Cracking the SAT Chemistry Subject Test, 1 by Princeton Review

Author:Princeton Review [Review, The Princeton]
Language: eng
Format: epub
ISBN: 978-0-8041-2569-7
Publisher: Random House Children's Books
Published: 2015-02-16T16:00:00+00:00

Acid-Base Indicators

An indicator is just the conjugate pair of a weak acid or base, where each conjugate is a different color. Here’s an example:

Note that the full chemical formula of the indicator compound is not important, hence the abbreviation as “H-Indicator” above. All this means is that the hydrogen ion is part of the indicator itself, and when the indicator dissociates, it dissociates into a hydrogen cation and the indicator anion. Since only a trace amount of an indicator is used in a titration, the acid-base dissociation doesn’t impact the solution’s overall pH. Instead, the indicator’s dissociation equilibrium is shifted one way or another depending upon the solution’s pH, according to Le Chatelier’s principle. If the indicator above were used in a titration, then in acidic solutions, the indicator would be driven to the conjugate acid form (red), and in basic solutions, it would be driven to the conjugate base form (yellow).

All you really need to know for this test is that a chemical acid-base indicator is a substance that changes color in a pH range ±1 of its pKa.

For example, thymol blue, which has a pKa = 2, undergoes a red-to-blue color change in the pH range 1 to 3. Also, litmus paper changes color at about pH 7. At pH’s lower than 7, the paper is red, while at pH’s greater than 7, the paper is blue. Keep these things in mind when selecting an appropriate chemical indicator.

Incidentally, the test writers will want you to know how we add acid to base or base to acid, and the answer is to use a buret. A buret is a little measuring device that allows us to drop small, known amounts of liquid into a container.

Review this section on acids and bases, and then try the following questions. Answers can be found in Part III.

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