Ostwald's law of dilution

The Ostwald dilution law (discovered by Wilhelm Ostwald ) describes the dissociation of weak electrolytes , so the fraction of free particles in a solution , using the law of mass action . According to this relationship, the degree of dissociation α increases with decreasing sample concentration c ₀ (i.e. with increasing dilution ), so even weak electrolytes are practically completely dissociated when sufficiently diluted:

{\ displaystyle K_ {d} = {\ frac {c ({\ text {K}} ^ {+}) \ cdot c ({\ text {A}} ^ {-})} {c ({\ text { KA}})}} = {\ frac {\ alpha ^ {2}} {1- \ alpha}} \ cdot c_ {0}}

With

K _d	Dissociation constant (possibly also protolysis constant K _p )
α	Degree of dissociation (possibly also degree of protolysis )
c (A ^- )	Concentration of anions
c (K ⁺ )	Concentration of the cations (possibly the oxonium ions )
c (KA)	Concentration of the non-dissociated (possibly also non-protolyzed) electrolyte

If a solution is diluted by adding water, i. H. If the sample concentration of the substance is reduced, the degree of dissociation increases, since the dissociation constant must remain the same. Conversely, with further addition of substance, i.e. H. when the sample concentration is increased, the proportion of ions in the solution and thus the equivalent conductivity decrease.

conductivity

Wilhelm Ostwald derived the law from conductivity studies: if one sets the degree of dissociation of the above equation:

{\ displaystyle \ alpha = \ Lambda _ {\ rm {c}} / \ Lambda _ {0},}

so it results:

{\ displaystyle \ Rightarrow K_ {d} = {\ frac {\ Lambda _ {c} ^ {2}} {(\ Lambda _ {0} - \ Lambda _ {c}) \ cdot \ Lambda _ {0}} } \ cdot c}

With

${\ displaystyle \ Lambda _ {c}}$	Equivalent conductivity
${\ displaystyle \ Lambda _ {0}}$	Limit conductivity
c	Concentration of the electrolyte.

This concentration dependence of the equivalent conductivity is attributed to two effects:

on the hindrance of the movement of the ions by the strong Coulomb forces of attraction , which are noticeable at high concentrations,
to the incomplete dissociation of molecules, which for this reason are called weak electrolytes.

Ostwald made the following simplifying assumptions for determining the dissociation constants:

1. In the case of weakly dissociated molecules
The term changes only slightly with the dilution ( ), therefore the expression depending on the dilution factor can be used roughly . ${\ displaystyle \ Lambda _ {0} - \ Lambda _ {c}}$ ${\ displaystyle \ Lambda _ {0} - \ Lambda _ {c} \ approx \ Lambda _ {0}}$ ${\ displaystyle K_ {d} = \ Lambda _ {c} ^ {2} / \ Lambda _ {0} ^ {2} \ cdot c = \ alpha ^ {2} \ cdot c}$

2. The determination of two weakly dissociated particles
Here the corresponding ratios of the above expression must be used.

3. With strong electrolytes
Here, the reciprocal value of is determined with increasing dilution. ${\ displaystyle \ Lambda _ {0} - \ Lambda _ {c}}$

The determination of the limiting conductivity of ions of weak organic acids and bases , which is necessary for the determination of the dissociation constants, presented a problem . The alkali salts of the acids or the hydrohalic acids of the bases can, however, be easily determined, so that after subtracting the alkali or halogen limit conductivities, the limit conductivities of very weakly dissociated anions and cations can also be determined.

Ostwald's theory was improved by the Debye-Hückel law .

literature

Max Le Blac: Textbook of Electrochemistry , Verlag Oskar Leiner, Leipzig 1922