Polyprotic Titrations

Okay, we're not going to lie to you. Polyprotic titrations are not easy. In fact, they're the hardest material on the Death Test. But they are doable. Just as in any other titration problem, the key is staying organized. As you already know, polyprotic acids and polyteric bases are acids and bases that can react multiple times with water and give away or recieve more than one proton (H3PO4, for example). The trick to doing a polyprotic titration is to know what reactions are going on in any given region. To do this, it helps to have a titration curve, as you'll see in the example problem. You draw the curve, and then you figure out what you have in solution in each region of the curve. The reactions going on in each region are the reactions in which all of the reactants are present in solution. Here are your steps:

  1. Write out all of the possible reactions between the acid or base and water, including all of the intermediate acids and bases, as well as any reactions involving the acid or base reacting with itself.
  2. Find the equilibrium constants for all of the reactions.
  3. Find the equivalence points and max buffer points.
  4. Draw a nifty titration curve.
  5. Figure out what's in solution, and which reactions are going on.
  6. Figure out which reaction is the dominant reaction in each region of the curve (in other words, which reaction has the biggest K).
  7. Go through each region, doing the math as it relates to the dominant reaction.

Sounds like a lot of work, huh? Well, it is! This is why the Death Test lasts two periods. Of course, everything always seems so much clearer after we do a problem, doesn't it?

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