This method for the determination of radon (Rn-222) in water solutions is a special
case of the well known de-emanation method primarily developed for radiochemical
The Henry-Dalton distribution law describes the distribution of gases between the
liquid and the gas phase. This law leads to a partition coefficient for radon
called Ostwald’s partition coefficient aRn that depends on the liquid and the temperature.
The low value for radon in water allows its determination by indirect measuring methods p.e. the
de-emanation method using scintillation chambers to detect radon and its progenies.
Experiments have shown that a five- to tenfold liquid volume of inactive gas results in
quantitative de-emanation success for water at nominal temperatures.
Depending on the user’s needs with respect to detection limit and measuring time exist three methods of determining the radon concentration in a water solution:
The user must think of the high volatility of radon gas. From this reason an open handling of the sample should be avoided. The best way is to evacuate a bottle and fill it with the sample under water. The rest pressure in the bottle to half or two-thirds fill it with sample is to be determined by experiments in laboratory. This air cushion is necessary to have a partitioning of radon in air and liquid. The filling without evacuated bottles also leads to good results. Bubbling during the bottle filling procedure must be avoided.
An equipment like the one in the picture should favourably be used for the
de-emanation procedure. The method is the same as for
1. Evacuation of apparatus
Attach a nitrogen flushed scintillation chamber as well as the sample tube to the
apparatus. With the vacuum pump on open
2. De-Emanation
Slowly open valve 2 until pressure equalisation visualised by the instrument.
Pressure value will reach about
3. Volume determination
Measure the sample volume in the tube.
The chamber is counted with the analysis ´Radon in
The handling during this procedure is described for both: flexible and rigid sample bottles:
1. Evacuation of apparatus
Attach a nitrogen flushed scintillation chamber as well as the sample bottle to the
apparatus. With the vacuum pump on open
2. First De-Emanation
Slowly and carefully open valve 2. For flexible bottles: until liquid fills completely the bottle. It will be deformed by atmospheric pressure. Pressure equalisation between chamber and sample bottle is not necessary. Essential is the complete transport of the gas phase activity to the chamber. For glass bottles: Until pressure equalisation is reached. Close all valves. Fill the chamber with nitrogen to normal pressure.
3. Second De-Emanation
Let nitrogen fill completely the bottle by opening valve 1.
Close it after pressure equalisation. Wait to reach equilibrium between gas and liquid phase.
After attaching a second evacuated chamber to the apparatus and opening of
4. Volume determination
Measure the sample volume in the bottle.
Count the chambers with analysis ´Radon in liquid 1´ (for the first chamber) resp. ´Radon in liquid 2´ (the second one). After termination of the measurements both values can be combined by use of the ´Radon in liquid´ menu item from the menu ´Measurements´. The single values are only intermediate results.