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How can I measure Fe in cleaning/acid tanks w/o an AA unit?

How can I measure Fe+++ in the cleaner tanks, especially in acid tank other than AA procedure.

Mortez Aghaebrahim
Plating lab manager - Richmond,VA, USA
2007

AAS ⇦this on eBay or Amazon [affil links] will necessarily give you the total of Fe++ and Fe+++. If you know that there is no Fe++ in your solution, that's fine. But if there's Fe++ as well as Fe+++, then AAS cannot separate the oxidation states and you would be reading the total of Fe++ and Fe+++. To separate them, you need to use some wet chemistry.

Option 1
If there is no Fe++ in the solution, so that all the iron is present as Fe+++, you can use AAS or you can do it in one chemical determination which uses preliminary reduction of the Fe+++ to Fe++ then titration with potassium dichromate to quantitatively oxidise the Fe++ to Fe+++.

If there IS any Fe++ in the solution and you want the Fe+++ only, you need two separate determinations, one that gives you total Fe (i.e. Fe++ plus Fe+++) and one that gives you Fe++ only. The first is the same as outlined in the first paragraph above, and the second is a direct titration without the preliminary reduction. Simple subtraction then tells you the original Fe+++ content.

The procedure is simple enough to do for anybody with basic lab skills and apparatus, but the written description of the procedure is quite lengthy.

Option 2
Although an unorthodox procedure, it would be possible to use careful adjustment of the pH of the test solution to precipitate the Fe+++ as ferric hydroxide without precipitating the ferrous ion, then filtering off the ferric hydroxide and redissolving it in acid to give a test solution whose total Fe content is in fact only the Fe+++ from the original test solution. That could then be analysed for Fe by AAS and the result would be the Fe+++ content of the original. But given that there is a bit of wet chemistry in doing that, it would I think be easier to instead do the bit of wet chemistry required for the potassium dichromate procedures.

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

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2007

Sir:
Some acids like sulfuric and hydrochloric are "reducing" acids and dissolved iron is in the reduced state (lower oxidation number of +2). Other acids like nitric acid are oxidizing acids and dissolved iron is in the oxidized state of +3.
If you could state which acid is being used, then a test method may be determined.
For a reducing acid simple dropper bottle testing using potassium dichromate (hex chrome) is very easy and quick.
Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA

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This question comes up on a regular basis. My suggestion is to add an excess of EDTA under acid conditions, then bring to pH 10 with aqua ammonia ⇦this on eBay or Amazon [affil links] and back titrate to a blue eriochrome black t ⇦this on eBay or Amazon [affil links] endpoint with a standard Zn or Mn solution. Then, it doesn't matter what oxidation state the Fe is in. The EDTA/Fe complex is 1:1 on a molar basis either way.

Dave Wichern
Consultant - The Bronx, New York
2007

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Dave, for determination of total iron, your procedure is certainly an alternative to my recommendation of prior reduction to Fe++ then titration with dichromate.

However, the original post requested a procedure for determining Fe+++ and that is the request that I addressed.

The original post gives no information about the test solution that could tell us whether or not any Fe++ is present, but the fact that Mortez specifically refers to Fe+++ suggests that both oxidation states are present in the solution - if he wanted total Fe he would probably have said so. With both oxidation states present, determination of Fe+++ must be done by determining total Fe, and separately determining Fe++, and then subtracting Fe++ from total Fe to give Fe+++

Since a dichromate titation is necessary for Fe++ in the presence of Fe+++ it seems to me more sensible and convenient and cost effective to use that same titrant and indicator combination for the total Fe as well.

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