LAB #2.2: %Fe in Iron Tablet (Redox Titration)

Introduction

A common task in analytical chemistry is the determination of the amount of a substance present in a sample or product. A chemical analysis that is performed primarily with the aid of volumetric glassware (e.g., pipets, burets, volumetric flasks) is called volumetric analysis. For a volumetric analysis procedure, a known quantity of one substance reacts with a to-be-determined amount of another substance. The known quantity is either a primary standard (a substance that has a known high degree of purity, a relatively large molar mass, is nonhygroscopic, and reacts in a predictable way) or a standard solution (a solution having a very well known concentration of a solute).

Redox titration is a process used to determine the concentration of an ion in an unknown solution by reacting it with another ion in a solution having a known concentration. The equivalence point is reached when the total number of electrons lost in the oxidation reaction is equal to the total number of electrons gained in the reduction reaction. In this lab, a solution of an oxidizing agent will first be standardized and then used to determine the amount of iron in an iron pill.

Background

As biochemical research becomes more sophisticated, we are learning more about the role of metallic elements in the human body. For example, copper and zinc are present in enzymes, and trace amounts of molybdenum and selenium are vital in regulating internal oxidation-reduction reactions. Iron is necessary for oxygen transport in the bloodstream. Many people gain these essential elements through their diets, or by taking multivitamin tablets. The iron that is present in these tablets is in the form of water-soluble Fe2+ ions.

Part 1, Standardization

During the standardization part of the lab (Part 1), ferrous ammonium sulfate (FAS) will be oxidized to iron (III) ions in acidic solution according to the following unbalanced equation:

MnO4- (aq) + Fe2+ (aq) → Mn2+(aq) + Fe3+ (aq) (acidic medium)

For this titration, the equivalence point occurs when the exact number of moles of Fe2+ ions has been added to react completely with all the permanganate ions in the solution. Instead of phenolphthalein or some other acid-base indicator, the indicator for this titration is the permanganate ion itself. The permanganate ion is purple (violet) in solution. At the endpoint of the titration, the solution changes from light pink to colorless.

You will need to balance the equation above to determine stoichiometrically how many moles of permanganate (MnO4-) react with iron(II) (Fe2+).

Get help with balancing redox reactions here.

Part 2, Experimental Set up

The following video provides the concept of the lab that you are going to do. Please make sure that you follow our written procedure provided to you, which is also posted below. Please not what you see in the video is very similar but not 100% exact (step-by-step).

Lab Handouts, Analysis, Calculations & Discussion Questions

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Preview: The following are the sections contained in the document below:

1. Lab Handout

2. Class Data Spreadsheet

3. Pre-Study Worksheet

4. Pre-Lab Questions

5. Sample Data

6. Sample Calculations


7. Analysis/ Discussion Questions

8. Possible Sources of Error

9. Links to Similar Labs

10. Previous AP FRQs

11. Related AP Style Questions

12. Lab Prep/ Instructor Notes

05 - % Fe [HAC]