium with the carbon dioxide in the air has a conductivity of about 0.8 × 10⁻⁶ siemens cm⁻¹. Repeated distillation in a vacuum can bring the conductivity down to 0.043 × 10⁻⁶ siemens cm⁻¹ at 18°C (sometimes called conductivity water). The limiting conductivity is due to self ionization: H₂O ⇌ H⁺ + OH⁻. See also deionized water.