The procedure for analysis of sulphur dioxide, sulphation rate, hydrogen sulphide, oxides of nitrogen, oxidants, fluorides, and carbon monoxide are explained below. These methods have been adopted by the Bureau of Indian Standards, New Delhi (India), which are given in IS: 5182. (They are reproduced briefly with kind permission of Bureau of Indian standards).
Estimation of Sulphur Dioxide from Atmospheric Air:
Theory and Principles:
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Even though several methods available for the estimating of sulphur dioxide in air, following two methods are popular:
1. Hydrogen peroxide method, which is simple to use and normally operate over periods of 12 to 72 hours. The principle involved in this method is the measurement of acidity increase due to sulphuric acid, which is estimated by titration using an indicator. When air (containing sulphur dioxide) is passed through a dilute hydrogen peroxide at pH 5, then sulphur dioxide in air is absorbed and oxidised to sulphuric acid.
2. Sodium tetra chloromecurate method, which is less simple to operate than hydrogen peroxide method. But, it is suitable for short- term sampling (5 minutes to 6 hours). The principle involved is that, when (air containing sulphur dioxide) is passed through absorber solution of sodium tetra chloromecurate. It forms a stable dichlorosulphito mecurate.
If p-rosaniline hydrochloride is added, a colour is produced and the intensity of colour is proportional to the sulphur dioxide present in air. The colour is estimated by using spectrophotometer and the values are reported to the nearest 0.005 ppm at concentration below 0.15 ppm and to the nearest to 0.01 ppm above 1.5 ppm.
If ozone and nitrogen dioxide present in air with concentrations more than sulphur dioxide, they interfere in estimation of sulphur dioxide. Hence interferences of nitrogen dioxide is eliminated by using 0.06 % sulphamic acid in the absorbing reagent. Heavy metals especially iron salts also interfere in oxidizing dichloro-sulphito mercurate during sample collection. So, interference is eliminated by adding ethylene diamine tetra acetic acid in the absorbing reagent.
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Arrange a sampling train which consists of absorbers, trap to protect flow device, flow control and metering devices, temperature and vacuum gauges, and air pump. They are arranged in the same order. The suction pump and gas meter should be capable of drawing 1.5 lpm. The gas meter should be protected be a moisture trap which contain proper desiccant.
Calibration Curve Preparation:
A calibration curve is prepared by using 0.5, 1.0, 1.5, and 2.0ml of standard sulphite solution into a 100 ml volumetric flask and dilute to mark with absorbing reagent. The final solution in the flask contains 0.75, 1.5, 2.25, 3.0 microlitres of sulphur dioxide per millilitres. Plot the absorbance (optical density) as the ordinate against the microliters of sulphur dioxide per 10 ml of absorbance solution on a rectangular co-ordinate paper and draw best fitting straight line.
Procedure:
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First record the gas meter reading. Pipette exactly 10 ml of absorbing reagent into the absorber. Aspirate the air sample through the absorber at a rate of 0.2 to 2.5 litres per minute. After sampling period (a few minutes to 24 hrs.), the sample contain 2 to 4 microliter of sulphur dioxide in 10 ml of absorbing reagent. Stop the pump and read the final reading of the gas meter.
If necessary, filter the sample to get clarity and make up with distilled water up to 10 ml mark. Add 1.0 ml of p-rosaniline solution and 1.0 ml of the formaldehyde solution and mix well. After 20 minutes, find the absorbance at 560 nm wavelength in a spectrophotometer and read the value from the calibration chart.
Calculation and Results:
The air is sampled at ambient conditions. Hence, the volume of air is corrected to Standard Temperature and Pressure (STP) conditions, which is as follows –
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Based on Boyle’s and Charle’s law,
P1· (V1/T1) = P2 · (V2/T2) (4.3)
Where,
P = Pressure; V = Volume and T = Temperature
Suffixes 1 and 2 are the two different conditions.
Similarly, the volume of air at STP (25 °C and 760 mm Hg) is obtained as follows –
Estimation of Sulphation Rate from Atmospheric Air:
The measurement is made by using lead peroxide and it reacts with sulphur dioxide gives sulphur trioxide. The lead peroxide is pasted over the surface area (100 square centimeter only) of a plastic or glass tube candle. After drying, it is kept at the observation site for a period of 30 days. The sulphation rate is estimated by prescribed gravimetric method. The results are expressed as sulphation rate, as mg of SO2/100 cm2/day.
