Critical relative humidity


The critical relative humidity of a salt is defined as the relative humidity of the surrounding atmosphere 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 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.
The critical relative humidity of several fertilizer salts is given in table 1:
SaltCritical Relative Humidity
Calcium nitrate46.7
Ammonium nitrate59.4
Sodium nitrate72.4
Urea72.5
Ammonium chloride77.2
Ammonium sulfate79.2
Diammonium phosphate82.5
Potassium chloride84.0
Potassium nitrate90.5
Monoammonium phosphate91.6
Monocalcium phosphate93.6
Potassium sulfate96.3

Table 1: Critical relative humidities of pure salts at 30°C.
Mixtures of salts usually have lower critical humidities than either of the constituents. Fertilizers that contain Urea as an ingredient usually exhibit a much lower Critical Relative Humidity than Fertilizers without Urea. Table 2 shows CRH data for two-component mixtures:
---Ammonium nitrateUreaAmmonium sulfatePotassium chloride
Ammonium nitrate---18.162.367.9
Urea18.1---56.460.3
Ammonium sulfate62.356.4---71.3
Potassium chloride67.960.371.3---

Table 2: Critical relative humidities of mixtures of salts at 30°C.
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.