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Analysis of Trivalent Chrome


Q. I find difficulty in analyzing precisely and accurately trivalent Chrome in Chrome Plating Bath, where its permitted maximum concentration is very low, i.e. 4 g/l max, in a 250~g/l Chromic Acid solution. According to the Iodometric method of analysis we use, above low concentration of trivalent Chrome corresponds to only 0.4 ml of titrant (very small volume) which does not assure repeatability and accuracy of method. Could you suggest a more precise, repeatable and accurate method?

Angela Nikolaou
Hellenic Aerospace industry - Athens, Greece


A. A couple of ways to improve your present method.
1. use a titrant that is half as strong, this would get you up to 0.8 mL. 2. You might consider a larger sample size. 3. My favorite would be to use a mini pipet- -Hach sells a nice one that is fairly cheap. Accuracy to 0.01 mL.

James Watts
- Navarre, Florida


Q. Dear James,

Thanks for your answer. 1. We have tried in the past your suggestion but the results were not repeatable. We also suspect that the oxidation of Cr+3 to Cr+6 is not complete with our method (we oxidize with Sodium Peroxide, boiling for about 30 minutes). Whats your opinion? 2. Do you have in mind (or in the bibliography) another method of analysis for above bath? 3. We would like to check the effectivity of our method preparing a std solution in our lab, similar to our bath. The problem is that we cannot find in the market a Cr+3 std solution. We tried to prepare a std solution in the lab out of Cr2O3 solid compound but we could not dissolve it. Please comment.

Thank you in advance,

Angela Nikolaou
Hellenic Aerospace Industry (HAI) - Tanagra, Greece

July 26, 2012

A. Hach makes a Cr3+ standard that is available from their website.

Scott Merritt
- Eastman, GA, USA

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