Looking for a titration method for ferric chloride

A. Hi Kaushik. While awaiting responses, you may wish to read the earlier entries on this thread. You may also find Barlow Campano's article "Kleingarn Regenerated Spent Acid at Increasing Ferrous & ferric chloride ⇦this on eBay or Amazon [affil links] Content" in our on-line reference library interesting.

Luck & Regards,

A. phenolphthalein is not a good indicator for your acid titration because of the high pH of 9 (plus or minus). At an alkaline pH, you are getting a false reading as metals start to precipitate out as hydroxides, using part of your titrant. I think that you can see the endpoint of methyl red ⇦this on eBay & Amazon [affil links] , pH of about 5. methyl orange ⇦this on eBay or Amazon [affil links] would probably be tough to see in a ferric chloride ⇦this on eBay or Amazon [affil links] solution.

A. Hmmm. It's a tough problem because the acidity doesn't change, just the solution metal loading.

You could try this: take a suitably sized sample (trial and error testing would be necessary to determine this) and dilute it up to 100 ml with water, then add aq. NH3 to pH 11 or so. Filter out precipitated metal hydroxides to get a solution that should only contain the nickel. I'm assuming you're processing a stainless steel with nickel in it - most do.

Now, add some murexide [affil links] indicator, and titrate with 0.1 M EDTA to a violet endpoint. This, after a simple arithmetical calculation, will give you the nickel loading, which should be correlatable to your etch test.

Or, wait a few days. Someone who actually does this type of work themselves may have a method all worked out. In that case, you should listen to them.

A. I'm not going to look up the fudge factors, etc., but if you do an acid/base titration for HCl and a total chloride titration (titrate w/AgNO3, chromate indicator), you could then subtract out the chloride from HCl. What's left comes from the ferric chloride ⇦this on eBay or Amazon [affil links] .

Were I in your shoes, though, I'd just use an ORP controller.

A. Reduce your ferric chloride ⇦this on eBay or Amazon [affil links] (Fe3+) down to ferrous chloride (Fe2+) using tin chloride (SnCl2). Keep adding the tin chloride until the solution turns clear. Then precipitate out the excess tin chloride with mercury chloride (HgCl). Be careful because mercury can be toxic. Add the mercury chloride to the solution until it doesn't dissolve anymore. Let this solution settle out until you have a clear liquid on top, and precipitate at the bottom.

For the titration, siphon off a known quantity of the clear liquid and titrate with potassium permanganate ⇦this on eBay or Amazon [affil links] (KMnO4). Measure the ORP as you titrate - you should start out around 400-500 with Fe2+, and add KMnO4 until you hit 900 mV or so (Fe3+). Use the quantity of KMnO4 it took to hit 900 mV ORP as your titration quantity, and multiply the # moles of this additive by 5 to figure out how many moles of iron was in your original solution