Conductimetry

Conductivity σ of an ionic solution

The value of the conductance G of an ionic solution depends on the nature of the solution, as well as geometry of the measuring cell. It can be given by the relation:

G = \ frac {\ sigma \ cdot S} {L}

with G in mho (S), S in square meter (m²), L measures some (m) and σ in mho per meter (S.m^-1).

In addition the conductance is the reverse of resistance: $G = \ frac {1} {R}$ with G in mho (S) and R in Ohm S $(\ Omega)$ .

One thus can using a simple cell, of a generator of tension U and of a Ampèremètre connected in series, to deduce the conductance using the Loi from Ohm: $U = R \ cdot I = \ frac {I} {G}$ with U in Volt S (V), R in ohms $(\ Omega)$ , I in amp S (A) and G in mho (S). One can also write: $G = \ frac {I} {U}$ .

One calls σ (sigma) the conductivity of the solution. This size is characteristic of the solution. It depends:

of the nature of the ions which compose it,
their concentrations,
of the temperature of the solution.

A conductimeter, calibrated beforehand, makes it possible to directly post the value of conductivity σ of the solution.

Conductivity checks the following equality: $\ sigma = K \ cdot G$ or $\ sigma = \ frac {G \ cdot L} {S}$

σ in S.m-1, K constancy of cell, G in S, L spaces between the two cells of the conductimeter immersed in the solution in m, S surfaces these cells in m2.

Ionic molar conductivity λ_i

Monochargées species

The value of conductivity σ can be calculated starting from ionic molar conductivities λ_i of the ions which compose this solution (see table given below as an indication), as well as theirs concentration:

\ sigma = \ sum \ lambda_i \ cdot

with σ in S.m^-1, λ_i in S.m².mol^-1 and mol.m^-3.

One notices that the ions H₃0⁺ and HO^- have, in aqueous solution, an ionic molar conductivity more important than that of the other ions. These two ions being derivative of water their mobility in water is indeed very important.

However, in the case of pure water, their concentration is very weak (10^-7 mol. L^-1) and their contribution are thus negligible: a pure water solution conducts only very little electricity.

Example: if one poses the conductivity of a solution of Sodium chloride of concentration C = = = 2,00 mol.m^-3 is equal to:

σ = λ_Cl^-. + λ_Na⁺. : σ = 7,63.10^-3 X 2,00 + 5,01.10^-3 X 2,00 = 2,53.10^-2 S.m^-1

Polychargées species

If the ions carry several loads, the tables of values generally give specific molar conductivities, i.e. brought back to the unit of load.

$\ sigma = \ sum z^i \ lambda_i \ cdot$ where zⁱ, is the number of loads carried by the ion, independently of their sign.

Conductimetric methods

Measurements of conductimetry make it possible to determine the concentration ions contained in this solution. They are very much used in chemistry for:

of the Proportioning S,
of the determinations of Kinetic chemical,
of the constant determinations of of thermodynamic balances (constant of solubility for example).

Notes and references of the article

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