Study of ozone generation
Contents
1 Measurement of ozone concentration 2
2 Chemical method 2
3 Photochemical method 4
4 Plasma characteristics 5
5 Assignments 5
1
1 Measurement of ozone concentration
With the increasing number of water treatment facilities, and facilities for
oxidation of gaseous and liquid exhalations, the process of ozone generation
is becoming one of the most investigated plasmochemical reactions. recently
the ozone is used in textile, paper and ceramics fabrication, replacing the
past use of chlorine. In both ozone generation process and any applications
in industry the determination of its concentration is fundamental.
There were many papers about the ozone concentration measurement
published recently. These analytical methods involve chemical oxidation,
absorption of the UV radiation, catalytic decomposition, chemiluminiscence,
fluorescence or breaking the double bonds. Most of these techniques are not
ozone specific and will generally determine the amount of any oxidation
agents. They can be divided into two main subgroups:
• chemical methods
• photochemical methods
Among the chemical methods, iodometric titration is one of the most
widely used and the most refined with the main advantage being the absolute
measurement of ozone concentration in oxygen. The tradeoff is, that it is
not possible to provide continuous (in-situ) measurements.
In majority of recently published works, the photochemical method,
based on the UV radiation absorption, is used. This methods’ main advantage
is the possibility of continuous measurement of ozone concentration,
which can be utilized for the automatic regulation of the ozone production
by ozonizer.
It should be noted that for ozone concentration measurement, combining
these two methods without further considerations or supplements is only
possible when the ozone is generated from the pure oxygen. In different cases
(e.g. ozone generated from air) however, contribution to the UV radiation
absorption from other molecules has to be taken into account (mainly N2,
N2O and NO). Fortunately, when the mercury lamp is used as the source
of UV radiation, these contributions are negligible, as the it involves only
excited metastables.
2 Chemical method
The principle of the iodometric titration (or iodometry) is the reaction of
ozone with alcaline iodide solution
2 KI + O3 + H2O −→ I2 + 2 KOH + O2 (1)
2
Figure 1: Experimental set-up: 1 – oxygen (or pressurized air), 2 - flow
meter, 3 - ozonizer, 4 - UV radiation absorption measurement, 5 - washer
with KI solution 0.2 M
during which iodine is extracted from the iodide. The solution then
turns yellow or even brown. The amount of iodide is then determined via
the sodium thiosulfate titration in the acidic environment.
I2 + 2 Na2S2O3 ←→ 2 Γ + Na2S4O6 (2)
where the reduction of iodine to iodide dims the yellow tint in the liquid.
For higher sensitivity of the method, a starch solution is added, at the point
where the yellow tint has almost vanished. The diluted starch then reacts
with the remaining traces of iodine coloring the solution into blue. The
subsequent desaturation of blue tint is more apparent to human eye than
the almost vanished level of the yellow tint.
The entire measurement will be as follows:
The gas is pumped from the discharge region (ozonizer) into the washer
with 100 ml 0.2 M solution of KI (see Figure 1). The generated ozone reacts
with the iodide, producing iodine. The reaction time suitable for sufficient
amount of generated ozone is 5 minutes. Afterwards the solution with the
extracted iodine is spilled over?? into titration flasks, acidified? with 10ml
2 M HCl and titrated using the 0.5 M solution of Na2S2O3. The sensitivity
of the measurement is then improved by adding the starch solution before
the end of the titration. The content of ozone is calculated from the amount
of used thiosulfate (1 ml of the 0.05 M solution Na2S2O3 is equivalent to 1.2
mg of ozone).
3
3 Photochemical method
The most common photochemical method is based on the absorption of
the UV radiation transmitting through the substance. The absorption of a
species at the given wavelength can be characterized either by the absorption
cross-section σ(ν) or the absorption coefficient k(ν). The absorption crosssection
is defined as usually (Beer-Lambert law):
I(ν) = I0(ν)e−σ(ν)N
(3)
where N is the number of absorbing species (molecules or atoms) in
the cylinder, which has base area of 1cm2, I0 is the intensity of the radiation
incident on the gas column, I(ν) is the intensity of the radiation
transmitted through the gas and σ(ν) is the absorption cross-section. The
absorption cross-section σ is often expressed in Megabarn (Mb) units, where
1Mb = 10−18 cm2. The Beer-Lambert law can also be written using the absorption
coefficient k(ν):
I(ν) = I0(ν)e−k(ν)l
(4)
where l is the distance traveled by the light in the gas. The relation between
absorption cross-section σ(ν) and the absorption coefficient k(ν) can
be determined using the Loschmidt constant N0 = 2, 687 · 1019 molecules · cm−3,
which is the number of molecules inside 1cm3 at standard conditions (i.e.
at the temperature T0 = 273, 15K and the pressure p0 = 101325Pa).
Should we measure the absorption in an environment under different
conditions than T0 and p0, the real distance l in the Beer-Lambert formula
has to be replaced with the reduced thickness x.
x =
pTO
p0T
l (5)
As for the absorption in gas mixtures, where the i-th species has the
concentration ci and
n
i=1
ci = 1, the Beer-Lambert law must be used in the
form:
I(ν) = I0(ν)e
−x
n
i=1
ciki
(6)
In case of ozone generated from oxygen using UV light with the wavelengths
in the range 220-280 nm, the equation (6) reduces to:
I(ν) = I0(ν)e−xcO3
k(ν)
(7)
since the absorption coefficient for molecular and atomic oxygen is zero
in this region.
4
Figure 2: Absorption coefficient O3 at 0oC as a function of wavelength.
The absorption coefficient for ozone as a function of wavelength is
shown in Figure 2. It is apparent that the mercury lamp and its 253.7 nm
line can be used as a perfect source of the UV radiation. In this measurement
this wavelength is picked using the interference filter, which is placed behind
the absorbing environment.
4 Plasma characteristics
The Becker parameter is used for the expression of plasma efficiency. It is
the amount of the discharge energy transferred from the discharge to the
the volume of the flowing gas. It is calculated as:
η =
I · U
Q
=
P
Q
[W · s · dm−3
] (8)
where I is the total discharge current, U is the voltage across the electrodes,
Q is the oxygen flow rate and P is the discharge power
5 Assignments
1. Perform the calibration of the float flow meter (rotameter) measuring
the rate of gas filling up the known volume.
5
2. Calibrate the device for the measurement of ozone concentration from
the UV radiation absorption using the iodometric titration. Use the
appropriate absorption coefficient (reading from the Figure 2). Perform
separate calibrations for technic oxgen and for air.
3. Measure the concentration of the generated ozone as functions of the
air flow rate and the power supplied to the ozonizer.
4. Measure the concentration of the generated ozone as functions of the
technic oxygen flow rate and the power supplied to the ozonizer.
5. Determine the Beck parameter for the plasma in ozonizer.
6