Fundamental Relationships in Microwave Energy Absorption

Microwave energy absorption can be expressed in a fundamental thermodynamic relationship which relates energy absorption to the specific heat capacity, mass in the microwave field. 

In addition, a quartic model has been developed (Equation 2.4), which allows the prediction of the energy absorbed in closed vessels by a quantity of mineral acid used in decomposition. This equation is valid within the mass range of 25 to 1000 g of reagent. Coefficients A through E are reported for specific concentrations of HNO3, HCl, HE, H2SO4 and H2O. Using this method the specific power absorption for other acid concentrations and mixtures can be derived as well. 

Equation 2.4 allows the calculation of the energy absorption with accuracy between 4% to 10% depending on the kind and concentration of the mineral acids used. 

The ability of a solvent to absorb microwave energy and to convert it into heat is given by the dissipation factor (tan5). The dissipation factor is given by the following equation ° °  

Unlike mineral acids, which are 100% ionized in solution and have a permanent dipole moment which is affected by microwaves, some organic solvents such as hexane are nonpolar and therefore not heated when exposed to microwaves. Selected physical parameters, including dielectric constants and dissipation factors, are shown in Table 2.4 for solvents which are used in MAE 

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