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Cr3+ determination in plastic etching solutions

Dear Sirs

I work in the departement of quality control and I would appreciate very much if you could help me.

For at least 5 years, we plate plastics: etching, neutralization, activation, activation, electroless nickel, then chrome. We've always controlled the concentration of Cr6+ and Cr3+ with the test method suggested in the Metal Finishing Guidebook at the lab, and with the porous pot in the bath. Suddenly, the test method gives the same result for the concentration of Cr6+ and for Cr3+, as if there where no Cr3+. But I am sure there is Cr3+ present because the solution works very well.

We operate at 72 C, 390 g/l of chomic acid, 20% W/W sulfuric acid, air agitation; Cr3+ should be m·x 45 g/l. What could be happening?

Thank you very much for your advise.

Mariangela Terlizzi
- Caracas, Venezuela
2005

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Hi Mariangela,

Can you tell us the methods you used? I am doing very similar analysis and would be most grateful if we could exchange analysis methods. I read from the book that the concentration of Cr6+ can be determine by using ammonium bifluoride, conc. HCl, 10% KI and starch solution with Na₂S₂O₃ as titrant. Similarly, concentration of Cr3+ could be determine by treament with Na₂O₂ followed by heating and using the same titration method as Cr6+. Are you using similar methods? Have you try ion-chromatography for such analysis?

Tommy Yuen
- China
October 13, 2009

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Ed. note: letter 44293 may help.