Estimation of Hydrogen Sulphide from Atmospheric Air:
Theory and principles – The hydrogen sulphide in air is estimated by colorimetric method. The principle involved is that the hydrogen sulphide reacts with N, N-dimethyl-p-phenylene diamine, sulphate ferric sulphate and sulphide ions and produce a methylene blue colour. The intensity of colour is proportional to the hydrogen sulphide present in air.
The values are reported in the range of 6 to 600 micrograms per cubic meter. Reducing substances like sulphite and thiosulphate prevent the formation of colour. To eliminate them, sufficient amount of ferric sulphate is added. Oxides of nitrogen also interfere, which is eliminated by adding sulphamic acid after sampling.
Sample Collection:
Using the sampling train equipment, the sample of air (containing hydrogen sulphide) is collected into an impinger which contain 20 ml of absorbing solution at a rate of 1 lpm for 30 min or 0.5 lpm for one hour. After the sampling, cover the impinger with aluminium foil to avoid the exposure to atmosphere.
Preparation of Calibration Curve:
Take 11 tubes of having capacity of 50 ml and filled with 15 ml absorbing solution. Add dilute sodium sulphide to each tube, which is sufficient to maintain 1 to 25 micrograms. Also add 1 ml of sulphamic acid, 0.6 ml of N, N-dimethyl- p-phenylene-diamine sulphate solution and 0.05 ml of ferric sulphate solution to each tube and shake them well.
Make up the tubes upto 20 ml with distilled water and mix thoroughly. Wait 30 min for colour development and measure the intensity of colour in a spectrophotometer at 670 nm on transmission scale. Draw the curve based on to transmission values versus micrograms of hydrogen sulphide.
The same procedure for calibration curve is adopted for sample. The colour of the sample is measured with spectrophotometer and find the micrograms of hydrogen sulphide from calibration chart and its value is expressed as –
Estimation of Oxides of Nitrogen in Atmospheric Air:
Theory and Principles:
The principle involved in this method is the collection of nitrogen dioxide by bubbling the air (which contain nitrogen dioxide) through an absorber (sodium hydroxide) solution. It gives a stable sodium nitrite solution. Such nitrite ion produced is made to react with phosphoric acid, sulphanilamide, and N (1- naphtyl) ethylene diamine dihydrochloride, which produce colour. This colour is measured colorimetrically by using spectrophotometer at 540 nm wavelength. The intensity of colour is proportional to the nitrogen dioxide present in the sampled air.
The interference caused by sulphur dioxide is eliminated by treating with hydrogen peroxide which converts sulphur dioxide to sulphuric acid.
Preparation of Calibration Curve:
Prepare solution containing 25 microgram of nitrogen dioxide per ml of absorbent reagent.
Pipette 1, 2, 5, and 15 ml from that solution into 50, 100, and 250 volumetric flasks and dilute to mark with absorbent reagent, which contains 0.5, 1.0, 1.25, and 1.5 micrograms of nitrogen dioxide per ml. After gaining colour, measure absorbance at 540 nm wavelength of spectrophotometer and plot absorbance versus micrograms of nitrogen dioxide per ml.
Procedure:
By using the sampling train, the sample is collected in a 50 ml of absorber reagent which is kept in an impinger. 10 ml of collected sample is put into a test-tube and add 1 ml of hydrogen peroxide solution and 10 ml of sulphanilamide and 1.4 ml of NEDA solution.
In each addition of these reagents, through mixing is done. After 10 minutes colour will be developed and that colour is measured in terms of absorbance at 540 nm wavelength of spectrophotometer. The values are read as micrograms of nitrogen dioxide per ml from calibration curve using absorbance values.
The results are expressed as follows:
Estimation of Oxidants in Atmospheric Air:
Theory and Principles:
The principle involved in estimation of oxidants (low concentration of ozone plus nitrogen dioxide, chlorine, peroxy-acetyl nitrate) in air is that the oxidants are made to absorb in potassium iodide solution buffered to a pH of 6.8. Ozone and other organic oxidants liberate iodine, and form trioxide. It produces colour, which is determined by spectrophotometer by measuring absorbance of trioxide ion at 352 nm wave length. Not only ozone but also many oxidizing substances like chlorine, nitrogen dioxide, PAN will liberate iodine.
Reducing gases like sulphur dioxide also interfere negatively (increase the value). Nitrogen dioxide is estimated to account the interference. The interference of sulphur dioxide is very high, which is eliminated by using a chromic acid paper absorber in the upstream of absorber of sampling train. This method may be used to determine oxidants concentrations in the atmosphere in the range of 0.01 and 10 ppm as ozone.
