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Measurements & Applications of Equilibrium Moisture Content

When heated, moisture is made up of all those substances that vaporize and cause a material's weight to decrease.

Equilibrium Moisture Content

When heated, moisture is made up of all those substances that vaporize and cause a material's weight to decrease. A balance determines the weight and interprets that as moisture content. A moisture content definition includes not only water content, but also other mass losses, such as those resulting from solvent evaporation, alcohol loss, grease losses, oil losses, aromatic compound loss, as well as that resulting from combustion and decomposition. A moisture analysis of pharmaceutical products is a very important analysis, also known as a moisture assay. Parallels to the moisture content of pharmaceuticals, the water activity also plays an important role in determining quality and stability.

Based on the type of bonding with solids, moisture in products can exist in different forms. See the figure below for an example. A substance is said to contain "free water" if there is water on its surface and it retains its shape, "absorbed water" if it is absorbed into large pores, cavities, or capillaries of a substance, and "water of hydration" if it is contained within lattice ions or crystallization coordinates.





The moisture content of a solid that contains free moisture (water) will be higher than the equilibrium moisture content. Free water does not evaporate when a solid is dried out. Based on the vapor concentration above a solid, one can calculate its free moisture content (FMC). Equilibrium moisture content (EMC) in the gas phase refers to the amount of water a solid retains at equilibrium with given partial pressures of vapor. Also called Critical Moisture Content (CMC), this is the moisture content at which a rise-and-fall drying period ends, and begins a fall-and-rise drying period. When a constant rate of drying takes place, the amount of moisture evaporated per unit area of drying surface remains constant, while when a falling rate of drying takes place, the amount continues to decrease.

As the air temperature reaches the saturation temperature for water vapour, the EMC of materials increases rapidly. As the material passes through these stages, it undergoes more than just adsorption. As water condenses within the pores of the materials, it condenses as water vapor. A very long period of exposure to 100% saturated air will result in condensed moisture filling all pores of the material. This is the equivalent moisture content (EMC) of a material's saturation moisture content (SMC). This process, however, cannot be performed at a rate much greater than that of the capillary saturation moisture content (CSMC), as this is often much lower than the saturation moisture content referred to above.

Measurements

Several direct and indirect methods are used to determine moisture content.

Direct Methods

Among direct methods, thermogravimetric methods are the most common. An oven method can be used to determine moisture content directly. Weighing the solid and drying it, and then weighing it again in accordance with standard procedures is performed. Thermogravimetric methods always separate moisture from solids. Therefore, water is not distinguished from readily volatile components of products. To determine the moisture content, a representative sample must be collected. A sealed container should be used to store the sample from the point of collection to the point of measurement in order to maintain the moisture content of the product. By continuously measuring the mass of a sample during controlled heating, thermogravimetric techniques can be used.

Depending on its molecular environment, water evaporates at a certain temperature. Water that is free to evaporate begins to evaporate at a lower temperature than water that is bound. It is often possible to detect the amount of water in a molecular environment by observing the mass of a sample as it loses water during heating. Granules, in particular, are subject to this method for many solids. A hot air oven can be used to determine the moisture content of granules by heating them until their weight is constant. The boiling point of liquids is lowered in the oven by applying vacuum to heat-sensitive materials.

Indirect Methods

Rapidly determining the moisture content is achieved through indirect methods. Infrared, microwave, ultrasound, and spectroscopy are just a few of the modern measurement technologies available. In order to determine moisture content quickly, non-destructively, and precisely, these methods have been developed. Direct moisture determination is generally slower than indirect methods. In contrast, when applied correctly, they are less precise and accurate than the direct methods. 

Although indirect methods are more accurate and precise, they must be prepared and analyzed carefully in order to establish a reliable calibration curve. Equipment investment is a major concern with indirect methods of calibration. The accuracy of calibration curves and standard preparation must be verified through a specific direct method in order to establish an indirect method of instrumentation that can predict with accuracy and precision.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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