This article throws light upon the top fourteen techniques used for monitoring pollutants. Some of the techniques are:- 1. Digital Titration System 2. Digital Radiometer 3. Pulsed Fluorescence Technique 4. Paper Tape Analyser 5. Chemical Sensing Electrodes 6. A Mercury Substitution Ultraviolet Absorption Analyzer 7. Correlation Spectroscopy 8. Laser Techniques and Others.
Technique # 1. Digital Titration System:
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Is one button operated, microcomputer controlled system for automatic potentiometer titrations.
Technique # 2. Digital Radiometer:
Is titration system used for all types of titrations — Acid-base, redox, etc. We have Radiometer model A/S, from 72 Emdrup vef, DK- 2400 Copenhagen NV, Denmark.
Technique # 3. 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 through which a narrow band filter impinges upon the photo multiplier tube.
The emitted light is linearly proportional to concentration of SO2 molecules in the sample. The signal is amplified and recorded by the recorder. Now-a-days digital pulsed fluorescence techniques give the direct results.
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The techniques 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.
Technique # 4. Paper Tape Analyser:
This is a technique used for the monitoring of SO2, NOx or CO based a chemical reaction which takes place on test paper which has been impregnated with suitable chemicals to obtain specificity for the concerned pollutants.
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 a easy technique and is used in mobile labs for monitoring gas parameter.
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Technique # 5. Chemical Sensing Electrodes:
The technique is used for monitoring of SO2, NOx and CO. Here a known volume of air is sampled with a pH buffered absorbing solution. The solution containing 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 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.
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Technique # 6. A Mercury Substitution Ultraviolet Absorption Analyzer:
The principle of UV — Fluorescence analyzer for SO2 monitoring is based on the measurement of intensity of the fluorescence in the ultraviolet light of SO2 which is excited by Zn 213.8 nm or Cd 228 nm line.
This is based on the generation of mercury vapour from mercuric oxide after reacting with CO. The generated vapour is detected by UV absorption — H analyzer.
Technique # 7. Correlation Spectroscopy:
The technique is generally used for the monitoring of NOx and 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 photo multiplier tube will observe a minimum when the mask is shifted off.
The difference in the light intensities seen by the photo multiplier is a measure of the SO2 or NO2 concentration between the light sources and the instrument. Artificial sources like Quartz Iodine or Zenon lamp with a defined distance is used. This technique is used in open labs for the analysis of air.
Technique # 8. 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 of air pollutants. Here sample is excited by an intense monochromatic light source such as laser and frequency is analaysed with a grating monochromator. The detection system uses a highly sensitive photon-counting technique.
Technique # 9. Non-Depressive – UV Visible Absorption:
This technique is mainly utilized for the monitoring of various oxidants present in environment. The ultraviolet region is important for analysis.
Technique # 10. A Chemiluminescent System for O3 and NOx:
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- absorbed 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 is combined with Rhodamin — B to avoid the effect of moisture and extend the disc life and permits continuous monitoring of O3. It can be used in Range of 10 ppb to 3.5 ppm.
The technique has also been successful for the detection of nitric oxides and NOx. The chemiluminescent reaction of nitric oxide and ozone is used as given here.
NO + O3 = NO2* + O2
NO2*= NO2 + hv-Detected by photomultiplier tube.
A pulsed ozone generator gives directly AC Signal (proportional to NO concentration) which can be easily amplified. This is unaffected by interference from SO2, H2O, CO, CO2 and HC and so the technique can be used in presence of SO2and H2O.
Technique # 11. Non-Dispersive Infra-Red Photometric System for CO:
Carbon monoxide as an air contaminant is uniquely suited to this method of analysis, as its absorption characteristic and typical concentration makes possible direct sampling.
A typical analyzer consists of a sampling system, two infrared sources, sample and references gas cells, detector, control unit and amplified 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 frequencies 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.
