The Difference Between Pure Water and Ultra-Pure Water

Pure water and ultra-pure water are two commonly used types of high-purity water in laboratories, the electronics industry, and medical fields. Although they both appear as colorless and transparent liquids, they have significant differences in purity, preparation processes, and application scenarios. Understanding the distinctions between them is crucial for the correct selection and use.

Pure water typically refers to water that has undergone preliminary purification processes, removing most impurities. Its conductivity generally ranges from 1-10 μS/cm (microsiemens per centimeter), and its resistivity is approximately 0.1-1 MΩ·cm (megohm·centimeter). Common pure water standards include China's GB/T 6682-2008's Grade 3 water, as well as the U.S. ASTM's Type III water. Pure water is mainly produced through processes such as reverse osmosis (RO), ion exchange, or distillation, and can meet general laboratory cleaning and routine reagent preparation needs.

Ultra-pure water is a higher purity form of water, with a conductivity of less than 0.055 μS/cm (18.2 MΩ·cm), containing almost no ions, organic matter, microorganisms, or particulates. It meets the standard reference of China's GB/T 33087-2016 or the International Organization for Standardization (ISO) Type I water. The preparation of ultra-pure water requires a combination of multiple technologies such as reverse osmosis, electro-deionization (EDI), ultra-filtration, and UV oxidation, and must be completed in a dust-free environment for the final filtration.

Key Performance Indicators Comparison:

Resistivity/Electric Conductivity

The resistivity of pure water is typically below 1 MΩ·cm, whereas ultra-pure water needs to reach 18.2 MΩ·cm (theoretical maximum at 25℃). The lower the conductivity, the fewer the ions present in the water.

2. Organic Content (TOC)

Total Organic Carbon (TOC) in pure water may exceed 50 ppb (parts per billion), whereas ultra-pure water requires TOC < 5 ppb, and the high-end semiconductor industry demands < 1 ppb.

Microorganisms and Particles

Ultra-pure water requires stricter control over microorganisms (such as bacteria, endotoxins) and particulate matter (≥0.1 μm).

Dissolved Gas

Ultra-pure water requires the removal of dissolved oxygen, carbon dioxide, and other gases to prevent affecting high-precision experiments or wafer manufacturing.

The core of pure water production is the removal of ions and certain organic matter, common processes include:

Pre-treatment: Sand filtration and activated carbon adsorption to remove suspended particles and chlorine.

Primary Treatment: Reverse Osmosis Membrane (Desalination Rate 95%-99%) or Distillation;

Post-treatment: Further ion residue reduction through ion exchange resins.

Ultra-pure water systems require additional technology overlays:

1. Primary Purification: Reverse Osmosis or EDI as the foundation;

2. Deep Purification:

Mixed-bed ion exchange (polishing resin) removes residual ions.

Ultraviolet sterilization (dual wavelengths of 185nm/254nm) breaks down organic matter.

Ultrafiltration membranes (0.01 μm) retain colloids and microorganisms.

3. Terminal Processing: Nitrogen-sealed circulation systems prevent air pollution; non-silicone material pipes are required in some applications.

Application Scenario Differentiation:

Typical Uses of Pure Water

Laboratory ware washing and autoclave water usage.

Solvent Preparation in Chemical Production

Companion water for general analytical instruments (such as pH meters, UV spectrophotometers).

2. The Irreplaceability of Ultra-Pure Water

Semiconductor Manufacturing: The wafer cleaning process is highly sensitive to metal ions (Na⁺, K⁺) in water, exceeding limits can lead to circuit short circuits.

Life Sciences: Avoid DNA/RNAase contamination in PCR, gene sequencing, etc.

In the medical field, water used for hemodialysis must control endotoxins to less than 0.25 EU/ml.

Precision Analysis: Ultra-low background interference required for the mobile phase in Liquid Chromatography (HPLC) and Mass Spectrometry (MS).

Caution for Use and Storage:

Pure water can be stored short-term in polyethylene drums, but it is important to avoid long-term exposure to air to prevent the dissolution of carbon dioxide (which reduces pH).

The "ultra-pure" characteristic of ultra-pure water can degrade over time.

Resistance may drop by 50% within one hour after contact with air.

Recommend immediate use or the adoption of a closed-loop system.

Storage containers must be flushed with high-purity nitrogen and are made of low-extraction PVDF or PFA.

Choose between pure water and ultra-pure water based on actual needs, weighing the cost against performance. Pure water is sufficient for general physical and chemical experiments, whereas fields requiring molecular-level precision must rely on ultra-pure water. Users should regularly test water quality (such as resistivity, TOC) and establish a comprehensive quality management system to ensure the stability and reliability of water quality.