The core difference between different types of water used in laboratories (such as ultrapure water, deionized water, RO water, etc.) lies in the preparation process and the degree of impurity removal. The type of water affects its applicability in different experiments. Deionized water and distilled water are most commonly used in laboratory bottle washers to clean various glassware. Ultrapure water is also chosen for experiments with higher requirements. What are the differences between these water sources? This article will detail the differences between commonly used water in laboratories.

Classification of Laboratory Water

Natural water

Natural water typically contains five types of impurities:

  1. Electrolytes: These include charged particles. Common cations include H+, Na+, K+, NH4+, Mg2+, Ca2+, Fe3+, Cu2+, Mn2+, and Al3+, among others; common anions include F-, Cl-, NO3-, HCO3-, SO42-, PO43-, H2PO4-, and HSiO3-, among others.
  2. Organic substances, such as organic acids, pesticides, hydrocarbons, alcohols, and esters.
  3. Particulate matter.
  4. Microorganisms.
  5. Dissolved gases, including N₂, O₂, Cl₂, H₂S, CO, CO₂, CH₄, etc.

Water purification involves removing these impurities. The more thoroughly the impurities are removed, the purer the water quality becomes.

Drinking water

Drinking water refers to water that can be consumed directly, produced through methods such as electrodialysis, ion exchange, reverse osmosis, distillation, and other appropriate processing techniques. It generally contains no harmful substances or bacteria, including organic pollutants, inorganic salts, additives, or various impurities. Pure water typically has an electrical conductivity of 1.0–0.1 µS/cm, a resistivity (at 25°C) of (1.0–10.0) MΩ·cm, and a salt content of <1 mg/L.

Pure water

Pure water refers to H₂O that contains no impurities. From an academic perspective, pure water is also known as high-purity water, which refers to water with extremely high chemical purity. It is primarily used in fields such as biology, chemistry and chemical engineering, metallurgy, aerospace, and power generation. However, it has very high requirements for water quality purity, so its most common application is still in the electronics industry.
Preparation method: Filtered through reverse osmosis membranes (pore size approximately 0.1–1 nm), utilizing high pressure to force water to permeate from the high-concentration side to the low-concentration side, removing 95–99% of ions (e.g., Na⁺, SO₄²⁻), large organic molecules (e.g., proteins), and bacteria.

Purity indicators:

  1. Resistivity: 0.055–1 MΩ·cm (approximately 10–100 times that of tap water);
  2. TOC: 100–1000 ppb (small organic molecules may pass through the membrane);
  3. Microorganisms: <100 CFU/mL (the membrane can retain most bacteria).
  4. Main impurities: Small organic molecules (e.g., methanol), dissolved gases (e.g., CO₂), and some monovalent ions (e.g., Cl⁻).

Typical applications:

  1. Basic laboratory water (e.g., cleaning glassware, preparing non-critical reagents);
  2. Pre-treatment for ultra-pure water/deionized water (reducing the load on subsequent purification systems);
  3. Experiments with low water quality requirements (e.g., preparing standard culture media).

Ultrapure water

Ultrapure water, also known as UP water, refers to water with a resistivity of 18 MΩ*cm (at 25°C). This water contains virtually no impurities other than water molecules, and is free of bacteria, viruses, chlorinated dioxins, and other organic compounds.
Preparation method: Using RO water/deionized water as a base, multiple purification steps (ion exchange + ultrafiltration + UV oxidation + 0.22 μm filtration) are employed to remove ions, organic compounds, microorganisms, and particles.

Purity specifications:

  1. Resistivity: ≥18.2 MΩ·cm (at 25°C, the theoretical maximum value for pure water);
  2. Total Organic Carbon (TOC): <5 ppb (trace amounts of organic matter);
  3. Microorganisms: <1 CFU/mL (sterile);
  4. Particles (≥0.2 μm): <1 per mL.
  5. Major impurities: Virtually none (only trace amounts of dissolved gases such as O₂ and CO₂).

Typical Applications:

  1. High-sensitivity analysis (e.g., HPLC-MS, ICP-MS, capillary electrophoresis);
  2. Molecular biology experiments (e.g., PCR, gene sequencing, primary cell culture);
  3. Mobile phase or diluent for precision instruments (e.g., mass spectrometers, atomic absorption spectrometers).

Deionized Water

Deionized water has a wide range of applications in modern industry. Since the ion content in deionized water can be artificially controlled, its physical, chemical, and pathological parameters—such as resistivity, solubility, corrosion, and viral/bacterial content—are effectively regulated.
Preparation method: Ion exchange resins (cation exchange resin + anion exchange resin) are used to adsorb cations and anions (e.g., Na⁺, Cl⁻, Ca²⁺) from water, but they cannot effectively remove organic matter, microorganisms, or particles.

Purity specifications:

  1. Resistivity: 1–18 MΩ·cm (depending on resin exchange capacity);
  2. TOC: 50–500 ppb (may contain organic matter not adsorbed by the resin);
  3. Microorganisms: May contain trace amounts (resins are prone to bacterial growth).
  4. Main impurities: Organic matter (e.g., humic acid), microorganisms, particles.

Typical applications:

  1. General chemical experiments (e.g., preparing non-critical buffers, cleaning glassware);
  2. Experiments sensitive to ions but not to organic matter (e.g., routine spectrophotometry);
  3. Pre-treatment for ultra-pure water (further purification is required to remove organic matter).