The ionic strength of a solution is a measure of the

concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', '' number concentration'', ...

of ions in that solution. Ionic compounds, when dissolved in water,

dissociate Dissociation in chemistry is a general process in which molecules (or ionic compounds such as salts, or complexes) separate or split into other things such as atoms, ions, or radicals, usually in a reversible manner. For instance, when an aci ...

into ions. The total

electrolyte An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble Salt (chemistry), salts, acids, and Base (chemistry), bases, dissolved in a polar solven ...

concentration in solution will affect important properties such as the

dissociation constant In chemistry, biochemistry, and pharmacology, a dissociation constant (''K''D) is a specific type of equilibrium constant that measures the propensity of a larger object to separate (dissociate) reversibly into smaller components, as when a complex ...

or the

solubility In chemistry, solubility is the ability of a chemical substance, substance, the solute, to form a solution (chemistry), solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form su ...

of different

salts In chemistry, a salt or ionic compound is a chemical compound consisting of an assembly of positively charged ions ( cations) and negatively charged ions (anions), which results in a compound with no net electric charge (electrically neutral). ...

. One of the main characteristics of a solution with dissolved ions is the ionic strength. Ionic strength can be molar (mol/L solution) or molal (mol/kg solvent) and to avoid confusion the units should be stated explicitly. The concept of ionic strength was first introduced by Lewis and Randall in 1921 while describing the activity coefficients of strong electrolytes.

Quantifying ionic strength

The molar ionic strength, ''I'', of a solution is a function of the

of ''all'' ions present in that solution. :

I = \begin\frac\end\sum_^ c_i z_i^

where one half is because we are including both

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s and

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

s, ''c''_i is the

molar concentration Molar concentration (also called molarity, amount concentration or substance concentration) is the number of moles of solute per liter of solution. Specifically, It is a measure of the concentration of a chemical species, in particular, of a so ...

of ion i (M, mol/L), ''z''_i is the charge number of that ion, and the sum is taken over all ions in the solution. For a 1:1

such as

sodium chloride Sodium chloride , commonly known as Salt#Edible salt, edible salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. It is transparent or translucent, brittle, hygroscopic, and occurs a ...

, where each ion is singly-charged, the ionic strength is equal to the concentration. For the electrolyte MgSO₄, 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 (+2)^2+c(-2)^2 = \frac c + 4c = 4c

Generally multivalent ions contribute strongly to the ionic strength.

Calculation example

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

\\ & = 0.17 M \end

Non-ideal solutions

Because in non- ideal solutions volumes are no longer strictly additive it is often preferable to work with

molality In chemistry, molality is a measure of the amount of solute in a solution relative to a given mass of solvent. This contrasts with the definition of '' molarity'' which is based on a given volume of solution. A commonly used unit for molality ...

''b'' (mol/kg of H₂O) rather than molarity ''c'' (mol/L). In that case, molal ionic strength is defined as: :

I = \frac\sum_^ b_z_^

in which :''i'' = ion identification number :''z'' = charge of ion :''b'' = molality (mol solute per Kg solvent)

Importance

The ionic strength plays a central role in the Debye–Hückel theory that describes the strong deviations from ideality typically encountered in ionic solutions. It is also important for the theory of double layer and related electrokinetic phenomena and electroacoustic phenomena in colloids and other heterogeneous systems. That is, the Debye 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 or valence of the counterions compresses the double layer and increases the

electrical potential Electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as electric potential energy per unit of electric charge. More precisely, electric potential is the amount of work neede ...

gradient In vector calculus, the gradient of a scalar-valued differentiable function f of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p gives the direction and the rate of fastest increase. The g ...

. Media of high ionic strength are used in stability constant determination in order to minimize changes, during a titration, in the activity quotient of solutes at lower concentrations. Natural waters such as

mineral water Mineral water is water from a mineral spring that contains various minerals, such as salts and sulfur compounds. It is usually still, but may be sparkling ( carbonated/ effervescent). Traditionally, mineral waters were used or consumed at t ...

and

seawater Seawater, or sea water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has approximat ...

have often a non-negligible ionic strength due to the presence of dissolved salts which significantly affects their properties.

External links

Ionic strength

Ionic strength introduction at the EPA web site

References