Water Critical relative humidity

Anhydrous sodium tripolyphosphate is slow to hydrate in contact with the atmosphere under normal ambient conditions and generally remains free-flowing. If the relative humidity is below a critical relative humidity, which is different for both anhydrous forms of STP and dependent on temperature, hydration does not take place. For prolonged storage at room temperature, relative humidities above ca 60% in the air result in water absorption. For shorter periods, high levels of humidity can be tolerated. However, even at higher humidities, the amount of water absorbed is small. The heats evolved from vapor hydration of STP-I and -II have been estimated at 343 and 334 kj /mol (82.0 and 79.9 kcal/mol), respectively (25). 

A deliquescent material takes up moisture freely in an atmosphere with a relative humidity above a specific, well-defined critical point. That point for a given substance is defined as the critical relative humidity (RH0). Relative humidity (RH) is defined as the ratio of water vapor pressure in the atmosphere divided by water vapor pressure over pure water times 100% [RH = (PJP0) X 100%]. Once moisture is taken up by the material, a concentrated aqueous solution of the deliquescent solute is formed. The mathematical models used to describe the rate of moisture uptake involve both heat and mass transport. 

The effect of relative humidity and temperature on the physical and structural properties of the 1 1 isopropanol solvatomorph of warfarin has been studied [58], Below the critical relative humidity of 60-68% the solid is not hygroscopic, but becomes deliquescent at higher values of relative humidity without exchange of water for isopropanol. Storage of the solvate-morph at elevated temperatures causes formation of an amorphous solid owing to loss of isopropanol, which may proceed through an intermediate crystalline phase. 

It is the objective of this chapter to discuss the various mechanisms whereby water can interact with solid substances, present methodologies that can be used to obtain the necessary data, and then discuss moisture uptake for nonhydrating and hydrating crystalline solids below and above their critical relative humidities, for amorphous solids and for pharmaceutically processed substances. Finally, transfer of moisture from one substance to another will be discussed. 

Recent reports about the microdroplets formation in the starting periods of atmospheric corrosion [15-18] show that the idea of a thin uniform water layers is not completely in accordance with the reality. It has been observed that when a water drop is on the metallic surface, formed in the place where a salt deposit existed before, microdroplets are formed around this central drop. The cathodic process takes place in these surrounding microdroplets, meanwhile the anodic process takes place in the central drop. This idea is not consistent with the proposal of an uniform water layer on the surface and it is very probable that this situation could be obtained under indoor conditions. It has been determined that microdrops (about 1 micron diameter) clusters are formed around a central drop. An important influence of air relative humidity is reported on microdrops formation. There is a critical value of relative humidity for the formation of microdroplets. Under this value no microdroplets are formed. This value could be considered as the critical relative humidity. This situation is very similar to the process of indoor atmospheric corrosion presence of humid air, deposition of hygroscopic contaminants in the surface, formation of microdrops. Water is necessary for corrosion reaction to occur, but the reaction rate depends on the deposition rate and nature of contaminants. 

One of the best known examples of electrochemical corrosion is atmospheric rusting. For this to occur, a certain critical relative humidity of between 60-80% or higher (depending upon whether salts are present) is required. At such a relative humidity, every object is covered with a coherent film of water which serves as an electrolyte. Electrochemical corrosion also occurs when an iron object is partly or completely immersed in water. 

AS is a white to brownish-gray crystalline salt that is soluble in water, but is only slightly soluble in ethanol. AS has excellent storage properties in bags and in bulk, and it does not require any anti-caking conditioners. This is because AS is resistant to moisture absorption and has a critical relative humidity of 79% at 30°C244. AS can be irritating when in contact with the skin or mucous... 

If corrosion rate is plotted against humidity, then a curve such as that in Fig.3 would be obtained. Here, corrosion rate is low until, over a narrow range of humidity, the rate suddenly begins to increase. This point is termed the critical humidity and its value will depend upon the metal and nature of any dissolved species in the water film. For example, iron in a sulphur dioxide polluted atmosphere will have a critical relative humidity of above 75%, whereas a copper surface polluted with iodide will reach a critical relative humidity of about 35%. 

The stepwise character demonstrated in this profile is characteristic of a substance capable of forming a hydrate. At humidities below 60% RH, the moisture content of the solid remains virtually unchanged. When the activity of water reaches a critical relative humidity of 60%, water is sorbed by the solid as the anhydrous form converts to the monohydrate. For this example, the moisture content associated with the complete transition to the monohydrate is 10% (e.g., 1 mole of water = 18g 1 mole of the substance = 180g). 

