Saturated salt solutions

In a closed container, a saturated salt solution (with excess salt present) produces a constant water vapour pressure at a given temperature. Relative humidity (RH) in the container is calculated by:

where A and B are constants, and T is the temperature in kelvin (Wexler, 1997). The values of A and B as well as the valid temperature range are given in Table A2.1 for various salts. For example, RH of saturated KCl solution at 25°C is equal to 49.38 X exp(159/298) = 84.2 (%). Calculated values are generally accurate within ± 2%. RHs of commonly used salt solutions between 5 and 40°C are compiled in Table A2.2. RHs of additional salt solutions at 25°C are given in Table A2.3. These data are taken from earlier works of Rockland (1960), Winston and Bates (1960) and Young (1967), tested and corrected by the author. Users are advised to avoid salts that release salt vapours into the atmosphere. When autoclaving is required, the decomposition temperature of a salt should be checked. After autoclaving, the closed container should be allowed sufficient time to equilibrate (i.e. avoiding condensation and supersaturation). The supersaturation in the liquid phase and the condensation of water on the wall in the container affect RH significantly.

Table A2.1. Commonly used salts, their vapour pressure constants A and B, and the valid temperature range. Data are taken from Wexler (1997).

(°C)

Relative humidity at 25°C

A

B

NaOH.H2O

15-60

6

5.48

27

LiBr.2H2O

10-30

6

0.23

996

ZnBr2.2H2O

5-30

8

1.69

455

KOH.2H2O

5-30

9

0.014

1924

LiCl.H2O

20-65

11

14.53

-75

CaBr226H2O

11-22

16

0.17

1360

LiI.3H2O

15-65

18

0.15

1424

CaCl226H2O

15-25

29

0.11

1653

MgCl2.6H2O

5-45

33

29.26

34

NaI.2H2O

5-45

38

3.62

702

Ca(NO2) 2.4H2O

10-30

51

1.89

981

Mg(NO3)2.6H22O

5-35

53

25.28

220

NaBr.2H2O

0-35

58

20.49

308

nh4no3

10-40

62

3.54

853

KI

5-30

69

29.35

254

SrCl2.6H2O

5 -30

71

31.58

241

NaN2O3 2

10-40

74

26.94

302

NaCl 3

10-40

75

69.20

25

NH4Cl

10-40

79

35.67

235

KBr

5-25

81

40.98

203

(NH4) 2SO4

10-40

81

62.06

79

KCl

5-25

84

49.38

159

Sr(NO3)2.4H2O

5-25

85

28.34

328

BaCl2.2H2O

5-25

90

69.99

75

CsI

5-25

91

70.77

75

KNO3

0-50

92

43.22

225

K2SO4

10-50

97

86.75

34

Table A2.2. Equilibrium relative humidities of saturated salt solutions at different temperatures. Data are taken from Rockland (1960), Winston and Bates (1960) and Young (1967).

Saturated salt solution

Temperature

5°C

10°C

15°C

20°C

25°C

30°C

35°C

40°C

h2so4

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

ZnCl2

5.5

-

5.5

-

5.5

-

5.5

-

NaOH

6.0

-

6.0

6.0

7.2

-

7.5

-

LiBr

9.0

-

8.0

-

7.0

-

7.0

-

KOH

13.0

-

9.0

-

8.0

-

8.0

-

LiCl.H2O

14.0

13.5

13.0

12.5

12.0

11.5

11.5

11.0

CaBr2

23.0

-

20.0

18.5

16.5

-

15.0

-

KAc

24.8

24.0

23.5

23.0

23.0

23.0

22.0

23.0

MgBr2

32.0

31.0

31.5

31.0

30.5

30.0

30.0

30.0

MgCl2

34.0

33.5

33.5

33.0

32.5

32.0

32.5

32.0

CaCl2

40.0

38.0

35.0

32.5

30.0

-

30.0

-

K2CO3

43.0

47.0

44.0

44.0

43.0

42.0

41.5

40.0

Nal

43.5

-

38.0

38.5

38.0

36.0

34.0

32.5

Zn(NO3)2

45.0

43.0

40.7

40.0

32.5

24.0

21.0

19.0

KCNS

54.0

52.0

50.0

47.0

46.5

43.5

41.5

41.0

Mg(NO3)2

55.0

53.0

53.7

53.0

52.5

52.0

50.5

51.0

Na2Cr2O7.H2O

59.5

60.0

56.5

54.5

53.0

52.5

51.0

50.0

NaBr

60.5

58.0

58.0

57.8

57.2

57.0

57.0

57.0

Ca(NO3)2

61.0

66.0

58.0

56.0

52.2

51.0

45.5

46.0

NaBr.2H2O

63.0

61.0

59.0

57.5

56.0

54.5

53.0

51.5

CuCl2

66.7

68.0

68.0

68.5

67.0

67.0

67.0

67.0

LiAc

72.0

72.0

71.0

70.0

68.0

66.0

65.0

64.0

nh4no3

73.0

75.0

70.0

65.5

62.5

59.5

56.8

53.0

NaCl

76.0

75.8

75.5

75.3

75.1

75.0

75.0

75.0

NaNO3

79.0

77.5

76.5

76.0

74.0

72.5

71.0

70.5

(NHJ2SO4

81.7

81.2

80.0

79.8

79.7

79.5

79.2

79.0

NH4Cl

82.0

79.0

79.5

79.0

78.0

77.5

75.5

74.0

U2SO4

84.0

84.0

84.0

85.0

85.0

85.0

85.0

81.0

KBr

85.0

86.0

85.0

83.5

83.0

82.0

81.0

80.0

KCl

87.8

86.7

86.0

85.3

85.0

83.5

83.0

83.0

K2CrO4

89.0

89.0

88.0

88.0

87.0

86.0

84.0

82.0

BaCl2

95.0

93.0

92.0

90.7

90.0

89.0

88.0

87.0

ZnSO4

95.0

93.0

92.0

90.0

88.0

86.0

85.0

84.0

KNO3

96.5

95.5

95.0

94.0

92.5

91.5

89.5

88.5

K2SO4

98.0

97.0

97.0

97.0

97.0

97.0

96.0

96.0

Na2HPO4

98.0

98.0

98.0

98.0

97.0

96.0

93.0

91.0

Pb(NO3)3

99.0

98.5

98.5

98.5

96.2

95.5

95.2

94.7

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