Importance of Using High Purity Water in Laboratory Analysis

Complete Article from Edition 45 APG eNewsletter

Article written by Kristel Fijolek, Quality Control Chemist, APG

There are three types of reagent grade water ranging from Type I to Type III. The type of water selected should be based upon the compound or compounds being analyzed for.  Water requirements may be different for inorganic, organic, and microbiological analysis. Reagent grade water should not contain substances that could interfere with the analytical method. Table 1 summarizes laboratory water parameters and specifications by type from the American Society for Testing and Materials (ASTM). 

Table 1. Water Quality Specifications

Type I water is the purest and should be used when test methods require maximum precision, such as analysis by HPLC, ICP, and IC among others. Type I water is essentially free of dissolved and ionic compounds. Type I water is usually prepared by further treating Type II water by distillation or reverse osmosis followed by passing it through a mixed-bed deionizer and a 0.2 μm pore filter.  Type I water is often pre-treated or post-treated with activated carbon to remove organic contamination and chlorine.  Since it is free of bacteria, this type of water is suitable for microbiological use. Type I water should not contain any traces of the compound or compounds being analyzed for at the detection limits of the analysis method.

Type II water contains very low concentrations of inorganic and organic contaminants. This water type is suitable for analysis such as AAS and wet chemistry work.  It is usually prepared by multiple distillations, ion exchange, or reverse osmosis.  Type II water is suitable for reagent preparation and analysis in which the presence of bacteria is not an issue.

Type III water has the lowest purity of the three, and it is suitable for glassware washing and qualitative analysis.  Type III water is usually prepared by a single distillation.

Reagent grade water can be additionally classified when bacterial levels need to be kept under control.  Table 2 shows the ASTMs further breakdown of water types.

Table 2.

It is important to choose the correct type of water for the analysis method and the analytes to be analyzed, and review your analytical methods for requirements. Laboratory instruments are increasingly sensitive, and it is not uncommon to analyze for compounds in the ppb range. Proper water selection will help to ensure accurate and reproducible results.  It is also good practice to verify the quality of water being delivered from your in-house system on an annual basis using the limits specified in Table 1 and 2 where applicable.