What are the differences between common types of water used in laboratories?

What are the differences between common types of water used in laboratories? (Ultrapure water, deionized water, RO water, distilled water, double-distilled water)

The core distinctions between different types of laboratory water (such as ultrapure water, deionized water, RO water, etc.) lie in their preparation processes and levels of impurity removal, which directly impact their suitability for various experiments. The following comparison across three dimensions—“Preparation Method - Purity Indicators - Typical Applications”—will help you quickly grasp the selection logic:

I. Ultrapure Water (UP Water)
Production Method: Based on RO water/deionized water, undergoes multi-stage purification (ion exchange + ultrafiltration + UV oxidation + 0.22μm filtration) to remove ions, organic compounds, microorganisms, and particles.
Purity Indicators:

Resistivity: ≥18.2 MΩ·cm (25°C, theoretical maximum for pure water);
Total Organic Carbon (TOC): <5 ppb (trace organic matter);
Microorganisms: <1 CFU/mL (sterile);
Particles (≥0.2μm): <1/mL.
Primary Impurities: Virtually none (only trace dissolved gases like O₂ and CO₂).
Typical Applications:
High-sensitivity analysis (e.g., HPLC-MS, ICP-MS, capillary electrophoresis);
Molecular biology experiments (e.g., PCR, gene sequencing, primary cell culture);
Mobile phase or diluent for precision instruments (e.g., mass spectrometers, atomic absorption spectrometers).
II. Deionized Water (DI Water)
Preparation Method: Removes cations (e.g., Na⁺) and anions (e.g., Cl⁻, Ca²⁺) from water via ion exchange resins (cation resin + anion resin), but cannot effectively remove organic matter, microorganisms, or particulates.
Purity Specifications:

Resistivity: 1–18 MΩ·cm (depending on resin exchange capacity);
TOC: 50–500 ppb (may contain organic matter not adsorbed by resin);
Microorganisms: May contain trace amounts (resins are prone to bacterial growth).
Primary Impurities: Organic matter (e.g., humic acids), microorganisms, particulates.
Typical Applications:
General chemical experiments (e.g., preparing non-critical buffers, cleaning glassware);
Experiments sensitive to ions but not organic matter (e.g., routine spectrophotometry);
Pre-treatment for ultrapure water (requires further purification to remove organic matter).
III. RO Water (Reverse Osmosis Water)
Preparation Method: Filtered through reverse osmosis membranes (pore size ~0.1-1 nm), utilizing high pressure to force water from the high-concentration side to the low-concentration side, removing 95-99% of ions (e.g., Na⁺, SO₄²⁻), macromolecular organic compounds (e.g., proteins), and bacteria.
Purity Specifications:

Resistivity: 0.055–1 MΩ·cm (approximately 10–100 times that of tap water);
TOC: 100–1000 ppb (small-molecule organic compounds may permeate the membrane);
Microorganisms: <100 CFU/mL (membranes retain most bacteria).
Primary Impurities: Small-molecule organic compounds (e.g., methanol), dissolved gases (e.g., CO₂), some monovalent ions (e.g., Cl⁻).
Typical Applications:
Basic laboratory water (e.g., cleaning glassware, preparing non-critical reagents);
Pre-treatment for ultrapure/deionized water (reducing load on subsequent purification systems);
Experiments with low water quality requirements (e.g., standard medium preparation).
IV. Distilled Water (DW)
Preparation Method: Removes most inorganic salts, organic matter, and microorganisms through heating-vaporization-condensation (single distillation). However, volatile substances (e.g., NH₃, CO₂) may condense back into the water with the steam.
Purity Indicators:

Resistivity: 0.5–5 MΩ·cm (dissolved CO₂ forms H⁺ and HCO₃⁻, reducing resistivity);
TOC: 100–1000 ppb (volatile organic compounds may remain);
Microorganisms: <1000 CFU/mL (high temperatures kill most bacteria).
Primary Impurities: Volatile substances (e.g., ammonia, phenol), dissolved gases (CO₂), trace ions (e.g., HCO₃⁻).
Typical Applications:
Traditional chemical experiments (e.g., preparing common reagents, cleaning non-precision instruments);
Scenarios with lower water quality requirements (e.g., laboratory humidifier water);
Basic laboratory water in earlier stages (now gradually replaced by RO water).
V. Double Distilled Water (ddH₂O)
Preparation Method: Distilled water undergoes secondary distillation (sometimes with oxidizing agents like KMnO₄ added during secondary distillation to remove organic matter), further reducing impurities.
Purity Specifications:

Resistivity: 1–10 MΩ·cm (higher than single-distilled water, still affected by CO₂);
TOC: 50–500 ppb (secondary distillation removes some volatile organic compounds);
Microorganisms: <100 CFU/mL (secondary high-temperature distillation achieves more thorough bacterial elimination).
Primary impurities: Trace volatile substances (e.g., low-boiling-point organics), dissolved gases.
Typical Applications:
Basic molecular biology experiments (e.g., preparing PCR buffers, electrophoresis reagents);
Scenarios requiring slightly higher water quality than single-distilled water (e.g., plant tissue culture);
Replacing deionized water in experiments insensitive to organic compounds (lower cost).
VI. How to Select Suitable Water?
Experiment Type
Recommended Water Quality
Reason
High-precision analysis (HPLC-MS, ICP-MS, etc.)
Ultrapure water
Extremely low impurities to avoid interfering with mass spectrometry signals
Molecular biology (PCR, gene sequencing)
Ultrapure water/double-distilled water
Sterile, low organic content to prevent enzyme inhibition or DNA degradation
General chemical analysis (e.g., titration)
Deionized water/RO water
Low ion concentration sufficient for routine reactions
Cleaning glassware/basic reagent preparation
RO water/distilled water
Low cost, sufficient impurity levels for cleaning purposes
Key Reminder: Laboratories must select water quality based on specific experimental sensitivity requirements (e.g., detection of trace substances) and cost considerations (ultrapure water preparation costs approximately 5-10 times that of RO water). Avoid “over-purification” or “insufficient water quality” that may compromise experimental results.