Lecture 4 - Lecture 4 Chapter 10 Acid-Base Titrations...

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Anal Chem 251 Lect 4 1 Lecture 4 • Chapter 10: Acid-Base Titrations • Chapter 11: Polyprotic Acids and Bases
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Anal Chem 251 Lect 4 2 Acid-Base Indicators Common devices to measure pH in the lab are as follows: 1. pH Meter – lab 2. Acid-Base Indicator –lab An acid-base indicator is a weak organic acid (denoted as HIn) that has a different color than its conjugate base (In - ). Typically, one or both of the forms are intensely colored, so only a tiny amount of indicator is needed, far too little to affect the pH of the solution being studied. Choice of an Indicator 1. Choose one that changes color at the equivalence point 2. The indicator should react last after the titration
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Anal Chem 251 Lect 4 3 Acid- Base Indicator (Cont’d) Phenolphthalein Indicator – One of the most common indicators – changes from colorless in acid to pink in base
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Anal Chem 251 Lect 4 4 Acid-Base Indicators (Cont’d) Because the indicator molecule is a weak acid or weak base, the ratio of the two forms is governed by the [H3O] + of the test solution HIn (aq) + H2O (l) H3O + + In - Ka = H3O+] [In - ] [HIn] [H3O+] = Ka [HIn]/ [In - ] [H3O + ] determines the ratio of [HIn]/[In - ] Human eye sees pure acid form when [HIn]/[In-] = or > 10/1 Human eye sees pure base form when [HIn]/In-] = or < 1/10 Therefore, full acid form pH (acid color) = -log 10K a = pK a + 1 Indicator pH range = pKa + 1
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Anal Chem 251 Lect 4 5 Acid-Base Indicators (Cont’d) For example; bromothymol blue pKa = 7.1 6.1<7.1<8.1 (the change is 2 pK units) yellow blue 6.2<7.1<7.6 in actuality Therefore, an indicator has a color change that reflect a 100-fold range in the [HIn]/[In] which means that an indicator changes color over a range of about 2 pH units
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Anal Chem 251 Lect 4 6 The color change of the indicator bromthymol blue. Figure 19.6 acidic basic change occurs over ~2pH units
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Anal Chem 251 Lect 4 7 Titration Curve for a Strong Acid by a Strong Base:20.00 mL of 0.500 M HCl by 0.500 M NaOH Choice of Indicator – Which ones would you pick or avoid?
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Anal Chem 251 Lect 4 8 pH Colors and approximate pH range of some common acid-base indicators. Figure 19.5
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Anal Chem 251 Lect 4 9 Figure 19.7 Titration Curve for a strong acid-strong base (Titration of 40.00 mL of 0.1000 M HCl with 0.1000 M NaOH) Methyl red changes from red at pH 4.2 to yellow at pH 6.3 . Phenolphthalein changes from colorless at pH 8.3 to pink at pH 10.00. Neither color change occurs at pH 7.00, both occur on the vertical portion of the curve, where a single drop of base causes a large pH change .
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Anal Chem 251 Lect 4 10 Strong Acid – Strong Base Titration Curves Features of the Curve : There are three distinct regions of the curve, which corresponds to three major changes in slope: 1. The pH starts out low, reflecting the high [H 3 O + ] of the strong acid, and increases slowly as acid is gradually neutralized by the added base. 2. Suddenly, the pH rises steeply. This rise begins when the moles of OH - that have been added nearly equal the moles of H 3 O + originally present in the acid. An additional drop or two of base neutralizes the final tiny excess of acid and introduces a tiny excess of base, so the pH jumps 6 to 8 units.
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This note was uploaded on 04/27/2008 for the course CHEM 251 taught by Professor Vassilian during the Fall '06 term at Rutgers.

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Lecture 4 - Lecture 4 Chapter 10 Acid-Base Titrations...

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