Benson method

The Benson group contribution method, and more recent methodologies, allow the computation of heat of hydrogenation reactions, even for large molecules (note that Benson method gives the reaction enthalpy assuming each species to be a perfect gas ). Software and database (e.g., NIST) are also available. 

Color Reaction Based on the Formation of Nitrosophenols (Pearl-Benson Method)... 

The procedure for the Pearl-Benson method is described in detail in Chapter 2.2.4. This method has been used primarily to detect and estimate quantitatively... 

Of the methods for determining lignin in solution based on a specific chemical reaction, that involving nitrosation, the so-called Pearl-Benson method, has found the widest application. In this procedure, reaction of the phenolic units in lignin with acidified sodium nitrite leads to the formation of a nitrosophenol which, upon addition of alkali, is tautomerized to an intensely colored quinone mono-oxime. The absorbance of the latter structure is measured at 430 nm and related to lignin concentration by calibration with a standard lignin. The procedure described below is essentially that developed by Barnes et al. (1963), who modified the original Pearl-Benson method (Pearl and Benson 1940) to improve its sensitivity. 

Although developed originally and used principally for the determination of lignosulfonates in sea water, the applicability of the Pearl-Benson method has been widened to include the determination of not only lignosulfonates but of other lignins as well in fresh water. In its present form the sensitivity of the method is such that, in the absence of any extraneous absorbance, lignosulfonate concentrations as low as 0.2 to 0.5 ppm can be determined (Goldschmid 1971). 

The principal drawback of the method is its lack of specificity for phenols having the characteristic lignin structure. Thus, other phenolic impurities in water (e.g., tannins) would most likely be nitrosated under the conditions specified in the procedure and thereby contribute the absorbance reading. In addition, certain nitrogen-containing and inorganic substances commonly found in fresh- and sea water are also known to react with nitrous acid (Felicetta and McCarthy (1963). However, in comparison to other colorimetric and to UV spectrophotometric procedures, the Pearl-Benson method has been found to be less affected by interfering impurities (Jurkiewicz 1977). 

The group contribution parameters can be taken from Table 3.1. A more sophisticated method has been proposed by Domalski and Hearing (21, 22(. As well, the Benson method is widely used and further developed with industrial support [23, 24],... 

If one looks at a molecule such as heptane, for example, one can add all of the appropriate increments and calculate the heat of formation with acceptable accuracy by the method previously described. But there are a few things that are really not proper about that kind of calculation. Heptane in the gas phase at 25°C (where heats of formation are defined) is actually a complicated mixture (a Boltzmann distribution) of a great many conformations, most of which have different enthalpies and entropies. Additionally, each of these conformations is also a Boltzmann distribution over the possible translational, vibrational, and rotational states. The Benson method works adequately for many cases like this because these statistical mechanical terms can be lumped into the increments and averaged out, and they are not explicitly considered. By adjusting the values of the parameters in Eq. (11.1) or (11.2), much of the resulting error of neglecting the statistical mechanics can be canceled out, or at least minimized, in simple cases. But we would like for this scheme to work for more complex cases too. As the system becomes more complicated, errors tend to cancel out less well. So let us go back and approach this problem in a more proper way. 

This difference between the hypothetical point on the potential surface and the real molecule can be well described by statistical mechanics. With respect to heats of formation, the ways for calculating them (by quantum mechanical methods or by molecular mechanical methods) require either the explicit inclusion of statistical mechanics (the details of this procedure have been spelled out in fulf and will be outlined in the following) or else an implicit inclusion by lumping the statistical effects into the bond energies and hoping for the best, as in the Benson method. 

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