Formulation and analysis procedure for tri-acid etch solution

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 thiosulphate ⇦ this on eBay or Amazon [affil links] 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.

  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.

Regards,

A. Dear Julie,
You do not need HF concentration, not rough either exact. All you need is etching rate -- and love, of course :)
Good luck!