Common Problems with Ag, As, S, Ba, Pb and Cr

Chromium (Cr)

The major difficulty that I have experienced with Cr is that it often exists in forms that are difficult to put into solution. Chromite (FeO.Cr₂O₃), chromic oxide, pigments, stainless steel and ferro-chrome all present a challenge but the hexavalent chromium oxides are the most difficult. If the oxide has been ignited (pigments) the refractory nature is such that an analyst confronted with the task of bringing about solution will never forget the experience. The most common approach is to perform a fusion. Fusions that have been used include but are not limited to potassium and sodium bisulfate, carbonate (sodium or potassium), sodium peroxide, NaOH / KNO₃, and NaOH / Na₂O₂. In addition, the fusion will not be complete unless the chrome is finely divided and mixed with the flux.

• Know you sample to the fullest extent possible. The possible chemical forms of Cr should influence the sample preparation technique employed.
• If your sample is an inorganic pigment containing Cr then you know that you have an extremely refractory material to dissolve.
• If you are unfamiliar with your sample type a literature search is strongly suggested.
• Method validation using a CRM containing Cr in the suspected or known chemical form(s) is vital. The importance of CRMs prepared from 'real world' materials is critical (i.e., synthetic CRMs are likely to contain easily dissolved compounds).
• Avoid mixing water-soluble hexavalent chrome with Ba or Pb to avoid loss of Cr Pb and Ba as the insoluble chromates.

• Cr elemental data

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