What test determines the purity of reagent grade distilled water?

The conductivity test is crucial in determining the purity of reagent grade distilled water because it measures the water's ability to conduct electrical current, which is directly related to the concentration of ionic impurities present in the water. Pure distilled water is expected to have very low conductivity, indicating a minimal presence of dissolved salts and ions.

When the conductivity is measured, any increase from the expected low level suggests contamination from sources such as minerals or industrial pollutants, which would render the water less pure. Therefore, this test serves as an essential quality control measure, ensuring that the distilled water meets the required purity standards for laboratory use.

Other tests like the pH test, though relevant in other contexts, do not provide a comprehensive measure of water purity with respect to ionic contaminants. Hardness tests focus on specific ions (primarily calcium and magnesium) rather than the overall ionic content. The colorimetric test is used for detecting specific chemical substances rather than measuring general purity. Thus, the conductivity test stands out as the most effective method for assessing the suitability of distilled water in a laboratory setting.