The authoritative public forum
  for Metal Finishing since 1989
-----

Nitric passivation: measuring dissolved iron?

We will be passivating an electropolished tank with nitric acid (60% solution). We are looking to identify a way to verify if the nitric has become saturated with Fe, so that we could empty that batch and begin with a new fresh batch (nitric)? Any ideas?

Kevin O Smith
Aerospace - VAFB/Lompoc, Calif, USA
2007

---

2007

Kevin:

The iron concentration in nitric can be determined in a number of ways, including simple titration with the Iodometric method (oxidation followed by sodium thiosulphate ⇦this on eBay or Amazon [affil links] titration), colormetric determination and others. Use of nitric acid is generally kept below 2% total iron in the solution for quality passivation techniques.

Thanks,

Daryl L. Roll
Costa Mesa, California, USA

---

2007

Measure an appropriately sized sample (1 ml would be my guess) into a mixture of about 70 ml of water and 30 ml of 5 N sulfuric acid. Titrate this mixture to a persistent violet color with 0.1N potassium permanganate ⇦this on eBay or Amazon [affil links] solution.

Each equivalent of permanganate added equals an equivalent of iron in the sample. As they say, do the math.

Dave Wichern
Consultant - The Bronx, New York

---

First of two simultaneous responses --

Whoa there, Dave.

The iron is already +3, being in a nitric acid solution. You can't oxidise it with permanganate (or any other oxidant) when it's already oxidised.

Perhaps you'll need to raise the pH to precipitate the iron, filter it off, then dissolve the precipitate in dilute hydrochloric acid for quantitation (reduction by stannous chloride, removal of excess stannous chloride with mercuric chloride, titration with potassium dichromate).

Note incidentally that you can't use permanganate as titrant in the presence of hydrochloric acid, as the permanganate will oxidise chloride to chlorine and you will thus get a titration volume far in excess of what is correct for the iron itself.

Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.
2007

---

Second of two simultaneous responses --

Being a chemical engineer I can't follow your calculations, the permanganate ion drops 5 levels in oxidation, so to make it one equivalent to one equivalent then each iron must raise 5 levels in oxidation. I would expect iron to exist as Fe3+ in concentrated nitric acid, but I have certainly newer heard of Fe8+

I have always used atomic absorption ⇦this on eBay or Amazon [affil links] , but newer found it necessary to exchange the acid, the dragout kept down the iron content, if you replenish to keep you acid concentration.

Bo König
- Odense, Denmark
2007

---

2007

Holy gross error Batman!

These gentlemen are right. I apologize if I caused any confusion.

Another way to go about this: take that 1 ml sample, add about 50 ml of water, and an excess of EDTA. Heat it to near boiling for a few minutes.

Elevate the pH to about 10 with aqueous ammonia, add eriochrome black t ⇦this on eBay or Amazon [affil links] indicator, and titrate to a red endpoint with a 1 M solution of zinc or manganese nitrate. The sulphate salts should work as well.

Subtract the ml titrated from the ml it would take to equivalence with the EDTA alone. This represents the equivalents of iron present.

Dave Wichern
Consultant - The Bronx, New York