Various Types of Water in the Laboratory

1. Properties of Water

One water molecule (H₂O) is composed of one oxygen atom and two hydrogen atoms bonded together by bending. Because the centers of positive and negative charges are not aligned, it is a polar molecule. When two water molecules coexist, they are attracted to each other and maintain a certain distance through electrostatic interactions and hydrogen bonds. One water molecule can simultaneously bond with four other water molecules, forming a crystalline, orderly structure.

In water molecule aggregates, the network structure of hydrogen bonds partially breaks, resulting in a state of successive movement and change. Therefore, water as a whole exists as a liquid, and this structural change can occur up to 10¹² times per second.

Generally speaking, water tastes good if it contains appropriate amounts of sodium and potassium ions and silicates, while it tastes bad if it contains a large amount of residual salts, such as magnesium and calcium, which are not acid-base neutral salts. In other words, water contains many other components besides H₂O, and the types and amounts of these components determine the taste of the water.

Water readily dissolves salts. Even when cations and anions are strongly bound together by electrostatic interactions, electrolysis is easily achieved in water. This is because water molecules can combine with ions to form "hydrated ions." Ions have very small radii, and ions with large charges interact strongly with water molecules, causing the water molecules to align closely around the ions. At this point, cations interact with oxygen atoms carrying negative polarities, while anions form the opposite structure.

2. Impurities in Water

① Soluble inorganic substances: Inorganic salts, dissolved gases, heavy metals, hardness components (calcium, magnesium, etc.).

② Soluble organic substances: Lignin, tannins, humic acid, endotoxins, RNA-degrading enzymes, pesticides, chloroform, endocrine disruptors, surfactants, organic solvents.

③ Microparticles: Rust, colloids, suspended solids, solid particles.

④ Microorganisms: Bacteria, algae.

3. Requirements for Laboratory Water

An experiment is the act of verifying hypotheses derived from observed phenomena. Whether a hypothesis can be proven true is crucial to its reproducibility. The reproducibility of an experiment requires not only good technique but is also affected by the purity of the chemical reagents used and the precision of the analytical instruments. The purity of the chemical reagents used to prepare the solutions and the water used in the experiment is also very important. If contaminants in the water affect the experimental detection, these substances must be removed. Furthermore, to obtain good reproducible results, it is necessary to use pure water that maintains stable water quality.

With the increasing sensitivity of analytical systems used in experiments, higher requirements have been placed on the purity of water. In water, two electrodes with a surface area of 1 cm² are placed 1 cm apart and an electric current is applied to monitor the conductivity between the electrodes. The resistance between the electrodes can be determined by the applied voltage and the measured current. This value is usually called resistivity or specific resistance in water quality analysis, and its unit is expressed in MΩ·cm. The reciprocal of resistivity is called conductivity, expressed in μS/cm. These two parameters are the most commonly used parameters to express the purity of water.

Removing ions from tap water increases resistivity (decreases conductivity), but this increase is not unlimited. This is because some water molecules ionize into hydrogen and hydroxide ions, with a resistivity limit of 18.248 MΩ·cm (25°C). Furthermore, resistivity changes with the ionization constant of water and is therefore affected by water temperature. For example, ultrapure water at 25°C has a resistivity of 18.2 MΩ·cm, but it is 84.2 MΩ·cm at 0°C and 1.3 MΩ·cm at 100°C. Around 25°C, a 1°C increase in temperature will decrease the resistivity by 0.84 MΩ·cm. Therefore, the resistivity value compensated for at 25°C is often used as a benchmark.

In addition, parameters such as Total Organic Carbon (TOC), pyrogen and endotoxin content, bacterial content, particulate matter content, microbial content, and total dissolved solids (TDS) are also frequently used to supplement and describe important parameters of water quality. Therefore, water purity standards are usually described and graded by a combination of one or more of these parameters.

4. Grading Standards for Pure Water

Pure Water: The lowest level of purification, typically with a conductivity between 1-50 μS/cm. It can be produced using a single weakly basic anion exchange resin, reverse osmosis, or single distillation. Typical applications include glassware cleaning, autoclaving, constant temperature and humidity chambers, and water for cleaning machines.

Deionized Water: Conductivity typically between 1.0-0.1 μS/cm. Produced using mixed-bed ion exchange with a strong anion exchange resin, but it has relatively high levels of organic matter and bacterial contamination, meeting various needs such as cleaning, preparation of analytical standards, reagent preparation, and sample dilution.

Laboratory Grade II Pure Water: Conductivity <1.0 μS/cm, total organic carbon (TOC) content less than 50 ppb, and bacterial content less than 1 CFU/ml. Its water quality is suitable for a variety of needs, from reagent preparation and solution dilution to preparing nutrient solutions for cell cultures and microbial research. This pure water can be produced by double distillation, or by integrating multiple technologies such as RO and ion exchange/EDI, and can also be combined with adsorption media and UV lamps.

Ultrapure water: This level of pure water approaches the theoretical purity limit in terms of resistivity, organic matter content, particle content, and bacterial content. It is obtained by pre-purification through ion exchange, RO membranes, or distillation, followed by nuclear-level ion exchange purification. Typically, ultrapure water has a resistivity of up to 18.2 MΩ-cm, TOC < 10 ppb, filters out particles of 0.1 μm or even smaller, and has a bacterial content of less than 1 CFU/ml. Ultrapure water is suitable for various precision analytical experiments, such as high-performance liquid chromatography (HPLC), ion chromatography (IC), and ion capture-mass spectrometry (ICP-MS). Low-pyrogen ultrapure water is suitable for biological applications such as eukaryotic cell culture. Ultrafiltration technology is typically used to remove large molecular bioactive substances, such as pyrogens (resulting in <0.005 IU/ml) and undetectable nucleases and proteases.

Currently, the most widely used pure water standards in the world include: the International Organization for Standardization (ISO), the American College of Clinical Pathology (CAP) Standard for Pharmaceutical Grade Water, the American Society for Testing and Materials (ASTM), the National Committee on Clinical Trial Standards (NCCLS), and the American Pharmaceutical Association (USP). my country also has corresponding pure water standards: the Chinese National Standard for Electronic Grade Ultrapure Water GB/T 11446-1997 and the Chinese National Standard for Laboratory Water GB/T 6682-2008. Therefore, the vast majority of pure water systems on the market, whether imported or domestically produced, are designed according to these standards.

The above information on laboratory water applications was compiled by the marketing department of Sichuan Youpu Ultrapure Technology. For more details, please call our 24-hour nationwide customer service hotline at 400-884-6567.