Formulation and analysis procedure for tri-acid etch solution
A discussion started in 2005 but continuing through 2018(2005)
Q. Dear Sir:
I would like to know the formula to calculate the concentration of chromic acid, nitric acid, ammonium bifluoride (etch rate) in tri-acid etch solution in oz/gal and g/l.
Thank you,Stephanie Tran
finishing shop - Ontario, Canada
May 8, 2012
A. The most straightforward method, I believe, would be to first iodometrically titrate for hexavalent chromium by adding potassium iodide to a diluted solution of your sample that has had acid added to it to lower the pH, adding starch indicator, and titrating with sodium thiosulfate to a clear endpoint.
Next, you can titrate for the combination of nitric acid and chromic acid using sodium hydroxide to an endpoint of pH 3.2, which should be a point at which strong acids are neutralized and weak acids (e.g., HF) are not.
There are numerous threads in the forum discussing fluoride analyses using an ion-specific electrode or thorium nitrate titration, but I've heard that you can also get a rough value of HF concentration by continuing the total acid titration to a pH of 5.8.
I'd love to hear if more experienced finishers have comments on this procedure.
- Long Island, New York
October 18, 2018
Q. Did you ever get an answer to this? I would like to know if you got feedback or the method below your question worked.Julie Nguyen
- California, USA
Hi Julie. From this and several other threads here on the same or similar questions, it seems like the question really comes down to whether Joshua's comment about getting a rough value of HF from titration actually works. The answer seems to be that whether or not you can get a rough value, you probably won't get a useful value :-)
But I'm no chemist, and if anyone feels that there is a way to get a useable fluoride value without cesium/thorium titration or specific-ion electrodes, I'm sure we'd all like to hear it.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"
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