Conditioning Silica Gel with a Saturated Salt Solution

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Saturated salt solutions can be used to condition silica gel to a specific relative humidity; however, salts are not generally recommended for use in exhibit cases.

How are saturated salt solutions used in museums?

Saturated solutions of commercially available salts maintain specific, but different relative humidities in their immediate surroundings. These solutions supply water vapor to maintain a specific RH as long as there is even a small amount of liquid remaining in the solution. In the past, salt solutions have been used directly within exhibit cases to maintain a target level of RH, but this is no longer recommended because:

  • the threat of spillage
  • salts are hard to contain within a vessel
  • some salts degrade over time and can emit gaseous pollutants

Silica gel can be conditioned by exposure to a saturated salt solution maintaining the appropriate relative humidity. The solutions, however, can only be used to add moisture to silica gel and can- not desiccate gel.

Using saturated salt solutions, however, is not simple and is seldom recommended. Their use is also dependent upon the following important factors:

  • Purity of the salt and the water: Use only chemically pure salts and distilled water.
  • Preparation of the salt solution: The solution must be saturated; there should be a slush-like consistency with excess, undissolved crystals. Too much or too little water can create a significantly higher or lower humidity. To prepare a solution add distilled water slowly, constantly stirring until half of the salt crystals are dissolved. A significant amount of heat is produced when the salt solution is first prepared; allow sufficient time for the solution to cool down before placing in a covered container.
  • Water vapor equilibrium rate between the liquid and the vapor: Provide as large a surface area of the solution as is practical. Use the salt solution in as small a chamber as possible, and insure air circulation over the solution either by convection or a fan.
  • Temperature equilibrium between the liquid and vapor: Temperature has a direct effect on the level of humidity achieved. A difference of 2°F can cause a humidity level error of approximately 5% RH.
  • Presence of hygroscopic materials within the vapor space: Any materials that are hygroscopic (or absorb moisture), like wood, rubber, paper, textiles, etc., will throw off the desired humidity adsorption of the silica gel and should not be used to construct the conditioning chamber. Use rigid acrylic sheeting, glass or metal-enamel containers.

What salts can be used?

The table below provides a list of potentially useful salt solutions and their respective humidity levels at three temperatures. Selection of the salt to be used is based on the desired relative humidity levels— e.g., for a low RH of 33%, magnesium chloride would be selected.

Relative Humidity % at Selected Temperatures
Saturated Salt Solution Chemical Formula 68°F 77°F 86°F
Lithium Chloride LiCl H2O 12. 4% 12. 0% 11.8%
Potassium Acetate CH3CO2K 23. 3% 22. 7% 22.0%
Magnesium Chloride MgCl2 6H2O 33. 6% 33. 2% 32.8%
Potassium Carbonate K2CO3 2H2O 44. 0% 43. 8% 43.5%
Potassium Nitrate KNO3 49. 0% 48. 1% 47.2%
Magnesium Nitrate Mg(NO3)2 6H20 54. 9% 53. 4% 52.0%
Sodium Nitrate NaNO3 65. 3% 64. 3% 63.3%
Sodium Chloride NaCl 75. 5% 75. 8% 75.6%
Ammonium Sulfate (NH4)2SO4 80. 6% 80. 3% 80.0%
Potassium Nitrate KNO3 93. 2% 92. 0% 90.7%
Potassium Sulfate K2SO4 97. 2% 96. 9% 96.6%

Table obtained from Technical Bulletin No. 5, Creating and Maintaining Humidities by Salt Solutions. Hygrodynamics, Inc.

Products, Manufacturers, and Suppliers

Mention of a product, manufacturer, or supplier by name here is for information only and does not constitute an endorsement of that product or supplier. Listed materials have been used successfully in past applications. It is suggested that readers also seek alternate product and vendor information to assess the full range of available supplies and equipment.

Salts for making solutions
Fisher Scientific, Silver Spring, MD 20910