Memorandum
Date: October 10, 1997
To: All RISchemtesting crew
From: Dr. Carl
I have just received a note from Dr. Vijay, of Veggiepros Inc. I have not included a copy of his note as he uses rather forceful language; he is rather upset. His company has been accused by the Thai Authority of Food and Drug Inspection (TAFDI) of false advertising in the sale of iron supplement tablets. The label on the bottle of these tablets indicates that every tablet contains 200 mg of iron as iron(II) sulfate. The TAFDI claim to find no more than 160 mg per tablet.
Both the TAFDI and Veggiepros have agreed to hire us to settle this dispute. It is our job to determine the quantity of iron in each of the tablets.
I would like you to work in the groups that you established during the training for the titrational analysis. Your first step is to develop as a group a titration procedure to determine the amount of iron in the tablets. I would like to see a copy of this procedure and a list of required materials by the end of this chemistry meeting. Please consider the following points when developing your procedure:
* in this titration reaction the Fe2+ ions react with permanganate ions, MnO4- and acid, H+, according to the following equation:
MnO41-(aq) + 8 H1+(aq) + 5 Fe2+(aq) Mn2+(aq) + 5 Fe3+(aq) + 4 H2O(l)
Purple reactant Colourless product
* the purple MnO4- ion becomes colourless when it reacts with the Fe2+ ions. The endpoint of the titration (that is, when all the Fe2+ ions have been used up) is that point when the purple colour of MnO4- no longer disappears after addition to the iron solution.
* the iron tablet also contains chalk powder, sucrose, and other minor ingredients. In order to prepare this tablet for analysis you will need to grind it and transfer it to a volumetric flask. I recommend using 2 tablets and in addition, the tablets should be dissolved in 1 M H2SO4. I suggest using some of the acid to help you transfer your tablet to the flask. The final solution should have a final volume of 100 mL.
* analyze 10.00 mL samples of your iron solution. Titrate with 0.005 M potassium permanganate. Remember that you have reached the endpoint of the titration when the purple colour of the permanganate just begins to stay - this indicates that all of the Fe2+ ions have reacted. Do at least three determinations although you may have to do more to ensure that your values are within 0.3 mL of each other
* present your data in the form of a table to record the volumes of MnO4- that you have used during the titrations. I recommend that across the top of your table you make at least three columns, one for each of the titrations you have performed. You can label the columns as "Trial I", "Trial II" and "Trial III". Down the left-hand-side of your table you will need to indicate 3 rows: "final reading MnO4- (mL)"; initial reading MnO4- (mL); volume of MnO4- used (mL).
* the following steps will help you in your calculations
1. Determine the number of moles of MnO4- used to react with 10.0 mL Fe2+ solution (you know the volume and concentration).
2. Calculate the number of moles of Fe2+ that reacted (use the reaction equation).
3. Calculate the number of moles that would have reacted in 50.0 mL of Fe2+ solution (since this contains 1 tablet).
4. The number of moles of Fe2+ is the same as the number of moles of FeSO4. Calculate the mass of FeSO4 in 1 tablet.
5. Convert the mass to milligrams.
I would like your group to hand to me a report at the end of the lab period with your conclusions about whether you feel Veggiepros is guilty of false advertising. You should make this conclusion based on your data and how confident you feel about your results. In order to determine your level of confidence you need to ask yourself what are the sources of error and are they large enough to account for differences in the experimental mass of iron and the advertised mass of iron.
Submitted by:
Dr. Carl Doige
Ruamrudee International School
Bangkok, Thailand