Finally, the critical relative humidities are dependent on the nature of the solid. For example, the spontaneous dissolution process has been observed for many water-soluble substances at relative humidities significantly below that associated with a saturated solution of the substance in water.Van Campen, Amidon, and Zograf have examined the moisture sorption kinetics of deliquescent solids at relative humidities above what they term the critical relative humidity (RHo), where adsorbed water takes on the character of condensed water and serves as a solvent. It is important to recognize that a highly undesirable process such as deliquescence can occur when it may not be expected (e.g., when RHo < RH < RHs). 

The method for the determination of the critical relative humidities of some extremely water-soluble compounds is described in a research publication. The determinations by the reported method are in good agreement with that of the conventional method. 

Kontny MJ, Grandolfi GP, Zografi G. Water vapor sorption of water-soluble substance Studies of crystalline solids below their critical relative humidities. Pharm Res 1987 4 104-112. 

Many compounds and salts are sensitive to the presence of water vapour or moisture. When compounds interact with moisture, they retain the water by either bulk or surface adsorption, capillary condensation, chemical reaction and, in extreme cases, a solution (deliquescence). Deliquescence is where a solid dissolves and saturates a thin film of water on its surface. It has been shown that when moisture is absorbed to the extent that deliquescence takes place at a certain critical relative humidity, the liquid film surrounding the solid is saturated. This process is dictated by vapour diffusion and heat transport rates (Kontny et al. 1987). 

Atmospheric corrosion rates are commonly related to a critical relative humidity , above which the corrosion rate increases significantly and below which the rate is insignificant for many practical purposes. Depending on metal and exposure conditions, critical relative humidities have been reported in the range from 50 to 90%. The critical relative humidity is associated with the point of deliquescence of deposited aerosol particles, above which the aerosols rapidly absorb water until a saturated solution is obtained. For a single-phase aerosol, there is a well-defined critical relative humidity, whereas for a mixture of phases (the common situation in natural outdoor environments) the critical relative humidity is lower than those of the single phases. 

The relationship between the equilibrium vapor pressures on a concave meniscus and that on a flat surface is given by the Kelvin equation Pr = Pf x exp (-A/r), where r is the radius of curvature of the concave meniscus. In the case of a crevice of constant width, under fixed environmental conditions, r is constant, and hence, there is a critical relative humidity of the atmosphere, at which the crevice is empty and above which the crevice is filled with water. In the case where, instead of a spherical particle on a flat surface, the radius of curvature of the meniscus at the sphere-plane point of contact tends to zero, and hence, the critical relative hnmidity also tends to zero, there does not exist a minimum value below, which, under the particle, there is no water. By increasing the relative humidity of the atmosphere, both the radius of curvature of the meniscus and the amount of water below the particle increase (Figure 12.26). 

From previous discussions, it is apparent that, in an uncontaminated atmosphere at constant temperature, appreciable corrosion of a pure metal surface would not be expected at any value of relative humidity below 100%. Practically, however, because of normal temperature fluctuations (relative humidity increases on decrease of temperature) and because of hygroscopic impurities in the atmosphere or in the metal itself, the relative humidity must be reduced to values much lower than 100% in order to ensure that no water condenses on the surface. In very early studies, Vernon discovered that a critical relative humidity exists below which corrosion is negligible [21]. Experimental values for the critical relative humidity are found to fall, in general, between 50% and 70% for steel, copper, nickel, and zinc. Typical corrosion behavior of iron as a function of relative humidity of the atmosphere is shown in Fig. 9.3. In a complex or severely polluted atmosphere, a critical humidity may not exist [22]. 

Since saturated NaCl brine has a vapor pressure approximately 75% of that of water, a relative humidity greater than 75% will cause condensation onto the salt. This is known as the critical humidity. KCl solutions have higher vapor pressures and therefore higher critical humidity. Because KCl also has a higher temperature coefficient of solubility, the concentration of saturated solutions increases more rapidly with temperature. The critical humidity therefore is not so nearly constant as is the case with NaCl, and it decreases with increasing temperature. At most ambient temperatures, the critical humidity of KCl is 85% or higher. 

A current issue which has not been adequately explained is that there is a critical water concentration (or critical relative humidity (r.h.)), below which structural adhesive joints in metals are not weakened above it they are progressively weakened. 

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