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