Methods are available to identify and determine total arsenic, arsenite, and arsenate. Unpolluted fresh water normally does not contain organic arsenic compounds, but may contain inorganic arsenic compounds in the form of arsenate and arsenite. The electrothermal atomic absorption spectrometric method (3113) is the method of choice in the absence of overwhelming interferences. The hydride generation-atomic absorption method (3114) is preferred when interferences are present that cannot be overcome by standard electrothermal techniques (e.g., matrix modifiers, background correction). The silver diethyldithiocarbamate method (3500-As B), in which arsine is generated by reaction with sodium borohydride in acidic solution, is applicable to the determination of total inorganic arsenic when interferences are absent and when the sample contains no methylarsenic compounds. This method also provides the advantage of being able to identify and quantify arsenate and arsenite separately by generating arsine at different pHs. It is reprinted from the 20th Edition. The inductively coupled plasma (ICP) method (3120) is useful at higher concentrations (greater than 50 µg/L) while the ICP/mass spectrometric method (3125) is applicable at lower concentrations if chloride does not interfere. When measuring arsenic species, document that speciation does not change over time. No universal preservative for speciation measurements has been identified. Editorial revisions consist of new references to the QC practices considered to be an integral part of each method.