By Paul Gaines, Ph.D.
Many chemists often ask us about Ag chemistry. The fact is that silver is a relatively trouble-free element, provided it is kept away from HCl. We tend to keep Ag+ solutions in nitric acid because these solutions are stable to photo-reduction. However, when HCl is added, two things can occur. First, you may get precipitation if modest amounts of HCl are present. Second, with higher amounts of HCl you form a soluble chloride complex which will undergo photo-reduction to the metal. This occurs at a rate that is proportional to the amount of light exposure.
If you must use an HCl matrix, here are some important tips:
These lower levels of Ag+ are typically not a problem since modern ICP-OES instrumentation is so very sensitive and Ag+ itself is also quite sensitive. If you use glassware exposed to Ag+ + HCl, you should keep it isolated to avoid Ag+ contamination when using HNO3matrices. These simple precautions along with the analytical properties listed below should make your analyses of Ag+ go much smoother.
Chemical Compatibility: Stable in HNO3 and HF. Avoid basic media. Ag forms more insoluble salts than any other metal. It is also subject to photochemical reduction to the metal in HCl media, although 10 µg/mL solutions in 10% HCl [AgClx1-x] are commonly used in the analytical laboratory. The most common solubility problems exist with arsenate, arsenite, bromide, chloride, iodide, carbonate, chromate, cyanide, iodate, oxalate, oxide, sulfate, sulfide, tartrate, and thiocyanate in aqueous media. The addition of nitric acid renders many of these salts soluble.
Stability: 2-100 ppb levels stable for 75+ days when mixed with equivalent levels of all other elements, including the precious metals (where chloride is present), when in 1% HNO3/LDPE container. 1-10,000 ppm solutions chemically stable for years in 1-5% HNO3/LDPE container.
Ag Containing Samples (Preparation & Solution): Metal (soluble in HNO3); Oxides (soluble in HNO3); Ores (digestion with conc. HNO3).
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