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Sample Preparation - Silver (Ag) precipitation using EPA Method 200.7

 
At times we are required to use EPA method 200.7 to analyze water samples. The digestion technique uses 0.5% HCl and 1.0% HNO3, but Ag concentrations greater than 100 ppb precipitate. An equivalent ASTM method D1976-07 uses 5.0% HCl and 1% HNO3. Do you have a feeling for what concentration of HCl would be best to keep Ag in solution? I believe the ASTM method would be better to use, but I need to convince others in our workgroup. In general, I believe the 200.7 method is out of date, and prohibits the use of many features available with the more modern instrumentation. Have you used 200.7, and what is your opinion of 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 Tl2Oto 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.

 
Posted: 09/06/18 03:27:35

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