Acid-Base TitrationsAcid-Base Titrations

Strong Base to Weak AcidStrong Base to Weak Acid• Consider adding a strong base (e.g. NaOH) to a

solution of a weak acid (e.g.KHP).

Acid-Base TitrationsAcid-Base Titrations

Strong Acid-Base TitrationsStrong Acid-Base Titrations

•The plot of pH versus volume during a titration is a titration curve.

Acid-Base TitrationsAcid-Base Titrations

Weak Acid-Strong Base Weak Acid-Strong Base TitrationsTitrations

ProblemWhat is the pH of a solution prepared by mixing exactly 30.0 ml of 0.200 M HC2H3O2 with 15.0 ml of 0.400 M KOH.

Strong acid: [H3O+] = concentration of acidso: pH = -log [H3O+] = -log[acid]

Weak Acid: one must calculate the [H3O+] from an equilibrium ionization expression.

HA + H2O H3O+ + A-

Ka = _ [H3O+][A-] [HA]

These are equilibrium concentrations.

Titration Curve of a Weak Acid with a Strong Base

0

2

4

6

8

10

12

14

0 5 10 15 20 25 30 35 40 45 50

Volume of Strong Base Added

equivalence pointbuffer region note volume ratiobetween these points

25.0 mL of 0.10M Acetic acid Ka=1.8 x 10-5

0.10 M NaOH

1. Starting pH, only acid and water.

2

34

1. pKa = pH at this point (halfway point)

This is also the “buffer region”

Some base has been added

Titration Curve of a Weak Acid with a Strong Base

0

2

4

6

8

10

12

14

0 5 10 15 20 25 30 35 40 45 50

Volume of Strong Base Added

equivalence pointbuffer region note volume ratiobetween these points

1

2

3

4

[H+]=[A-]

[H+][A-]

pOH=-Log[OH-]

[OH-]=[HA]

]HA[

]A][H[K a

]HA[

]A][H[K a

]A[

]HA][OH[Kb

Approximations:

Titration Curve of a Weak Acid with a Strong Base

0

2

4

6

8

10

12

14

0 5 10 15 20 25 30 35 40 45 50

Volume of Strong Base Added

equivalence pointbuffer region note volume ratiobetween these points25.0 mL of 0.10M Acetic acid Ka=1.8 x 10-5

0.10 M NaOH

1.

2.

34

Some base has been addedThis is also the “buffer region”

pKa = pH at this point (halfway point)

The color change of the indicator bromthymol blue

acidic

basic

change occurs over ~2pH units

Curve for a strong acid-strong base titration

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Strong Acid-Strong base Strong Acid-Strong base TitrationTitrationpH starts low and increases gradually as acid pH starts low and increases gradually as acid is neutralized by the added baseis neutralized by the added base

Close to equivalence point pH rises steeplyClose to equivalence point pH rises steeply

Beyond this, pH increases slowly with Beyond this, pH increases slowly with addition of more baseaddition of more base

Equivalence pointEquivalence point

– the mol OHthe mol OH1- 1- = mol H= mol H1+ 1+

–The pH = 7The pH = 7

Curve for a weak acid-

strong base titration

Titration of 40.00mL of 0.1000M HPr with 0.1000M NaOH

[HPr] = [Pr-]

pH = 8.80 at equivalence point

pKa of HPr = 4.89

methyl red

Curve for a weak base-strong acid

titration

Titration of 40.00mL of 0.1000M NH3 with 0.1000M HCl

pH = 5.27 at equivalence point

pKa of NH4+

= 9.25

pKa = 7.19

pKa = 1.85

Curve for the titration of a weak polyprotic acid.

Titration of 40.00mL of 0.1000M H2SO3 with 0.1000M NaOH