Application and Preparation of Pure and Ultrapure Water
With the rapid development of science and technology, our understanding of natural phenomena has become more detailed and microscopic. In many experiments and tests, the requirement for reagents and the purity of the culture environment has reached the parts per billion (ppb) level, and in some cases even the parts per trillion (ppt) level. For example, in life science research, water quality is extremely critical, as even small amounts of heavy metals or organic compounds can significantly affect experimental results. Similarly, high-performance liquid chromatography (HPLC) and other advanced techniques require ultrapure water to ensure accurate and reliable outcomes. To meet these stringent requirements, various organizations have established water quality standards. These include China's 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), the American Society for Testing and Materials (ASTM), the National Committee for Clinical Laboratory Standards (NCCLS), and the United States Pharmacopeia (USP). Now, let’s take a closer look at the main applications of pure and ultrapure water: 1. **Reverse Osmosis Pure Water**: Used for cleaning lab equipment, preparing buffers and chemical reagents, making microbial culture media, operating hydrogen generators, humidifiers, and autoclaves, and even as drinking water for people or animals. 2. **Ultrapure Water**: Essential for plant and cell culture, medical biochemical analyzers, hemodialysis, drug dilution, physiological and toxicological experiments, hospital laboratories, atomic absorption spectroscopy, in vitro fertilization, and HPLC or ion chromatography. It also supports various other specialized laboratory and medical applications. Today, the most reliable and convenient method for producing pure and ultrapure water is through an integrated water purification system. Over time, the design and production of such systems have evolved with a focus on delivering the best and most stable water quality. **First, Achieving the Best Water Quality** 1. **Common Impurities in Natural Water** These include dissolved inorganic substances, organic matter, particulates, microorganisms, and gases. A pure/ultrapure water system aims to remove these impurities as effectively as possible. 2. **Main Purification Processes** Common methods include distillation, reverse osmosis, ion exchange, filtration, adsorption, and UV oxidation. The process is typically divided into three stages: pretreatment (to produce pure water), ion exchange (to achieve 18.2 MΩ·cm resistivity), and post-treatment (to meet specific ultra-pure water requirements). 3. **Pretreatment** This stage includes pre-filtration and reverse osmosis units. Pretreated water must be free from contaminants that could damage the RO membrane, such as large particles, chlorine, and calcium/magnesium ions. Customized solutions are often needed depending on the source water quality. Many manufacturers lack expertise in this area, which can lead to inefficiencies and reduced lifespan of key components like the RO membrane. 4. **Reverse Osmosis** This process uses a high-pressure pump to push water through a semi-permeable membrane, removing up to 99% of contaminants, including inorganic ions. It is highly efficient and often used as the first step in water purification, helping to extend the life of deionization columns. Due to its importance, it's recommended to choose systems with built-in protection for the RO membrane. 5. **Ion Exchange** Ion exchange involves exchanging positive and negative ions in water with H+ and OH- ions from resin. This process can theoretically remove nearly all ions, achieving a resistivity of 18.2 MΩ·cm at 25°C. However, it is not effective against organic matter or microorganisms, which can grow on the resin and become a heat source. Therefore, it is often combined with other purification steps. 6. **Post-Treatment** This stage ensures the final water meets special requirements, such as low organic content or minimal endotoxins. Techniques include ultrafiltration to remove heat sources, dual-wavelength UV oxidation to reduce total organic carbon (TOC), and microfiltration to eliminate bacteria. Ultrafiltration membranes act as molecular sieves, reducing endotoxin levels to below 0.001 EU/ml. UV oxidation can lower TOC to under 5 ppb. **Second, Ensuring the Most Stable Water Quality** While producing high-quality ultrapure water is essential, maintaining that quality over time is equally important. Many manufacturers emphasize routine maintenance and consumable replacement, but few focus on long-term stability. Users need systems that not only produce clean water but also sustain its purity consistently. This requires thoughtful design, reliable components, and regular monitoring to ensure optimal performance and longevity.
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