Purified water plays a significant role across various industries and in daily life, ensuring systems function correctly and processes yield accurate results. However, the term “purified water” encompasses different types, often leading to confusion regarding their specific characteristics and applications. This article aims to clarify the distinctions between two commonly encountered forms of purified water: distilled water and deionized water.
Understanding Distilled Water
Distilled water is created through distillation, an age-old purification method. This process involves heating water until it vaporizes into steam, leaving impurities behind. The steam is then cooled and condensed back into liquid, resulting in distilled water.
This method effectively removes a broad spectrum of contaminants, including minerals, salts, heavy metals like lead and iron, bacteria, viruses, and most organic compounds. However, it may not effectively remove volatile organic compounds (VOCs) with boiling points similar to or lower than water, nor dissolved gases like carbon dioxide. Distilled water is commonly used in car batteries, humidifiers, medical equipment sterilization, and laboratory procedures where mineral buildup or biological contaminants are undesirable.
Understanding Deionized Water
Deionized water (DI water) has almost all mineral ions removed. Its purification involves passing water through specialized ion-exchange resins. These resins attract and bind to dissolved ions, exchanging them for hydrogen (H+) and hydroxyl (OH-) ions, which combine to form pure water (H2O).
This ion-exchange process effectively removes charged impurities, such as salts and minerals like calcium, magnesium, sodium, chloride, sulfates, and nitrates. However, deionization does not remove non-ionic substances like sugars or organic molecules, nor does it eliminate bacteria, viruses, or particulates. It is widely used in industries requiring high ionic purity, such as electronics manufacturing, laboratory rinsing, pharmaceutical production, cosmetics, and cooling systems.
Key Distinctions and Practical Uses
The fundamental difference between distilled and deionized water lies in their purification methods and the types of impurities they target. Distillation is a thermal process removing a wide range of contaminants—ionic, non-ionic, and biological—by physical separation through boiling and condensation. Deionization, conversely, is a chemical process using ion-exchange resins primarily to remove dissolved ionic contaminants.
While distilled water generally removes a broader range of contaminants, deionized water achieves extremely low ionic conductivity, meaning it has very few electrically charged particles. Water’s ability to conduct electricity relates directly to ion presence; deionized water, stripped of these, exhibits very low conductivity, often below 0.1 µS/cm. Distilled water, while also having low conductivity (typically 0.5 to 3 µS/cm), can still contain dissolved gases like carbon dioxide, which can make it slightly acidic with a pH around 5.8.
Distilled water might retain volatile organic compounds, while deionized water may still contain uncharged organic molecules, bacteria, or particulates. From a production standpoint, distillation is more energy-intensive and costly than deionization, which is quicker and more energy-efficient. Therefore, the choice depends on the specific application’s purity requirements. Deionized water is often preferred for electronics where ionic purity is paramount, while distilled water is used in medical settings where organic and biological contaminants must be absent.
Purity Levels and Considerations for Use
The concept of “purity” for both distilled and deionized water is specific to the removal of certain contaminants, yet it does not imply universal suitability. Neither distilled nor deionized water is recommended for regular drinking. This is because purification removes essential minerals like calcium and magnesium, beneficial for human health. Long-term consumption of water lacking these minerals can lead to electrolyte imbalances and a flat, unappealing taste.
Proper storage is also a consideration for both types of purified water. Once exposed to air, they can reabsorb impurities like carbon dioxide, altering their pH and conductivity. Additionally, water can leach trace elements from storage containers over time. Therefore, the selection between distilled and deionized water should always be guided by the precise needs of the intended application, ensuring the water’s purity profile aligns with required standards.