Determination of the Percentage of Iron in a Sample
The concentrations of redox-active species can be determined by redox titrations. In a redox titration, a measured sample of the unknown is titrated against a standard solution of a substance that will oxidize or reduce the unknown. In the present experiment you will take a sample containing iron, add acid to dissolve it [thereby converting all the iron to iron(II)], then use a solution containing permanganate ion, MnO4-, to oxidize this Fe2+ to Fe3+ ion. The percent of iron in the sample will be calculated from the amount of permanganate needed to oxidize fully all the Fe2+ ions. A solution of permanganate ion in sulfuric acid efficiently oxidizes Fe2+ to Fe3+ MnO4- + 5 Fe2+ + 8 H+ ( Mn2+ + 5 Fe3+ + 4 H20
The permanganate ion acts as its own indicator, as MnO4- is highly colored while Mn2+ is essentially colorless. The product of oxidation, the Fe3+ ion, is itself, slightly colored. To avoid any possible interference with the equivalence point determination a little phosphoric acid, H3PO4, is added so as to complex Fe3+ to a completely, colorless ion.
I. Standardization of permanganate solution
Use distilled water at all times throughout the experiment. (1)
Weigh three clean dry labeled 125 mL erlenmyer flasks on an analytical balance. Place about 0.135 grams of oxalic acid dihydrate, H2C204.2H20, into each of the three separate flasks and reweigh the flasks containing the acid.
Set up a buret with KMnO4 solution to be standardized by titration.
Dissolve each acid sample in about 25 mL of distilled water. Again don't mix up the samples. Take one flask and add 1-2 mL of concentrated sulfuric acid.
CAUTION! Concentrated sulfuric acid is dangerous; don't spill or splash any. Always slowly add acid to water, never the other way around.
The solution to which the acid has been added should get quite warm, but, since the titration is to be done at elevated temperatures to prevent side reactions, this is desirable. Heat the solution further to 70oC; during the titration the solution should be kept between 60 and 80oC.
Read the level in the permanganate buret (to hundredths of a mL) -- the initial reading -and then add the solution slowly from the buret into the flask with the warmed acid sample with constant stirring. The equivalence point is the first appearance of a pink color (excess MnO4-) that lasts, with stirring, for 30 seconds. When this is obtained, read the buret again -- the final reading.
Take a second flask with oxalic acid in it and add 1-2 mL of concentrated sulfuric acid. Repeat steps 7 and 8 above with this sample. Do a third trial with the third flask in the same manner.
II. Determination of iron
Have at least 10 mL of 85% H3PO4 ready for use in step (5). (1)
Weigh out accurately three samples of your unknown mixture containing iron and put each sample in a separate, clean (but not necessarily dry) Erlenmeyer flask. The amount of the sample you take depends on what the unknown mixture is; consult instructor. First weigh the vial containing the unknown iron sample. Use the centigram balance to approximate the weight of iron sample in the flask, then, on the analytical balance, re-weigh the vial.
Fill a buret with standardized KMnO4 solution.
Dilute 6 M H2SO4 (called dilute sulfuric acid) to 1 M H2SO4 by adding 25 mL of the 6 M acid to 125 mL of water (acid into water, never the other way). Stir well.
Put one-third of this 1 M H2SO4 solution (50 mL) into only one of your Erlenmeyer flasks and dissolve the iron sample quickly and completely. Take an initial reading of the buret and, with stirring, as speedily as possible, start titrating by slowly adding the permanganate solution to the acidified sample.
When the solution turns a...
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