lecture 9 titration curves

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Schedule8:10-9:00 Questions9:00-9:45 Test on Acids and Bases9:45-12:00 Buffers and titration curves

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Chapter 15 – Titration Curves

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Titration

• Strong acid/strong base• Strong acid/weak base• Strong base/weak acid

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Setup to Do pH Titration of an Acid or Base

Photo © Brooks/Cole, Cengage Learning Company. All rights reserved.

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Titration of 50ml of 0.20M HNO3 with 0.100M NaOH: No NaOH Added

Of solution in the beaker (called analyte)

From the burette, called the titrant

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Titration of 50ml of 0.20M HNO3: 10.0 mL of 0.100 M NaOH Added

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Titration of 50ml of 0.20M HNO3: 50.0 mL (total) of 0.100 M NaOH Added

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Titration of 50ml of 0.20M HNO3: 100.0 mL (total) of 0.100 M NaOH Added

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Titration of 50ml of 0.20M HNO3: 150.0 mL (total) of 0.100 M NaOH Added

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Titration of 50ml of 0.20M HNO3: 200 mL (total) of 0.100 M NaOH Added

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Titration of 50ml of 0.20M HNO3 with 0.100 M NaOH

Note: at the equivalence point the number of moles of OH¯ added = the number of moles of H3O+ titrated

The terms equivalence point and end point mean the same thing.

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Strong base/weak acid titration

Unlike the titration of a strong acid, the pH of a weak base is buffered at pH valued of pKa ± 1.

][

][log

acid

basepKpH a

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][

][log

acid

basepKpH a

The endpoint of the titration is at 50 mL of added OH¯. The midpoint is halfway, at 25 ml of added OH¯.

At the midpoint, pH = pKa.

In other words, the pKa can be read directly from the titration curve.

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Below is shown the titration of a weak base with HClO4.

For this buffer it shows how the pH changes with added strong acid.

It can be thought of as a buffer profile.

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Each point in this titration is the result of a separate experiment.

Point by point is shown buffer pH for a given buffer composition. In sum, it shows how the pH changes with added strong acid.

It gives you the pKa of the weak acid/base buffer pair.

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What percent of the NH3 has been converted to NH4+ after

25ml of HClO4 have been added?

Titration of 100 ml of 0.05M NH3

NH3 + H3O+ NH4+ + H2O

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What is the pKa of the conjugate weak acid? a. 11 b. 3 c. 9.3 d. 4.7 e. lost

][

][log

acid

basepKpH a

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Titration of 100 ml of 0.05M NH3

What percent of the NH3 has been converted to NH4+ after

20ml of HClO4 has been added?

After 40ml of HClO4 has been added?After 50 ml?

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What are the major species in solution at the end of the

titration?

a. ClO4¯, NH3, H3O+ b. HClO4, NH3 c. ClO4¯, NH4

+, H3O+ d. HClO4, NH4

+

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If the volume of the analyte NH3 is 100 mL, what was its

concentration at the start of the titration?

a. 0.10M b. 0.20M c. 0.05M d. Lost!

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Titration of 100 ml of 0.05M NH3

What is buffering region here, in terms of mL HClO4 added?

Does a buffering range of pH = pKa ± 1 seem about right?

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How would you calculate the pH at the endpoint of this titration of 100 ml of 0.05M NH3?

Hint: first you need to know what will react with water, and its concentration!

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What is the pH at the midpoint of a HOCl titration (Ka = 3.5x10-8)1. 7.52. 6.53. Lost

What is the pH at the midpoint of a OCl¯ titration (Kb = 2.9x10-7)4. 7.55. 6.56. lost

Consider the titration of 0.20M HOCl. What is the concentration of OCl¯ at the midpoint of the titration (ignoring dilution)?1. 0.20M2. 0.10M3. Not possible to determine without more information4. lost

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Titration of a polyprotic acid: H3PO4

• pKa values• Species at 5ml

intervals• pH of

amphoteric salts

H3PO4

H2PO4¯

HPO4-2

PO4-3

pKa1 = 2.1 pKa2 = 7.2 pKa3 = 12.3

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How to prepare a buffer

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How to prepare a bufferSuppose you wanted to make a pH 7.4 phosphate buffer

Given that pKa1 = 2.12, pKa2 = 7.21, and pKa3 =12.3, what chemicals would you use from this list:H3PO4, KH2PO4, K2HPO4, K3PO4

What would be the ratio of Base/Acid that you would use?

Suppose you wanted a 0.20M, pH 7.4 phosphate buffer. How much base and how much conjugate acid would you use to make 1 liter of buffer?

KH2PO4, K2HPO4

][

][log2.74.7

acid

base

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The steps for a buffer preparation calculation:

1. Determine the weak acid/weak base couple using the pH of the buffer as a guide

2. Determine the ratio of base to acid in the buffer using pH = pka + log(B/A)

3. Determine the actual concentrations of B and A to be used based on the desired buffer concentration and the B/A ratio.

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Example: Make a 0.15M pH 6.7 carbonate buffer.

1. Determine the weak acid/weak base couple using the pH of the buffer as a guide

For H2CO3, pKa1 = 6.4, pKa2 = 10.3

Choose the weak acid with pKa1 = 6.4, and its conjugate base: H2CO3/HCO3¯

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Example: Make a 0.15M pH 6.7 carbonate buffer.

2. Determine the ratio of base to acid in the buffer using pH = pka + log(B/A)

6.7 = 6.4 + log([HCO3¯]/[H2CO3])

[HCO3¯]/[H2CO3] = 2.0

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Example: Make a 0.15M pH 6.7 carbonate buffer.

3. Determine the actual concentrations of B and A to be used based on the desired buffer concentration and the B/A ratio.

[HCO3¯]/[H2CO3] = 2.0

[HCO3¯] + [H2CO3] = 0.15

Solve these two equations for the two unknowns:

[HCO3¯] = 0.10M [H2CO3] = 0.05M 30

Try a problem: What would you need in solution to have a 0.30M, pH 7.1, hypochlorous buffer?HClO: pKa= 7.41. Determine the weak acid/weak base couple

using the pH of the buffer as a guide2. Determine the ratio of base to acid in the buffer

using pH = pka + log(B/A)3. Determine the actual concentrations of B and A

to be used based on the desired buffer concentration and the B/A ratio.

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Could you prepare this buffer using only HClO and KOH?

How many moles of HClO would you use to prepare 1.0L of 0.30M, pH 7 buffer?1. 0.12. 0.23. 0.34. lost

About how many moles KOH would you expect to need to make 1.0L the buffer?5. 0.16. 0.27. 0.38. lost

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