Saturday, May 16, 2009

Just one little drop...

A chemist stands, swirling an Erlenmeyer flask with a small sample of liquid in the bottom, holding the flask up to a Buret that drips liquid into it. It drips at a rate of about 1 drop every 2 seconds, and Oddly, the two liquids combining are clear, but each time the liquid from the Buret comes into contact with the liquid in the Erlenmeyer flask, a bright pink liquid evolves, and disappears in the swirling motion of the chemicals. Then, the chemist notices how the pink liquid dissipates at a slower rate, so he slows the rate in order to observe the reaction of every drop. One drop is added. The chemist swirls, and the pink disappears. Another drop is added. The solution in the Erlenmeyer begins to fully turn pink, but with a little swirling, it slowly goes clear. Almost there, the chemist allows half of a bead of liquid to form, and stops the rate entirely. He pulls out a stirring rod, and grabs the half-bead with it, then swirls it into the Erlenmeyer solution. The solution turns a mild shade of pink, but does not dissipate. The experiment is over -- but what just happened, and why was the chemist interested in it?

This was a description of a Titration experiment. This is an analytical tool for determining how much chemical stuff is dissolved in a known amount of liquid, and the bane of students in General Chemistry II. It combines the ideas of atoms, ions, dissolution, chemical reactions, and moles -- that's a lot of theory, and while I realize everyone knows what an atom is, I don't know if everyone will remember what an ion is, or why salt dissolves in water. Nonetheless, I don't think its absolutely necessary to fully understand these concepts to get the gist of what a Titration is all about.

Take some baking soda (do it!), and put some of it into two different cups -- I put 1/2 teaspoon into two coffee cups. Then fill one of the cups with water halfway, and the other one quarter of the way up with water. Break out the household vinegar, and a tablespoon, and drop one tablespoon of vinegar into each glass, and watch what happens. Keep doing this. (this can get messy, so it might be best to do this in the sink) This is a little rough, but when I did it at home, it seemed to illustrate the point -- you'll notice that bubbles stop forming with roughly equal amounts of vinegar, even though one cup has twice the amount of water in it. This suggests that the water has nothing to do with the reaction, just the baking soda and the vinegar.

That's because it's true -- the water is the medium through which the chemical reaction takes place, or in chemical parlance the "solvent". It's where the chemicals you're interested in float around, find each other, and react.

Just so you know, there is a chemical reaction going on between the water and the chemicals you're interested in, but it has nothing to do with the one that evolves the bubbles, and is called "Dissolution". Basically, it's what happens when you put sugar in your milk, or when you mix salt and water. The water molecule separates all the molecules packed together in that grain of sugar or salt and surrounds them, which is what renders them "invisible", since molecules are too small to see all by themselves.

Now, back to titration -- that's basically what you performed in the kitchen! But there are a few differences.For starters, chemist's use instruments that are known to produce better accuracy and precision. This is primarily a quantitative experiment, which leads to the next point of difference: Normally, the liquid in the Erlenmeyer flask has an unknown amount of chemicals you're interested in in it. You usually know that it's in water, or can easily tell, and you can tell what type of chemical you need to react with it with litmus paper. Further, when preparing a chemical to react with the unknown, you have complete control: You can measure the amount of chemical you dissolve into water. Then, you react a known amount of chemical with an unknown amount of chemical, and when the reaction doesn't happen anymore, you know that all of the unknown amount has reacted and that that unknown amount is equal to the amount of chemical you prepared to react with it.

One other little snag in this is, how do you know when a chemical reaction is complete? Most of them aren't as dramatic as the reaction between baking soda (Sodium Bicarbonate) and Vinegar (Acetic Acid). This is what the pink color was all about in the story above -- this is another chemical present in the mixture. It doesn't interfere with the reaction between the two chemicals you're interested in, but it changes color whenever your prepared chemical comes into contact with it. This way, if the pink disappears, you know that all of your prepared chemical reacted with the unknown chemical, and you continue. If pink stays, even just a little bit, then you know you've reached what is generally termed the "Equivalence Point" -- the point where a solution's pH changes dramatically from acidic to basic with the addition of a small amount of either an acid or a base.

This is why the chemist was taking so much care near the end of his experiment. It doesn't take a lot to accidentally go past the equivalence point. Even one little drop can add too much of your prepared chemical, and then your calculation for how much chemical amount was added will be off far enough that you'll have only a very rough idea of how much chemical amount was in the unknown, rather than a good idea.

Now, this can get much more complex, but the general idea holds: You have some unknown amount of molecules floating around in some water, and you want to know how many molecules are there. So you throw in a chemical that will react with those molecules, and when they're done reacting, you do a little math and figure out the unknown -- pssh, who says chemistry is hard to understand? It's just colors, numbers, and bubbles.

1 comment:

  1. my dad used to take me into his lab when I was little and do titrations--it really does look like magic!