Critical relative humidity

The critical relative humidity (CRH) of a salt is defined as the relative humidity

Relative humidity

Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...

 of the surrounding atmosphere (at a certain temperature) at which the material begins to absorb moisture from the atmosphere and below which it will not absorb atmospheric moisture.

The critical relative humidity (CRH) of a salt is defined as the relative humidity

Relative humidity

Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...

 of the surrounding atmosphere (at a certain temperature) at which the material begins to absorb moisture from the atmosphere and below which it will not absorb atmospheric moisture.

When the humidity of the atmosphere is equal to (or is greater than) the critical relative humidity of a sample of salt, the sample will take up water until all of the salt is dissolved to yield a saturated solution. All water-soluble salts and mixtures have characteristic critical humidities; it is a unique material property.

The critical relative humidity of most salts decreases with increasing temperature. For instance, the critical relative humidity of ammonium nitrate decreases 22% with a temperature from 0°C to 40°C (32°F to 104°F).

The critical relative humidity of several fertilizer salts is given in table 1:

Salt	Critical Relative Humidity (%)
Calcium nitrate Calcium nitrate Calcium nitrate, also called Norgessalpeter , is the inorganic compound with the formula Ca2. This colourless salt absorbs moisture from the air and is commonly found as a tetrahydrate. It is mainly used as a component in fertilizers but is found other applications...	46.7
Ammonium nitrate	59.4
Sodium nitrate Sodium nitrate Sodium nitrate is the chemical compound with the formula NaNO3. This salt, also known as Chile saltpeter or Peru saltpeter to distinguish it from ordinary saltpeter, potassium nitrate, is a white solid which is very soluble in water...	72.4
Urea Urea Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....	72.5
Ammonium chloride Ammonium chloride Ammonium chloride NH4Cl is an inorganic compound with the formula NH4Cl. It is a white crystalline salt that is highly soluble in water. Solutions of ammonium chloride are mildly acidic. Sal ammoniac is a name of natural, mineralogical form of ammonium chloride...	77.2
Ammonium sulfate Ammonium sulfate Ammonium sulfate , 2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen as ammonium cations, and 24% sulfur as sulfate anions...	79.2
Diammonium phosphate Diammonium phosphate Diammonium phosphate is one of a series of water-soluble ammonium phosphate salts which can be produced when ammonia reacts with phosphoric acid...	82.5
Potassium chloride Potassium chloride The chemical compound potassium chloride is a metal halide salt composed of potassium and chlorine. In its pure state, it is odorless and has a white or colorless vitreous crystal appearance, with a crystal structure that cleaves easily in three directions. Potassium chloride crystals are...	84.0
Potassium nitrate Potassium nitrate Potassium nitrate is a chemical compound with the formula KNO3. It is an ionic salt of potassium ions K+ and nitrate ions NO3−.It occurs as a mineral niter and is a natural solid source of nitrogen. Its common names include saltpetre , from medieval Latin sal petræ: "stone salt" or possibly "Salt...	90.5
Monoammonium phosphate	91.6
Monocalcium phosphate Monocalcium phosphate Monocalcium phosphate is a chemical compound with the formula Ca2. It is commonly found as the monohydrate, Ca2·H2O.-Fertilizer:Phosphorus is an essential nutrient and therefore is a common component of agricultural fertilizers...	93.6
Potassium sulfate Potassium sulfate Potassium sulfate is a non-flammable white crystalline salt which is soluble in water...	96.3

Table 1: Critical relative humidities of pure salts at 30°C.

Mixtures of salts usually have lower critical humidities than either of the constituents. Table 2 shows CRH data for two-component mixtures:

---	Ammonium nitrate	Urea	Ammonium sulfate	Potassium chloride
Ammonium nitrate	---	18.1	62.3	67.9
Urea	18.1	---	56.4	60.3
Ammonium sulfate	62.3	56.4	---	71.3
Potassium chloride	67.9	60.3	71.3	---

Table 2: Critical relative humidities of mixtures of salts at 30°C (values are percent relative humidity).

As shown, the effect of salt mixing is most dramatic in the case of ammonium nitrate with urea. This mixture has an extremely low critical relative humidity and can therefore only be used in liquid fertilisers (so called UAN

UAN

UAN is a solution of urea and ammonium nitrate in water used as a fertilizer. The combination of urea and ammonium nitrate has an extremely low critical relative humidity and can therefore only be used in liquid fertilizers...

-solutions).

See also

• Deliquescent
• Hygroscopy
  Hygroscopy
  Hygroscopy is the ability of a substance to attract and hold water molecules from the surrounding environment. This is achieved through either absorption or adsorption with the absorbing or adsorbing material becoming physically 'changed,' somewhat, by an increase in volume, stickiness, or other...
  
  
• Humidity
  Humidity
  Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...