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Chromium VI, III and total Chromium analysis using Atomic Flame absorption

I am analysing chromium VI for some time, but now I want to analyse total chromium(VI and III) in the same sample. Is there any special procedures to do that?
Usually I analyse Cr (VI) with K2Cr2O7 standard diluted in 0.1M HNO3 calibration range of 10 to 100 mg/l, at 429 nm and samples also diluted in 0.1M HNO3. The equipment is an old Varian SpectrAA 250 Plus FAA.
Now I found that with a Cr(VI) calibration, a CrIII solution returns a very high value of absorvance (almost double!).
Any expert would like to help?
Thank you

Madalena Vieira
researcher - Braga, Portugal
February 4, 2008

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Cr analysis by AA is very tricky. It is very dependent on flame conditions, and other ions (esp. chloride) present in the sample.

I suggest using as lean a flame as is consistent with the sensitivity you require. Also, the addition of 0.25% w/v ammonium chloride to all samples/standards often helps to smooth out inconsistencies.

If you want to run Cr6+ individually, you could try precipitating it with Ba or Pb ion, then determining the result by difference of the two Cr results gotten via AA. Or, you could do the Cr6+ colorimetrically.

Dave Wichern
Consultant - The Bronx, New York
February 23, 2008