Observation, qualitative versus quantitative

We started this review with a brief discussion of some apparent dilemmas that we face in chemistry qualitative versus quantitative approaches, observables versus non-observables, structural criteria versus properties as criteria for characterization of aromaticity, chemical graph theory versus quantum chemistry, Clar 6/j rule versus Huckel An+ 2 rule, and hydrocarbons versus heteroatomic systems. As we have seen, most of the mentioned dilemmas are man-made and reflect inbred biases of different circles of chemists. It is not uncommon to come across critics with strong opinions and weak arguments, and it would be a waste of time to try to point out to them the other side of the coin . Max Planck apparently experienced difficult times before his quantum constant was accepted, as is reflected in the following quotations ... 

Based on experimental observations, the theoretical models considered throw light on the origin of simple periodic phenomena and complex oscillations, including chaos, in biochemical systems. Besides illustrating the variety of molecular mechanisms producing oscillations at the cellular level, the models allow us to delineate in a qualitative and quantitative manner the conditions in which sustained oscillations occur. The theoretical approach thereby contributes to a better understanding of the physiological roles of such rhythms. Furthermore, in such an approach the relative frequency of occurrence of simple versus complex patterns of oscillations becomes amenable to quantitative assessment. Finally, theoretical models for biochemical and cellular... 

For speetroseopie analysis (e.g. XRF, XRD, IR, Raman, Mossbauer etc.), a plot of the observed intensity versus the eorresponding wavelength or frequeney (or some other related parameter) is ealled the spectrum of that particular analytical method. The spectrum or the data obtained from such experiments contain information about nature of the interactions, atomic and molecular energy levels, chemical bonds, crystallographic information and other related processes. When only the item of interest is identified, it is called qualitative analysis and when the amoimt present is estimated, it is known as quantitative analysis. The effect of heat on a sample is reflected through its variation of thermodynamic properties. Such studies are done by thermal analysis. 

There are a number of aspects of lead neurotoxicology presented in this chapter. The first is the clear distinction between Pb toxicity observed in young children qualitatively and quantitatively and what comprises the typical case for older children and adults. That is, the nature of Pb-associated neurotoxic injury is different and the onset of neurotoxic injury in children is known to occur at lower exposures than in adults and older children. This chapter is consequently divided into sections dealing separately with childhood versus adult Pb toxicity in humans. 

The observed catalytic effect of the crown ether appears to be dependent on the nucleophile employed in both polymerization and corresponding model reactions. Not surprisingly, it appears that the stronger the nucleophile employed, the smaller the catalytic influence of the crown ether. For example, with potassium thiophen-oxide yields of polymer or model products were almost quantitative with or without catalyst. By contrast, the reaction of PFB with potassium phthalimide, a considerably weaker nucleophile, affords 6 in 50% with catalyst and in 2-3% without catalyst under identical conditions. However, it may be that this qualitative difference in rates is, in fact, an artifact of different solubilities of the crown complexed nucleophiles in the organic liquid phase. A careful kinetic study of nucleophilicity in catalyzed versus non-catalyzed reactions study is presently underway. 

Formation of black silver sulfide is visible on silver wool when 10 mg of silver wool was placed in about 1.5 g of the reference gasoline containing 2.6 ppm elemental sulfur and heated as described above. No visible formation of silver sulfide occurred with silver wool in the reference fuel prior to the addition of elemental sulfur. This is illustrated in Fig. 5, where A represents silver wool heated in the reference fuel versus B, which represents silver wool heated in the reference fuel that contained 2.6 ppm elemental sulfur. Even though the pictures in Fig. 5 were obtained under magnification, the silver corrosion was readily apparent to visual inspection without magnification. The surface of the silver wool became even darker when exposed to higher levels of elemental sulfur as one would expect, and it appeared that the entire surface became coated at about the 10-15 ppm level. Further experimentation was not pursued in this area, as it was demonstrated that the more qualitative silver wool test and quantitative GC analysis appear to be correlated. One could expect to find an optimum ratio between the amount of sample and silver wool to maximize the visible contrast that is observed, however. 

The bifurcation scenario discussed above was actually observed in the experiment. Although a good qualitative agreement between theory and experiment was found [40], there are quantitative discrepancies. In the experiment, the measured onset of the nutation-precession motion turns out to be about 20% lower than predicted by theory. Moreover, the slope of the precession frequency versus intensity predicted by theory turned out to be different from that observed in the experiment. One of the two possible reasons could be the use of finite beam size in the experiment (that is typically of the order of the thickness of the layer), whereas in theory the plane wave approximation was assumed. Actually, the ratio 5 between diameter of the beam and the width of the layer is another bifurcation parameter (in the plane wave approximation, 6 oo) and was shown to play crucial role on the orientational dynamics [13]. There and in [44] the importance of the so called walk-off effect was pointed out which consists of spatial separation of Pointing vectors of the ordinary and extraordi-... 

There is much more experience with validation than with scientific assessment. This validation can either be qualitative, for example by means of scatterplots of observed versus modeled concentration, or more quantitative, usually by means of statistical measures of the difference between the two. 

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