Air quality measurement are generally done by two methods:

a) Continuous automatic analyserb) Conventional laboratory analysis of discrete sample

Gaseous pollutant measurement procedure are generally classified as:

a) Involving chemical change: Pollutant being measuredundergoes chemical transformations and the product is thenanalysed with an appropriate analytical technique.

a) Physical principle: Physical properties of the gas areexploited such as the ability of the gas to absorb infraredradiation. Amount of radiation is then detected.

Wet chemical analysis:

Pollutant is absorbed in a liquid for a given period of timewhich is then treated with a reagent resulting in a change ofcolour or in the formation of another product.

The colour intensity or product concentration is related to theoriginal pollutant concentration.

Analysis of atmospheric samples

The methods used can be divided into three basic groups:

• Chemical Methods

• Instrumental Methods

• Biological Methods

• Chemical Methods

– Gravimetric Method

– Volumetric Method

– Colorimetric method

– Turbidimetric and nephelometric method

– Chromatographic method

(a) Gravimetric Method

• Weight of substance (Pollutant) is determined.

• Can be used for (1) Determination of gas (sensitivity: approx. 1 mg.)(2) Determination of dusts and soot collected in

sedimentation jars. (Sensitivity: approx. 1 mg.)(3) Determination of particulate matter dispersed in the air

(b) Volumetric Method

In this, components are not weighted but determine by means of reagents of known concentrations

1. Acidimetric or Alkalimetric Method

2. Oxidation and reduction method

3. Precipitation method: Suitable for halogens. (Silver nitrate solution of known strength, which precipitates the halogen in the form of silver halides.)

Acidimetric or Alkalimetric Method

This is for the determination of acids and bases using solution ofa base or an acid respectively in the presence of an indicatorwhich undergo a colour change according to acidity or alkalinityof the medium.

Oxidation/ Reduction:

These are based on the oxidation or reduction of the compoundto be analysed by means of standardised substance.

The amount of O2 given up or consumed being measuredaccording to colour change developed in the reaction in thepresence or absence of indicator.

(c) Colorimetric method

• A very large number of elements, radicals and organiccompounds form colored complexes with special reagentsand such colorimetric reactions are sensitive to detect eventhe most minute amount of the compound.

• Apparatus used: Filter Photometer

• Use for analysis of gas and particulate matter

(d) Turbidimetric and nephelometric method

Precipitate obtained by means of specific reaction areanalyzed by optical measurement of light passing through thecorresponding suspension

Advantages:• Suitable for gas and particulate matter

• Require only small air sample

• Highly suitable for determination of Cl2, SO3

(e) Chromatographic method

The separation and identification of many substances bymeans of adsorption on adsorption columns or on speciallysensitized paper, followed by elution of the variouscompounds adsorbed with selective solvents.

Advantages:Sensitivity is very high

Suitable for metals and voc

Instrumental Methods

• Emission Spectrometric method• Absorption spectrometric method (Spectrophotometry)• X ray diffraction method• Mass spectrometric method• Polarographic method• Method using microscopy• Refractometric method• Thermal conductivity method• Radioactivity method• Sound absorption method• Atomic absorption spectroscopy

Biological Methods

By studying effects on plant and animals

Sulphur Dioxide

Ambient Air Measurement

• The most common methods are Colorimetry, Iodimetry or turbidimetry.

• Method for determination of Sulphur Dioxide in Air: Modified West and Gaeke Method

Principle

• Sulphur dioxide from air is absorbed in a solution ofpotassium tetrachloromercurate (TCM) to form a stable andnonvolatile dichlorosulphitomercurate complex.

• Once formed, this complex is stable to strong oxidants suchas ozone and oxides of nitrogen and therefore, the absorbersolution may be stored for some time prior to analysis.

• The complex is made to react with pararosaniline andformaldehyde to form the intensely colored pararosanilinemethylsulphonic acid.

• The absorbance of the solution is measured by means of asuitable spectrophotometer.

• The color intensity is proportional to the conc. of SO2 over arange of 0.002 to 5 ppm.

Interferences

• Interferences by oxides of nitrogen are eliminated bysulphamic acid which destroy the nitrate ion prior to colorformation.