Collection of Sample:
The oxidants containing air is sampled by using sampling train and also by placing chromium trioxide paper on the upstream of absorber. A 10 ml of absorbing solution is placed in an impinger and sampled the air at a rate of 0.5 to 3.0 lpm for 30 min. It gives 2 micrograms of ozone by absorbing solution at an atmospheric concentration of 0.01 ppm.
Procedure:
Prepare 0.1 to 1.0 ml of 0.0025 N iodine solution in 25 ml volumetric flasks. Dilute to the mark with absorbing solution and mix thoroughly. After 30 min, colour will be developed and its absorbance is measured by using the spectrophotometer at 320 nm wave length. A calibration curve is drawn between absorbance and iodine solution. Similarly % absorbance of the sample is read from the calibration chart.
The results are expressed as –
Estimation of Total Fluorides in Atmospheric Air:
Theory and principles – Fluorides may be found in air as gas or as particulate matter, which are discharged into the atmosphere through the gaseous effluents of industries like fertilizer, aluminium which use fluoride containing compounds in the manufacturing processess.
The Bureau of Indian Standards recommended following two methods for the determination of total fluorides in ambient air.
1. Zirconium SPANDS method
2. Selective Ion Electrode method
1. Zirconium SPANDS Method:
In this method, the fluorides present in air react with metal ion of metal dye complex like Zirconium SPANDS reagent. This reaction will fade the absorbance of the dye solution which is directly proportional to the fluorides present in air. Zirconium SPANDS reagent obey’s Beer’s law over the range from 0-1.4 micrograms of fluorides per ml, with a detection limit of 0.02 micrograms per ml. Substances like aluminium, iron, phosphate and sulphate ions are causing interference. Hence they have to be eliminated.
Preparation of Calibration Curve:
Prepare a series of fluoride containing solution ranging from 0.1 to 35 micrograms are placed into 25 micro ml volumetric flasks. Add 5 ml of Zirconium-SPANDS reagent to each flask make up to mark and mix the contents thoroughly. Wait for 30 min for colour gaining and measure the colour in terms of absorbances by using spectrophotometer at 570 nm. Draw a curve between fluoride concentration in micrograms and absorbance values.
Procedure:
The procedure used for calibration curve may be used for the sample also to get absorbance value.
Results:
The concentration of fluorides of the sample are calculated as follows:
Estimation of Carbon Monoxide in Atmospheric Air:
Theory and Principles:
In the literature several methods are available for the estimation of carbon monoxide in air. In the polluted atmosphere, the carbon monoxide concentration would be around 1 to 100 ppm. No method is reliable to estimate CO for this range of values. Each method is reliable for a certain range of values. Hence, depending on the range of values required, proper method is selected to get accurate values.
High values of CO is determined by volumetric gas analysis method, in which CO is oxidised to carbon dioxide with hot copper oxide or cuprous salts. Low values are determined by colorimetric methods. But oxidation with iodine pentoxide and colourimetric method by using palladous salts, or ammonium molybdate are widely used methods. However, now a days, Non-dispersive infrared Absorption (NDIA) method and gas chromatography methods are preferred for accurate results.
Bureau of Indian Standards recommends four methods. They are as follows:
1. The Indicator tube method is used for routine conditions. CO present in air reduces silicomolybdate (yellow in colour) to lower oxidizing power, which changes yellow to green and green to blue, depending on CO present in air. Interfering substances like hydrogen sulphide, unsaturate hydrocarbons, etc. may present in air, which are eliminated by providing absorbents in the tubes. This processes is carried by drawing 250 ml of sample of air at a rate of 40 to 50 ml per minute.
2. Iodine pentoxide method
3. Non-dispersive Absorption method, and
4. Gas chromatographic method
Indian Standard for measurement of carbon monoxide in atmospheric air.
The last two methods are briefly given below:
NDIA Method:
The air sample containing CO is made to pass through the sources of infrared energy and detectors, which are separated by optical cell, where the specific spectral absorption of the component of CO is determined. This arrangement is made in an equipment called ‘Infrared Gas Analyser’.
This apparatus is suitable for temperature range of 15-40 °C and a relative humidity range of 0 to 99%. The minimum volume of air sample is 21 and it is collected by sampling train. The results are expressed in units of concentration, which are read from the standard scale.
Gas Chromatography Method:
The sample of air containing CO is injected into the gas chromatograph. It is arranged to move one column to other column end. During its movement, the components of air sample are identified by plotting distribution curves and distinct peaks to each gas.
Based on peak, the quantity of CO may be estimated. The results are expressed and estimated by comparing peak areas of the sample with peak areas of known amount of constituent of gas chromatograph. The lower detection limit of this method is 10 ppm and the sample size should be 10 ml.