A conductivity meter is a device for measuring the conductivity σ (expressed in S.m^-1) or the resistivity ρ (expressed in Ω.m) of a solution.
In practice, the device measures a conductance G (expressed in Siemens Ω^-1) or a resistance R (expressed in Ω) (R = 1/G).

The property of electrical conductance (or resistance) relates to the movement of ions (ion mobility u_i).

Why measure conductivity?

• To follow up a titration in addition to potentiometric, pH-metric or colorimetric monitoring (acetic titration in vinegar, basicity of Destop®) or to determine the concentration of ions in solution (Cl- in milk, SO₄^2- in mineral water).
• To check the quality of water: the conductivity of ultrapure water is about 0.055 mS/cm at 25°C.
• To determine kinetic constants or thermodynamic constants (pK_A, pK_S, E°).

Water quality

Water conductivity gives an indication of its quality as illustrated below.

What is the conductivity of pure water?

Pure water is made of H₂O and a small amount of H₃O⁺ and HO^- ions. There are no other ions in solution.

As seen in the sheet on pH, the autoprotolysis reaction leads to [H₃O⁺] = [HO^-]=10^-7 mol.L^-1.
Pay attention to the units: 10^-7 mol.L^-1 = 10^-10 mol.cm^-3 and 1 mS.m².mol^-1 = 10⁴ mS.cm².mol^-1

σ = (35 + 20) x 10⁴ x 10^-10 mS.cm^-1 σ = 55 x 10^-6mS.cm^-1 σ = 5.5 x 10^-8S.cm^-1 σ = 0.055 x 10^-6S.cm^-1 σ = 0.055 x μS.cm^-1 = 5.5 μS.m^-1

λ°_H3O⁺ = 34.98 mS.m².mol^-1
λ°_HO^- = 19.86 mS.m².mol^-1