Sample Preparation - Silver (Ag) precipitation using EPA Method 200.7
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As a general rule we propose a minimum of 1% (v/v) HCl for every ppm of Ag in solution in order to promote long term stability (e.g., 10 ppm Ag would require at least 10% HCl). This is based upon diluting ~37 wt.% concentrated HCl, and regardless the Ag will be photosensitive so exposure to light should be minimized. We tend to not recommend HCl concentrations lower than ~2% regardless of the Ag concentration, and at higher Ag levels the amount of acid required decreases somewhat. For example, 50 ppm Ag is stable in 30% HCl, 100 ppm Ag is stable in 40% HCl, etc. We have not used 200.7 directly, but would agree that the recommended digestion matrix is not suitable for Ag except at low ppb levels, and that the recommended use of a TlNO3 material for thallium is not appropriate for the same reason (i.e., possible TlCl precipitation; we’d recommend using Tl2O3 to create Tl+3solutions). We have given technical presentations on the preparation of stable and compatible calibration standards using 200.7 as an example, and would be happy to discuss those points in greater detail if desired. |