There are many SO2 analysis and
measurement techniques in this research I will mention about four main analysis
and measurement techniques.
1 The Flame Photometric Detector (FPD)
An integrated gas sample is extracted from the
stack. The SO2 is removed selectively from the sample using a citrate buffer
solution. The TRS compounds are then thermally oxidized to SO2 and analyzed as
SO2 by gas chromatography (GC) using flame photometric detection (FPD).
Emission of
radiation can be stimulated thermal-chemically by a gas/hydrogen-flame chemiluminescent
interaction. The emission can be detected by photometric techniques. If sulfur
compounds (SO2, H2S, CS2, CH3SH) were introduced into the hydrogen flame, a
emission between 300 to 420 nm wavelength can be detected. Total sulfur can be
measured and combined with GC provides the capability to separate and measure
each sulfur compound. It has a highly sensitive, 1 - 10 ppb.
FDP is still the most commonly used
physiochemical transducer for gaseous sulfur-containing compounds despite its
notable limitations. Various FPD response optimization studies have been
performed, but problems with regard to detectability, response time,
selectivity and misunderstanding of its exponential response remain. FPD
designs include fast response burners a dual flame burner and a catalytic,
nonflame device. Other reported sulfur-selective transducers are a dc plasma,
the electrolitic conductivity detector, ozone-sulfide chemiluminescence and
F2-induced chemiluminescence.
Sulfur Dioxide (SO2) is a colorless, nonflammable
gas that has a strong suffocating odor.
SO2 originates from
fuel containing sulfur (mainly coal and oil) burned at power plants and during
metal smelting and other industrial processes.
High levels of SO2 can result in
temporary breathing impairment for asthmatic children and adults who are active
outdoors. Long-term exposure to high
levels of SO2, in the presence of high levels of
particulate matter, may aggravate existing cardiovascular disease and
respiratory illness.
The
Teledyne-Advanced Pollution Instrumentation (T-API) model 100AS combines proven
detection technology for the determination of trace levels of SO2. This SOP will detail the operation,
preventive maintenance, cautions and health warnings.
The Detection Limit (DL) for
a non-trace level SO2 analyzer is 10
parts per billion (ppb) (Code of Federal
Regulations, Volume 40, Part 53.23c, or, in the shortened format used
hereafter, 40 CFR 53.23c)3.
However, the T-API model 100AS has an estimated DL of 100 parts per
trillion (ppt), which is accomplished by an increased detector sensitivity, as
well as increasing the length of the standard instrument’s optics bench. This document will discuss the Trace Level
(TL) operating procedures in detail.
The Model 100AS Trace Level
operating principle is based on measuring the emitted fluorescence of SO2 produced by the absorption
of ultraviolet (UV) light. The UV lamp
emits ultraviolet radiation which passes through a 214 nm band pass filter,
excites the SO2 molecules,
producing fluorescence which is measured by a photomultiplier tube (PMT) with a
second UV band pass filter. SO2 absorbs in the 190 nm – 230
nm region free of quenching by air and relatively free of other
interferences. The equations describing
the above reactions are as follows:
SO2 + hν1 Ia → SO2*
The excitation ultraviolet
light at any point in the system is given by:
Ia =I0[1-exp(-ax(SO2))]
where:
I0 = UV light intensity,
a = the absorption coefficient of SO2
x = the path length
SO2 = concentration of SO2
The excited SO2 decays back to the ground
state emitting a characteristic fluorescence:
SO2* kF→ SO2 + hν2
When the SO2 concentration is relatively
low, the path length of exciting light is short and the background is air, the
above expression reduces to:
F = k(SO2)
where:
F = amount of
fluorescent light given off
k = rate at which SO2* decays into SO2
The 100AS instrument operates in the following fashion:
In sample mode,
the sample is drawn into the analyzer through the SAMPLE bulkhead. The sample flows through a hydrocarbon
“kicker,” which operates on a selective permeation principle, allowing only
hydrocarbon molecules to pass through the tube wall. The driving force for the hydrocarbon removal
is the differential partial pressure across the wall. This differential pressure is produced within
the instrument by passing the sample gas through a capillary tube to reduce its
pressure and feeding it into the shell side of the hydrocarbon kicker. The SO2 molecules pass through the
hydrocarbon “kicker” unaffected.
The sample
flows into the fluorescence chamber, where UV light is focused through a narrow
214 nm band pass filter into the reaction chamber, exciting the SO2
molecules; the molecules then give off their characteristic decay
radiation. A second filter allows only
the decay radiation to fall on the PMT.
The PMT transfers the light energy into the electrical signal which is
directly proportional to the light energy in the sample stream being analyzed. The preamp board converts this signal into a
voltage which is further conditioned by the signal processing electronics.
The UV light
source is measured by a UV detector.
Software calculates the ratio of the PMT output and the UV detector in
order to compensate for variations in the UV light energy. Stray light is the background light produced
with zero ppb SO2. Once this
background light is subtracted, the CPU will convert this electrical signal
into the SO2 concentration which is directly proportional to the
number of SO2 molecules.
