These three terms are shorthand for deionized water (diH₂O), distilled water (dH₂O), and double-distilled water (ddH₂O). They are the most common types of purified water that are used in laboratories. Although similar, there are slight differences in preparation and usage between them.

diH₂O water, as the name suggests, has had the charged molecules (ions) in the solution removed by filtration through a bed of ion exchange resins. These small plastic beads with a positive or negative charge, attract the undesirable molecules and replace them with either a H⁺ ion or an OH^- ion, which combine to form pure water. The process does not remove anything without a charge, including viruses and bacteria. diH₂O, due to the lack of ions, has very low electrical conductivity. The process is relatively inexpensive, as there is minimal hands-on time, the plastic beads can be "recharged" multiple times with strong acids and bases, and the tanks require infrequent cleaning.

dH₂O is produced by heating water to boiling and gathering the resulting water vapor in a clean container. Any contaminants and undesirable molecules are left behind in the original container as precipitated solids. The boiling process sterilizes the water, meaning viruses and bacteria are killed during the procedure. It is the oldest method of purification and is still in common usage. However, some dissolved impurities can have a lower boiling temperature than water and may be gathered in the secondary container along with the purified water vapor. The distillation process requires more hands-on time and cleaning of the solid contaminants out of the distillation chamber after every procedure, which can be challenging or even hazardous depending on the initial cleanliness of the water. ddH₂O is traditionally considered the "most pure" type of laboratory-grade water. This is simply the secondary distillation of the water vapor of a prior distillation procedure. This process is the most time-consuming out of the three and does not necessarily avoid the risks of potential impurities being carried along with the vapor.

Table 1. Summarized comparison of water purification procedures.

These three types of laboratory-grade water are often produced in combination with other purification methods, such as reverse osmosis, UV radiation and microfiltration, to alleviate some of the shortfalls or vulnerabilities of any single procedure.

The AAT Bioquest Buffer Preparation and Recipes interactable tool will specify which type of water to use for mixing buffers. For other questions on experimental design and protocols regarding water purity with our products, contact us.