Water used in scientific and medical environments must undergo extensive purification and categorization because minute impurities can compromise the integrity of experiments and analyses. To ensure reliable and repeatable results, laboratories use a stringent classification system to select the appropriate grade of water for specific needs. Type 2 water serves as a widely used standard in everyday laboratory settings.
Understanding the Purity Grading System
Contaminants like dissolved ions, organic molecules, and microorganisms interfere with sensitive chemical reactions and analytical instruments. Standardized systems, such as those established by the American Society for Testing and Materials (ASTM), classify laboratory water into different types based on purity specifications. This classification creates a hierarchy that guides scientists toward the correct water quality for any given procedure.
Type 1 water represents the highest grade, often called “ultrapure,” and is reserved for the most sensitive analytical techniques, such as high-performance liquid chromatography. Type 3 water is the lowest grade of purified water, typically used for initial rinsing of glassware or as a feed source for higher-purity systems. Falling between these two extremes, Type 2 water is the workhorse of the modern laboratory, offering a high level of purity suitable for general applications.
Defining the Quality Metrics
The quality of Type 2 water is defined by specific, measurable parameters that ensure its suitability for general laboratory use. The primary metric is resistivity, which measures the water’s resistance to electrical flow and indicates its ionic purity. For Type 2 water, this must be a minimum of \(1.0 \text{ M}\Omega\cdot\text{cm}\) at \(25^\circ\text{C}\). This high resistance corresponds to a very low concentration of dissolved salts and minerals.
Another defining characteristic is the Total Organic Carbon (TOC) content, which quantifies carbon-containing impurities. Type 2 water must have a TOC level no greater than \(50 \mu\text{g/L}\) (parts per billion) to minimize interference in organic-sensitive procedures. Standards also place limits on specific contaminants, requiring sodium, chloride, and total silica levels to be below \(5 \mu\text{g/L}\), \(5 \mu\text{g/L}\), and \(3 \mu\text{g/L}\), respectively.
To meet biological requirements, Type 2 water may also be classified with a maximum heterotrophic bacteria count, often limited to fewer than 100 colony-forming units per milliliter. These metrics ensure the water is clean enough for many applications without the expense and complexity of producing the absolute highest grade.
How Type 2 Water Is Produced and Applied
The creation of Type 2 water typically involves a multi-stage purification sequence to remove different types of contaminants. The process begins with pre-treatment, including filtration to remove sediment and activated carbon to eliminate chlorine and larger organic molecules. This pre-treated water then flows through a Reverse Osmosis (RO) unit, which pressurizes the water through a semi-permeable membrane to remove 90-99% of dissolved ions and organics.
Following the RO stage, the water is polished using ion exchange resins in a Deionization (DI) step, or sometimes with Electrodeionization (EDI), to remove remaining ionic impurities. Additional treatments, such as ultraviolet (UV) light exposure, may be incorporated to control bacteria growth and break down residual organic compounds. This comprehensive process yields water that meets the \(1.0 \text{ M}\Omega\cdot\text{cm}\) resistivity standard.
Type 2 water is utilized across a broad spectrum of laboratory functions due to its balance of high purity and cost-effectiveness. It is the preferred choice for general tasks and serves as the primary source water for systems that produce Type 1 ultrapure water.
Common applications for Type 2 water include:
- Preparing non-critical chemical reagents.
- Making microbiological buffers.
- Preparing culture media.
- Rinsing non-critical glassware.
- Feeding equipment like autoclaves and water baths.