48072

Chromic anodise analysis problem [UK] 

March 6, 2008

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 acidty, 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 chrmoic acid anodising bath?

Paul Fulford
Chemist - Southampton, UK

FREE CHROME ANALYSIS?

Use AAS, it is easy.

Jose Castellanos
- Minneapolis, MN, USA

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 0.1 N sodium thiosulfate 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 - Bronx, NY, USA