Application and Preparation of Pure and Ultrapure Water
As science and technology continue to advance, our understanding of natural phenomena is becoming more detailed and microscopic. In many laboratory experiments and testing processes, the requirements for reagents and the purity of the culture environment have reached the parts per billion (ppb) level, and in some cases even the parts per trillion (ppt) level. For instance, in life science research, water quality is crucial as it is highly sensitive to contaminants such as heavy metals and organic compounds. Similarly, high-performance liquid chromatography (HPLC) and other advanced analytical techniques require ultrapure water.
To meet these stringent demands, various professional organizations have established water quality standards. These include the Chinese National Standard GB6682-92 "Water Specifications and Experimental Methods for Analytical Laboratories," GB/T 11446.1-1997 "Electronic Water Specifications and Experimental Methods," the American Chemical Society (ACS), ASTM International, the Clinical and Laboratory Standards Institute (CLSI), and the United States Pharmacopeia (USP). This article will provide an overview of the main applications of pure and ultrapure water in different fields.
1. Reverse Osmosis (RO) Pure Water:
- Final cleaning of lab equipment
- Preparation of buffers and chemical reagents
- Production of microbial culture media
- Water for hydrogen generators, humidifiers, and autoclaves
- Drinking water for humans or experimental animals
2. Ultrapure Water:
- Plant and cell culture solutions
- Water for medical biochemical analyzers and hemodialysis systems
- Dilution of reagents and pharmaceutical preparations
- Water for physiological, pathological, and toxicological experiments
- Purified water for hospital laboratories and central facilities
- Water for atomic absorption spectroscopy
- Water for in vitro fertilization (IVF) procedures
- Water for HPLC and ion chromatography
- General laboratory and medical water needs
Currently, the most reliable and convenient method for producing pure and ultrapure water is through a dedicated water purification system. Since the introduction of the first ultrapure water system, the design and development of such systems have continuously focused on achieving the "best and most stable water quality."
First, the goal of achieving the best water quality:
1. Common Impurities in Natural Water:
These include dissolved inorganic substances, organic matter, particulate matter, microorganisms, and dissolved gases. A pure/ultrapure water system aims to remove these impurities as thoroughly as possible.
2. Main Processes for Water Purification:
Common methods include distillation, reverse osmosis, ion exchange, filtration, adsorption, and UV oxidation. The purification process can generally be divided into three steps: pretreatment (to produce pure water), ion exchange (which can generate 18.2 MΩ·cm ultrapure water), and post-treatment (to meet specific ultra-pure water requirements). The choice of method depends on the quality of the incoming water and the desired output quality.
3. Pretreatment:
This step typically includes a pretreatment unit and a reverse osmosis (RO) unit. Since RO further purifies the water, it's essential to remove impurities that could damage the RO membrane, such as large particles, chlorine, and calcium/magnesium ions. It’s important to tailor the pretreatment based on the source water quality, as improper setup can lead to reduced efficiency and shorter lifespan of critical components like the RO membrane.
Reverse osmosis uses a high-pressure pump to apply pressure greater than the osmotic pressure, forcing water molecules through a semi-permeable membrane to the low-concentration side. This process can remove 90% to 99% of pollutants, including inorganic ions. Due to its high efficiency, reverse osmosis is often used as the first stage in water purification systems, helping to extend the life of deionization columns. Given the high cost of RO membranes, it's recommended to choose a system with built-in protection for the RO membrane.
4. Ion Exchange:
Ion exchange involves exchanging positive ions in the water with H+ ions from the resin and negative ions with OH- ions. This process effectively removes almost all ionic species, theoretically achieving a resistivity of 18.2 MΩ·cm at 25°C. However, this method is not effective against organic matter or microorganisms, which can grow on the resin and become a source of contamination. Therefore, it should be combined with other purification methods during post-treatment.
5. Post-Treatment:
Post-treatment is designed to meet specific customer requirements, such as low organic content or low endotoxin levels. Techniques like ultrafiltration (UF) can remove endotoxins, while dual-wavelength UV oxidation reduces total organic carbon (TOC) to below 5 ppb. Microfiltration is also used to eliminate bacteria. UF membranes act as molecular sieves, filtering out larger molecules while allowing smaller ones to pass through, reducing endotoxin levels to less than 0.001 EU/ml.
Second, ensuring the most stable water quality:
While producing high-quality ultrapure water is essential, maintaining consistent water quality over time is equally important for users. Many manufacturers focus on maintenance details and regular replacement of consumables, but there is still room for improvement in system design to ensure long-term stability. By optimizing every stage of the process, users can achieve both high performance and reliability in their water purification systems.
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