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Iron ppt from nickel bath

Dear expert,

I am doing a project about reducing iron concentration from an electroplating bath. It was a nickel plating into steel process, the bath contain Nickel sulphate, nickel chloride, Boric acid, sulfuric acid.... hydrogen peroxide is added to make the nickel surface be brighter.

It was found that the iron ion will form ppt in a right amount of hydrogen peroxide and pH. Additionally the ppt can remove by a filter easily. I just want to know what kind of reaction is taking place, because I have try putting hydrogen peroxide into a sample solution but no ppt come out. From inorganic chemistry I learn that when alkali such as sodium hydroxide added into iron sulphate solution, then ppt will form. But the thing is hydrogen peroxide is an acid...

Chi Cheng
- Baltimore, Maryland
2000

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Two things happen, both of which act in the same direction. First, ferric ions are precipitated at a much lower pH than ferrous ions, so if your bath is in the right pH range (say, 4-5), the result of the oxidation process will be not ferric ions in solution but rather a hydrated ferric oxide. Secondly, it so happens that the reaction of hydrogen peroxide with ferrous ions generates hydroxide:

H₂O₂ + 2Fe⁺⁺ ---> 2OH^- + 2Fe⁺⁺⁺

The acidity of hydrogen peroxide is irrelevant here. It is too weak an acid (pKa about 12), not much stronger than water, so it cannot help keep the solution pH down around 2-3 where it should be to prevent precipitation of ferric ions.

Emanuel Cooper
- Yorktown Heights, New York
2000

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2000

Mr Cooper,

Thanks for your reply, so does it mean when the pH is over 4, then it should form ppt? the pH does change a lot from the previous data, but never get over pH. But these data is taken when the bath is not operating. Is that possible that the bath will jump into a higher pH during electroplating?

If I want to carry out an lab experiment to show that the solution will form ppt when an amount of hydrogen peroxide is added, can you suggest me how to do that?

There is some more questions,

So, what exactly the ferric ppt are? Fe2O3? or Fe(OH)3? or else?

and, if the solution contain ferric oxide which is hydrated, does it mean it can't be removed? and if yes, do you think it will be a large amount compare with the other ferric ppt?

thanks

Chi Cheng
- Baltimore, Maryland

Chi, yes the pH rises during electroplating. I cannot answer the rest of your question.

Bill Hemp
tech svc. w/ chemical supplier - Grand Rapids, Michigan
2000