Saturday, May 9, 2009

Preparation of pH buffer solutions



The different names for phosphate salts. Standardization buffers pH 4 and pH 7. Ph range of some buffer systems. Making up buffer solutions by adding an adjuster solution (acid or base) to a known volume and concentration of a primary salt solution. Potassium hydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, potassium hydrogen phthalate, sodium acetate,sodium tetraborate, tris aminomethane.

Phosphates

Phosphate salts are known by several names and the correct phosphate must be used to prepare buffer solutions. One phosphate cannot be substituted for another phosphate. Check formula of salt to be certain.

Formula

Name of salt

Other names

KH2PO4

potassium dihydrogen
phosphate

potassium dihydrogen orthophosphate
monobasic potassium phosphate
monopotassium phosphate
acid potassium phosphate
potassium biphosphate

K2HPO4

potassium hydrogen
phosphate

dipotassium hydrogen orthophosphate
dipotassium hydrogen phosphate
dibasic potassium phosphate
dipotassium phosphate

K3PO4

potassium phosphate

tribasic potassium phosphate
tripotassium phosphate

Standardization buffers

For pH=7.00 :
Add 29.1 ml of 0.1 molar NaOH to 50 ml 0.1 molar potassium dihydrogen phosphate.

Alternatively :
Dissolve 1.20g of sodium dihydrogen phosphate and 0.885g of disidium hydrogen phosphate in 1 liter volume distilled water.

For pH= 4.00 :
Add 0.1 ml of 0.1 molar NaOH to 50 ml of 0.1 molar potassium hydrogen phthalate .

Alternatively :
Dissolve 8.954g of disodium hydrogen phosphste.12 H2O and 3.4023g of potassium dihydrogen phosphate in 1 liter volume distilled water.

Range of common buffer systems ¹


Buffering system


Useful buffering pH range @ 25°C

Hydrochloric acid/ Potassium chloride

1.0 - 2.2



Glycine/ Hydrochloric acid

2.2 - 3.6



Potassium hydrogen phthalate/ Hydrochloric acid

2.2 - 4.0



Citric acid/ Sodium citrate

3.0 - 6.2



Sodium acetate/ Acetic acid

3.7 - 5.6



Potassium hydrogen phtaalate/ Sodium hydroxide

4.1 - 5.9



Disodium hydrogen phthalate / Sodium dihydrogen orthophospate

5.8 - 8.0



Dipotassium hydrogen phthalate / Potassium dihydrogen orthophospate

5.8 - 8.0



Potassium dihydrogen orthophosphate / sodium hydroxide

5.8 - 8.00

Barbitone sodium / Hydrochloric acid

6.8 - 9.6



Tris (hydroxylmethyl) aminomethane / Hydrochloric acid

7.0 - 9.00



Sodium tetraborate/ Hydrochloric acid

8.1 - 9.2



Glycine/ Sodium hydroxide

8.6 - 10.6



Sodium carbonate/ Sodium hydrogen carbonate

9.2 - 10.8



Sodium tetraborate/ Sodium hydroxide

9.3 - 10.7



Sodium bicarbonate / Sodium hydroxide

9.60 - 11.0



Sodium hydrogen orthophosphate / Sodium hydroxide

11.0 - 11.9



Potassium chloride/ Sodium hydroxide

12.0 - 13.0

Preparing a Buffer Solution ²

This page gives tabulated info on the preparation of buffers by mixing adjusters
with a known volume of the primary salt solution.

BUFFERS 1.00 - 9.00

Buffer A :
pH 1.0 - 2.2

Buffer B :
pH 2.2 - 4.00

Buffer C :
pH 4.10 - 5.90

Buffer D :
pH 5.8 - 8.00

Buffer E :
pH 7.0 - 9.00

50 ml 0.2 M KCl + mls of 0.2 M HCl

100 ml 0.1 M potassium hydrogen phthalate + mls of 0.1 M HCl.

100 ml 0.1 M potassium hydrogen phthalate + mls of 0.1 M NaOH

100 ml 0.1 M KH2PO4 + mls of 0.1 M NaOH.

100 ml 0.1 M tris (hydroxymethyl) aminomethane + mls of 0.1 M HCl.

