This part of ISO 13165 specifies the determination of radium-226 (²²⁶Ra) activity concentration in non-saline water samples by extraction of its daughter radon-222 (²²²Rn) and its measurement using liquid scintillation counting.

Radium-226 activity concentrations which can be measured by this test method utilizing currently available liquid scintillation counters goes down to 50 mBq l⁻¹. This method is not applicable to the measurement of other radium isotopes.