Selective values defined

The general theoretical approach to the selectivity observed in the hydrogenation of acetylene has been discussed in Sect. 2.3, where it was noted that the observed selectivity may be dependent upon both thermodynamic and mechanistic factors. A possible explanation of the operation of a mechanistic factor has been discussed in Sect. 4.3. The selectivity values, defined as S = Pc2h4/(Pc2h4 + Pc2h6) observed for various metal catalysts are shown in Table 15. Selectivities have been observed to... 

For the evaluation of the effect of composition of the reaction mixture on the hydrogenation process, and above all of the effect of solvents on the relative reactivities of two substrates, the selectivity values defined by Eq. (8) appear to be those best suited usually, they are determined with great accuracy because both compounds react in competitive hydrogenation under exactly the same conditions. Most authors, however, prefer to find out to what extent changes in the relative reactivity are produced by changes in... 

If two variables are dependent, the value chosen in the simulation for the dependent variable can be linked to the randomly selected value of the first variable using the defined correlation. 

Possible numerators include the gross income net pretax income net after-tax income gross profit, ie, gross income minus book depreciation cash flow or net income. An average return value is selected by defining a typical or mature proof year as the basis of calculation. The denominator can be the original total investment, depreciated book-value investment, lifetime averaged investment, or fixed capital investment. 

TTie pH signal is fed via a pH meter to a potentiometric recorder and to a neutrahzation controller, which compares the electrode e.m.f. with pre-selected values and opens or closes gas-control valves accordingly. Depending on the pH, it is possible to obtain a flow of air, ammonia or a mixture of both gases. By careful adjustment of the potentiometers and the flow-rates of ammonia and air, it is possible to control the final pH and also to keep the digest within a fairly closely defined pH range during the neutrahzation process. 

For the activity tests an 8-fold batch reactor system (reactor volume 20 ml) with magnetic stirring which allows the measurement of hydrogen uptake at constant hydrogen pressure was used. Analysis of substrates and products was performed offline by GC for determining selectivity values. Activity values were derived from hydrogen up-take within a defined time interval. Hydrogenation of both cinnamic acid and dibenzylether were carried out at 10 bars and 25°C. 

The products were diluted with hexane (1 mg product / 200 ml hexane), separated using a DB5-MS column, and analyzed using an HP 5890 GC-MS Series II Plus. Random errors associated with GC-MS concentration measurements were less than 5%, and the reproducibility of conversion measurements was 15% of the reported values. Selectivity is defined as the percentage of biphenyl (the preferred HDS product from dibenzothiophene) divided by the percentage of dibenzothiophene converted times 100. 

This is another publication in the series of Monographs for Teachers which was launched in 1959 by the Royal Institute of Chemistry. The initial aim of the series was to present concise and authoritative accounts of selected well-defined topics in chemistry for those who teach the subject at GCE Advanced level and above. This scope has now been widened to cover accounts of newer areas of chemistry or of interdisciplinary fields that make use of chemistry. Though intended primarily for teachers of chemistry, the monographs will doubtless be of value also to a wider readership, including students in further and higher education. 

For only weakly acidic alcohols (e.g. methanol-water and ethanol—water mixtures), typical values of S are often close to unity, but can vary from ca. 0.1 to ca. 10 [41]. When S < 1, selectivity may be said to be inverse in this event, if selectivity were defined using log S, the selectivity value would become negative. Note also that, if the reactions were simply competing second-order bimolecular processes with the solvent, S would not depend on the solvent composition, so mechanistic information can be obtained from variations in S. 

Selectivity describes the degree of spectral interferences, and several measures have been proposed. Most definitions refer to situations where pure-component spectra of the analyte and interferences are accessible [19-21, 28]. In these situations, the selectivity is defined as the sine of the angle between the pure-component spectrum for the analyte and the space spanned by the pure-component spectra for all the interfering species. Recently, equations have been presented to calculate selectivity for an analyte in the absence of spectral knowledge of the analyte or interferences [25-27], These approaches depend on computing the NAS, defined as the signal due only to the analyte. Methods have been presented to compute selectivity values for N-way data sets (see Section 5.6.4 for the definition of N-way) [29, 30],... 

When k values have been determined, the concept of selectivity can be introduced and a separation factor calculated. The degree of separation between two band centers after elution is called the selectivity or separation factor. It is the ratio of the k values defined by equation (8-6) ... 

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