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Testing for Fluoboric Acid in Pickle Using Phenolphthalein Indicator

I've been getting some conflicting information in regards to the phenolphthalein [on eBay & Amazon affil links] 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 21, 2008

simultaneous replies April 22, 2008

Most references would have you use methyl orange [on eBay or Amazon affil links] . 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.
Try using a weaker titrant solution and see what happens. I do not like 1N.
On a practical note, you can sort out your problem by seeing what happens to the color after you record your reading and add a few drops more titrant. If the color comes back, then your solution was not at equilibrium. Then take and add a few more drops of the indicator. If you get color that disappears in several seconds, add another couple of drops. If this is the case, then the indicator is entering the reaction more than it should. Most indicators will fade if they were minimal and are left for a long period of time. Some a lot faster than others.

James Watts
- Navarre, Florida

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.

What are you pickling? Frequently, a significant fraction of the titrant is consumed by dissolved metal ions; in the anodizing solutions I mentioned, this makes necessary a separate determination for "free" and "total" acid.

It is fairly arbitrary, but I always used 30 seconds persistence of the pink color as my criterion.

dave wichern
Dave Wichern
Consultant - The Bronx, New York
April 23, 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
April 25, 2008

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