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Yellow Precipitate in H2SO4 Titration
March 24, 2011
Recently I have encountered a yellowish precipitate while titrating for free acid in one of my Type II anodize tanks.
The other Type II tank nor the Type III tank get this yellowish precipitate. During both of those titrations the precipitate stays white until the endpoint is reached.
Another thing is that instead of the color change being instantaneous and dramatic, the color change is slow. More resembling the color change during the titration for total acid.
Has anyone ever seen this?
My method is as follows:
40 mls DI water
5 mls tank solution
20 mls 10% potassium fluoride
[affil links] solution
Titrate with 1.0N NaOH [1N NaOH on
Amazon]
until a stable, pink endpoint.
The yellow color is definitely in the precipitate because I filtered it. Something in the bath is
Tim Hamlett, CEF
- West Palm Beach, Florida, USA
Hmmmm .... NaOH in your titration of passivation solution ...
Iron likes pH 4-6 to appear
Maybe check your iron content
Industrialist - Toronto Ont Canada
April 7, 2011
April 25, 2011
You need to check out all elements that form a yellow hydroxide. You may be anodizing an alloy of aluminum that has that metal, and over time builds to a level that will form a visible yellow precipitate, when made alkaline. The contaminating element may or may not be harmful to your anodize bath, or coating.
Presuming you are using C.P. Grade Sulfuric Acid to make the solution, request a Q.C. analysis for that batch from your vendor, to see if it meets spec. Are you using a Tech Grade Sulfuric Acid? Some poor grades might be akin to "Mine Acid," and I'd never use them to make or replenish my anodize tanks. Some that I've seen are black, with floaters in them. Not the water-white acid that should be used in a quality shop.
Ed Budman [dec]
- Pennsylvania
With deep sadness we advise that our good friend Ed passed away Nov. 24, 2018
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