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PROFESSOR CIMA: OK, solubility and pH-- I think I'm going to put this up next time.
If this is some solid that is a sparingly soluble salt like you've
studied, it'll have some K_sp. Now this anion can react with water.
If it does, this becomes a proton acceptor. So we call this a base.
And what it produces, of course, is a conjugate acid.
Now if the anion is the conjugate base of a weak acid, when I add hydrogen
ion to this solution containing the sparingly soluble salt, I will drive
this reaction that way. So I add acid.
It consumes hydroxyl ions. It moves this reaction this way.
And what do you think it does to the solubility? STUDENT: It increases.
STUDENT: It's reduced. PROFESSOR CIMA: It increases it.
So this sparingly soluble salt can become soluble in acid, if the anion
it produces is the conjugate base of a weak acid.
If it's a conjugate base of a strong acid, it goes this way.
And nothing you can do on this side is going to pull it that way.
So I'm going to go to a little slide set. STUDENT: Can you say that again?
PROFESSOR CIMA: Yeah. OK, so here we have some sparingly soluble
salt, like all the things you're doing for your homework problems.
Things that have these ridiculously small solubilities.
If the anion it produces reacts with water according to this equation, that
is, it is the conjugate base of a weak acid, then this reaction can be moved
that direction by adding acid, right? I add acid, I reduce this.
I force this reaction that way. As I force this reaction that way, I force
this reaction that way, because I'm consuming this.
So I can drive up the solubility dramatically, orders of magnitude.
All right, I wanted to end with-- so here's an interesting story.
So in your stomach, you know you have acid. Right before an exam, it's really bad, right?
And you might wonder how does it get there. How does it have, literally, concentrations
of-- we're talking about a pH of around 0.8.
It's like mineral acid. Really.
So how does it get concentrated? That's the interesting thing.
How do I move those protons uphill? Well, in the wall of your stomach are these
molecular machines called the proton pump.
And what it does, it pumps protons from the rest of your
body into your stomach. And it does that by simultaneously pulling
potassium out. And this guy is responsible for getting that
acid in. Now you want to get rid of that acid in your
stomach. What you do is you go after Prilosec.
This is called Omeprazole. This funny looking molecule goes into that
proton pump and shuts it down. It just jams up the work.
And it's highly specific. The problem with it is it falls apart in acid.
Yeah. So you eat this stuff.
And you just ate this. It would just completely destroy itself before
it did anything. So you have to engineer the answer.
So a Prilosec tablet has the drug inside. And it's coated with a polymer.
And it makes use of the fact that you have the acid in your stomach.
But in your intestine there's a pH gradient. pH in your intestine is high.
And this coating here is a polymer that has an acid
group on it like this. So what do you think its solubility is in
acid? STUDENT: Not very high.
PROFESSOR CIMA: Low. So as soon as this pellet gets past your stomach
into your intestine, now the solubility goes up.
And it releases its payload. And then you jam your proton pump systemically.
The drug goes into your bloodstream and, from the other side, jams the
proton pump. So, had to engineer the material to do that.
Prilosec, by the time it went off patent, was a $8 billion a year drug.
Hard to imagine. And so it wouldn't have worked without engineering
that polymer.