NITROGEN TRICHLORIDE DETECTION IN INDOOR SWIMMING POOL AIR
PRINCIPLE OF THE METHOD
Nitrogen trichloride (NCl3) measurement is performed by bubbling the indoor swimming pool air in a water solution containing DPD (diethyl-p-phenylenediamine) and Potassium Iodide (KI). The sample is analyzed as combined chlorine, according to Palin’s method (1967).
- Air sampling pump (flow rate at list 1 l/min)
- A couple of glass bubblers with a minimum reservoir of 50 ml and fritted glass stem with a porous septum (101 – 160 µm)
- One flask with stopcocks (involved as security tank)
- Silicone hoses
- Photometer for DPD analyses
- 25 ml graduated flasks
- Reagent grade water (Type 1 conductivity > 18 MΩ)
- Commercial DPD 1 tablets for spectrophotometers analyses
- Potassium Iodide (reagent grade)
- 50 ml graduated cylinder
- One glass rod
- 50 and 100 ml beakers
Chose a proper site and install the sampling train as described in the following scheme:
1st bubbler – 2nd bubbler – Security tank – Pump (see the picture)
Using the graduated cylinder, add 15 ml of reagent grade water to each bubbler
For each bubbler, using a glass rod, dissolve one DPD1 tablet and add 200 mg of Potassium Iodide
Switch on the pump and set the flow rate at 1000 ml/min
Sample at least 100 litres of swimming pool air (it will takes 100 minutes)
At the end of the sampling session, switch off the pump and remove the hoses from the bubblers
Transfer the 2nd bubbler solution (the one closer to the pump) into the 25 ml graduated flask. Wash properly, with reagent grade water, inside and the porous septum of the bubbler, until to obtain the final volume.
Mix well and suddenly analyze the sample by using the photometer (A value). Read the reagent grade water as blank solution.
Do the same procedure for the first bubbler (B value).
The 2nd bubbler verifies if NCl3 has saturated completely the first bubbler and passed over. Thus, in the second bubbler should be no presence of NCl3. Values > 0,1 mg/l Cl should be considered as reagent blanks and needed to be subtracted from the value obtained from the 1st bubbler.
The final value is the difference between the two analytical readings (C value):
C value = B value –A value
The instrument shows the residual chlorine concentration value expressed in mg/l Cl; so the following steps are requested:
Cl Atomic Mass = 35.5
NCl3 Molecular Weight = 120.5
Ratio 120.5 / 35.5 = 3.4
mg/l Cl x 3.4 = mg/l NCl3
mg/l = µg/ml
(assuming that the final volume is 25 ml, in order to obtain the microgram’s in the air litres sampled, multiply for 25)
µg/ml x 25 = total air µg
(due to obtain the concentration value expressed in µg/m3 NCl3, divide by the number of air litres sampled and multiply for 1000)
total µg / aspirated litres x 1000 = µg/m3 NCl3