Understanding the Fundamentals: Defining Demineralized vs. Purified
At a glance, both demineralized and purified water seem like clean, high-quality water, and are often confused due to their shared attribute of having impurities removed. However, the term "purified water" is a broad category that can be achieved through multiple methods, while "demineralized water" refers to a very specific type of purification. Demineralization specifically targets and removes dissolved mineral salts, or ions, from the water. Purified water, by contrast, is water that has been processed to remove a wide range of contaminants, which can include minerals, chemicals, pathogens, and suspended solids. Understanding these fundamental differences is crucial for selecting the right type of water for a specific task, from household use to sensitive industrial applications.
What is Demineralized Water?
Demineralized water, also known as deionized water or DI water, is water that has had its mineral ions removed. The most common method for achieving this is ion exchange. The process involves running water through beds of synthetic resin that are packed with either positively charged ions (cations) or negatively charged ions (anions).
- Cation Exchange: In this stage, resins with hydrogen ions (H⁺) are used. As water passes through, positively charged mineral ions like calcium (Ca²⁺) and magnesium (Mg²⁺) cling to the resin, and are exchanged for H⁺ ions.
- Anion Exchange: Next, resins with hydroxyl ions (OH⁻) are used. Here, negatively charged ions such as chloride (Cl⁻) and sulfate (SO₄²⁻) are captured and exchanged for OH⁻ ions.
- Result: The H⁺ and OH⁻ ions combine to form pure water (H₂O). The end result is water with very low electrical conductivity because the charged mineral ions have been removed.
It is important to note that while demineralization effectively removes minerals, it does not reliably remove uncharged molecules, bacteria, or viruses. For this reason, it is not recommended for drinking unless it undergoes further purification and is remineralized.
What is Purified Water?
Purified water is a broad designation for any water that has been treated to remove contaminants and is fit for a specific purpose, such as drinking. Unlike demineralization, purification can be achieved through a variety of methods, often in combination, to achieve a desired level of purity. Some common purification techniques include:
- Reverse Osmosis (RO): This process uses pressure to force water through a semi-permeable membrane that filters out most dissolved solids, organic materials, heavy metals, and some microbes.
- Distillation: This is one of the oldest methods of purification, involving boiling water and collecting the condensed steam, leaving most contaminants behind. Distillation is highly effective but can be energy-intensive.
- Filtration: Various types of filters, such as activated carbon or membrane filters, can be used to remove specific impurities, including sediment, chemicals, and microorganisms.
- UV Sterilization: This uses ultraviolet light to kill bacteria and viruses that may be present in the water.
High-grade purified water, often used in pharmaceuticals and electronics, may involve a multi-stage process that includes RO followed by deionization (demineralization) to achieve extremely low conductivity and ultra-high purity levels.
Demineralized vs. Purified: A Side-by-Side Comparison
To clarify the differences, here is a breakdown comparing demineralized water and purified water across several key criteria.
| Criteria | Demineralized Water | Purified Water |
|---|---|---|
| Purification Method | Ion exchange resins primarily used to remove mineral ions. | Broader term; includes processes like reverse osmosis, distillation, filtration, and UV sterilization. |
| Contaminants Removed | Specifically targets charged mineral ions (salts like calcium, sodium, magnesium, chloride). | A wider range of impurities including chemicals, pathogens (bacteria, viruses), suspended solids, and dissolved minerals. |
| Purity Level | Very low mineral content (low conductivity), but may still contain bacteria, viruses, and uncharged molecules. | Generally higher overall purity because multiple contaminant types are removed. |
| Suitability for Drinking | Not recommended for long-term consumption due to lack of essential minerals; can have a "flat" taste. | Generally safe for drinking, though some mineral content may be removed depending on the method. Often remineralized for taste and health. |
| Primary Uses | Industrial processes (e.g., boiler feed water), laboratories, cosmetics manufacturing, automotive applications. | Drinking water, medical applications, food and beverage industry, electronics manufacturing, laboratories. |
| Primary Goal | To achieve low ionic content (low conductivity). | To eliminate harmful contaminants and meet specific safety or quality standards. |
Which Water Should You Use?
The choice between demineralized water and purified water depends entirely on the application.
Industrial and Laboratory Applications
For industrial processes like boiler feed water in power plants or manufacturing where mineral deposits can damage equipment, demineralized water is often the most cost-effective choice. The absence of scale-forming minerals prevents corrosion and build-up. In a laboratory setting, demineralized water is useful for experiments where mineral interference is a concern. However, for applications requiring a broader removal of contaminants, such as in the pharmaceutical industry, higher grades of purified water are necessary to ensure product safety and quality.
Household and Everyday Use
For general consumption, purified water is the intended choice. Bottled purified drinking water, for example, is treated to remove harmful substances and made safe for drinking, though it may lack some of the beneficial minerals found in spring water. Many consumers choose home filtration systems, such as reverse osmosis, to create their own purified water. Demineralized water, on the other hand, is typically used for specific non-consumable household tasks.
- Steam Irons and Humidifiers: Using demineralized water prevents mineral scale from building up and clogging appliances.
- Car Batteries and Cooling Systems: In car batteries, demineralized water is essential to prevent the accumulation of minerals that can shorten the battery's lifespan.
- Cleaning: It can be used for cleaning windows or cars to prevent spots and streaks from mineral residue.
Health Considerations for Drinking
The World Health Organization (WHO) has highlighted potential health risks associated with drinking demineralized or low-mineral water over long periods. The body relies on water not just for hydration but also for a minor intake of essential minerals like calcium and magnesium. Exclusive, long-term consumption of water that has been stripped of these minerals can, in extreme cases, affect overall mineral balance. For this reason, many commercial purified water products that remove these minerals often add them back in (remineralization) or market versions with mineral enhancement. For drinking water, opting for a reliable source of purified and potentially remineralized water, rather than industrial-grade demineralized water, is the safer and healthier option.
Conclusion
While often used interchangeably, the terms demineralized water and purified water represent distinct products with different purification goals. Demineralized water is a specific, narrowly defined type of purification focused on removing mineral ions via ion exchange, making it ideal for specific industrial and laboratory tasks. Purified water is a broad category, and the final product, which may or may not be demineralized, is produced through a combination of processes to remove a wide range of contaminants. For most general and drinking purposes, purified water is the correct choice, while demineralized water is reserved for technical applications. The key takeaway is to always consider the intended use before deciding which type of water is appropriate.
