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Section: 15 | Constant Humidity Solutions |
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John R. Rumble, ed., CRC Handbook of Chemistry and Physics, 103rd Edition (Internet Version 2022), CRC Press/Taylor & Francis, Boca Raton, FL.
If a specific table is cited, use the format: "Physical Constants of Organic Compounds," in CRC Handbook of Chemistry and Physics, 103rd Edition (Internet Version 2022), John R. Rumble, ed., CRC Press/Taylor & Francis, Boca Raton, FL.

CONSTANT HUMIDITY SOLUTIONS

Anthony Wexler

An excess of a water soluble salt in contact with its saturated solution and contained within an enclosed space produces a constant relative humidity and water vapor pressure according to

RH = A exp(B/T)  (1)

where RH is the percent relative humidity (generally accurate to ±2%), T is the temperature in kelvin, and the constants A and B and the range of valid temperatures are given in the table below. The vapor pressure, p, can be calculated from

p = (RH/100) × p0  (2)

where p0 is the vapor pressure of pure water at temperature T as given in the table in Section 6 titled “Vapor Pressure and Other Saturation Properties of Water.” Column definitions for the table are as follows.

Column heading Definition
Mol. form. Molecular formula of water soluble salt

Name

 Name of water soluble salt
Temp. range Range of temperatures for which equation 1 is valid, in °C
A Constant A in equation 1
B Exponential constant B in equation 1
RH(25 °C) Relative humidity, in %, of saturated solution at 25 °C as calculated from equation 1; table arranged by RH(25 °C) values

References

  1. Wexler, A. S. and Seinfeld, J. H., Atmospheric Environment, 25A, 2731, 1991. [https://doi.org/10.1016/0960-1686(91)90203-J]
  2. Greenspan, L., J. Res. National Bureau of Standards, 81A, 89, 1977. [https://doi.org/10.6028/jres.081A.011]
  3. Broul, et al., Solubility of Inorganic Two-Component Systems, Elsevier, New York, 1981.
  4. Wagman, D. D. et al., J. Phys. Chem. Ref. Data, Vol. 11, Suppl. 2, 1982.
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Coefficients in the Equation for Relative Humidity above Saturated Aqueous Salt Solutions



Mol. form.NameTemp. range/°CABRH(25 °C)/%
Continued on next page...
NaOH·H2OSodium hydroxide monohydrate15–605.48276
LiBr·2H2OLithium bromide dihydrate10–300.239966
ZnBr2·2H2OZinc bromide dihydrate5–301.694558
KOH·2H2OPotassium hydroxide dihydrate5–300.01419249
LiCl·H2OLithium chloride monohydrate20–6514.53-7511
CaBr2·6H2OCalcium bromide hexahydrate11–220.17136016
LiI·3H2OLithium iodide trihydrate15–650.15142418
CaCl2·6H2OCalcium chloride hexahydrate15–250.11165329
MgCl2·6H2OMagnesium chloride hexahydrate5–4529.263433
NaI·2H2OSodium iodide dihydrate5–453.6270238
Ca(NO3)2·4H2OCalcium nitrate tetrahydrate10–301.8998151
Mg(NO3)2·6H2OMagnesium nitrate hexahydrate5–3525.2822053
NaBr·2H2OSodium bromide dihydrate0–3520.4930858
NH4NO3Ammonium nitrate10–403.5485362
KIPotassium iodide5–3029.3525469
SrCl2·6H2OStrontium chloride hexahydrate5–3031.5824171
NaNO3Sodium nitrate10–4026.9430274
NaClSodium chloride10–4069.202575
NH4ClAmmonium chloride10–4035.6723579


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