Lewis-Randall, ionic strength

In activity studies in multicomponent systems, G. N. Lewis and M. Randall found in 1923 that in the case of dilute solutions, when a foreign electrolyte is added, the activity change of the substance studied depends only on the concentration and valence type of the substance added, not on its identity. For a quantitative characterization of solutions, they introduced the concept of ionic strength / of a solution (units mol/L),... 

Lewis and Randall stated that in dilute solutions the activity coefficient of a strong electrolyte is the same in all solutions of the same ionic strength this statement was confirmed in thermodynamic deductions of activity coefficients. The molality version of 7 can be applied in a fully analogous way and allows a more straightforward treatment of solution properties. [Conversion of molality into molarity requires the solution densities e.g., for a solute of molar mass M and a solution of density q we have... 

Activity coefficient vary with the concentration especially in the presence of added electrolyte. Lewis and Randall introduced the quantity called ionic strength which is a measure of the intensity of the electric field due to the ions in a solution. It is defined as the sum of the terms obtained by multiplying the molarity (concentration) of each ion present in solution by the square of its valence... 

The concept of ionic strength, which allows individual ionic activity coefficients to be estimated, was developed by Lewis and Randall (1921). The ionic strength of a solution is given by... 

The mean salt method derives implicitly from the concept of ionic strength, as expressed by Lewis and Randall (1921). It is assumed that, within the range of the ionic strength of interest, for a standard uni-univalent electrolyte such as KCl, the following assumption is valid ... 

Tn the equation (V-58) the quantity [i is of considerable importance, called by Lewis and Randall the ionic strength of the solution, and equals onc-half the sum of products of molarities of all the ions present in the solution and the square of their valences ... 

The activity coefficient varies with concentration. This variation is rather complex the activity coefficient of a particular ion being dependent upon the concentration of all ionic species present in the solution. As a measure of the latter, Lewis and Randall (1921) introduced the quantity called ionic strength, /, and defined it as the half sum of the products of the concentration of each ion multiplied by the square of its charge. With mathematical symbols this can be expressed as... 

We first call attention to the concept of ionic strength introduced by Lewis and Randall in 1921. This quantity is defined as... 

As the particle ionizes, the number of particles increases. Thus, it is not a surprise that activity coefficient is a function of the number of particles in solution. The nnmber of particles is characterized by the ionic strength p. This parameter was devised by Lewis and Randall (1980) to describe the electric field intensity of a solntion ... 

Ionic strength—A term devised by Lewis and Randall to describe the electric field intensity of a solution. 

It was pointed out by Lewis and Randall that, in dilute solutions, the activity coefficient of a given strong electrolyte is approximately the same in all solutions of a given ionic strength. The particular ionic strength may be due to the presence of other salts, but their nature does not affect the activity coefficient of the electrolyte under consideration. This generalization, to which further reference will be made later, holds only for solutions of relatively low ionic strength as the concentration is increased the specific influence of the added electrolyte becomes manifest. 

Qualitative Verification of the Debye-Hiickel Equations.—The general agreement of the limiting law equation (54) with experiment is shown by the empirical conclusion of Lewis and Randall (p. 140) that the activity coefficient of an electrolyte is the same in all solutions of a given ionic strength. Apart from the valence of the ions constituting the particular electrolyte under consideration, the Debye-Hiickel limiting equation contains no reference to the specific properties of the salts that may be present in the solution. It is of interest to record that the... 

An important concept that aided in the development of electrolyte theory was the ionic strength, I, introduced by Lewis and Randall (1921) ... 

For dilute electrolyte solutions, Lewis and Randall observed that the mean activity coefficient of a strong electrolyte does not depend on the kind of ion, but only on the concentration and charge numbers of all ions present in solution. So, the individual properties of the ions are not decisive for interionic interactions in dilute electrolyte solutions. These observations paved the way for the introduction of the concept of ionic strength / ... 

Activity coefficients for uncharged, molecular species generally obey the empirical Setchenow equation up to high ionic strengths (cf, Lewis and Randall 1961 Miller and Schreiber 1982). Such species include dissolved gases, weak acids, and molecular organic species. The Setchenow equation is... 

Lewis and Randall introduced a term called ionic strength, denoted by (i in order to represent the variation of activity coefficient of an electrolyte with concentration, in presence of other ions in a fixture. The ionic strength is the measure of the effective influence of all the ions in the mixture. It is obtained by multiplying the molality of each ion present in the solution with the square of its valency and dividing the sum of these products by two. Hence mathematically... 

Lewis, Randall and Bronsted showed through experiments in diluted solutions, that each mean activity coefficient depends on all the different kinds of ions in the electrolyte, through the ionic strength, /s ... 

This expression can now incorporate Lewis and Randall s (3) expression for ionic strength ... 

The work of the Danish chemists was based on Lewis and Randall s introduction of the concept of the ionic strength and on the Debye-Hlickel theory. Two years before Debye and Hilckel had formulated their theory Lewis and Randall (39) had introduced the idea of the ionic strength of a solution, which they defined as... 

Lewis and Randall (Ref. [99]) introduced the term ionic strength, defined by this equation, two years before the Debye-HUckel theory was published. They found empirically that in dilute solutions, the mean ionic activity coefficient of a given strong electrolyte is the same in all solutions having the same ionic strength. 

The Lewis and Randall rule that strong electrolytes at the same ionic strength exhibit similar ionic effects is valid to S 0.1. Solutions of higher strength... 

We have just seen that the activity coefficient of ions was influenced by the presence of other ions. The experiment shows that it is not necessarily the concentration that is influenced but a variable introduced by Lewis and Randall the ionic strength. 

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