Will Cast Iron dissolve in dil. or conc. acid?

[dfx]

Hi,
I have searched rigorously for a solution to this problem but I seem to get inconclusive answers. I want to find the percentage purity of iron in cast iron and the best way to go about this is a redox titration. The first step is to reduce the Iron to Fe2+ . I intend to do this by reacting it with Sulphuric Acid and forming FeSO4. The only thing is, will cast iron react with dilute sulphuric acid (<1M) or does it require concentrated acid (>1M)? I know with conc. acid the resulting products are FeSO4 + SO2 + H2O and just FeSO4 + H2 with dilute acid. Any advice/feedback would be much appreciated.

 

[mrjeffy321]

The first step is to reduce the Iron to Fe2+

Well, technically, that would be Oxidation to Fe+2.
LEO GER Loose Electrons Oxidation, Gain Electrons Reduction.

Cast Iron should react with both concentrated and dilute Sulfuric acid to produce an aqueous solution of Iron (II) Sulfate.
Since both conc. and dil. H2SO4 will produce FeSO4 (aq) which is what your aiming for, you might as well use dilute since you will not have to deal with the Sulfur Dioxide gas being produced in the reaction.

 

[Borek]

I would rather try to dissolve iron in HCl, avoiding oxidation at all. Even then Fe²⁺ solutions tend to get oxidized with air oxygen so just before titration you should add some SnCl₂ to reduce Fe³⁺ to Fe²⁺, then some HgCl₂ to remove excess Sn²⁺:

SnCl₂ + 2HgCl₂ -> SnCl₄ + Hg₂Cl₂

(note Hg₂Cl₂ is weakly soluble and doesn't interfere during manganometric titration).

Check details in some handook, I am sure Vogel should list some methods.

 

[dfx]

I would rather try to dissolve iron in HCl, avoiding oxidation at all. Even then Fe²⁺ solutions tend to get oxidized with air oxygen so just before titration you should add some SnCl₂ to reduce Fe³⁺ to Fe²⁺, then some HgCl₂ to remove excess Sn²⁺:

SnCl₂ + 2HgCl₂ -> SnCl₄ + Hg₂Cl₂

(note Hg₂Cl₂ is weakly soluble and doesn't interfere during manganometric titration).

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Hello, thank you for your reply. That is a great addition which will score valuable marks. My only problem with using HCl is that after oxidation to Fe2+ I intend to titrate against MnO4- thus oxidizing further to Fe3+. Won't Cl- from the HCl interfere as it is a reducing agent?

 

[dfx]

Well, technically, that would be Oxidation to Fe+2.
LEO GER Loose Electrons Oxidation, Gain Electrons Reduction.

Cast Iron should react with both concentrated and dilute Sulfuric acid to produce an aqueous solution of Iron (II) Sulfate.
Since both conc. and dil. H2SO4 will produce FeSO4 (aq) which is what your aiming for, you might as well use dilute since you will not have to deal with the Sulfur Dioxide gas being produced in the reaction.

Ah yes my bad it is oxidizing.. posted in a hurry! Thank you very much I'm glad that's sorted. What about the impurities in cast iron will they also dissolve in dilute acid?