Catalytic concentration, definition

In many instances a selected and agreed reference measurement procedure forms the basis of the definition of the quantity and thereby represents the top of the calibration hierarchy. This is particularly true for establishing reference systems for the catalytic concentrations of enzymes. In 1999 members of the IFCC working group and some enzyme reference laboratories decided to establish new 37 C measurement procedures as IFCC reference procedures on the basis of the existing 30 C IFCC procedures and to certify enzyme reference materials for AFT, AST, GGT, CK, FD and amylase in collaboration with the IRMM. The enzymes having IFCC reference measurement procedures have catalytic concentra-... 

These definitions are valid only when the concentration of the enzyme is very small compared with that of the substrate. Moreover, they apply only to the initial rate of formation of products in other words, the rate of formation of the first few percent of the product, before the substrate has been depleted and products that can interfere with the catalytic reaction have accumulated. 

In 1920 Pease and Taylor (23) proposed a definition of promoter action that is still appropriate today. Promotion occurs in those cases in which a mixture of two or more substances is capable of producing a greater catalytic effect than can be accounted for on the supposition that each substance in the mixture acts independently and in proportion to the amount present. Hence better catalyst performance implies an improvement beyond that expected from a simple additive rule. The phenomenon is often characterized by a steep rise in activity to a sharp maximum at a low concentration of promoter followed by a decline to a lower or zero level of activity, depending on whether or not the promoter alone is capable of acting as a catalyst. 

On the basis of their findings they contend that the effect of almost any compound -hydrocarbons, alcohols, aldehydes, acids, amines, nitro-compounds, H20, H2S, S02, NH3 - can be co-catalytic or inhibitory, according to its concentration [66]. They extend quite unnecessarily the concept of co-catalyst to cover any substance which enhances the DP, and they thereby confuse and debase the originally perfectly precise meaning of the term co-catalyst a substance the presence of which is essential for the functioning of the catalyst [22, 71]. It follows of course from this definition that evidence on co-catalytic activity can be obtained only from rate measurements, and never from studies of DP. 

The effective molarity (EM) is formally the concentration of the catalytic group (RCOO- in [5]) required to make the intermolecular reaction go at the observed rate of the intramolecular process. In practice many measured EM s represent physically unattainable concentrations, and the formal definition is probably relevant only in reactions (which will generally involve very large cyclic transition states) where the formation of the ring or cyclic transition state per se is enthalpically neutral, or in diffusion-controlled processes. For the formation of small and medium-sized rings and cyclic transition states the EM as defined above contains, and may indeed be dominated by, the enthalpy of formation of the cyclic form. This topic has been discussed briefly by Illuminati et al. (1977) and will be treated at greater length in a future volume in this series. 

The quantitation of a protein that has a specific biological function, a hormone, for instance, may not give a true indication of its biological activity owing to the inactivation of some of the protein. For proteins that have definite biological functions the choice is between chemical quantitation and bioassays. For this reason the catalytic activity of an enzyme is more frequently measured than is its protein concentration. 

Kjaer (K9) gives a very comprehensive study of concentration and temperature profiles in fixed-bed catalytic reactors. Both theoretical and experimental work is reported for a phthallic anhydride reactor and various types of ammonia converters. Fair agreement was obtained, but due to the lack of sufficiently accurate thermodynamic and kinetic data, definite conclusions as to the suitability of the dispersed plug flow model could not be reached. However, the results seemed to indicate that the... 

Two important ways in which heterogeneously catalyzed reactions differ from homogeneous counterparts are the definition of the rate constant k and the form of its dependence on temperature T. The heterogeneous rate equation relates the rate of decline of the concentration (or partial pressure) c of a reactant to the fraction / of the catalytic surface area that it covers when adsorbed. Thus, for a first-order reaction,... 

It is usually difficult to express the enzyme concentration in molar unit because of difficulties in determining enzyme purity. Thus, the concentration is sometimes expressed as a unit, which is proportional to the catalytic activity of an enzyme. The definition of an enzyme unit is arbitrary, but one unit is generally defined as the amount of enzyme that produces 1 pmol of the product in 1 min at the optimal temperature, pH and substrate concentration. 

Extending the definition of n-type and p-type reactions, as defined by Vol kenshtein (21) to the electron transfer step, it would seem that the only reaction given by Equation 1 is a p-type reaction. This reaction would be accelerated by the increase in the value of free hole concentration. On the other hand, all other reactions besides the one given by Equation 1 are n-type and would be accelerated by the increase in free electron concentration. Hydrocarbon oxidation reactions catalyzed by solid oxides are accompanied by oxidation and reduction of the catalyst and the degree of the stoichiometric disturbance in the semiconductor changes. The catalytic process in the oxidation of 2-methylpropene over copper oxide catalyst in the presence of Se02 can be visualized as ... 

Tire speed of the reaction is thus proportional to the concentration of arsenious acid and to the square of the concentration of nitrous acid present. The effect of the presence of mercuric salts on the velocity of this oxidation is peculiar 8 at a concentration of 7-7 x 10 6 mol. per litre the reaction is completely inhibited, but with diminishing concentration the inhibiting effect becomes less marked until, at 7-7 x 10-9 mol. per litre, there is a definite positive catalytic effect which becomes more marked at the extremely low concentration of 7-7 x 10 u mol. per litre. The retarding effect of the mercuric salt may be overcome by the addition of a small amount of halogen acid to the nitric acid.9 The... 

Few definitive studies have been reported for the reduced catalyst. One of the earliest studies which was made attempted to relate activity for aldol condensation of n-butylaldehyde to Mo5+ concentration on reduced Mo/Al catalysts (39). The catalysts contained varying amounts of Mo and were prereduced under identical conditions. Aldol conversions were approximately linear with Mo5+ signals (by ESR) up to about 10% Mo. Morris et al. (101) also report a correlation between Mo5+ and catalytic activity for ethylene polymerization over reduced Mo/Al catalysts. Unfortunately, in both studies, tests were not done on a catalyst having the same Mo content but prereduced under different conditions (to give different Mo5+ values). Therefore, the correlation obtained was not unequivocal since other parameters besides Mo5+ might also be proportional to Mo level. 

Thus, firstly, the choice of the pure solvent as the reference state for the definition of activities of solutes in fact impairs a fair comparison of the activity of dilute solutes such as general adds to the activity of the solvent itself. Secondly, the observed first-order rate constants k or k0 for the reaction of a solute with the solvent water are usually converted to second-order rate constants by division through the concentration of water, h2o = oA iho, for a comparison with the second-order rate coefficients HA. Again, it is questionable whether the formal h2o coefficients so calculated may be compared with truly bimolecular rate constants kUA for the reactions with dilute general acids HA. It is then no surprise that the values for the rate coefficients determined for the catalytic activity of solvent-derived acids scatter rather widely, often by one or two orders of magnitude, from the regression lines of general adds.74... 

At the initial oxidation stages (in the induction period) active sites are accumulated. When their concentration reaches a definite level required for a jump-like acceleration of oxidation, either chain or catalytic transformation of the substrate begins at a high rate. 

In the well-known mechanism for the hydrogen-bromine reaction one may for instance not be inclined to consider the bromine atoms as a catalyst. As a matter of fact their concentration can be increased at will within limits by irradiation of the mixture (16) which causes a definite increase of the velocity of the overall reaction, an effect which may very well be called catalytic. 

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