This article throws light upon the ten instrumental techniques used for monitoring industrial pollutants. Some of the techniques are: 1. A Chemiluminescent System for O3 and NOx 2. Non-Dispersive Infra Red Photometric System for Co 3. Conduct Metric Analyser 4. Paper Tape Analyser 5. Pulsed Fluorescence Technique 6. Chemical Sensing Electrodes 7. A Mercury Substitution Ultraviolet Absorption Analyser and Others.
Techniques for Monitoring Industrial Pollutants:
- A Chemiluminescent System for O3 and NOx
- Non-Dispersive Infra Red Photometric System for Co
- Conduct Metric Analyser
- Paper Tape Analyser
- Pulsed Fluorescence Technique
- Chemical Sensing Electrodes
- A Mercury Substitution Ultraviolet Absorption Analyser
- Correlation Spectroscopy
- Laser Techniques
- Non-Dispersive—UV—Visible Absorption Technique
Instrumental Technique # 1. A Chemiluminescent System for O3 and NOx:
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It is sensitive and specific system for determining ambient levels of O3 and is based on chemiluminescent reaction between O3 and disc coated with Rhodamin B-adsorbed on silica gel. The total resultant emission is detected by phototube.
The resulting current is directly related to the mass of O3 glowing over the dye in unit time. Silicon resin is combined with Rhodamin—B to avoid the effect of moisture and extends the disc life and permits continuous monitoring of O3. It can be used in the Range 10 ppb to 3.5 ppm.
The technique has also been used successfully for the detection of nitric oxides and NOx. The chemiluminescent reaction of nitric oxide and ozone is used as given here.
NO + O3 = N02* + 02
N02* = N02 + hv— Detected by photomultiplier tube.
A pulsed ozone generator gives directly AC signal (proportional to NO concentration) which can be amplified. This is unaffected by interference from SO2, H2O, CO, CO2 and HC and so the technique can be used in presence of SO2 and H2O.
Instrumental Technique # 2. Non-Dispersive Infra Red Photometric System for Co:
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Carbon monoxide as an air contaminant is uniquely suited to this method of analysis, as its absorption characteristics and typical concentration make possible direct sampling.
A typical analyser consists of a sampling system-two infra red sources, sample and references gas cells, detector, control unit and amplifier along with recorder. The reference cell contains a non-infra red absorbing gas, while the sample cell is continuously flushed with the sample atmosphere.
The detector system consists of a 2 compartment gas cell (both filled with carbon monoxide under pressure) separated by a diaphragm whose movement causes a change of electrical capacitance in an external circuit, and ultimately gives an amplified electrical signal which is detected by the detector.
In the analysis part, the reference and sample cell is exposed to the infrared sources. At the frequency imposed by the chopper, a constant amount of infrared energy passes through the reference cell to one compartment of the detector cell, while a varying amount of infrared energy, inversely proportional to the carbon monoxide concentration in the sample cell, reaches the other detector cell compartment.
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These unequal amounts of residual infrared energies reaching the two compartments of the detector cell cause unequal expansion of the detector gas. This unequal expansion causes variation in the detector cell diaphragm movement resulting in the electrical signal described above. It is a simple technique used for the detection of CO directly by the digital instrument.
Instrumental Technique # 3. Conduct Metric Analyser:
This technique measures the conductance of an absorbing solution into which SO2 from the sample has been dissolved by contact of the solution with the sample. As we know that an increase in conductance is caused by ions formed as SO2 combines with the solution.
The two kinds of solutions used are demonized or distilled water and dilute acidified hydrogen peroxide solution. The observed increased in the conductivity is proportional to the SO2 concentration in the air if there are no interferences. So with the help of digital instrument, we can read directly the SO2 concentration.
Instrumental Technique # 4. Paper Tape Analyser:
This is a technique used for the monitoring of SO2, NOx or CO based on a chemical reaction which takes place on test paper which has been impregnated with suitable chemicals to obtain specificity for the concern pollutant.
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The result of this reaction is coloured stain which is monitored photo-electrically. The test paper is in form of a continuous motor driven reel of paper tape which allows continuous monitoring. This is an easy technique and is used in mobile labs for monitoring gas parameters.
Instrumental Technique # 5. Pulsed Fluorescence Technique:
This is a technique used for the monitoring of SO2 and H2S from air samples. A given gas sample is placed to a source of pulsed ultraviolet (UV) through a monochromatic filter. The SO2 molecules energized by the high intensity pulsed light source emit an SO2 specific illumination which through a narrow band filter impinges upon the photomultiplier tube.
The emitted light is linearly proportional to concentration of SO^ molecules in the sample. The signal is amplified and recorded by the recorder. Now-a-days digital pulsed fluorescence technique gives the direct results.
The technique can also be used successfully for H2S monitoring. In this, the sample is first scrubbed to remove the SO2 content and then passed through a converter which converts H2S molecules into SO2 which is measured directly by the digital arrangement.
Instrumental Technique # 6. Chemical Sensing Electrodes:
The technique is used for monitoring SO2, NOx and CO. Here a known volume of air is sampled with a pH buffered absorbing solution. The solution containing the dissolved gas pollutant (SO2 or NOx) then passes to an ion selective electrode where the ion concentration proportional to the pollutant concentration is measured potentiometrically.
