Purity and Classification of Laboratory Water

Distilled water, pure water, various cleaning solutions, grade I, II, and III water—laboratory water comes in all its forms and is frequently used in analytical testing. But are you using it correctly…?

I. Required Purity of Laboratory Water

The reproducibility of an experiment depends not only on good technique but also on the purity of the chemical reagents used and the precision of the analytical instrument. The purity of the chemical reagents used to prepare solutions and the water used in the experiment is also crucial. If contaminants in the water affect the experimental results, these substances must be removed. Furthermore, to obtain good reproducible results, using pure water that maintains stable water quality is necessary.

As the sensitivity of analytical systems used in experiments increases, the requirements for water purity become even higher. The reciprocal of resistivity is called conductivity, expressed in μS/cm. These two parameters are the most commonly used parameters for expressing water purity.

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℃). Furthermore, resistivity changes with the ionization constant of water and is therefore affected by water temperature.

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 often used to supplement and describe important parameters of water quality. Therefore, water purity standards are usually comprehensively described and graded based on one or more of these parameters.

II. Grading Standards for Pure Water

Laboratory pure water can be divided into four conventional grades: pure water, deionized water, laboratory Class II pure water, and ultrapure water.

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

2. Deionized water: Its conductivity is typically between 1.0-0.1 μS/cm. It is produced using mixed-bed ion exchange with strong anion exchange resin, but it has relatively high levels of organic matter and bacterial contamination. It can meet various needs, such as cleaning, preparing analytical standards, preparing reagents, and diluting samples.

3. Laboratory Grade II pure water: Its quality is suitable for a variety of needs, from reagent preparation and solution dilution to preparing nutrient solutions for cell cultures and microbiological research. This pure water can be produced by double distillation, or by integrating RO and ion exchange/EDI technologies, and can also be combined with adsorption media and UV lamps.

4. 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 pre-purified through ion exchange, RO membranes, or distillation, and then further purified by nuclear-level ion exchange to obtain ultrapure water. Ultrapure water typically has a resistivity of up to 18.2 MΩ·cm, TOC < 10 ppb, filters out particles as small as 0.1 μm, and has a bacterial content below 1 CFU/ml. Ultrapure water is suitable for various precision analytical experiments, such as high-performance liquid chromatography (HPLC) and ion chromatography (IC). Low-pyrogen ultrapure water is suitable for biological applications such as eukaryotic cell culture. Ultrafiltration technology is commonly used to remove large bioactive molecules, such as pyrogens (resulting in < 0.005 IU/ml) and undetectable nucleases and proteases.

III. Laboratory Water Standards