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	Iodimetry§ A lab exercise in "redox" and titration. (Quantitative Chemistry)
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This lab exercise is to teach you simultaneously titration, use of an indicator, and a type of "redox" reaction. "Redox" is a combo-name for reduction/oxidation. You will be using thioglycolate, as shown on the left, to reduce iodine, I₂. Thus, the thioglycolate is the reducing agent, and the I₂ is the oxidizing agent. To be confusing, the reducing agent becomes oxidized, and the oxidizing agent becomes reduced! Got that?

This is the general reaction that will occur. You have the privilege of using your learning to balance this reaction! Once you have that done, for further practice, see if you can make a balanced reaction between sulfate and thiosulfate, as shown on either sides of the title. They are two other common redox reagents.

MATERIALS AND METHODS

You will first make a fast and imprecise titration to get a rough idea of how much care you must take in procedure. Then you will do a much more precise measurement.

1. Fast, imprecise range-finding procedure.
   1. Your teacher will show you how to set up a titration apparatus - called a buret (don't say it in French: pronounce the final "t").
   2. Very precisely measure out 5.0 ml of standard iodine solution into a beaker or flask.
   3. Add 3 or 4 drops of dissolved starch; the solution should turn dark blue or black.
   4. Load the buret with the "unknown" concentration of thioglycolate
   5. Take an initial buret reading; write it down!
   6. Let 1.0 ml of the unknown go into the dark solution. Swirl.
   7. Keep adding 1.0 ml volumes of unknown into the iodine until the dark color totally disappears.
   8. Record your final reading on the buret.
   9. What is the difference between the final and original readings? Double it and take off 3.0 milliliters. This will be your "speed number."
   10. THOROUGHLY rinse your beaker or flask

2. Precise method for data collection
   11. PRECISELY add 10 ml of iodine solution into a clean beaker or flask. (Notice that the volume of iodine is double that of the previous, "fast" procedure.)
   12. Add a few drops of dissolved starch.
   13. Load your buret and write down the initial reading. (Then add the "speed number" to that!)
   14. With constant swirling of the black solution, allow the buret to quickly go down to that number and STOP!
   15. From here onwards you will add only a drop at a time until you and your partner agree that the black is gone. (You might invent methods for adding half-drops, etc.)
   16. After subtracting the initial number from the final number, you have your first data point.
   17. Re-do the titration process several more times so that you have at least three data points to average together.

3. CALCULATIONS
   18. Determine how many millimoles of I₂ were in the beaker or flask.
   19. Looking at the equation you have balanced, you know that it took 3 moles of I₂ to oxidize 1 mole of thioglycolate. Thus you added exactly a third as many millimoles of thioglycolate as there were I₂ in the beaker or flask.
   20. Use your "CV" equation:
       C_{thioglycolate} x V_{thioglycolate} = 1/3 C_iodine x V_iodine;
       Since you know the values for C_iodine, V_iodine and V_{thioglycolate}, solve for C_{thioglycolate}
   21. Report your "unknown's" result to your instructor.

   Perhaps your teacher has already told you about the concept of "equivalents" and "normality." You might then attack the above calculations using "milli-equivalents" (meq), and you might express the concentrations of the initial iodine and thioglycolate in terms of "normality" such as 1 mM I₂ = 2 mN, and 1 mM thioglycolate = 6 mN. (Hopefully, you can "see" the six oxidation states through which the sulfur must rise as it goes from -SH to -SO₃H.

   You might also use this "normal" concept when comparing acids, for example. 1 M H₂SO₄ = 2 N (because it has 2 hydrogens to give off); but 1 M HCl = 1 N (it only has one hydrogen to give off)

   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?
   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?

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   § How in the world does one pronounce "iodimetry"?-!
       Try this: "He gave me a bush, and I owed him a tree."

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