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topic 32777

Sulfuric acid titration indicator



I work for a company that zinc plates steel strip in a continuous process line. We pickle with sulfuric acid. I am wondering if anyone knows a good indicator to use in a sulfuric acid titration. The acid obviously has dissolved iron in it. I have tried:

Methyl orange
Bromocresol Purple
Bromophenol blue

with sodium hydroxide as the titrant.

With all of these, I have encountered the same result - the solution goes to a deep gold colour and the solution begins to flash green as the endpoint is approached. The solution then gradually starts to go to a dark murky green/brown. No distinct endpoint is visible. I am no expert in chemistry but I presume that the colour change is due to iron coming out of solution as the pH rises. Can anyone help? Could a different titrant provide better results?

p.s. Please do not suggest that we should be pickling with HCl rather than sulfuric! :)

Daile Coey-Braddon
Electroplating - Brisbane, QLD, Australia

First of three simultaneous responses -- (2004)

Any of the indicators will work. Things that will help, 1.titrate with 0.1N hydroxide 2. titrate slowly 3. use very strong mixing, preferably with a mixer and a proper sized teflon spin bar (manual is a pain in the ---), 4. try diluting your sample with trice as much DI water as you are currently using. Finally, try the titration with clean sulfuric acid so that you recognize the approach to the endpoint and the final endpoint. All of the above is meant to prevent the precipitation of iron in the very high pH of the titrant. It will dissolve, but darn slow. You may want to go to a nice lab pH meter. Just remember that they are not instantaneous-You need to give the meter time to equilibrate as you get above pH of 2 to 3.

James Wtts
- FL

Second of three simultaneous responses -- (2004)

To reduce the effect of the iron, try diluting the acid to (say) 10% of its original strength. This will also reduce the colour masking effects of the iron and the dilution can be taken into acount in the calculations.

Also, but perhaps a bit inconventional, but try titrating the acid against the standard sodium hydroxide, using phenolphthalein as the indicator. That is, have a known volume and concentration of sodium hydroxide in the beaker, add phenolphthalein, which will go pink, and then titrate the acid until the sodium hydroxide goes clear.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

Third of three simultaneous responses -- (2004)

Daile, I would stick with methyl orange for this titration. You are spot on when you say that the murky brown is precipitated iron hydroxide. The green colour you can see is the change in colour from the indicator. Call the end point the stage at witch all the orange colour is gone.

If your still not happy you could titrate with a p.H. probe to a p.H of 7.0 in the presence of methyl-o and observe the colour, or maybe even try phenolphthalein for a distinct clear to pink colour change at a p.H. of about 8 which is just past the equlivance point.

At the end of the day don't stress too much about this as I doubt there is a need for super accurate control, keeping the tank within a few percent should be fine. Perhaps check the iron level ocassionally.

Peter Van de Luecht
Macdermid Australia - Melbourne, Vic, Australia
supporting advertiser

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