Titration Curves

Chem 18.1 Experiment 8

Titrationmethod of analysis that will allow

you to determine the equivalence point of a reaction and therefore the precise quantity of reactant in the titration flask

Titrantsolution in the buretteusually a strong base or acid

Analytesolution being titratedoften the unknown

Titration Set-up

Titration Curvesare the graphs obtained by

plotting the pH of the reaction mixture against the volume of base (or acid) added during the titration of either an acid by a base or vice versa.

If the pH of an acid solution is plotted against the amount of base added during a titration, the shape of the graph is called a titration curve. All acid titration curves follow the same basic shapes.

Equivalence pointThe volume of titrant and pH at

which the amount of acid equals the amount of base present in the original solution, or the amount of base equals the amount of acid present in the original solution.

Not the same as end point

Results and Discussion

Titration of Weak Acid with Strong Base

0 3 5 9 12 16 19 21 23 25 32 38 40 48 51 53 55 600

2

4

6

8

10

12

Titration of 25mL of 0.1M HOAc with O.1M NaOH

mL of titrant

pH

Equivalence point: volume: 50.5 mL8.96-pH

0 7 16 22 32 44 530

2

4

6

8

10

12

Titration of 25mL of 0.1M HOAc with

O.1M NaOH

mL of titrant

pH

Discuss the titration curves obtained in Part A

0 7 16 22 32 44 530

2

4

6

8

10

12

Titration of 25mL of 0.1M HOAc with

O.1M NaOH

mL of titrant

pH

• Curve begins at low pH• Near the equivalence point there is a rapid increase in pH• Equivalence point higher than 7, indicating the presence of basic salt•Curve ends at high point

Titration of Strong Acid with Strong Base

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 77 78 800

2

4

6

8

10

12

14

Titration of 25mL of 0.1M HCl with 0.1M NaOH

mL of titrant

pH

Equivalence point: volume: 77.5 mL 7.5-pH

0 10 20 30 40 50 60 70 77 800

2

4

6

8

10

12

14

Titration of 25mL of 0.1M HCl with 0.1M

NaOH

mL of titrant

pH

Solution of strong acid will always have a lower initial pH

Before reaching eq. pt, pH increases slowly but as it approaches the said point, solution experiences steeper rise in pH

Theoretically, the eq. pH of a strong acid-strong base titration is 7

0 15 30 45 60 75 800

2

4

6

8

10

12

14

Titration of 25mL of 0.1M HCl with 0.1M

NaOH

mL of titrant

pH

Titration of Weak Base with Strong Acid

0 3 5 6 7 8 9 10 110123456789

10

Titration of 25mL of 0.1M NH4OH with 0.1M HCl

mL of titrant

pH

Equivalence point: volume: 6.5 mLpH: 5.355

0 3 5 6 7 8 9 10 110

2

4

6

8

10

Titration of 25mL of 0.1M NH4OH with

0.1M HCl

mL of titrant

pH

Pattern of weak base-strong acid titration quite similar with that of weak acid-strong base, except that the pH of the solution decreases as titration continues

Equivalence point less than 7

Cation of weak base (NH4

+) will undergo hydrolysis

0 3 5 6 7 8 9 10110

1

2

3

4

5

6

7

8

9

10

Titration of 25mL of 0.1M NH4OH with 0.1M HCl

mL of titrant

pH

Weak Acid and Weak Base

•Curve begins at higher acidic pH and ends at low basic pH.•There is not a great pH change at the equivalence point (pH ~ 7) making this a very difficult titration to perform.

Guide QuestionsConstruct the theoretical titration curves

for the following by plotting pH versus the volume of the titrant computed after every 5mL of the titrant has been added.

A. 25 mL 0.1M HOAc with 0.1 M NaOHB. 25 mL 0.1M HCl with 0.1 M NaOHC. 25 Ml 0.1M NH4OH with 0.1 M HClShow sample computations.

25 mL 0.1M HOAc with 0.1 M NaOH

mL of HOAc

mL of NaOH

total vol

(mL)

pH

25 0 25 2.87

25 5 30 4.14

25 10 35 4.56

25 15 40 4.74

25 20 45 5.35

25 25 50 8.72

25 30 55 11.95

25 35 60 12.22

Calculations:

A. 25 Ml 0.1M HOAc with 0.1M NaOH

25 mL 0.1M HCl with 0.1 M NaOH

mL of

HCl

mL of NaOH

total vol

(mL)

pH

25 0 25 1.0025 5 30 1.1825 10 35 1.3025 15 40 1.4825 20 45 1.7825 25 50 7.0025 30 55 11.9

525 35 60 12.2

2

Calculations

B. 25 mL 0.1M HCl with 0.1M NaOH

25mL 0.1M NH4OH with 0.1M HCl

mL of

NH4OH

mL of

HCl

total vol (mL

)

pH

25 0 25 11.13

25 5 30 9.86

25 10 35 9.43

25 15 40 9.08

25 20 45 8.65

25 25 50 5.28

25 30 55 2.04

25 35 60 1.78

Calculations

C. 25 mL 0.1M NH4OH with 0.1M HCl