Several Measurement 3.10. Classification Methods

One classification method treats a large molecule as aromatic if it has a single benzene ring regardless of the other content. Another method considers the fraction of each molecule that is aromatic, naphthenic, or paraffinic. Obviously, in either case the analysis procedure is tedious. A third classification method simply measures the specific gravities of several fractions separated by distillation and attempts to relate chemical structure to specific gravity. 

All pattern classification methods listed group items by similarity. Measurement of similarity differs depending upon the method, and therefore different methods yield different results. Table 12 describes several similarity metrics and why they produce different results for the same data set. 

The solvent triangle classification method of Snyder is the most enduring approach to solvent characterization used by chromatographers, but in several respects is not entirely satisfactory [568,575,576]. Snyder classified solvents based on their interactions with three prototypical solutes determined by their gas-liquid distribution constants corrected for differences in solvent size, polarizability and dispersion interactions (assumed identical to the interactions of a hypothetical n-alkane with the same molar volume). Each value was then corrected empirically to give a value of zero for the polar distribution constant for saturated hydrocarbon solvents. Snyder chose the solutes nitromethane, ethanol and dioxane as probes for a solvent s capacity for dipole-type, hydrogen-bond base and hydrogen-bond acid interactions, respectively. The sum of the three polar distribution constants provides a measure of the solvent strength P ) and the ratio of the individual polar distribution constants to their sum a measure of selectivity (xn, and Xd). 

A large number of electrochemical methods exist which are or have the potential to be useful in the study of reactive intermediates. The methods are conveniently categorized according to the quantity measured, usually the current, potential, or some optical property of the reactants or the intermediates. A further classification arises from the manner in which experiments are conducted, i.e. transient or steady state measurements. In this brief survey only those techniques which have been reduced to useful practice are discussed and even then the coverage is not exhaustive. More detailed discussion can be found in several excellent references sources (Bard, 1966-present MacDonald, 1977 Bard and Faulkner, 1980). 

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