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Analysis for total aluminium in anodizing solutions

Q. This is a precise technique. Lots of the volumetric methods described for Al involve adding fluoride, the retitrating via acid base. Not precise at all, maybe ± 10%. The technique described here is precise - done right, perhaps ±0.5%. I once used it in part to reverse engineer Fleishmann's double acting baking powder.

Put 75 ml DI water in a flask. Add 25.00 ml 0.1 M EDTA solution. Add 1.00 g etch solution. Warm to about 70 C, then bring the pH to 3 with 1:1 HCl, with good stirring. Wait 5 minutes. Then, bring the pH to 10 with 1:1 aqua ammonia. Let cool. Add a little eriochrome black t ⇦ this on eBay or Amazon [affil links] . Then, titrate to a pink endpoint with 1.00 M Zn2+ or Mn2+ solution. Then:

25.00 - ml titrated = mol/kg Al.

For anodizing acid, procedure is the same, except one omits the HCl addition.

I've been trying to figure out how to get to the aluminate concentration (yes, that again) but the procedure doesn't distinguish between aluminate Al and Al complexed by the anti-scaling additive. I suppose one could assume that all the anti scale present is complexed to Al, and get there by calculation.

Dave Wichern
Consultant - The Bronx, New York
September 18, 2015