An electrical conductivity meter is a device for determining the electrical resistance R of a solution located between two platinum plates of a conductivity cell. An alternating voltage (100-1000Hz) of low amplitude (to avoid electrolysis and electrode polarization) is applied between the two plates. Resistance is then determined thanks to a complex electrical circuit (Wheatstone bridge). Conductance is defined as the inverse of the resistance R and is noted as G:
The conductivity noted σ is proportional to G with k a proportionality factor called cell constant which depends on the conductivity cell used:
where k = I/S (IUPAC recommendation) is the cell constant related to the physical characteristics of the conductivity cell (I the distance between the two plates and S the actual surface area of the two plates).
Electrical resistance R of a solution
If we consider that the solution between the two platinum plates behaves like a homogeneous filamentary conductor (stream tube), we can establish a relation linking the resistivity 𝜌 to the resistance as:
where I is the distance between the two plates and S is the actual surface area of the two plates.
A conductivity cell is made of two platinised platinum cells (platinum coated with finely divided platinum black particles) facing each other (distanced by a length l) and having a surface area S. The use of platinised platinum allows the contact between the plates and the solution to be analyzed to be optimized.