1. Occurrence and Significance
Arsenic (As) is the third element in Group VA of the periodic table. It has an atomic number of 33, an atomic weight of 74.92, and valences of 3 and 5. The average abundance of As in the earth’s crust is 1.8 ppm; in soils it is 5.5 to 13 ppm; in streams it is less than 2 μg/L, and in groundwater it is generally less than 100 μg/L. It occurs naturally in sulfide minerals such as pyrite. Arsenic is used in alloys with lead, in storage batteries, and in ammunition. Arsenic compounds are widely used in pesticides and in wood preservatives.
Arsenic is nonessential for plants but is an essential trace element in several animal species. The predominant form between pH 3 and pH 7 is H2AsO4−, between pH 7 and pH 11 it is HAsO42−, and under reducing conditions it is HAsO2(aq) (or H3AsO3). Aqueous arsenic in the form of arsenite, arsenate, and organic arsenicals may result from mineral dissolution, industrial discharges, or the application of pesticides. The chemical form of arsenic depends on its source (inorganic arsenic from minerals, industrial discharges, and pesticides; organic arsenic from industrial discharges, pesticides, and biological action on inorganic arsenic).
Severe poisoning can arise from the ingestion of as little as 100 mg arsenic trioxide; chronic effects may result from the accumulation of arsenic compounds in the body at low intake levels. Carcinogenic properties also have been imputed to arsenic compounds. The toxicity of arsenic depends on its chemical form. Arsenite is many times more toxic than arsenate. For the protection of aquatic life, the average concentration of As3+ in water should not exceed 72 μg/L and the maximum should not exceed 140 μg/L. The United Nations Food and Agriculture Organization’s recommended maximum level for irrigation waters is 100 μg/L. The U.S. EPA primary drinking water standard MCL is 0.01 mg/L.1
2. Selection of Method
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 (Section 3113 B) is the method of choice in the absence of overwhelming interferences. The hydride generation atomic absorption method (Section 3114 B) is preferred when interferences are present that cannot be overcome by standard electrothermal techniques (e.g., matrix modifiers, background correction). The silver diethyl-dithiocarbamate 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. The inductively coupled plasma (ICP) emission spectroscopy method (Section 3120) is useful at higher concentrations (greater than 50 μg/L) while the ICP-mass spectrometric method (Section 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.