SO2 + hv1 = SO2*
SO2* = SO2 + hv2
(by fluorescence)
SO2* + M = SO2 +
M (by quenching)
SO2* = SO + O
(by dissociation)
This method is
sensitive, specific and has a wide linear range. It is good for detecting
ambient and industrial sources. The emission analyzers can measure from 0 -
5000 ppm. Ambient atmospheric analyzers can go down to 0 - 0.5 ppm. The
detection limit is around 0.02 ppm. This method needs calibration gas to
perform a multi-point calibration.
2 Gas Chromatographic Method
Gases migrate
differentially in a porus sorptive medium(consist of an absorption column and a
detection unit). Air sample is aspirated into the sample loop of a gas sampling
valve. The valve is then switched to inject the sample on to a GC column where
SO2 is separated from other gaseous compounds. The effluent passes into a FPD
(flame photometric detector) and the detector signal amplified and recorded.
Minimum detection limit is 5 -10 ppb.
Reagents;
To prepare mercury stabilizing solution,
mercuric chloride (27.2 g) and sodium chloride (11.7 g) were dissolved in water
and made up to 1 L. Sodium bisulfite stock solution was made by dissolving 10 g
of sodium bisulfite (AR grade) in water and making up to 1 L. The solution was
assayed iodimetrically for SO2 content (1) and further diluted to contain 300
mg/ L SCh. The concentrated stock solution is stable and lasts for at least a
week. The 300 mg/L SCh solution must be prepared daily before use. Sample
Preparation Free SCh was prepared by taking beer (20 ml in a 100-ml volumetric
flask) and adding 11.0 ml of phosphoric acid. This was mixed and subsequently
brought to volume with distilled water. Total SCh was measured in 10 mi-samples
of beer to which was added 2 ml of mercury-stabilizing solution. This was mixed
well in the 100-ml volumetric flask, and 15.0 ml of 0.10 NaOH was pipetted into
the flask. The contents were swirled for sufficient mixing and subsequently
allowed to stand for at least 1 min.
Phosporic acid (11.0 ml of 85%) was added and
the contents brought to volume with distilled water.
Standards;
To a series of nine 100-ml volumetric flasks
each containing 2 ml of mercury stabilizing solution, 0-0.8 ml aliquots of
sodium bisulfite stock solution were added in 0.1-ml increments. The solutions
were acidified with 11 ml phosporic acid and made up to volume with distilled
water. The resulting SCh concentrations ranged from 0 to 2.4 mg/L,
respectively.
Chromatographic Determination;
For
headspace analysis, a 50-ml test sample was placed in a 100-ml vial and sealed
with a butyl rubber septum. Equilibration was achieved by shaking the vial for
1 5 min on a wrist action shaker and standing 10 min at room temperature. The
headspace sample (5.0 ml) was withdrawn using a gas-tight syringe, and it was
injected into the gas chromatograph. The chromatography conditions were as
follows: instrument, Tracer with flame photometric detector; column, glass (2 m
X 4 mm i.d.) packed with Carbopak B HT 100, 60/80 mesh. Temperatures were
injector 100°C, detector 175°C, and column isothermal at 45° C; gas flow rates
were hydrogen 55 ml/min, air 100 ml/min, and nitrogen 50 ml/min.
Quantitation;
With the aid of a microcomputer statistical
software program, a second-order regression equation was obtained by plotting
the square root of the peak area (or peak height) against the SO2 concentration
of the standards. Using this equation, the free and total SO2 concentrations in
the treated test samples were calculated. Taking into account the dilutions in
preparation of the test samples, free and total SO: in the original beer sample
were determined.
3 West-Gaeke colorimetric
analysis
West-Geake
Method for SO2 showed no significant NH3 interference. Dasgupta et al. have
described another method that uses a 0.02% solution of formaldehyde at a pH of
4 to absorb and stablize SO2 as hydroxymethane sulfonate. A fluorometric
technique or a more conventional colorimetric technique can be used with this
latter SO2 method.
The
divices contains a rechargeable battery and a constant volume adjustable air
pump.
An air
sample is continuously drawn into the unit and any formaldehyde present is
scrubbed
with a sodium tetrachloromercurate solution that contains a fixed quantity of
sodium
sulfite. Acid bleached pararosaniline is added and the intensity of the
resultant
color is
measured at 550 nm by a colorimeter and displayed on a digital readout. A
recorder
output is
also provided and both formaldehyde in air and liquid samples can be analyzed.
4 Fluorescent Analyzers
Fluorescent
Analyzers is a popular method which is
conducted for SO2 determination. Several such commercial instruments are
avaliable for this method. Certain design of fluorescent SO2 analyzers have
interference from aromatic hydrocarbons and water vapor. Because of this
response to aromatic hydrocarbons, devices called cutters were added to these
fluorescent SO2 analyzers to reduce this interference. Smith and Buckman
examined the performance of these cutters and suggested that their test
procedure should be employed on a routine basis to verify effective cutter
operation. Interferences due to water are minimized by the use of discrete line
source or gas drying via Nafion permeation dryers.
Other SO2
methods include oxidation-induced chemiluminescence, ion chromatography and
ultraviolet (UV) absorption spectrometry.
