Ionic strength definition

In many situations, the actual molar amount of the enzyme is not known. However, its amount can be expressed in terms of the activity observed. The International Commission on Enzymes defines One International Unit of enzyme as the amount that catalyzes the formation of one micromole of product in one minute. (Because enzymes are very sensitive to factors such as pH, temperature, and ionic strength, the conditions of assay must be specified.) Another definition for units of enzyme activity is the katal. One katal is that amount of enzyme catalyzing the conversion of one mole of substrate to product in one second. Thus, one katal equals 6X10 international units. 

The problem of the definition of charge and consideration of the sizes of reactants (the diameters of the reactant ions and the activated complex are assumed equal in the derivation of (2.184)) is most acute with reactions of metalloproteins. Probably the most nsed expression for the effect of ionic strength on such reactions is ... 

The rate constants for the reaction of a pyridinium Ion with cyanide have been measured in both a cationic and nonlonic oil in water microemulsion as a function of water content. There is no effect of added salt on the reaction rate in the cationic system, but a substantial effect of ionic strength on the rate as observed in the nonionic system. Estimates of the ionic strength in the "Stern layer" of the cationic microemulsion have been employed to correct the rate constants in the nonlonic system and calculate effective surface potentials. The ion-exchange (IE) model, which assumes that reaction occurs in the Stern layer and that the nucleophile concentration is determined by an ion-exchange equilibrium with the surfactant counterion, has been applied to the data. The results, although not definitive because of the ionic strength dependence, indicate that the IE model may not provide the best description of this reaction system. 

This definition for K holds only for monovalent ions and depends on the aqueous dielectric constant (c), Boltzmann s energy (k T), and the electronic charge (e). The Debye length varies inversely with the ionic strength of the solution and thus measures the ability of the ions to screen the protein/interface interactions. 

Table 6.1 Theoretical estimation of the types of structures of protein self-assembly induced by interactions of food proteins (0.5 % w/v) with food-grade surfactants in aqueous solution (pH = 7.2, ionic strength = 0.05 M, 293 K) below the cmc (CITREM cmc = 15 mg/L SSL cmc = 3.5 mg/L PGE cmc = 1.0 mg/L (IFin el al., 2005)). See the text for definitions of k and k2.

Since some reactants are usually ionic, one must decide on some mixing rule. Usually the concentration of an individual medium ion or the ionic strength is kept constant. (The mixing rule is indicated by 3M Na(ClQ ), 3M (Na)C104 in the Tables of Stability Constants (18).) Whichever rule one uses, this will not change the definitions below. 

The activity definition (17) on the infinite dilution scale, paH = —log- H+ > = —log yH —log[H+] was experimentally studied by Bjerrum and Unmack (4). This definition is not rigid because of the intrinsic uncertainty in defining individual ionic activities. Depending on the precision required, it is usually stated that the approximations used to estimate yH breakdown at some ionic strength between 0.01 and 0.2M. 

The derivation of the law of mass action from the second law of thermodynamics defines equilibrium constants K° in terms of activities. For dilute solutions and low ionic strengths, the numerical values of the molar concentration quotients of the solutes, if necessary amended by activity coefficients, are acceptable approximations to K° [Equation (3)]. However, there exists no justification for using the numerical value of a solvent s molar concentration as an approximation for the pure solvent s activity, which is unity by definition.76,77... 

We also determined the effect of ionic strength on the formation of 3-MPA and 3-MPN in NaCl solutions at pH 8.0 and 40°C. Figure 6 shows plots of log k vs. I1/2, where k is the overall rate constant (units M day1), calculated as ki/fH ]. The rate of 3-MPA formation shows definite increase (slope from regression is 0.22) with ionic strength, which is in agreement with the Bronsted-Bjerrum equation. The formation of 3-MPN also shows an increase with ionic... 

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