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Testing for Fluoboric Acid in Pickle Using Phenolphthalein Indicator
April 21, 2008
I've been getting some conflicting information in regards to this acid-base titration. Specifically, the endpoint is in question in regards to the length of its duration- 15 seconds? 1 minute? More? In a new bath free of dissolved metals, what creates the endpoint interference in the first place? The answers don't seem to be available in the literature I have here (maybe time to upgrade my library).Rob Belter
QA Analyst - El Cajon, California, USA
April 22, 2008
Most references would have you use methyl orange. You need to have it at a pH that will not form metal hydroxides as you titrate with NaOH. You should be slowing down the rate of titrant as you approach the end point so the solution has enough time to completely reach equilibrium. I found out that in an instrumental analysis class when the points on a cal graph did not form a straight line.
- Navarre, Florida
April 23, 2008
I had this problem before when analyzing sulfuric acid anodizing electrolytes. It is caused by the gradual hydrolysis of dissolved metal species that consumes hydroxide ions, causing the pH to decrease, and making your indicator turn colorless again.
Consultant - The Bronx, New York
April 25, 2008
Actually we use two separate tanks, one for kovar, and and another for BeCu. Whether the fluoboric is being used as an activator or a pickler / de-scaler, I understand the hydrolysis potential, it's the brand new bath @ 40% by volume that scratches my head (the Fluorine itself?). I'll double check that the reaction has reached equilibrium in the meantime.Rob Belter
- El Cajon, California, USA