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:

**σ = kG**

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).

For a new conductivity cell, the value for k given by the manufacturer is often close to the unit (k = 1).

## 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:

R=ρl/S =kρ

where I is the distance between the two plates and S is the actual surface area of the two plates.