Summary for chromic acid anodise analysis Discussion
Chromic acid anodise analysis became unreliable after a chrome replenishment unit was installed, because the user’s pH titration no longer reflected the bath and the ferrous ammonium sulphate (FAS) method seemed to read high even as free chrome was dropping. Jose Castellanos suggested using AAS for free chrome analysis, calling it easy. However, another contributor disagreed and said titration is the better approach, recommending an iodometric method: add excess KI under acid conditions, then titrate with 0.1N sodium thiosulphate to an emerald green endpoint using fresh starch indicator. That answerer also suggested adding more sulfuric acid during the FAS titration and warned that FAS must be restandardized virtually every time because air oxidation changes its strength. The thread therefore presents differing views, but the practical advice centers on titration methods and careful reagent control.
Q. We process parts in a chromic acid anodise solution. We recently had a chrome replenishment unit installed to reduce the chromic acid additions we make. However, it is causing problems with our analysis of the solution. We have two methods, both of which do not appear to be reliable. A pH titration method now no longer works as the chrome recovery unit does not give back any acidity, just free chrome. Our method using ferrous ammonium sulphate also gives unreliable results as it gives us higher results when the free chrome is clearly depleting (ie, when the unit has been out of order and coating weight values are decreasing).
Are there any other reliable methods for free chrome analysis in a chromic acid anodising bath?
Paul Fulford
Chemist – Southampton, UK
March 6, 2008
A. FREE CHROME ANALYSIS?
Use AAS
⇦ on
or
Amazon [affil link] , it is easy.
Jose Castellanos
– Minneapolis, MN, USA
A. I must respectfully differ with the last poster.
The way to do this analysis is via a titration. Diluting by a factor of 10,000 and then using a trace analysis technique is a shortcut that I’d use as a last resort.
You might try an iodometric method – add an excess of KI under acid conditions, then titrate to an emerald green endpoint with 0.1N sodium thiosulphate
⇦ on
or
Amazon [affil link]
solution. Use plenty of fresh starch indicator solution.
You might also try adding more sulfuric acid to the mix when you do your FAS titration. Also…very important! FAS has to be restandardized virtually each time it is used, because the air oxidizes it and it loses titer no matter how carefully you seal it up.
Dave Wichern
Consultant – The Bronx, New York