These unequal amounts of residual infrared energy reaching the two compartments of the detector cell cause 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.
Technique # 12. Conductometric 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 an increase in conductance is caused by ions formed as SO2 combines with the solution. The two kinds of solutions used are deionised by distilled water and dilute acidified hydrogen peroxide solution.
Technique # 13. Suspended Particulate Monitoring Methods:
The term ‘suspended particulate mater’ refers to particulates less than 20 microns (µm) in size, which can remain suspended in air for significant periods of time, ranging from a few minutes for the larger particles through to several days for very fine material (ca.< 0.1 µm).
These particles can effect visual air quality and can have effects on human health. Traditionally this material was measured by sucking air through a filter and determining the weight of dust collected. The equipment used was known as a High-Volume Air Sampler, and collected all particles below 20 µm plus a proportion of larger particles as well. The results were referred to as total suspended particulate (TSP).
In more recent times the equipment has been modified to collect only particulates below 10 µm, which are the ones most likely to be inhaled and therefore have an effect on respiratory health. This measurement is known as inhalable particulate, or PM-10.
There are four methods currently being used in New Zealand for the measurement of PM-10. The most common is the High-Volume Air Sampler, fitted with a size selective inlet. Other system is in use are the β-attenuation tape sampler and the Tapered-Element Oscillating Microbalance (TEOM).
The High Volume Air Sampler operates by drawing air at a rate of about 1.5 m3/min through a 25 cm × 20 cm glass fibre filter, which is weighed before and after sampling under conditions of constant humidity. Samples are normally collected over 24 hours.
The β-attenuation unit operates by drawing air at a rate of 15 to 20 litre per minute through a continuous glass-fibre or Teflon tape. A source of β-particles is used to sense the build-up of particles on the tape by changes in the amount of absorption. Measurements are normally averaged over one hour to obtain sufficient sensitivity, and the tape is advanced either at the end of each cycle or some other pre-set interval.
In the TEOM monitor air is drawn through a filter which is attached to a sensitive oscillating microbalance. Changes in the frequency of oscillation are directly related to the mass of material on the filter, and this is computed electronically once every few minutes. The sampling rate is 16.7 litre per minute and the unit operates continuously. The micro-filters need to change every 1 to 4 weeks depending on particles loadings.
Technique # 14. Galvanic Analyzer:
A variety of electrochemical procedures has been developed for O3 and oxidants, such as the amperometric-coulometric and the galvanic detection methods. In the former a platinum wire helix electrode is wetted by an iodine solution, circulated by a pump.
The iodine set free is continuously reduced by an applied cathodic potential and the current flowing between the cathode and the wire loop anode is assumed to be a linear function of the ozone concentration.
A scheme of the latter is shown in Figure 7. Air is divided into two equal streams, bubbling into a recalculating neutral buffered electrolyte (bromide with traces of iodide). By selective removal of reducing agent in both sections and ozone in only one section, the differential galvanic current required to reduce galvanically the halogen set free at a platinum electrode is measured.
Besides instruments of the first generation the development of a second-generation instrument to monitor ambient level of ozone is worth mentioning. A set-up similar to the one described for nitrogen oxides can be used by reversing the roles of O3 and NO or by replacing nitrogen oxide with an olefin.
Alternately, a highly sensitive and specific procedure can be realized by making use of the chemiluminescent reaction between ozone and rhodamine B. A disc coated with this dye and Gallic acid in the presence of ozone yields a luminescence detected by a phototube and the current generated is directly related to the mass of ozone per unit time flowing over the dye.
The determination of oxidants and their precursors in the atmosphere, which are believed to be the nitrogen oxides and certain organic compounds, namely unsaturated hydrocarbons, is becoming of increasing importance as the concentration of oxidants can be taken as measurement of photo-chemical smog, i.e. the smog which occurs under oxidizing condition.
This type of pollution, well recognized in certain areas, is a matter of great concern and might have a definite effect upon the air quality.