ln12s08 - Lecture 12: Modeling the titration experiment-the...

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Lecture 12: Modeling the titration experiment—the monoprotic case. The classic acid-base titration, with its nostalgic use of out-dated oversized glassware, stills finds itself the center of attention in lectures on water chemistry. This lecture will attempt to explain the fondness for the experiment. First, the word titration. As a general definition, it is the idea that one would compare some unknown amount of material to a standard set of known amounts. That more general idea will never disappear. There will always be unknown amounts of stuff, and there will always be utility in finding out how much unknown you have by comparing it to known amounts of stuff. In the classic experiment which you watch me do in class, an amount of unknown is dissolved in a known volume, V 1 , to produce an unknown concentration, M 1 . To this unknown, a known volume, V 2 , of a known concentration of titrant, M 2 , traditionally from a piece of glassware called a buret which allows for easy measurement of volume. If there is a relationship between the known and unknown material that can be expressed, for example, a stoichiometric relationship, then it must be true that at some point the amount of the known material will be equal to the amount of the unknown material. moles 1 = moles 2
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Or expressed as concentrations and volumes, M 1 V 1 = M 2 V 2 and since we know three of the four values, we can solve for the unknown. Indicators. Of course the trick is having some indication of when the equality is true. There are many approaches to this. The most common is to use a dye molecule that changes color at different pH. I have shown two examples in class: Phenolphthalein was the indicator used in the experiment in which two students blew CO 2 into water to acidify the solution. Phenolphthalein turns from clear below pH 8 to pinkish purple above pH 8 as the two protons shown on the OH groups are removed colorless below pH 8 when protonated pinkish purple above pH 8 when deprontonated Bromthymol Blue was the indicator used in all our titrations including the indicator tee-shirt demo. Bromthymol blue turns from blue in basic solution above pH 7 to yellow in acid solutions below pH 7. There are many other pH indicators that can be used depending on the specific pH at which a titration changes color—examples of them and their pH region of color change are shown in the table below. We shouldn’t be surprised about the variety-- - -
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indicators are just acids and their conjugate bases that happen to be colorful. Note that we assume that indicators are present in such small concentrations that we don’t correct for them in our calculation on unknown concentrations. Modern America—the pH meter.
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ln12s08 - Lecture 12: Modeling the titration experiment-the...

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