Electro-chemical cell analysers avoid the use of wet chemistry of traditional conductometric, colorimetric and amperometric analysers by using sealed modules-the electro-chemical cells inside which all chemical reactions occur.
The gas pollutant to be detected diffuses through a semi-permeable membrane into the cell. The rate of diffusion and hence cell current is proportional to the pollutant concentration and hence the concentration of an unknown species is measured within no time.
Instrumental Technique # 7. A Mercury Substitution Ultraviolet Absorption Analyser:
The principle of UV-Fluorescence analyser for SO2 monitoring is based on the measurement of intensity of the fluorescence in the ultraviolet of SO2 which is excited by Zn 213.8 nm or Cd 223.8 nm line. This is based on the generation mercury vapour from mercuric oxid after reacting with CO. The generated vapour is detected by UV absorption—Hg analyser.
Instrumental Technique # 8. Correlation Spectroscopy:
The technique is generally used for the monitoring SO2 from air. Is this technique, we use either skylight or artificial light for the measurement of SO2 or NOx. A correlation spectrometer for remote sensing collects skylight by a telescope which is then collimated and dispersed by a prism or grating and focused into a correlation mask.
The patterns of the mask are formed by depositing aluminium on glass and then removing slits of aluminium corresponding to absorption lines of the incident spectrum, then the photomultiplier tube will observe a minimum when the mask is shifted off.
The difference in the light intensities seen by the photomultiplier is a measure of the SO3 or NO2 concentration between the light source and the instrument. Artificial source like Quartz Iodine or Zenon lamp with a defined distance is used. This technique is used in open labs for the analysis of air samples.
Instrumental Technique # 9. Laser Techniques:
These techniques are mainly used for the remote monitoring of air pollutants. Raman scattering and resonance scattering have a great promise for monitoring air pollutants. Here sample is ‘excited’ by an intense monochromatic light source such as laser and frequency is analysed with a grating monochromator. The detection system uses a highly sensitive photon-counting technique.
Instrumental Technique # 10. Non-Dispersive—UV—Visible Absorption Technique:
This technique is mainly utilized for the monitoring various oxidants present in environment. The ultraviolet region is important for qualitative and quantitative determination of organic compounds.
The common terms are chromophore, auxochrome, bathochromic effect and hypsochromic effect.
The first two terms apply to wavelength region in 200 nm and above. Chromophores can be defined as functional groups which absorb near ultraviolet or visible radiation. Chromophores have unsaturated bonds. Auxochromes are functional groups such as —NH2, —CI and —OH which have non-bonding valence electrons and do not absorb radiation at wavelength > 200 nm.
The bathochromic effect is a effect when auxochrome is attached to a chromophore due to which absorption band shifts to longer wavelength. The hypsochromic effect is a shift of absorption band to shorter wavelength.
Solvent:
The solvents used in ultraviolet spectroscopy are water, methanol, ethanol, carbon tetra-chloride and chloroform. Generally 95% ethanol is used. The solvent should be pure otherwise impurities may give absorption in the range of sample and likely to chance for erratic results.
Apparatus and Working:
The apparatus consists of:
(1) Radiation source;
(2) Monochromators;
(3) Sample cells;
(4) Detectors and
(5) Recording system.
Generally, mercury arc, hydrogen or deuterium lamps are used. In all sources, excitation is done by passing electrons through a gas and the collisions between electrons and gas molecules give vibrational, rotational and electronic excitation in gas molecule. If pressure is high, we get continuous band spectra but if pressure is low only line spectra is obtained.
The radiation from the source passes via mirror to the monochromator (which allows only a narrow range of wavelengths to pass through an exit slit) and then to rotating sector.
This rotating sector divides beam into two parts—one for passing through sample cell and another for passing through reference. The light beam after passing through reference cell and sample cell is focused onto the detector which is connected with amplifier to give any change in transmission.
The amplifier transmits the signals to the recorder which is followed by movement of pen on chart Thus, finally we get a spectrum.
Applications:
(1) Qualitative Analysis:
Identification of compounds can be done by comparing absorption spectrum with known compounds. A curve is plotted between wavelengths (A.) and degree of absorption (e). A typical spectrum of benzene is given below:
(2) Quantitative Analysis:
The technique is also used for quantitative analysis of compounds.
This is based on Beer’s Law:
A = log IO/It = £cI
where e = extinction coefficient It is independent of concentration of absorbing species,
c = concentration and
I = length of the cell used in UV spectrophotometer.
In the determination of concentration of an unknown compound, wavelength of maximum absorption for compound is selected. Then optical densities are measured for some known compounds. Now the optical density is plotted against concentration of solute over a range of concentrations. A straight line is obtained and form this graph, the concentration of unknown is evaluated.
(3) Chemical Kinetics:
The technique is helpful for studying the kinetics of a reaction. Here change in concentration of either a reactant or product with time is measured. An absorbance is proportional to concentration hence technique can be used to follow the course of reaction.
(4) Molecular Weight Determination:
Molecular weights of compounds can easily be determined by this technique.