• Ozone is made to decompose by allowing the solution tostand for sometime prior to analysis.

• The interference of trace metals (lowering the apparent SO2conc.) may be eliminated by the addition of ethylene diaminetetra-acetic acid (EDTA) to the absorbing solution prior tosampling.

SO2 + Sodium tetrachloromercurate solution (TCM)

(Dichlorosulphito mercurate complex + Pararosaniline and formaldehyde)

Pararosaniline methyl sulphonic acid (magenta)

Automatic instruments for SO2 monitoring

Colorimetric Analyser:

• for measuring 0.1-10 ppm

• 2H2O + SO2 + Br2 SO4-2 + 4H+ + 2Br –

Bromocolourimetric analyser

• Air containing SO2 is drawn continuously through anelectrolytic cell which contains oxidised bromine solution andtwo set of electrodes as shown in figure.

• The indicator reference set of electrodes is used to detect thebromine concentration while other set comprising thegenerating electrodes and the auxiliary electrode is used togenerate any bromine necessary to maintain the properbalance.

Principle of Colorimetric analyser

Bromocolourimetric analyser

• SO2 in the air sample is oxidised by bromine causing areduction in bromine concentration according to the reaction

2H2O + SO2 + Br2 SO4-2 + 4H+ + 2Br -

• This reduction in bromine concentration change the oxidation-reduction potential of the reagent

• A voltage is developed between the indicator electrode andreference electrode. This voltage is compared to a referencevoltage.

• The difference between two voltages is sensed by the otherelectrode system causing an electric current to flow betweenthem and generating sufficient bromine to maintain theoriginal concentration according to the reaction.

2 Br - Br2 + 2e-

• This current flow necessary to maintain proper balance is ameasure of SO2 concentration in air stream.

Greiss – Saltzmann Method

• Most versatile method

• Uses N- (1-naphthyl) ethylene diamine dihydrochloride.

• Greiss- Saltzmann method is not suitable for the collection of24 hour samples in field and subsequent analysis in the labbecause the dye complex formed is fairly unstable.

• Hence more stable technique is required.

Nitrogen oxides

Nitrogen oxides

1) Colorimetric method (Greiss- Saltzmann):

• NO2 + sulphanilic acid diazonium salt coupled with N-ethylene diamine dihydrochloride pink coloured dye complex (15 min at room temperature) spectrophotometer (550nm)

• NO is oxidised to NO2 by passing through a Potassium Permanganate solution.

2) Jacobs- Hochheisser method:

• NO2 + 0.1 N NaOH nitrite ion reacted withsulphanilamide and n-ethylene diamine in phosphoric acid toform azo dye colorimetrically

3) Phenoldisulphonic acid method:

• Used for NOx between the range 15 to 1500 ppmNOx 3% H2O2 and 0.1 N sulphuric acidNitric acid phenol disulphonic acid (Ammoniumhydroxide) yellow compound

4) Chemiluminescent analyser:

• NO + O3 NO2+ + O2

NO2* NO2 + hv

• Low cost

• Used for NOx in the range 0.004 – 1000 ppm

Chemiluminescent Analyser

Carbon monoxide

1) Electrochemical analysis :

• The gas sample is passed over hot iodine pentoxide at1500 C.

I2O5 + 5CO 5CO2 + I2

• The liberated iodine is absorbed by an electrolyte and isreduced at the cathode of an amperometric cell by theelectrode reaction:

I2 + 2e- 2I-

• The current produced is a quantitative measure of the COcontent. Minimum detectable concentration is 0.1 ppm.

2) Ultraviolet mercury replacement method:

• It is based on reduction of mercuric oxide by CO at 210 0C toproduce CO2 and mercury vapour.CO + HgO (s) CO2 + Hg (g)

• The mercury vapour formed is determined through itsabsorption of radiation from a mercury lamp.

• The range of applicability of this method is from 0.025 to 10ppm of CO.

3) Non-dispersive infrared analyser

• Can detect CO concentrations from 1 ppm upto 25 to100 ppm depending on path length, operating pressures and electronic amplification.

• Interferences: Water vapour

• Non-absorbing gas is N2

Non-dispersive infrared analyser