Determine percentage of iron(II) and iron(III) in a solution

  • Thread starter Thread starter willidietomorrow
  • Start date Start date
  • Tags Tags
    Titration Zinc
Click For Summary

Discussion Overview

The discussion revolves around a method to determine the percentage of iron present as iron(III) in a solution containing both Fe3+ and Fe2+ ions. Participants explore experimental approaches, chemical reactions involved, and calculations necessary for the analysis.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant outlines a plan involving the use of zinc to reduce Fe3+ to Fe2+, along with potassium dichromate and dilute sulfuric acid for titration.
  • Another participant points out the difficulty of reducing Fe2+ to elemental iron with zinc, while noting that reducing Fe3+ to Fe2+ is more straightforward.
  • Several participants discuss the titration process against dichromate to determine the amount of Fe2+ and subsequently infer the amount of Fe3+ present.
  • There is a suggestion that knowing the amount of dichromate used allows for the calculation of Fe2+ ions, leading to the conclusion that the titrated solution will contain only Fe3+ ions.
  • Concerns are raised about the direct determination of Fe3+ ions, with one participant questioning the reliability of using zinc for this purpose.
  • Another participant proposes that the amount of Fe3+ can be inferred from the amount reduced by zinc, assuming only Fe3+ is reduced.
  • Participants express uncertainty about controlling the reaction with zinc and emphasize the importance of measuring the dichromate used for accurate calculations.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of directly determining Fe3+ ions using zinc, with some supporting the idea while others raise concerns about the method's reliability. The discussion remains unresolved regarding the best approach to accurately quantify Fe3+ ions.

Contextual Notes

Participants highlight limitations in controlling the reaction with zinc and the dependence on accurate measurements of dichromate used in titrations. There are unresolved questions about the assumptions made in the proposed methods.

willidietomorrow
Messages
12
Reaction score
1
Please post this type of questions in the HW section using the template.
Hi I am stuck on the following question.
Outline a plan of an experiment to determine the percentage of iron present as iron(III) in a
solution containing Fe3+(aq) and Fe2+(aq) ions. You are provided with zinc, a standard
solution of potassium dichromate(VI) and dilute sulphuric acid. Zinc can reduce Fe3+(aq) to
Fe2+(aq).
Write equations for all the reactions that occur. Explain how you would use the zinc and
how you would calculate the final answer.​

I know the following equations are involved but don't know the method.
2Fe3+ + Zn --> 2Fe2+ + Zn2+
6 Fe2+ + Cr2O7 2- + 14 H+ → 6 Fe3++ 2 Cr3+ + 7 H2O​
 
Last edited:
Physics news on Phys.org
Reducing Fe(II) to Fe with zinc is not easy, as they both happily react with water. Reducing Fe(III) to Fe(II) on the other hand, is quite easy.
 
Borek said:
Reducing Fe(II) to Fe with zinc is not easy, as they both happily react with water. Reducing Fe(III) to Fe(II) on the other hand, is quite easy.
Yes, sorry that was a mistake. I changed the equation.
 
Assuming you titrate the original sample - what do you determine?
 
Borek said:
Assuming you titrate the original sample - what do you determine?
Titrating the original solution against Cr2O72-, I will find the amount of Cr2O72- I need to convert all Fe2+ ions into Fe3+ ions. Also titrating against zinc will give the amount of Fe3+ ions in the solution.
 
willidietomorrow said:
Titrating the original solution against Cr2O72-, I will find the amount of Cr2O72- I need to convert all Fe2+ ions into Fe3+ ions.

Sure, finding the amount of dichromate used is a part of the calculations, but that's not what I was asking about. Amount of which ions you will find in the end of the calculations?
 
Borek said:
Sure, finding the amount of dichromate used is a part of the calculations, but that's not what I was asking about. Amount of which ions you will find in the end of the calculations?
Well, since I know the amount of dichromate I can find the amount of Fe2+ ions that reacted (dichromate x 6), so the titrated solution will only contain Fe3+ ions.
 
OK, so let's assume now you know how much Fe(II) there were in the solution. Can you think of a way of determining the total amount of iron?
 
Borek said:
OK, so let's assume now you know how much Fe(II) there were in the solution. Can you think of a way of determining the total amount of iron?
Alright, so I know the amount of Fe2+ and can also determine the amount of fe3+ using zinc. So I can do (amount of Fe2+) / (amount of Fe2+ + amount of Fe3+) *100. Would that be a correct procedure?
 
  • #10
willidietomorrow said:
Alright, so I know the amount of Fe2+ and can also determine the amount of fe3+ using zinc.

Beware: what you wrote (while correct in general) may mean you are actually wrong in your thinking. Amount of Fe(III) can't be determined directly.

So I can do (amount of Fe2+) / (amount of Fe2+ + amount of Fe3+) *100. Would that be a correct procedure?

That's the definition of what you are expected to do, so yes, that's the correct approach (just see above).
 
  • #11
Borek said:
Beware: what you wrote (while correct in general) may mean you are actually wrong in your thinking. Amount of Fe(III) can't be determined directly.
That's the definition of what you are expected to do, so yes, that's the correct approach (just see above).
Hmm why is not possible to determine the Fe3+ ion directly, isn't the reaction with zinc suppose to help with that?
 
  • #12
willidietomorrow said:
Hmm why is not possible to determine the Fe3+ ion directly, isn't the reaction with zinc suppose to help with that?

Please describe step by step how you are going to determine amount of Fe(III) in the sample.
 
  • #13
Borek said:
Please describe step by step how you are going to determine amount of Fe(III) in the sample.
Well I was thinking just to look at the amount of Fe3+ ions reduced by the zinc (assuming that only Fe3+ ions will be reduced to Fe2+).
 
  • #14
willidietomorrow said:
Well I was thinking just to look at the amount of Fe3+ ions reduced by the zinc (assuming that only Fe3+ ions will be reduced to Fe2+).

How? You have no easy control over how much zinc reacted. Thing you can easily measure is the amount of dichromate used for Fe(II) oxidation.
 

Similar threads

Redox titration (calculating percent iron in sample)
  • · Replies 1 ·
Replies
1
Views
28K
Are My Redox Calculations Correct?
  • · Replies 2 ·
Replies
2
Views
9K
Solving Stoichiometry Problems: Titrating Fe(NH4)2(SO4)2*6H2O with KMnO4
  • · Replies 1 ·
Replies
1
Views
3K
MHB How Does Dilute Sulfuric Acid Affect the Titration of Ammonium Iron(II) Sulfate?
  • · Replies 1 ·
Replies
1
Views
3K
How Can You Determine the Percentage of Iron(ii) and Iron(iii) in a Mixture?
  • · Replies 2 ·
Replies
2
Views
6K
Solving the Electrochemistry Lab Problem: Determining Na2S2O3 Concentration
  • · Replies 8 ·
Replies
8
Views
5K
Redox Titration - Determining the % of Iron II and Iron III
  • · Replies 4 ·
Replies
4
Views
24K
Determining Iron Oxide Composition in a Sample using Redox Titration
  • · Replies 3 ·
Replies
3
Views
10K
How Do You Calculate the Mass Percent of Iron in an Ore Using Redox Titration?
  • · Replies 1 ·
Replies
1
Views
6K
Checking Answers of Chemistry Assignment
  • · Replies 1 ·
Replies
1
Views
8K