[peachyy]

In my research for a project, I have found a website that shows how an iodometric back titration can be used as a method to find the amount of caffeine. I have searched the forum, however I have not found a thread asking about this method; most threads are people using dichloromethane or ethyl acetate.

http://www.hkasme.org/News/818/Report_6B_Christiam%2520Alliance%2520SC%2520Chan%2520Memorial%2520Secondary%2520School_Chemical_test_for_caffeine.pdf

I am confused as to how the chemical equation on page 7 exactly works.

C8H10N4O2 + 2I2 + KI + H2SO4 → C8H10N4O2.HI.I4 + KHSO4

1. What is the purpose of the added sulfuric acid? Is there a neutralization here?
http://www.chemicalforums.com/index.php?topic=61568.msg219758#msg219758
- In this thread, it was said that caffeine needs acid to dissolve it in water, however, that there is already acid in tea? Is this a difference between the behavior of caffeine in tea versus coffee? If my sample is a sample of tea made with water, is there a need to add acid?
- If there is a neutralization, is the separated equation KI + H2SO4 → HI + KHSO4 ? If my sample is a sample of tea prepared with water, wouldn't HI form hydroiodic acid?
- Is water an omitted reactant, and H3O an omitted product within the general chemical equation?
2. Why did the 2I2 turn into I4?
3. Why are the caffeine, HI, and I4 joined with dots and not +? I know of hydrated ionic compounds, so are HI and I4 within the caffeine's structure, but not bonded to it? Why did a chemical reaction not occur between the three to create a new product?
4. Would KHSO4 be the "brown red" precipitate, or the other product? I cannot find a single other mention of C8H10N4O2.HI.I4 anywhere else on the internet.

Thank you to whoever offers guidance.

[Borek]

Apparently you are not the only one trying to learn details of the reaction, but quick googling shows only people asking, no answers (which is never a good sign in my experience).

Dot is sometimes used to separate acid from base in organic compounds, like writing C₂H₅NH₂·HCl for ethylamine hydrochloride. Sometimes it is also used just to group atoms in a particular way, like writing dolomite as MgO·CaO·2CO₂. My bet is that similar concept is used in the listed formula. I₄ is probably a variant of 2I₂ notation.

Speaking about neutralization and writing the reaction as KI + H2SO4 → HI + KHSO4 doesn't make much sense, as we are dealing with a solution and everything is almost 100% dissociated, so what you should be looking for is an equilibrium and net ionic reaction. It can be written as overall reaction, which makes the stoichiometry easier, but it is not what is really "happening" in the solution.

Caffeine solubility can be easily increased in low pH, as it gets protonated and converted into an ionic form. Lowering pH is a sure way to not have to worry about caffeine precipitating out of the solution.

Adding acids is a common trick in titrations, not only it easily keeps things dissolved (see above), it is also the simplest way of keeping pH reasonably constant and reproducible.

I would not expect KHSO₄ to precipitate out of the solution, its solubility is around 50 g/100 mL at STP. Plus, it is definitely white.

Not sure if it helps, but that's about as much as I can make about the problem before my first coffee.