Iodimetric Titration

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Iodimetry§
A lab exercise in "redox" and titration.
(Part of the "Science for Pennies" Series)
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The reagents for this experiment can be purchased in supermarkets around the world. You will need to buy three things: a small amount of starch powder (laundry starch would be best), antiseptic tincture of iodine (2% iodine), and permanent wave (hair) lotion, which is a source for the reducing agent thioglycolate (read the label to make sure it is a constituent).

This is the general reaction that will occur. You have the privilege of balancing the reaction! Balance-challenged? Click button.)

In keeping with the poorman's philosophy, we shall not be factidiously standarding our iodine solution because to do so would require that expensive reagent-grade reducing agent would need to be purchased - something like thiosulfate, which, if you happen to have a little of it, may be carefully weighed out and dissolved to make your standardization solution. Make it to 1.00 mMolar if you insist on going this route.

Otherwise, assume that the concentration of the IODINE is what the manufacturer says it is. Usually the antiseptic bottles say that it is 2%. That means it is 80 mMolar. Dilute it 40-fold with distilled water to make it 2 mMolar (if you have 25 ml of 2% add distilled water to it so that it is 40-times as much volume - 1,000 ml). This will be your oxidizing agent. (If you need to make your own iodine solution from solid iodine crystals, do this: place two or three large crystals of KI in a couple of drops of water. Dissolve the highly soluble crystals, then add the pre-weighed amount of iodine crystals (perhaps 0.01 mole). Once the iodine crystals are dissolved, add water up to 100 ml for a 0.1 M KI3 plus excess KI. Warning: you MUST start with the extreme concentrate!)

OTHER MATERIALS:

  1. Equipment will include burets, stands, beakers and flasks.
  2. Starch solution: To about 100 ml of COLD water, mix in a pinch of starch (it is very easy to add too much starch). Bring this to a boil, and the starch will dissolve. (Thou shalt not add the dry starch powder to hot water - try it to see what happens! Hint: any good cook knows how to make gravy.) As a quality-control check: to a few drops of this starch solution, add a few drops of your iodine solution. Do you get a dark blue or black color? If so, great! If not, add more powdered starch to your next attempt at making starch solution.
  3. The reducing reagent. Dissolve 2 ml of permanent wave lotion into a liter of water. Divide this into three batches (to form eventually three unknowns for the students).
    1. 400 ml (batch #1) is diluted to 1000 ml to make Unknown #1
    2. 300 ml (batch #2) is diluted to 800 ml to make Unknown #2
    3. 300 ml (batch #3) is diluted to 1000 ml to make Unknown #3

Their relative concentrations are known, so that any tittration results should be in those same proportions.

UnknownRel Conc.
11.00
20.75
30.60
Undiluted
Perm Lotion
500
(Hair thus gets a huge dose!)

THUS:

Iodine solution, with a little starch added as a color indicator, will be titrated with the thioglycolate unknowns. Most people think that it is easier to see the color change (end-point) with the starch and iodine in the beaker or flask below the buret, than if the starch is in the thioglycolate below the buret. Thus the flask will contain a known volume of iodine solution (plus a little starch), and the buret will be loaded with the unknown. You will thus be looking for the point when the dark color just disappears.

WARNING! Teacher, practice and rehearse this before going before your students. Don't "wing it!" Then, for good pedagogical practice, titrate a batch in front of the class. Remember that this might be the first time the students have ever seen a buret and its proper use. Remember that males and females learn slightly differently: the males tend towards learning by seeing, and females tend toward learning by being given oral instructions (landmarks, etc.). Yes, of course, there is a lot of overlap! Anyway, talk as you demonstrate making sure all can see what you are doing.

Teach the students how to save time by first measuring out a half quantity and very rapidly titrating - shooting in a milliliter of unknown at a time until the color change. For the data-gathering runs, they know when to slow down and do it dropwise or partial dropwise. Time is much more precious than are these inexpensive reagents.

QUESTIONS

  1. Which is the limiting reagent in this exercise?
  2. Which measurements must be made with precision?
  3. What is the starch doing in this reaction?
  4. Why do you think iodine titrations are so widely used to assay chemicals that are reducing agents? (Because you have the sharp end-point of the starch reaction!)
  5. What is a meniscus?
  6. Why is it important that the space between the stopcock and the tip of the buret be totally filled with liquid.
  7. What is the balanced equation for this "redox" reaction?
  8. What are all the possible oxidation states of the elements iodine and sulfur?
  9. Other than "perming" hair, what are some other redox reactions in daily living?


§ If you use thiosulfate in lieu of the much less expensive hair permanent wave lotion, you might be interested in knowing that you would be doing the latter half of the Winkler method for determining the amount of dissolved oxygen in a water sample such as used in environmental testing of ground water pollution. (What is not contained of the Winkler Method is its first half, which employs dissolved O2 and manganous sulfate, and reagents that require safety measures - strong base, and sulfuric acid.)


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