Chemical Changes

   A wide range of chemical changes are possible. For inorganic samples, controlling the pH can be useful in preventing chemical reactions. For example, metal ions may oxidize to form insoluble oxides or hydroxides. The sample is often acidified with HNO₃ to a pH below 2, as most nitrates are soluble, and excess nitrate prevents precipitation.

  Other ions, such as sulfides and cyanides,  are also pr eserved by pH co ntrol. Samples collected for NH₃ analysis are acidified with sulfuric acid to stabilize the NH₃as NH₄SO₄.  Organic species can also undergo changes due to chemical reactions.Storing the sample in amber bottles prevents photooxidation of organics (e.g., polynuclear aromatic hydrocarbons). Organics can also react with dissolved gases; for example, organics can react with trace chlorine to form halogenated compounds in treated drinking water samples. In this case, the addition of sodium thiosulfate can remove the chlorine. Samples may also contain microorganisms, which may degrade the sample biologically. Extreme pH (high or low) and low temperature can minimize microbial degradation. Adding biocides such as mercuric chloride orpentachlorophenol can also kill the microbes.