Strength specific ions

Reaction rates in enzyme systems are usually extremely sensitive to variation of experimental conditions, i.e., to pH, temperature, ionic strength, specific ions, solvent, etc. We now consider in some detail the influence of pH and temperature on enzymatic reactions. 

As mentioned above, units are defined for particular assay conditions. There are many conditions which must be controlled, including temperature, pH, ionic strength, specific ion concentrations, substrate concentrations, presence of activators, stabilizers, and inhibitors. The role of these factors will be illustrated for various enzymes in later sections. At this point it will merely be mentioned that there are no generalities that describe the effect of varying any of these conditions. Each enzyme must be studied as an individual case some are indifferent to conditions that effect others profoundly, and some environmental changes, as temperature, influence competing phenomena, as rate of catalyzed reaction and rate of enzyme destruction. 

Specific-Ion Electrodes In addition to the pH glass electrode specific for hydrogen ions, a number of electrodes that are selective for the measurement of other ions have been developed. This selectivity is obtained through the composition of the electrode membrane (glass, polymer, or liquid-liquid) and the composition of the elec trode. Tbese electrodes are subject to interference from other ions, and the response is a function of the total ionic strength of the solution. However, electrodes have been designed to be highly selective for specific ions, and when properly used, these provide valuable process measurements. 

The specific ion interaction approach is simple to use and gives a fairly good estimate of activity factors. By using size/charge correlations, it seems possible to estimate unknown ion interaction coefficients. The specific ion interaction model has therefore been adopted as a standard procedure in the NEA Thermochemical Data Base review for the extrapolation and correction of equilibrium data to the infinite dilution standard state. For more details on methods for calculating activity coefficients and the ionic medium/ ionic strength dependence of equilibrium constants, the reader is referred to Ref. 40, Chapter IX. 

Part Three describes a range of important specific examples of the interactions of individual biopolymers in the bulk aqueous medium of food colloids. Chapter six is devoted to the subject of the self-assembly of food biopolymers, and how this self-assembly is affected by conditions such as pH, ionic strength, divalent ions, cosolutes, etc. It is indicated how biopolymer self-assembly can form the basis of the bottom-up nano-biotechnological approach, which attempts to mimic Nature in the creation of new and varied structures with potential applications. It is... 

The main advantages of using this approach are that response is not affected by the ionic strength of the solution and that the development of specific ion-selective optodes is easily adapted from their analogous well-studied ion-selective electrodes [32 - 34], because the same selective reagents and membrane constituents are employed. Most of the ion-selective optodes... 

Adsorption-induced brittle fracture. This model is based on the hypothesis that adsorption of environmental species lowers the interatomic bond strength and the stress required for cleavage. This model of chemical adsorption can explain the fact that a certain alloy is susceptible to specific ions. An important factor in support of this mechanism is the existence of a critical potential below which the SCC does not occur in some systems, and this model underlines the relation between the potential value and the capacity of adsorption of the aggressive ion. It also explains the preventive action of SCC for some systems by cathodic protection. This model may interpret the rupture of plastic materials or glass. It is referred to as the stress-sorption model, and similar mechanisms have been proposed for HE and LME. In this model, the crack should propagate in a continuous way at a rate determined by the arrival of the embrittling species at the crack tip. The model does not explain how the crack maintains a sharp tip in a normally ductile material.156... 

---