Chemical Stability and Compatibility - Precious metals analysis in potassium iodide (KI) matrix

A 10% HCl matrix would be suitable for these elements (Pd, Au, and Ag), and the formation of KCl would be expected after mixing with the KI/I2; however, the KCl should not be an issue for the analysis (other than the matrix differences between samples and standards). A different concern is the formation of AgCl, though this reaction is reversible with the addition of more HCl. A general rule of thumb is that 10 ppm Ag is soluble in 10% (v/v) HCl, though the Ag will be photosensitive and exposure to light should be minimized for these solutions. We do have some other concerns with regard to the measurement though, in particular the stability of these elements in the KI/I2 matrix. The first is the potential of triiodide in solution to reduce the precious metals (in particular the Pd), causing an apparent ‘loss’ of the metal(s), and the second is the formation of the AgI complex, which should be significantly less soluble than the aforementioned AgCl complex. If possible, we would recommend using a standard additions approach, whereby you spike the Pd/Au/Ag standard directly into the sample and analyze immediately. If you follow this approach, and wish to create a high ppm Pd, Au, Ag standard (e.g., 100 ppm), you’ll need significant HCl to stabilize the Ag; in this example (100 ppm Ag) we’d recommend >30% (v/v) HCl.