Cadmium (Cd) is the second element in Group IIB of the periodic table. It has an atomic number of 48, an atomic weight of 112.41, and a valence of 2. The average abundance of Cd in the earth’s crust is 0.16 ppm; in soils it is 0.1 to 0.5 ppm; in streams it is 1 μg/L, and in groundwaters it is from 1 to 10 μg/L. Cadmium occurs in sulfide minerals that also contain zinc, lead, or copper. The metal is used in electroplating, batteries, paint pigments, and in alloys with various other metals. Cadmium is usually associated with zinc at a ratio of about 1part cadmium to 500 parts zinc in most rocks and soils.
The solubility of cadmium is controlled in natural waters by carbonate equilibria. Guidelines for maximum cadmium concentrations in natural water are linked to the hardness or alkalinity of the water (i.e., the softer the water, the lower the permitted level of cadmium). It is nonessential for plants and animals. Cadmium is extremely toxic and accumulates in the kidneys and liver, with prolonged intake at low levels sometimes leading to dysfunction of the kidneys. The United Nations Food and Agriculture Organization recommended maximum level for cadmium in irrigation waters is 10 μg/L. The U.S. EPA primary drinking water standard MCL is 10 μg/L.
The electrothermal atomic absorption spectrometric method (Section 3113 B) is preferred. The flame atomic absorption methods (Sections 3111B and C) and inductively coupled plasma methods (Sections 3120 and 3125) provide acceptable precision and bias, with higher detection limits. Anodic stripping voltammetry (Section 3130 B) can achieve superior detection limits, but is susceptible to interferences from copper, silver, gold, and organic compounds. When atomic absorption spectrometric or inductively coupled plasma apparatus is unavailable and the desired precision is not as great, the dithizone method detailed in the 19th Edition of Standard Methods is suitable.