Metal finishing Q&As since 1989

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Titration color (from dark brown to colorless end-point) of chromic acid

2002

I am just wondering about the dark brown color of my solution when I was doing the analysis of total and free chromic acid of our bath solution(anodizing). I was following the procedure presented at table X (Metal Finishing Guidebook & Directory, Vol 100, No. 1A++). I was suppose to have a blue solution after adding the chemicals needed in the procedure.

Will you help me figure out why there is difference in the color?

Thank you very much.

Nila O. Par
Cormer Group Ind. - Winnipeg, MB, Canada

It sounds as though you have forgotten to add any or enough starch indicator to your sample. The starch will turn the solution from brown or tan to a blue-black color, whereby increasing the sensitivity of the endpoint.

Ira Donovan, M.S.F.
Kansas City, Missouri
2002