Controlling iron content in hydrochloric acid

A. Gentleman,

Certainly, increase in Iron content will increase the pickling time because of the inhibition. As a rule of thumb 26 grammes per liter of iron in the pickling bath shall be fixed for discarding the bath.

A. In galvanizing it is possible to pickle in iron-acid (hydrochloric acid with iron-content even until 180-200 g/l), depending on the temperature of the pickling. In fact it becomes Iron chloride pickling. Iron chloride dissolves iron oxides better than low iron acid. Big advantage is that you can use the pickling much longer and it is even possible to sell the old pickling acid as iron chloride. This is already practice in Western Europe !

A. Like Dolf said, Fe content is not a problem. We start our "new" tanks with about 12% HCl and this is made with about 30% old acid to get the Fe content up high enough! (balance water) We run our acid up to 220 grams per litre Iron chloride, at which time the HCl content is as low as 1%. It still pickles well at this.

Hi Phil and other readers: Since the posting of this question, we've acquired a Hydrochloric Acid Pickling Chart, generously contributed by reader Barlow R. Campano, Chemical Engineer at HIDADA Galvanizing Plants 1 & 2, Jeddah, Saudi Arabia, which attempts to answer this question.
Luck & Regards,


Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. We use a titrating method to check both acid and Iron concentrations. Our Acid titrant is sodium carbonate / sodium hydroxide. Our Iron titrant is Potassium Dichromate.

A. Dear Akhtar,

We use methyl orange [on eBay or Amazon] as indicator in the titration to measure % HCl in pickling baths, and sodium hydroxide 1N as a titrant in this case. If the pickling bath has much iron dissolved, the end point is when the red color disappears and you can see a green precipitate (iron) and the sample of light-yellow color...

And for the iron, we use potassium permanganate [on eBay or Amazon] 0,1 N. This is not selective, if you have some oxidable matter in solution, you will measure it as iron and you will have an excess difference. But for qualitative matter, it is useful to know if the pickling bath can be used or must be discharged as waste.

I hope this will be useful. Best regards,

A. Yes, that endpoint is very hard to see. I have two suggestions:

1) Use very little indicator. 3 drops is plenty.

2) Titrate rapidly. The color change isn't very sharp and if you grossly overshoot you can always redo it.

A. Analytical procedure of Iron content in HYDROCHLORIC ACID BATH,

Take 1 ml of bath solution, Add 50 ml of D.M.WATER, Add 10 ml of 50% sulfuric Acid Titrate against with 0.1 N Potassium Permanganate. End point- Colourless to Light Pink Colour.

BURETTE READING * 5.56 = --------gm / ltr of Iron content

Hi Zainab. What is "y" or what is the value of "y"?
... and don't you dare tell us y is the free acid content divided by 7.3 :-)
Regards,

A. Hi Mutshidzi. Geoff explains that he simply starts with 30% old (used) pickling acid.

Luck & Regards,

A. Hi again Mutshidzi. I'm still not totally sure what you're trying to do ... but assuming your pickle acid is HCl it should be easy to add ferric chloride [on eBay or Amazon] or Ferrous Chloride if you don't have the old acid available that Geooff suggests.

Luck & Regards,

A. Hi again, Mutshidzi. We'll try to alert A.S.J. PETER of your posting, but his posting was from 9 years ago, so we may be unable to reach him. It's true that when a concentration is expressed as a percentage it should be labeled as "% by weight" or "% by volume" and whether that percentage is based on molecular weight or a commonly used and available dilution -- in fact just yesterday we printed a complaint about that on another thread.

I see that both 50% v/v and 50% w/w sulfuric acid are widely packaged, sold, and available -- so I agree that I do not know which Mr. Peter was referring to. If he was referring to 50% v/v, yes, g/l and % would be convertible.

Luck & Regards,

A. Hi Ted and Mutshidzi,
The 50% sulfuric acid is added only to maintain the mix very acidic. The results you have are expressed by volume, so it is g/l of Fe content.
You can express it in % m/v of Fe content just dividing by 10. But to express it in % m/m you must have the specific gravity of the solution.

We use the same procedure as A.S.J. PETER so this is my grain of sand :)
Best of luck!

Thanks as always Daniel!
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi Qohhar,
It's unlikely that Phil will see and be able to give you his method 17 years later, but Akhtar, Daniel, Dave, and A.S.J. offer their methods above.
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi Qohhar. Your postings don't acknowledge previous postings in response to yours, and you haven't acknowledged our e-mail either. So we don't know what you are seeing and not seeing on this page -- your web browser's cache may not be refreshing :-(
Your formula sounds okay for HCl titration, but you can't determine iron from that procedure. For iron content please view A.S.J.'s procedure, and Daniel's clarification. Please acknowledge that you are seeing these postings. Thanks!
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi Mansi.

• A fairly similar analytical procedure based on potassium dichromate is available in Analysis of Electroplating and Related Solutions [affil link on Amazon, ... on AbeBooks] if that book and that chemical are available.

• ASM Metals Handbook Vol. 5 offers this formula for a rough estimate:
D = 0.9971 + (4.46 x 10 ^-3) C_HCl + (8.15 x 10 ^-3) C_F2Cl2

• Thread 36286 offers a possibility based on nitric acid

• you can search the site with your search terms for more ideas on the subject.
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey