Mercury (Hg) is the third element in Group IIB in the periodic table. It has an atomic number of 80, an atomic weight of 200.59, and valences of 1 and 2. The average abundance of Hg in the earth’s crust is 0.09 ppm; in soils it is 30 to 160 ppb; in streams it is 0.07 μg/L, and in groundwaters it is 0.5 to 1 μg/L. Mercury occurs free in nature, but the chief source is cinnabar (HgS). Mercury is used in amalgams, mirror coatings, vapor lamps, paints, measuring devices (thermometers, barometers, manometers), pharmaceuticals, pesticides, and fungicides. It is often used in paper mills as a mold retardant for paper.
The common aqueous species are Hg2+, Hg(OH)20, Hg0, and stable complexes with organic ligands. Inorganic mercury can be methylated in sediments when sulfides are present to form dimethyl mercury, (CH3)2Hg, which is very toxic and can concentrate in the aquatic food chain. Mercury poisoning occurred in residents of Japan in the 1950s as the result of consumption of shellfish that had accumulated mercury. In times past, mercury was used in the haberdashery industry to block hats (the cause of the “mad hatter” syndrome).
Mercury is considered nonessential for plants and animals. The U.S. EPA primary drinking water standard MCL is 2 μg/L.
The cold-vapor atomic absorption method (Section 3112 B) is the method of choice for all samples. The inductively coupled plasma mass spectrometric method (Section 3125) also may be applied successfully in some cases, even though mercury is not specifically listed as an analyte in the method. The dithizone method detailed in the 19th edition of Standard Methods can be used for determining high levels of mercury (>2 μg/L) in potable waters.
Because mercury can be lost readily from samples, preserve them by treating with HNO3 to reduce the pH to <2 (see Section 1060). Glass storage containers are preferred to plastic, because they can extend the holding time to 30 d, rather than only the 14 d allowed in plastic containers.