The Ionic Strength of a Solution

The combined effect of the charges on ions and their concentrations is expressed as the ionic strength of a solution, defined as follows  

In calculating the ionic strength of a solution, the concentration of all the ions that are present in the solution and not on just the ionic species that are involved in a particular equilibrium, must be taken into account. As shown in Equation 3-7, the ionic strength of a solution is the sum of the individual contributions of each of the electrolytes present. It can be easily shown that the contribution to the ionic strength from each component electrolyte is proportional to its concentration multiplied by a proportionality factor, n, related to its charge t)q)e, i.e., 

To illustrate, in a 0.1 M in KCl aqueous solution, the concentration of both and Cl is 0.1 M. Since these ions both carry unit charges, the ionic strength of the solution is given by  

You will find it helpful to work out for yourself what this proportionality constant, n, is for electrolytes of other charge types. 

Care must be taken in evaluating the ionic strength contribution of weak electrolytes. For example, if any of the solutions above contained phosphoric or acetic acid also, the ionic strength would be essentially the same, because only the dissociated phosphoric or acetic acids contributes, and this is generally very small. If, on the other hand, H3PO4 is the only solute present, then an approximate equilibrium calculation must be carried out (see Example 3.3), an ionic strength calculated, and the process repeated until values of I remain constant. This may take one or two successive approximations. 

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Protein separation by hydrophobic interaction chromatography is dependent upon interactions between the protein itself, the gel matrix and the surrounding aqueous solvent. Increasing the ionic strength of a solution by the addition of a neutral salt (e.g. ammonium sulfate or sodium chloride) increases the hydrophobicity of protein molecules. This may be explained (somewhat simplistically) on the basis that the hydration of salt ions in solution results in an ordered shell of water molecules forming around each ion. This attracts water molecules away from protein molecules, which in turn helps to unmask hydrophobic domains on the surface of the protein. 

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... 

When the ionic strength of a solution is low, the denominator in equation 8.31 tends toward 1, and the individual ionic activity coefficient is well approximated by... 

Example 5.5. Calculate the ionic strength of a solution that is both 0.01 M in NaCl and 0.1M in Na2S04. 

In solution thermodynamics, the concentration (C) of ions is replaced by their activity, a, where a = Cy and y is the activity coefficient that takes into account nonideal behavior due to ion-solvent and ion-ion interactions. The Debye-Hiickel limiting law predicts the relationship between the ionic strength of a solution and y for an ion of charge Z in dilute solutions ... 

Which electrolytes are called strong, and which are called weak In what form can the law of mass action be applied to strong electrolytes Define the activity, the activity coefficient, and the ionic strength of a solution. 

Ionic strength (I) A value usually given in molal that describes the overall effect of the concentrations and charges of ions in an aqueous solution. The ionic strength of a solution is equal to one-half the sum of the molal concentration of each ion multiplied by its charge squared, that is, I = 0.5 J2 (m,z ). 

As shown in Equation (2.164b), the solubility of slightly soluble substances is simply related to the solubility product (Ksp). However, the solubility is dependent on the ionic strength of a solution. A slightly soluble salt dissolves in water ... 

What is the ionic strength of a solution containing 0.2 M NaCl and 0.5 M CaCl2 ... 

A useful descriptor of the overall strength of ionic forces in electrolyte solutions is given by the ionic strength / of a solution, defined as... 

Ionic strength measures the concentration of charges in solution. As the ionic strength of a solution increases, the activity coefficient of an ion decreases. The relationship between the ionic strength and the molarity of a solution of ionizable salt depends on the number of ions produced and their net charge, as sumniarized below. 

Only the nel charge on an ion is used in calculating ionic strength. Thus, un-ionized compounds (e.g., un-ionized acetic acid) or species carrying an equal number of positive and negative charges (e.g., a neutral amino acid) do not contribute toward the ionic strength of a solution. 

Activity coefficient, yx A unitless quantity whose numerical value depends on the ionic strength of a solution it is the proportionality constant between activity and concentration. 

The application of this model (Figs. 6 and 7) shows that the degree of distortion of a titration curve decreases upon an increase in the ionic strength of a solution, and we must obtain an intrinsic constant of pkjnt by extrapolating experimental values of pKa to the zero degree of ionization. 

It may be noted that in an ideal solution where all ions are completely noninteracting (a situation realized only under conditions of high dilution), the activity coefficient term would equal unity, and then K would be equal to Kc-The ionic strength of a solution is defined as ... 

Ionic strength is related to salinity. The ionic strength of a solution is a measure of the concentration of ions in that solution. The ionic strength, I, of a solution is a function of the concentrations of all the ions present in that solution (Atkins and de Paula, 2006)... 

The ionic strength of a solution can have an important effect on the rate of a reaction occurring in the solution. This is a matter of considerable importance with enzyme reactions, and it is therefore necessary to control the ionic strength. 

Effect of Ionic Strength Lagrange et al. (1994) suggested that the rate of the S(IV)-0, reaction increases linearly with the ionic concentration of the solution. The ionic strength of a solution, I, is defined as... 

The ionic strength of a solution generally alters the activity coefficients of all dissolved ionic species, thus influencing measured pK values. The Debye-Hiickel equations [Eqs. (13-17)] are required to correct for solution ionic strength in order to obtain thermod)mamic pK values ... 

Activity coefficients strongly depend on the ionic strength of a solution. Activity coefficients are responsible for influencing the magnitude of various equilibrium constants of interest to chemistry. Solute activities have minimal influence in TEQA for two reasons ... 

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