Demineralized water (DW) is water treated to remove mineral ions, which are electrically charged particles like dissolved salts and metals. Unlike simple filtration, which removes suspended solids, demineralization eliminates these dissolved ionic impurities. Standard tap water contains these ions, primarily calcium and magnesium, which cause water hardness. For specialized applications, the presence of these ions is problematic because they can cause scale buildup, interfere with chemical reactions, or damage sensitive equipment. Producing demineralized water aims to achieve high purity by stripping the liquid of these conductive mineral components.
Producing Demineralized Water Through Distillation
Distillation is a straightforward, heat-based process that mimics the Earth’s natural water cycle and is the most accessible method for making demineralized water at home. The process involves boiling water to create steam, leaving behind nearly all non-volatile impurities, including mineral salts, heavy metals, and microorganisms. This pure water vapor is then collected and cooled, causing it to condense back into liquid demineralized water.
A simple home setup requires a large stainless steel pot, an inverted lid, a glass collection bowl, and ice. The pot is filled halfway with tap water, and a glass bowl is placed inside to float without touching the bottom. The water is brought to a low, steady boil to generate steam.
The inverted lid is placed on top, and ice is positioned on its exterior. The cold surface rapidly cools the steam, causing it to condense into pure water droplets. These droplets are directed by the lid’s shape to drip into the floating collection bowl.
While effective at removing ionic solids, distillation has limitations regarding certain contaminants. Volatile organic compounds (VOCs) vaporize alongside the water, meaning distillation may not fully remove all VOCs. Commercial distillers often require additional post-filtration steps to achieve higher purity. The process is also time-intensive, taking a long period to produce a small volume of demineralized water.
Chemical and Filtration Methods
More advanced and industrial-scale demineralization relies on physical separation or chemical exchange instead of heat. Reverse Osmosis (RO) is a filtration process that uses pressure to force water through a semi-permeable membrane. This membrane has extremely small pores, often around 0.0001 microns, which allow water molecules to pass through while rejecting up to 99% of dissolved salts and other contaminants.
The applied pressure must overcome the natural osmotic pressure that would cause water to flow toward the higher concentration of solutes. RO is effective at removing a wide range of impurities, including salts, heavy metals, and bacteria. However, RO systems alone typically remove 95% to 99% of dissolved solids, which is usually not enough to meet the zero-ion standard required for true demineralized water.
Deionization (DI) is a chemical method that uses specialized ion exchange resins to achieve the highest levels of purity. Water is passed through beds of synthetic resin beads, composed of a cation resin and an anion resin. Cation resins attract positively charged ions (cations) and swap them for hydrogen ions (H+).
Anion resins attract negatively charged ions (anions) and swap them for hydroxyl ions (OH-). The resulting hydrogen and hydroxyl ions then combine to form a pure water molecule (H2O). DI is often used as a final “polishing” step after RO, which removes the bulk of the solids, extending the lifespan and efficiency of the DI resins. Systems combining RO and DI produce ultra-pure water with virtually no dissolved ions.
Measuring Purity and Appropriate Uses
Purity is quantified by measuring the Total Dissolved Solids (TDS) concentration, typically using a handheld TDS meter. The meter estimates dissolved material based on the water’s electrical conductivity. Since mineral ions carry an electrical charge, higher ion presence results in higher conductivity and a higher TDS reading, usually expressed in parts per million (ppm).
For demineralized water, the target is a TDS reading of 0 ppm, indicating the near-total absence of ionic solids. The sensor is dipped into the water sample until the reading stabilizes. Any reading significantly above 0 ppm suggests the demineralization process was incomplete or that the water became contaminated during handling.
Due to its lack of minerals, demineralized water is highly reactive and will attempt to dissolve substances it contacts, making proper storage important. It should be stored in clean, airtight, non-metal containers, such as glass or specific plastics, to prevent leaching of contaminants.
The primary applications are those where mineral buildup or chemical interference is unacceptable. Common uses include:
- Filling steam irons and ultrasonic humidifiers to prevent the white mineral residue that clogs and damages the devices.
- Topping off lead-acid batteries and use in cooling systems to prevent corrosion.
- Preparing laboratory solutions.
- Rinsing sensitive electronics.
- Use in high-pressure boiler feed systems where mineral scaling would cause catastrophic failure.