(BAT) Reference Document for the Production of Chlor-alkali ...

Chapter 4
species such as chlorine dioxide are frequently included in the measurement result depending on
the analytical method employed [ 67, Euro Chlor 2010 ].
Technical description
The absorption of gaseous chlorine in an aqueous solution is frequently carried out by using a
solution of sulphamic acid (H3NSO3), leading to the formation of stable chloramines. The level
of chloramines is subsequently determined with methods quantifying total chlorine such as
those using N,N-diethyl-1,4-phenylenediamine (DPD) (see Section 4.3.4.5). Chloramines may
also react with iodide in slightly acidic solution resulting in the formation of triiodide which can
be measured photometrically. These two methods are applicable to chlorine concentrations of
0.30 – 7.0 mg/m 3 (0.1 – 2.3 ppmv) with sampled air volumes of 15 l. Other oxidising species
such as bromine and iodine are included in the measurement [ 67, Euro Chlor 2010 ].
Alternatively, the chloramines react with iodide to form iodine and chloride with subsequent
quantification of the latter using a chloride-selective electrode. The limit of quantification using
this method is 0.14 ppmv when using sample volumes of 15 l with an overall measurement
uncertainty of ± 5 % in the range of 0.56 – 2 ppmv. Other oxidising species interfere with the
measurement [ 181, USDOL-OSHA 1991 ]. Chlorine and chlorine dioxide can be determined
simultaneously by absorption in a potassium iodide solution with subsequent titration using
thiosulphate (iodometry) [ 182, USDOL-OSHA 2011 ]. Chlorine dioxide alone can be measured
by absorption in a weakly basic potassium iodide solution. Under these conditions, chlorite
(ClO2 - ) is formed which can be quantified by ion chromatography (chlorite oxidises iodide only
in acidic solutions). The limit of quantification of this method is 1 ppbv when using
120-l-samples and 18 ppbv when using 7.5-l-samples with an overall measurement uncertainty
of ± 20 % in the range of 58 – 202 ppbv [ 183, USDOL-OSHA 1991 ].
Another possibility for measuring chlorine is its absorption in a solution containing
methylorange. The bleaching of the latter is measured with photometry. The methylorange
method can be used in the concentration range of 0.06 – 5 mg/m 3 with an overall measurement
uncertainty ± 10 % at 4.5 mg/m 3 [ 179, VDI 1979 ]. Alternatively, absorption is carried out in a
potassium bromide solution. The resulting bromine is quantified with iodometry. The
bromide-iodide method can be used in a concentration range of 0.7 – 250 mg/m 3 [ 180, VDI
1980 ].
Field and portable instruments for direct monitoring of chlorine in air are usually based on
electrochemical cells which measure all halogens. Some instruments are equipped with a pump
to bring the ambient air to the sensor while others are directly exposed to the air (mainly for
field analysers and personal monitors) without requirements for minimal air movement speed.
Electrochemical cells can measure chlorine concentrations from < 1 – 100 mg/m 3 . The limit of
detection is usually approximately 0.3 mg/m 3 with a measurement uncertainty of approximately
± 5 % and a response time of approximately 10 seconds. In case of saturation of the sensor,
some time is needed to recover the full sensitivity. Other oxidising species such as bromine and
chlorine dioxide may be included in the measurement [ 67, Euro Chlor 2010 ]. Electrochemical
cells are often used as detectors to ensure that chlorine concentrations at the point of
measurement do not exceed threshold values.
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Other techniques such as UV-spectrophotometry, mass spectrometry, ion mobility spectrometry
and papertape-based photometers may become of interest in the future [ 67, Euro Chlor 2010 ].
The calibration of the analytical techniques described above is usually based on molecular
chlorine (oxidation number 0). Because of the fact that chlorine dioxide contains only one
chlorine atom and its oxidation number is +IV, response factors differ. If both chlorine and
chlorine dioxide are converted to chloride during the analytical measurement, then 1 mol of
ClO2 gives 5 / 2 = 2.5 times as much signal as 1 mol of Cl2. Otherwise said, 1 ppmv of ClO2
leads to a measurement result of 2.5 ppmv of Cl2. Taking into account the molecular weights,
1 mg/m 3 of ClO2 therefore leads to a measurement result of (2.5 · 70.90 / 67.45 =) 2.6 mg/m 3 of
Cl2 [ 68, AkzoNobel 2007 ], [ 185, Infomil 2011 ].
214 December 2011 TB/EIPPCB/CAK_Draft_1