LAB #5: %Fe in Iron Tablet (Redox Titration)
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.
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).
Read and review carefully the procedure handout attached at the very bottom of this page. Questions about the procedure may be included on the Pre-Lab Quiz.
Frequently Asked Questions
As a questioning chemist, some of the steps written in the procedure may seem unusual to you. Below are some FAQs and the reasoning:
In this experiment why is dilute sulfuric acid used rather than DI water to dissolve the iron tablets?
If DI water were used, the Fe2+ in the tablets would be almost immediately oxidized to Fe3+. The sulfuric acid prevents this from occurring.
Why are burette readings taken from the top of the meniscus?
Because the very dark color of the manganate(VII) solution makes the meniscus difficult to see.
How is the end-point of the titration detected?
When the first permanent pale pink color that forms in the solution.
Why is a rough titration carried out?
To determine the approximate end-point. This can then be used to get accurate results in the subsequent titrations.
Why is more than one titration carried out subsequently?
To reduce experimental error, by getting the mean of the accurate trials.
If a brown precipitate appears during the titration, what does this indicate, and how can it be remedied?
Mn(IV) is formed, because of incomplete reduction of the Mn(VII). This should only happen if there is insufficient sulfuric acid in the flask. The remedy is to add more dilute sulfuric acid to the flask, or, preferably, to repeat the experiment with sufficient acid present in the flask.
This worksheet goes through the calculations of a similar experimental set-up. You will need to upload a copy of this worksheet to Canvas and manually input the answers on the pre-lab quiz.
After reviewing all the content on this page and any attachments and videos, complete the Pre-Lab TEST: