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In this article we will discuss about the procedure for analysing the quality of air.
High Volume Sampler is a basic instrument used to monitoring ambient air quality. The sampler provides, in general, devices for basic measurement of air-borne particles, gaseous pollutants like SO2, NOx and CO. In the sampler the air-borne particles are measured by passing air at a high flow rate of 1-1.5 m3/min through highly efficient filter paper, which retain the particulates.
The instrument is also capable of measuring the volume of air sampled so that a particular amount of sampled air can be passed through suitable absorbing solutions of SO2 and NOx, which can be measured by chemical analysis.
Requirements:
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(a) High volume air sampler
(b) Whatman’s EMP 2000 filter paper
(c) Chemicals for absorption of SO2 and NOx
(d) Ice tray
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(e) Spectrophotometer
Procedure:
The instrument is set up in a suitable site at a height of above 1 meter. Before starting the operation an oven-dried pre-weighed filter paper is placed in the position. Then 10 ml of absorbing solution for SO2 and NOx is taken in separate impinger and marked properly.
The totalizer instrument is set properly and initial reading of airflow and time of starting noted. Set the flow rate 1-2 L/min for the gaseous sample collection and connect the impingers taking some ice in the impinger tray to avoid evaporation. Run the sampler about 8 hours.
(i) Measurement of Suspended Particulate Matter (SPM):
After a period of 8 hrs. sampling the SPM is measured by weighing the filter paper.
Calculation for SPM:
Weight of SPM (W) = W2 – W1 (gms)
W1 = Initial weight of filter paper, W2 = Final weight of filter paper
Volume of air sampled (V) = Q.T (m3)
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Q = Average sampling rate (m3/minutes), T = Sampling time (minutes)
Q = (Q1 – Q2)/2
Q1 = Initial sampling rate, Q2 = Final sampling rate
Concentration of SPM = W/V (gms/m3)
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(ii) Determination of Nitrogen Oxides:
Principle:
Nitrogen oxides as nitrogen dioxide collected by bubbling the air through a sodium hydroxide solution to form a stable solution of sodium nitrate. The nitrate ion produced during sampling is determined colorimetrically in a spectrophotometer at 540 nm by reacting the exposed absorbing reagent with phosphoric acid, sulphanilamide, and N (1-napthyl) ethylenediamine di-hydrochloride (NEDA).
Reagent:
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(a) Absorbing reagent:
Dissolve 4 g of sodium hydroxide in 1 lit. distilled water.
(b) Sulphanilamide:
Dissolve 20 g of sulphanilamide in 700 ml of distilled water. Add 50 ml of conc. Phosphoric acid and dilute to 1 lit.
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(c) NEDA solution:
Dissolve 0.5 g of NEDA in distilled water.
(d) Hydrogen peroxide:
Dilute 0.2 ml of 30% H2O2 to 250 ml with distilled water.
(e) Standard nitrite solution:
Dissolve sufficient desiccated sodium nitrite in 1,000 ml distilled water so that the solution contains 1,000 µg Nitrogen dioxide/ml.
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Analysis:
Replace any water lost by evaporation during sampling by adding distilled water. Take 10 ml of collected sample; add 1 ml of H2O2 solution, 10 ml of sulphanilamide solution and 1.4 ml of NEDA solution with thorough mixing after addition of each reagent.
Prepare a blank in the same manner using 10 ml of absorbing reagent. After 10 minutes take the absorbance at 540 nm against the blank. Determine the concentration of Nitrogen dioxide from standard curve.
Calibration Curve:
Dilute 5 ml of the standard nitrite solution to 200 ml with absorbing reagent. This solution contains 25 µg Nitrogen dioxide/ml. Pipette 1, 2, 5 and 15 ml of the 25 µg Nitrogen dioxide/ml solution into 50, 50, 100 and, 250 ml volumetric flask and dilute to mark with absorbing reagent.
The solutions contain 0, 50, 1.0, 1.25 and 1, 5 µg Nitrogen dioxide/ml, respectively. Run standards as instructed. Plot absorbance against conc. of Nitrogen dioxide.
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Calculation:
Vol. of air sampled (V) = (F1 + F2) × T × 10-3/2
where, V = Vol of air sampled, m3, F1 = Flow rate before sampling, L/min
F2 = Flow rate after sampling, L/min, T = Time of sampling, min
Conc. of Nitrogen dioxide in µg/ m3 = (fig Nitrogen dioxide/ml) × 10 / (V × 0.35)
where, 10 = Vol. of absorbing reagent used in sampling
V = Vol. of air sampled
0.35 = overall average efficiency.
(iii) Determination of Sulphur Dioxide (West and Gaeke Method):
Principle:
When sulphur dioxide from the air stream is absorbed in a sodium tetra-chloro-mercurate solution it forms a stable di-chloro-sulphi-to-mercurate. The amount of sulphur dioxide is then estimated by the colour produced when p-rosaniline hydrochloride is added to the solution. The colour is estimated by reading from a spectrophotometer at 560 nm for which a calibration curve has already been prepared.
Reagents:
Absorbing solution:
(a) 0.1 M sodium tetrachloromercurate. Dissolve 27.2 g mercuric chloride and 11.7 g sodium chloride in 1 L of distilled water.
(b) P-rosaniline hydrochloride:
0.04 percent (w/v), acid bleached. Dissolve 0.2 g of p-rosaniline hydrochloride in 100 ml of distilled water and filter the solution after 48 hrs. This solution is stable for at least 3 months if stored in dark and kept cool. Pipette 20 ml of this into a 100 ml volumetric flask. Add 6 ml of conc. HCl, Allow standing for 5 minutes, then dilute to mark with distilled water. This solution should be pale yellow with a greenish tint.
(c) Formaldehyde solution 0.2%, Dilute 5 ml of 40% formaldehyde solution to 1 L with distilled water.
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(d) Sulfamic acid:
Dissolve 0.6 g of sulfamic acid in 100 ml distilled water.
(e) Standard solution for sulphur-dioxide:
Dissolve 640 mg of Sodium meta-bi-sulphate (assay 65.5% as SO2) in 1 L of water. This yields a solution of approximately 0.4 mg/ml as SO2. The solution should be standardized by titration with standard 0.01 N iodine with starch as an indicator. Then 1 ml is equivalent to 150 ml of SO2. Prepare and standardize freshly.
(f) Iodine Solution:
0.01 N, freshly standardised.
(g) Starch Solution:
0.25 g of starch dissolve in 100 ml boiling distilled water.
Preparation of Calibration Curve:
Take exactly 2 ml of standard sulphite solution into a 100 ml volumetric flask and dilute to mark with absorbing reagent. This final solution contains 3.0 µl of SO3 per milliliter.
Add accurately 0.5, 1, 1.5, and 2 ml portion of the standard sulphite solution to a series of 10 ml volumetric flask and dilute to mark with absorbing solution. Continue the analysis procedure given below. Plot a standard curve taking absorbance against the concentration of SO2 in a graph paper.
Analysis:
Take the sample after 8 hrs sampling, adjusted to 10 ml with distilled water to compensate for evaporation losses. Add 1 ml of sulphamic acid, 2 ml of p-rosaniline solution and 2 ml of formaldehyde solution and mix well. Treat a 10 ml portion of unexposed absorbing solution in the same manner for use as blank. After 20 minutes read the absorbance at 560 nm in a spectrophotometer with the blank as reference.
Calculation:
Concentration of SO2 (ng/m3) = µg of SO2 × 103/vol. of air sampled (m3)
