Various Indicators of Water Quality - Resistivity, Conductivity, Etc.

Resistance value of water

When measuring the conductivity of water, it is related to the resistance value of the water. If the resistance value is large, the conductivity is poor. If the resistance value is small, the conductivity is good. According to Ohm's law, when the water temperature is constant, the resistance value R of water is inversely proportional to the vertical cross-sectional area F of the electrode and directly proportional to the distance L between the electrodes.

The resistivity of water is related to the salt content in the water, the ion concentration in the water, the charge number of the ions and the movement speed of the ions. Therefore, the resistivity of pure water is very large, and the resistivity of ultrapure water is even greater. The purer the water, the greater the resistivity.

Conductivity of water

Since water contains various dissolved salts and exist in the form of ions, when a pair of electrodes are inserted into the water, after electricity is applied, the charged ions will move in a certain direction under the action of the electric field. The anions in the water move to the anode and the cations move to the cathode, making the aqueous solution conduct electricity. The degree of water's electrical conductivity is called conductivity S (or conductance). Conductivity reflects the salt content in water and is an important indicator of water purity. The purer the water, the less salt it contains, the greater the resistance and the lower the conductivity. Ultrapure water barely conducts electricity. The magnitude of conductance is equal to the reciprocal of resistance. That is, S=1/R, S=(1/ρ)·(F/L). 1/ρ is called conductivity, and its international unit is West·meter-1 (S·m-1)