pH

mls of 0.2M HCl added

pH

mls of 0.1M HCl added

pH

mls of 0.1M NaOH added

pH

mls of 0.1M NaOH added

pH

mls of 0.1 M HCl added

1.00

134.0

2.20

99.0

4.10

2.6

5.80

7.2

7.00

93.2

1.10

105.6

2.30

91.6

4.20

6.0

5.90

9.2

7.10

91.4

1.20

85.0

2.40

84.4

4.30

9.4

6.00

11.2

7.20

89.4

1.30

67.2

2.50

77.6

4.40

13.2

6.10

13.6

7.30

86.8

1.40

53.2

2.60

70.8

4.50

17.4

6.20

16.2

7.40

84.0

1.50

41.4

2.70

64.2

4.60

22.2

6.30

19.4

7.50

80.6

1.60

32.4

2.80

57.8

4.70

27.2

6.40

23.2

7.60

77.0

1.70

26.0

2.90

51.4

4.80

33.0

6.50

27.8

7.70

73.2

1.80

20.4

3.00

44.6

4.90

38.8

6.60

32.8

7.80

69.0

1.90

16.2

3.10

37.6

5.00

45.2

6.70

38.6

7.90

64.0

2.00

13.0

3.20

31.4

5.10

51.0

6.80

44.8

8.00

58.4

2.10

10.2

3.30

25.8

5.20

57.6

6.90

51.8

8.l0

52.4

2.20

7.8

3.40

20.8

5.30

63.2

7.00

58.2

8.20

45.8



3.50

16.4

5.40

68.2

7.10

64.2

8.30

39.8



3.60

12.6

5.50

73.2

7.20

69.4

8.40

34.4



3.70

9.0

5.60

77.6

7.30

74.0

8.50

29.4



3.80

5.8

5.70

81.2

7.40

78.2

8.60

24.4



3.90

2.8

5.80

84.6

7.50

82.2

8.70

20.6



4.00

0.2

5.90

87.4

7.60

85.6

8.80

17.0







7.70

88.4

8.90

14.0







7.80

90.6

9.00

11.4







7.90

92.2









8.00

93.4





BUFFERS 08 - 13

Buffer F: pH 8.0 - 9.10

Buffer G :
pH 9.2 - 10.80

Buffer H :
pH 9.60 - 11.00

Buffer I :
pH 10.90 - 12.00

Buffer J :
pH 12.00 - 13.00

100 mL 0.025 M Na2B4O7.10H2O (borax) + mls of 0.1 M HCl.

100 mL 0.025 M Na2B4O7.10H2O (borax) + mls of 0.1 M NaOH.

100 mL 0.05 M NaHCO3 + mls of 0.1 M NaOH.

100 mL 0.05 M Na2HPO4 + mls of 0.1 M NaOH.

50 mL 0.2 M KCl + volume indicated (in mL) 0.2 M NaOH.

pH

mls of 0.1M HCl added

pH

mls of 0.1M NaOH added

pH

mls of 0.1M NaOH added

pH

mls of 0.1M NaOH added

pH

mls of 0.2M NaOH added

8.00

41.0

9.20

1.8

9.60

10.0

10.90

6.6

12.00

12.0

8.10

39.4

9.30

7.2

9.70

12.4

11.00

8.2

12.10

16.0

8.20

37.6

9.40

12.4

9.80

15.2

11.10

10.2

12.20

20.4

8.30

35.4

9.50

17.6

9.90

18.2

11.20

12.6

12.30

25.6

8.40

33.2

9.60

22.2

10.00

21.4

11.30

15.2

12.40

32.4

8.50

30.4

9.70

26.2

10.10

24.4

11.40

18.2

12.50

40.8

8.60

27.0

9.80

30.0

10.20

27.6

11.50

22.2

12.60

51.2

8.70

23.2

9.90

33.4

10.30

30.4

11.60

27.0

12.70

64.4

8.80

19.2

10.00

36.6

10.40

33.0

11.70

32.4

12.80

82.4

8.90

14.2

10.10

39.0

10.50

35.6

11.80

38.8

12.90

106.0

9.00

9.2

10.20

41.0

10.60

38.2

11.90

46.0

13.00

132.0

9.10

4.0

10.30

42.6

10.70

40.4

12.00

53.8





10.40

44.2

10.80

42.4







10.50

45.4

10.90

44.0







10.60

46.6

11.00

45.4







10.70

47.6









10.80

48.5







Acetate buffer solutions pH 3 - 6 ³


Make up the following solutions
(1) 0.1M acetic acid
(2) 0.1M sodium acetate (tri-hydrate) (13.6g / l)
Mix in the following proportions to get the required ph

ph

vol. of 0.1M
acetic acid

vol. of 0.1M
sodium acetate

3

982.3 mls

17.7 mls


4

847.0 mls

153.0 mls


5

357.0 mls

643.0 mls


6

52.2 mls

947.8 mls


Phosphate buffer solutions ph 7 - 11


Make up the following solutions
(1) 0.1M disodium hydrogen phosphate (14.2g / l)
(2) 0.1M HCl
(3) 0.1M NaOH
Mix in the following proportions to get the required ph

pH

vol. of phosphate

vol. of 0.1M HCl

vol. of 0.1M NaOH

7

756.0 mls

244 mls

8

955.1 mls

44.9 mls

9

955.0 mls

45.0 mls

10

966.4 mls

33.6

11

965.3 mls

34.7

Addition of acid or base to a salt pH 3 - 11

Here, the primary salt is a solid and is weighed out in grams. A measured amount of 0.1M HCl or NaOH is added,
then made up to 1 liter to give the relevant buffer solution 4.

pH

Salt mixture
Dilute each mixture to 1 liter solution with distilled water

3

10.21g potassium hydrogen phthalate and 223ml of 0.01M HCl

4

10.21g potassium hydrogen phthalate and 1ml of 0.01M HCl

5

10.21g potassium hydrogen phthalate and 226ml of 0.01M NaOH

6

6.81g potassium dihydrogen phOsphate and 56ml of 0.01M NaOH

7

6.81g potassium dihydrogen phosphate and 291ml of 0.01M NaOH

8

6.81g potassium dihydrogen phosphate and 467ml of 0.01M NaOH

9

4.77g sodium tetraborate and 46ml of 0.01M HCl

10

4.77g sodium tetraborate and 183ml of 0.01M NaOH

11

2.10g sodium bicarbonate and 227ml of 0.01M NaOH

1. The Physical and Theoretical Laboratory, Oxford University.
2. "Electrolyte solutions" Robinson, R. A., and Stokes, R. H., 2nd ed., rev. London, Butterworths, 1968.
3. "Practical chemistry" J. Lambert and T.A. Muir, 3rd. Ed. Heineman, London.
4. pdf file, www.bc.ca/bcsc/resources/ (Canadian Teachers Federation).

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