Movatterモバイル変換

Ionic strength

From Wikipedia, the free encyclopedia

Quantification of the electrical interactions between ions in solution

Theionic strength of asolution is a measure of theconcentration ofions in that solution. Ioniccompounds, whendissolved in water,dissociate into ions. The totalelectrolyte concentration in solution will affect important properties such as thedissociation constant or thesolubility of differentsalts. One of the main characteristics of a solution with dissolved ions is the ionic strength. Ionic strength can bemolar (mol/L solution) ormolal (mol/kg solvent) and to avoid confusion the units should be stated explicitly.^[1] The concept of ionic strength was first introduced byLewis andRandall in 1921 while describing theactivity coefficients ofstrong electrolytes.^[2]

Quantifying ionic strength

[edit]

Themolar ionic strength,I, of a solution is a function of theconcentration ofallions present in thatsolution.^[3]

I={\begin{matrix}{\frac {1}{2}}\end{matrix}}\sum _{i=1}^{n}c_{i}z_{i}^{2}

where one half is because we are including bothcations andanions,c_i is themolar concentration of ion i (M, mol/L),z_i is thecharge number of that ion, and the sum is taken over all ions in the solution. For a 1:1electrolyte such assodium chloride, where each ion is singly-charged, the ionic strength is equal to the concentration. For the electrolyteMgSO₄, however, each ion is doubly-charged, leading to an ionic strength that is four times higher than an equivalent concentration of sodium chloride:

I={\frac {1}{2}}[c(+2)^{2}+c(-2)^{2}]={\frac {1}{2}}[4c+4c]=4c

Generallymultivalent ions contribute strongly to the ionic strength.

Calculation example

[edit]

As a more complex example, the ionic strength of a mixed solution 0.050 M in Na₂SO₄ and 0.020 M in KCl is:

{\begin{aligned}I&={\tfrac {1}{2}}\times \left[{\begin{array}{l}\{({\text{concentration of }}{\ce {Na2SO4}}{\text{ in M}})\times ({\text{number of }}{\ce {Na+}})\times ({\text{charge of }}{\ce {Na+}})^{2}\}\ +\\\{({\text{concentration of }}{\ce {Na2SO4}}{\text{ in M}})\times ({\text{number of }}{\ce {SO4^2-}})\times ({\text{charge of }}{\ce {SO4^2-}})^{2}\}\ +\\\{({\text{concentration of }}{\ce {KCl}}{\text{ in M}})\times ({\text{number of }}{\ce {K+}})\times ({\text{charge of }}{\ce {K+}})^{2}\}\ +\\\{({\text{concentration of }}{\ce {KCl}}{\text{ in M}})\times ({\text{number of }}{\ce {Cl-}})\times ({\text{charge of }}{\ce {Cl-}})^{2}\}\end{array}}\right]\\&={\tfrac {1}{2}}\times [\{0.050M\times 2\times (+1)^{2}\}+\{0.050M\times 1\times (-2)^{2}\}+\{0.020M\times 1\times (+1)^{2}\}+\{0.020M\times 1\times (-1)^{2}\}]\\&=0.17M\end{aligned}}

Non-ideal solutions

[edit]

Because in non-ideal solutions volumes are no longer strictly additive it is often preferable to work withmolalityb (mol/kg of H₂O) rather than molarityc (mol/L). In that case, molal ionic strength is defined as:

I={\frac {1}{2}}\sum _{{i}=1}^{n}b_{i}z_{i}^{2}

in which

i = ion identification number

z = charge of ion

b = molality (mol solute per Kg solvent)^[4]

Importance

[edit]

The ionic strength plays a central role in theDebye–Hückel theory that describes the strong deviations from ideality typically encountered in ionic solutions.^[5]^[6] It is also important for the theory ofdouble layer and relatedelectrokinetic phenomena andelectroacoustic phenomena incolloids and otherheterogeneous systems. That is, theDebye length, which is the inverse of the Debye parameter (κ), is inversely proportional to the square root of the ionic strength. Both molar and molal ionic strength have been used, often without explicit definition. Debye length is characteristic of the double layer thickness. Increasing the concentration orvalence of thecounterions compresses the double layer and increases theelectrical potential gradient.

Media of high ionic strength are used instability constant determination in order to minimize changes, during a titration, in the activity quotient of solutes at lower concentrations. Natural waters such asmineral water andseawater have often a non-negligible ionic strength due to the presence of dissolved salts which significantly affects their properties.

External links

[edit]

References

[edit]

This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed.
Find sources: "Ionic strength" – news ·newspapers ·books ·scholar ·JSTOR(October 2008) (Learn how and when to remove this message)

^Solomon, Theodros (2001). "The definition and unit of ionic strength".Journal of Chemical Education.78 (12): 1691.Bibcode:2001JChEd..78.1691S.doi:10.1021/ed078p1691.
^Sastre de Vicente, Manuel E. (2004). "The concept of ionic strength eighty years after its introduction in chemistry".Journal of Chemical Education.81 (5): 750.Bibcode:2004JChEd..81..750S.doi:10.1021/ed081p750.
^IUPAC,Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Ionic strength,I".doi:10.1351/goldbook.I03180
^Standard definition of molality
^Debye, P.; Huckel, E. (1923)."Zur Theorie der Elektrolyte. I. Gefrierpunktserniedrigung und verwandte Erscheinungen" [The theory of electrolytes. I. Lowering of freezing point and related phenomena](PDF).Physikalische Zeitschrift.24:185–206. Archived fromthe original(PDF) on 2013-11-02.
^Skoog, D.A.; West, D.M.; Holler, F.J.; Crouch, S.R. (2004).Fundamentals of analytical chemistry. Brooks/Cole Pub Co.ISBN 0-03-058459-0.