[sabrinasmith]

I am doing an experiment and received much lower results for the titrated iron content when using ferrous fumarate and bisglycinate versus ferrous gluconate. It was titrated using potassium permanganate. 

I need to be able to:

1. List the stoich reactions for each compound when they were reacted with dilute (1.0M) sulphuric acid to create Fe2+ ions

2. Explain why ferrous gluconate gave me the most accurate iron content results when compared to the listed amount on the bottle

Any insight is appreciated!

[Borek]

when using ferrous fumarate and bisglycinate versus ferrous gluconate.

Using them for what?

Following your description we won't be able to decipher what you did and what the experiment was, so there is no way to give any advice, you must post more details.

Also, per forum rules you have to show your attempts at answering the question/solving the problem to receive help, it is a forum policy.

[sabrinasmith]

My experiment was to determine the content of elemental iron in different brands of iron supplements.

What I did was:
Obtain two iron supplement tablets, and accurately weigh them.
Grind the tablets with a mortar and pestle until it is finely ground. 
The powder should resemble flour.
Dissolve the powdered tablet in about 20mL 1.0 M H2SO4 in the mortar. Transfer the powdered iron tablet and the acid to a 250 mL volumetric flask using a funnel. Use the 1.0 M H2SO4 to help you transfer your  powder to the flask, using more as needed to rinse the powder from the mortar and pestle. Add any remaining 1.0 M H2SO4  to the 250 mL volumetric flask. The total volume of H2SO4 used will be 50mL.
Swirl for 5 minutes to dissolve all powder.
Make up the final volume to 250 mL using distilled water and a 5mL pipet when approaching the line.
Using a seperate funnel, pour about 15 mL of KMnO4 solution into your buret. Rinse and discard in the 500mL waste beaker.
Fill up the buret with the KMnO4, and allow some to drain in order to fill the tip. Read the volume, from the bottom of the meniscus.
Using the pipet pump, withdraw about 5 mL of the iron supplement solution, and rinse inside of the 25mL pipet with it. Discard in 500mL waste beaker. Transfer 25.00 mL of the solution to a 150 mL Erlenmeyer flask. Place it under the burette with a white piece of paper underneath and if using a stirrer start it on low speed.
Titrate the Fe2+ solution with KMnO4 until the mixture has just turned purple.
When the purple colour starts to take a longer time to disperse, slow down the addition of the KMnO4 until you add it one drop at a time. On standing, the purple colour will disappear because of a secondary reaction; do not add any more KMnO4. Record the volume in the buret when the faint purple colour first stays in the flask.

For the results of this experiment, I received much lower iron content values than listed on the bottles for supplements that were ferrous fumarate and ferrous bisglycinate. I received accurate iron content values for bottles of ferrous gluconate.

I got these values using the basic equation:

5Fe2+ + MnO4- + 8H+  5Fe3+ + Mn2+ + 4H2O

However after seeing the difference in my values between different supplement compounds my teacher told me to assume that this was to do with the stoichiometric reactions between the different iron supplement compounds and the sulphuric acid. I need to figure out the stoich for these. My attempts are:

Ferrous gluconate: C12H22FeO14 + H2SO4 C12H24O14 + FeSO4
Ferrous fumarate: C4H2FeO4 + H2SO4  C4H4O4 + FeSO4
Ferrous bisglycinate: C4H8FeN2O4 + H2SO4 C4H10N2O8 + FeSO4

However, I do not know if these equations are accurate. I also do not know how they would effect the amount of Fe2+ titrated and why ferrous gluconate reacted the best and gave me the most accurate elemental iron content.

Hopefully this is enough details, I apologize for not adhering to forum rules! Thanks

[Borek]

Perfectly clear now

The redox reaction you listed is the one that should be used to calculate the titration result, no doubt about it.

To be honest I am not sure what is happening. Thera are two kinds of possible problems with these titrations. First - I suspect both fumaric and gluconic acid can be oxidized by permanganate, skewing the result - problem is, that would mean positive error (higher Fe²⁺ content than expected). Second - presence of chelating agents changes formal potential of the redox reactions of metal cations, which can be a source of error. It would require some calculations based on pKa and stability constants to estimate whether the problem really exists, and I don't have these constants at hand, but intuition tells me that should not matter around pH 1, when both acids are most likely almost 100% protonated.

Actually the only sure way of doing such an analysis is to roast the sample to get rid of all organics (alternatively: digest it by boiling in nitric acid to oxidize everything), then dissolve and reduce the iron to make sure it is present as Fe(II) only.