Indirect Methods and Special Cases

For the most part, the methods described allow direct observation of reactants, products, or intermediates by monitoring the change in some physical property, such as absorbance, electrical conductivity, NMR and so forth. In addition, there is a section on indirect methods and special cases to describe competition reactions and slow exchange reactions, that is, those reactions not amenable to study by NMR or ESR line broadening. 

In most large automated biochemistry analyzers electrode concentrations are evaluated in the diluted sample by ISEs, by the so-called indirect method. A special compartment for the determination of sodium, potassium, and chloride is integrated. The ISEs have replaced flame photometry as well as coulo-metry to a large extent. However, the compatibility as well as interpretability of the results is problematic in many cases (see above). Since the ion-selective assays strictly respond to molal single-ion activities in the aqueous phase, the comparability to direct measurements is weak. A volume displacement effect by lipids and proteins even affects the accuracy and comparability in diluted samples with buffered ionic strength. 

There are several other indirect and special methods that have not found any broader application for glycoside and saccharide synthesis yet but that might be useful for certain cases or might develop into more common procedures in the future. Such other indirect and special methods are discussed in this chapter. 

Fundamental Principles.—The determination of two or more sugars present in a mixture is usually carried out by indirect methods, based on (1) the rotatory powers, (2) the reducing powers,and (3) the transformation of certain sugars into others by inversion, which is effected by dilute acids or special enzymes. From the rotatory and reducing powers of the mixture, in some cases both before and after inversion, the quantity of each sugar in the mixture is calculated. 

In this chapter we describe methods that can be used for extrapolating between levels of biological organization, examine their assumptions, indicate their strengths and weaknesses, and look forward to new directions for research in this area. We start the chapter at the biochemical level and move upwards toward the ecosystem and landscape levels. Also, a few special cases of horizontal extrapolation are discussed direct versus indirect effects, and structure versus function. 

Indirect methods. These are specially used for the plasticizers and other less volatile substances. The application of the generalized expression of the first and the second laws of thermodynamics to heterogeneous equilibrium between condensed phase and vapor in isobaric conditions is given by the Clausius-Clapeyron equation. It links the enthalpy of vapor formation at the vapor pressure, P, and the temperature, T. In the case of a one-component system the Clausius-Clapeyron expression has the form ... 

If one has the choice to make direct or indirect calorimetric determinations one has to consider their strong and weak points. For analytical calorimetry, i.e. the proof if some process is proceeding and how fast, the direct approach is preferable, e.g. for temporal structures or periodicities in animal activity or for special developmental transitions like pupation and moulting of insects (section 5.3.). Although calorimeters are more expensive than most indirect techniques they are also advantageous for quantitative analysis of heat loss provided that the necessary corrections can be performed for the true heat dissipation. If one wants to know the reason(s) for an observed heat loss, indirect calorimetry may be the approach of choice. But in any case when both methods are at hand, it will be best to determine the possible heat loss in an indirect way and confirm it later with the direct one [39]. 

However, numerous questions remained unsolved in these works (1) the methods of prediction of possible product compositions for a given feed composition were absent, which does not allow to calculate minimum reflux mode (2) the methods of calculation were good only for two special splits direct and indirect ones, but these methods were not good for the intermediate splits (3) the peculiarities arising in the case of availability of a-lines, surfaces, and hypersurfaces that are characteristic of nonideal and azeotropic mixtures were not taken into consideration and (4) the sudden change of concentrations in the feed cross-section was not taken into consideration. 

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