Silver (Ag) is the second element in Group IB of the periodic table. It has an atomic number of 47, an atomic weight of 107.87, and valences of 1 and 2. The average abundance of Ag in the earth’s crust is 0.08 ppm; in soils it is <0.01 to 0.5 ppm; in streams it is 0.3 μg/L; in U.S. drinking waters it is 0.23 μg/L, and in groundwater it is <0.1 μg/L. Silver occurs in its native state and in combination with many nonmetallic elements such as argentite (Ag2S) and horn silver (AgCl). Lead and copper ores also may yield considerable silver. Silver is widely used in photography, silverware, jewelry, mirrors, and batteries. Silver iodide has been used in the seeding of clouds, and silver oxide to a limited extent is used as a disinfectant for water.
In acidic water Ag+ would predominate, and in high-chloride water a series of complexes would be expected. Silver is nonessential for plants and animals. Silver can cause argyria, a permanent, blue-gray discoloration of the skin and eyes that imparts a ghostly appearance. Concentrations in the range of 0.4 to 1 mg/L have caused pathological changes in the kidneys, liver, and spleen of rats. Toxic effects on fish in fresh water have been observed at concentrations as low as 0.17 μg/L. For freshwater aquatic life, total recoverable silver should not exceed 1.2 mg/L.
The atomic absorption spectrometric methods (Sections 3111 B and C) and the inductively coupled plasma methods (Sections 3120 and 3125) are preferred. The electrothermal atomization method (Section 3113 B) is the most sensitive for determining silver in natural waters. The dithizone method detailed in the 19th edition of Standard Methods can be used when an atomic absorption spectrometer is unavailable. A method suitable for analysis of silver in industrial or other wastewaters at levels above 1 mg/L is available.1
If total silver is to be determined, acidify sample with conc nitric acid (HNO3) to pH <2 at time of collection. If sample contains particulate matter and only the dissolved metal content is to be determined, filter through a 0.45-μm membrane filter at time of collection. After filtration, acidify filtrate with HNO3 to pH <2. Complete analysis as soon after collection as possible. Some samples may require special storage and digestion; see Section 3030 D.