NITROGEN TRICHLORIDE (NCl3) DETERMINATION IN
INDOOR SWIMMING POOLS: PROPOSAL FOR A NEW
Predieri G, Giacobazzi P, Fantuzzi G*, Righi E and Aggazzotti G
Department of Sciences of Public Health
University of Modena and Reggio Emilia – Italy
The aim of this study was to identify a new analytical method for NCl3 air determination that could be
used directly in swimming pool facilities for the human exposure assessment procedures. This new protocol
is based on a colorimetric reaction commonly employed to detect the total and free chlorine levels
in water. Particularly, it allows the entrapment of NCl3 in ambient air into a water solution containing
diethyl-p-phenylenediamine and potassium iodide. NCl3 from the air environment reacts with potassium
iodide realising iodine, which reacts with diethyl-p-phenylenediamine and produces a pink colour. The
intensity of the colour is proportional to the amount of NCl3 from the sampled indoor swimming pool air,
and can be easily measured using a portable photometer for DPD analyses. The proposed method has
been validated in terms of linearity, limit of detection (LOD) and repeatability. We tested the linearity by
creating an artificial swimming pool environments under a closed chemical safety cabinet, and reading
solutions of NCl3 in air sampling deriving from standards in water at different concentrations (5, 10, 20
mg/L of NCl3 standards). The actual concentration of NCl3 in the water was verified by adopting the same
DPD method used in the real swimming pools. This linearity test was repeated three times and showed R2
values of: 0.996; 0.998 and 0.996. The LOD measured as standard deviation was 3.6 μg/m3, while the LOD
measured according to the instrument sensitivity was 8.5 μg/m3. The repeatability showed a coefficient
of variation (CV%) equal to 1.7%. In conclusion, this method is easy to use, efficient and economical and
does not use toxic substances.
Keywords Nitrogen trichloride, ambient air, indoor swimming pools, new method.
It is well known that water disinfection treatments in indoor swimming pools generate a mixture of
potentially harmful disinfection by-products (DBPs) (WHO, 2006). Among them, nitrogen trichloride (NCl3),
a powerful irritant, has been linked with respiratory symptoms and asthma in swimmers, mainly in
children, and in occupationally exposed subjects (Jacobs et al., 2007, Weisel et al., 2009). Accurate NCl3
exposure assessment in indoor swimming pools is difficult, because NCl3 presence in air is influenced by
different factors, including the pool water quality and the number of swimmers in the pool. Moreover,
the analytical method usually adopted for the determination of NCl3 in air samples shows some problems
such as the use of particular filters for the air sampling procedures and the expensive instrumental
equipments for the laboratory quantification of this volatile DBPs (Hery et al., 1995).
The aim of this study was to develop a simple method for NCl3 air determination that could be used
directly in swimming pool facilities for the human exposure assessment procedures.
The proposed method is based on the entrapment of NCl3 in air (trough an air pump and impingers)
in a water solution where diethyl-p-phenylenediamine and potassium iodide, the reagents commonly
used to detect total and free chlorine levels in water according to the Palin method, were previously
added (Palin, 1957). The entrapment is performed by an active sampling procedure using an air pump
connected to two impingers. NCl3 in water solution reacts with potassium iodide realising iodine, which
reacts with diethyl-p-phenylenediamine and produces a pink colour. The intensity of the colour is proportional
to the amount of NCl3 from the sampled indoor swimming pool air, and can be easily measured
using a portable photometer.
Device and apparatus. Environmental air samples were collected by a simple apparatus consisting of
two glass impingers (Impinger 1 and Impinger 2), with a minimum reservoir of 40 ml and fritted glass
stem with a porous septum (101 – 160 μm) (INCOFAR – ITALY), a 250 ml flask with stopcocks, used as
security tank and, finally, an air sampling pump (PUMPTM Buck P.N. APB-921000, range 5-5000 mL/min
– SUPELCO – USA). The quantitative analyses of free and total chlorine in water were performed by a portable
photometer (PC Compact – Aqualytic).
Standards and reagents. For the determination of chlorine in water, diethyl-p-phenylenediamine
(DPD 1) and potassium Iodide (DPD 3) commercial tablets (Aqualytic) were adopted.
Given the lack of commercial NCl3 standard solutions, we prepared our standards according to the
protocol suggested by Shang and Blatchley III, (1999). The solutions were prepared from reagent-grade
chemicals and diluted to target concentration using reagent grade water.
Stock chlorine solution was prepared from 5% sodium hypochlorite (NaOCl), diluted to 4000–5000
mg/L as chlorine, and stored in aluminium foil-covered glass-stoppered flasks. Chlorine standard solutions
were prepared by dilution with phosphate buffer (pH 7). NCl3 standard solutions were prepared
daily by mixing a free chlorine solution with an ammonium chloride (NH4Cl) solution at a chlorine to
ammonia molar ratio of 3.15: 1.00 and standardised titrimetrically. Both solutions were adjusted to pH 5
with acetic acid (1 M) prior to mixing.
Sample collection. An active air sampling with a constant flow air sampling pump at a flow rate of
1000 ml/min for 100 minutes was performed to sample around 100 litres of swimming pool air, normalized
by 25 °C and 1013 hPa. In order to avoid contamination from free and combined chlorine in the water
aerosol, the sampling apparatus was located at 1.5 m from the floor surface and not less than 2 m from
the pool edge. Both impingers were filled with 15 ml of reagent grade water and a DPD1 and DPD3 tablet
(Aqualytic): the tablets were dissolved using a glass rod.
Determination of NCl3. To quantify the amount of NCl3 in the indoor swimming pool air, the solutions
derived from the two impingers were transferred into two 25mL graduated flasks, washing the inside
and the porous septums of the impingers. The obtained target volumes were mixed and immediately
analysed by using a photometer (PC Compact – Aqualytic). The solution from Impinger 2 was considered
as reagent blank as it verifies if NCl3 has completely saturated Impinger 1. The final value was the difference
between the two analytical readings. In order to convert the obtained residual chlorine concentration
(expressed in mg/L Cl) in micrograms per cube meter of NCl3, we applied a simple stoichiometric
The proposed method has been validated in terms of linearity, limit of detection (LOD) and repeatability.
The linearity was tested in our laboratory by creating an artificial swimming pool environment under a
closed chemical safety cabinet, and reading solutions of NCl3 in air sampling deriving from standards in
water at different concentrations (5, 10, 20 mg/L of NCl3 standards). The actual concentration of NCl3 in
the water was verified by adopting the same DPD method used in the real swimming pools. The linearity
test was repeated three times and showed R2 values of: 0.996; 0.998 and 0.996.
The limit of detection (LOD) was determined according to two procedures. Both as 3-fold the standard
deviation of the lowest NCl3 standard solution (5 mg/L) used in the analytical runs from the linearity
test, and considering the minimum instrumental reading: 0.01 mg/L per 100 litres of sampled air. The
NITROGEN TRICHLORIDE (NCl3) DETERMINATION IN