Moisture equilibria determinations

Inherent or equilibrium moisture is used for calculating moist, mineral-matter-free calorific values for the rank classification of high-volatile bituminous coals. It is also used for estimating free or surface moisture, since total moisture is equal to the sum of the inherent moisture and the free moisture and is considered the inherent moisture of the coal as it occurs in the unexposed seam, where the relative humidity is probably near 100%. However, due to physical limitations, equilibrium moisture determinations are made at 96 to 97% relative humidity and used as inherent moisture values. 

Equilibrium moisture content of a hygroscopic material may be determined in a number of ways, the only requirement being a source of constant-temperature and constant-humidity air. Determination may be made under static or dynamic conditions, although the latter case is preferred. A simple static procedure is to place a number of samples in ordinaiy laboratoiy desiccators containing sulfuric acid solutions of known concentrations which produce atmospheres of known relative humidity. The sample in each desiccator is weighed periodically until a constant weight is obtained. Moisture content at this final weight represents the equilibrium moisture content for the particular conditions. 

A sample dried at 110° C. was remoistened and then redried at a low temperature and a controlled low humidity to determine its equilibrium moisture content (regain). This value was compared with the regain of a remoistened control sample which was dried originally at a low temperature where decomposition was negligible (as proved by the... 

Equilibrium moistures (hygroscopic points) in AN determined by static or flow methods do not agree. In general, flow methods are recommended. To increase the rate of moisture exchange (at a given RH) the temp of system should be raised (Ref 11)... 

In practice, the diffusion constant is modified to reflect the complex nature of the biofilm. In the development of synthetic hydrogels, the hydrophilicity of the polymer in part defines the concept of equilibrium moisture. We discussed this concept earlier when we described determination of equilibrium moisture. The first practical application is the diffusion constant. As the equilibrium moisture approaches 100%, the diffusion constant approaches that of water. 

A hydrogel placed in an excess of water will absorb the liquid until it reaches a maximum. This ability is typically reported as the percent water in a fully swollen gel. The hydrophilicity of the polymer and the degree of cross-linking determine the degree to which the gel will absorb. Some hydrogels contain as much as 99% water. An acrylic acid gel will have a higher equilibrium moisture than a polyvinyl alcohol gel. This characteristic is not unlike the hydrodynamic volume factor described above. 

Isopiestic determination is one of the most commonly used methods for measuring food aw. In this method a sample of known mass is stored in a closed chamber and allowed to reach equilibrium with an atmosphere of known ERH (or equilibrate with a standard of known aw). In the first protocol (see Basic Protocol), a standard salt solution, for which aw is well established, is used to control this atmosphere. The aw of the sample is then determined by equilibration with the resulting atmosphere. In the second protocol (see Alternate Protocol), the isopiestic determination of aw is accomplished by equilibration of the sample with a reference material, for which the relationship between water content and aw is known. The condition of equilibrium is determined by reweighing the sample at intervals until constant mass is reached. The moisture content of the sample is then determined either directly or by calculation from the reference material s original moisture content and change in mass. Unsaturated salt solutions of known ERH can also be used to equilibrate the samples however, this requires estimation of the ERH of the jars at the end of the equilibration by measuring the exact concentration of the salt solution, which may be tedious. 

Analyses reported on a dry basis are calculated on the basis that there is no moisture associated with the sample. The moisture value (ASTM D-3173 ISO 331 ISO 589 ISO 1015 ISO 1018 ISO 11722) is used for converting as determined data to the dry basis. Analytical data that are reported on a dry, ash-free basis are calculated on the assumption that there is no moisture or mineral matter associated with the sample. The values obtained for moisture determination (ASTM D-3173 ISO 589) and ash determination (ASTM D-3174) are used for the conversion. Finally, data calculated on an equilibrium moisture basis are calculated to the moisture level determined (ASTM D-1412) as the equilibrium (capacity) moisture. 

Thus, to classify coal, the calorific value and a proximate analysis (moisture, ash, volatile matter, and fixed carbon by difference) are needed. For lower-rank coals, the equilibrium moisture must also be determined. To calculate these values to a mineral-matter-free basis, the Parr formulas are used (ASTM D-388). 

The in-place density (bank density) of coal is the means by which coal in the seam can be expressed as tons per acre per foot of seam thickness and/or tons per square mile per foot of seam thickness (Table 6.4). The in-place density must be determined on water-saturated samples (Berkowitz, 1979) to accommodate the equilibrium moisture (Chapter 3) that exists under the in-place (or seam) conditions. 

Salwin and Slawson (1959) found that stability in dehydrated foods was impaired if several products were packaged together. A transfer of water could take place from items of higher moisture-vapor pressure to those of lower moisture-vapor pressure. These authors determined packaging compatibility by examining the respective sorption isotherms. They suggested a formula for calculation of the final equilibrium moisture content of each component from the iso-... 

The relative wettabilities of hydrophobic pharmaceutical powders have been determined using a vacuum balance technique to measure the rates of w ater vapor uptake and the equilibrium moisture levels of the powder samples (1()5. ... 

In Eq. (14) the index is in the preexponential factor and con quently affects t>w less than the volume fraction dp which is in the exponent. This leads to a very important conclusion it is not the volume fraction of open cells, as usually considered, but the volume fraction of polymer and the type of packing of cells (through the value 7) which primarUy determine the extent of equilibrium moisture absorption of plastic foams. 

The nature of moisture content determinations is perhaps best exemplified by the fact that the Handbook of Pharmaceutical Excipients lists 31 separate versions of laboratory tests to determine moisture content and one to determine equilibrium moisture content. These methods were used to assess the moisture content in the compilation of 148 monographs, many of which do not deal with solids. 

The influence of sorbed moisture on chemical stability and the flow and compaction of powders and granulations is well established. The moisture content and hygroscopicity of excipients is particularly important as total product processing as well as finished product stability can be affected. Hygroscopicity, moisture-sorption isotherms, and equilibrium moisture content can be determined by thermogravimetric analysis and Karl Fisher titration methods. 

In summary the temperature of the wood surface is determined by the cooling effect of evaporation, rising from the initial wet-bulb temperature when the surface is wet to the dry-bulb temperature as the wood approaches the equilibrium moisture content. With permeable woods the surface temperature remains at the wet-bulb for a considerable proportion of the schedule. With impermeable woods the surface temperature soon begins to rise toward the dry-bulb temperature, as there is no mass flow of water from the interior to keep the surface moist and above fibre saturation. 

Adsorption and desorption isotherms may be determined by monitoring equilibrium moisture uptake or loss in samples stored in desiccators with different relative humidities (saturated salt solution yielding different percentage relative humidity). The amount of water present is determined by loss of drying (LOD), TGA, Karl Fischer titration, the coulometric method, or near-IR spectroscopy. 

Samples (1.0 tol.5 g) of the freeze-dried product were equilibrated over saturated salt solutions at 25°C, in order to achieve water activities between 0.11 and 0.90 (Spiess and Wolf, 1983). After equilibration (about 2 to 3 weeks), small samples were taken for DSC analyses and the remaining material was used to determine equilibrium moisture content. Phase transitions were determined by differential scanning calorimetry using a DSC TA2010 controlled by a TA5000 module (TA Instruments, Newcastle, USA). Samples of about 10 mg ( 0.01), conditioned in TA aluminum pans... 

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