The critical relative humidity (CRH) of a salt is defined as the relative humidity 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 larger 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 watersoluble salts and mixtures have characteristic critical humidities; it is an 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 percentage points at an increase in temperature from 0°C to 40°C (32°F to 104°F).

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

Table 1: Critical relative humidities of pure salts at 30°C. Calcium nitrate, also called Norgessalpeter (Norwegian saltpeter) and Kalksalpeter, is a soluble salt with the formula Ca(NO3)2. ... The chemical compound ammonium nitrate, the nitrate of ammonia with chemical formula NH4NO3, is a white powder at room temperature and pressure. ... Sodium nitrate (not to be confused with sodium nitrite) is a type of salt (NaNO3) which has long been used as an ingredient in explosives and in solid rocket propellants, as well as in glass and pottery enamel, and as a food preservative (such as in hot dogs), and has... Urea is an organic compound of carbon, nitrogen, oxygen and hydrogen, with the formula CON2H4 or (NH2)2CO. Urea is also known as carbamide, especially in the recommended International Non-proprietary Names (rINN) in use in Europe. ... Ammonium chloride or Sal Ammoniac (chemically ammonium chloride (NH4Cl); also nushadir salt, zalmiak, sal armagnac, sal armoniac, salmiakki, salmiak and salt armoniack) is, in its pure form, a clear white water-soluble crystalline salt with a biting taste. ... Ammonium sulfate, (NH4)2SO4, sometimes known as Mohrs Salt, is a chemical compound commonly used as a fertilizer. ... Diammonium phosphate (DAP) is one of a series of water-soluble salts which can be produced when ammonia reacts with phosphoric acid. ... The chemical compound potassium chloride (KCl) is a metal halide composed of potassium and chlorine. ... It has been suggested that this article or section be merged with Niter. ... Ammonium phosphate. ... Calcium dihydrogen phosphate (also called mono-calcium orthophosphate) Ca(H2PO4)2 is a chemical compound. ... This article needs to be cleaned up to conform to a higher standard of quality. ...

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

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-solutions). United American Nurses (UAN) is an American union affiliated with the American Nurses Association (ANA) and the AFL-CIO. Unlike other unions that some American nurses belong to, such as the Service Employees International Union, the UAN represents only registered nurses (RNs). ...

References

• Adams, J.R., A.R. Merz. (1929.). "Hygroscopicity of Fertilizer Materials and Mixtures. pp. 305-307,". Industrial and Engineering Chemistry 21 (4)..

See also

• Deliquescent
• Hygroscopy
• Humidity