The quality of water is often measured by its Total Dissolved Solids (TDS) level, which indicates the concentration of various substances dissolved within the liquid. Understanding the TDS value of distilled water is key to assessing its purity, as this measurement reveals how effective the distillation process is at removing impurities. Distilled water’s low TDS value serves as a benchmark for low-contamination water, making it relevant across industrial, laboratory, and household applications.
Understanding Total Dissolved Solids
Total Dissolved Solids (TDS) measures all inorganic and organic matter dissolved in water, excluding the water molecules themselves. These substances—composed of minerals, salts, metals, and ions—are picked up as water travels through the environment or plumbing systems. TDS is typically expressed in parts per million (ppm), equivalent to milligrams of dissolved solids per liter of water (mg/L). Common inorganic constituents include cations like calcium and sodium, and anions such as chloride and sulfate. While a high TDS reading is not inherently dangerous, it can affect the water’s taste and suitability for certain uses, acting only as a quantitative indicator of concentration.
The Specific TDS Value of Distilled Water
Theoretically, the perfect TDS of distilled water is 0 ppm because the purification method is designed to remove all dissolved matter. In practice, high-quality, freshly distilled water typically measures a TDS value between 0 and 5 ppm. This extremely low range confirms its classification as a highly purified water source. The slight deviation from zero is often due to the immediate absorption of atmospheric gases, such as carbon dioxide, which forms a weak acid that slightly increases conductivity. For comparison, the U.S. Environmental Protection Agency (EPA) suggests a maximum TDS level of 500 ppm for general drinking water.
How Distillation Achieves Low TDS
Distillation is a physical purification method that mimics the natural water cycle of evaporation and condensation. The process involves boiling the source water to create steam in a sealed chamber. As the water turns into vapor, nearly all dissolved solids—such as minerals, salts, and heavy metals—are physically left behind because they have significantly higher boiling points than water. The resulting steam, which is essentially pure water vapor, is directed into a cooling unit where it condenses back into liquid water, achieving up to 99.5% purity.
Practical Measurement of TDS
The TDS level of water is most commonly measured using a portable TDS meter. These handheld electronic devices do not measure the physical mass of the solids directly, which would require a time-consuming laboratory method. Instead, the meter measures the water’s electrical conductivity, as dissolved solids break down into electrically charged ions that increase the water’s ability to conduct a current. The meter measures this conductivity and applies a conversion factor to estimate the final TDS reading in ppm, providing a quick assessment of the total concentration of conductive substances.