Distilled or Deionized Water? What’s the Difference?
Regular water from the tap, though not unhealthy to consume, can cause a great deal of problems when used with highly sensitive lab equipment. It is important to remove ionic impurities and minerals from water to achieve precise results in any testing, formulations, calibrations or cleaning.
Impurities found in water include suspended particles, colloidal particles, dissolved organic and inorganic solids, dissolved gases, microorganisms, viruses and DNA.
Two of the most common processes used to remove ionic impurities from water are distillation and deionization (DI).
Distillation purifies liquid by boiling it, capturing the steam and then condensing the vapors. The condensed vapors are then returned to their liquid state finishing the distillation process. Solids and other contaminants, salt being the most important, remain in the original container with just pure water being reduced to vapor. It takes approximately five gallons of water to create one gallon of distilled water.
Deionization removes minerals and ions, both cations (positively charged ions) and anions (negatively charged ions), through a chemical process. DI uses specially manufactured ion-exchange resins which exchange hydrogen ions and hydroxide ions for dissolved minerals, which are then recombined to form water (this leaves DI in an unbalanced condition and with an electrical charge.) DI does not significantly remove uncharged organic molecules, viruses or bacteria. Because deionized water is unbalanced, it goes after any dissolvable or absorbable ions on contact trying to return to a balanced state.
So how do you know whether you should use distilled or deionized water in an application?
Because distilled water is pure, it’s valuable in research since it is a constant and does not interfere with any chemical processes. Distilled water is used primarily as a solvent for reagent preparation, as a calibration standard or analytical blank, for cleaning testing equipment and rinsing an analyte. Distilled water is used when making High Purity Water. It should be used in all equipment sterilization.
Deionized water is used when an application requires a soft solvent. DI water works best in cooling applications because of its lack of mineral deposits. Deionized water is also used in reagent preparation, transferring an analyte within a test procedure, as a calibration standard or analytical blank, and for cleaning lab equipment. DI water is best for washing glassware because of the absence of minerals and ions. DI is corrosive and should not be used when there will be extensive contact with certain metals.
As Drinking Water
Deionized water should never be consumed as drinking water since the deionized process does not remove bacteria or viruses (compared to municipally filtered drinking water.) The jury is still out about the benefits of drinking distilled water.
Can They Be Substituted?
Not always – distilled water can be substituted for deionized water, but deionized water should never be exchanged when distilled waster is called for in any application. Always check with the manufacturer’s instructions before speculating about whether distilled water or deionized water should be used. Your Biotechnical Services technician can always help you determine which one to use.