Measurement of SO2 Concentration in Ambient Air

 Measurement of SO2 Concentration in Ambient Air

        ô PURPOSE:

                    To describe the Laboratory standard operating procedure for measurement of ambient SO₂ concentration.

                     

ô SCOPE:

                    Applicable to the Laboratory for analysis of samples where measurement of ambient SO₂ concentration is required.

                     

ô RESPONSIBILITY:

Lab chemist	:	Analysis of sample for measurement of ambient SO2 concentration.
Technical Manager	:	Review of activity
Quality Manager	:	Implementation and compliance of SOP

 

ô PROCEDURE:

1.     Principle:

·   Sulphur dioxide is absorbed from air in a solution of potassium tetrachloromercurate (TCM).A dichlorosulphitomercurate complex which resists oxidation by the oxygen in air, is formed. This complex is stable to strong oxidants such as ozone and oxides of nitrogen and therefore the absorber solution may be stored for some time prior to analysis. The complex is made to react with para rosaniline and methylsulphonic acid. The absorbance of the solution is measured by means of a suitable spectrophotometer. 

2.     Reagents:

·       Distilled Water: Distilled water used shall be free from oxidants, particularly chlorine, which may not be removed by distillation. The criterion shall be observed whether water is prepared by distilling or deionizing or by using a combination of both techniques.

·       Potassium tetrachloromercurate (TCM) - 0.04M: Dissolve 10.86g mercuric chloride, 0.066g EDTA and 6.0g potassium chloride in distilled water & bring to the mark in a 1 litre volumetric flask. The pH of this reagent shall be approximately 4.0 but it has been that there is no appreciable difference in collection efficiency over the range of pH 5 to 3. The absorbing reagent is normally stable for 6 months. If a precipitate is formed, discard the reagent after recovering the mercury.

·       Sulphamic acid- 0.6%: Dissolve 0.6g of sulphamic acid in 100ml of distilled water. Prepare fresh, when needed.

·       Formaldehyde-0.2% approx.: Dilute 5ml of formaldehyde solution (36 to 38%) to 1 liter with distilled water. Prepare fresh, when needed.

·       Stock Iodine solution-0.1N: Take 12.7g iodine in a 250ml beaker; add40g of potassium iodide to 25ml of water. Stir to dissolve completely, then to 1 litre with distilled water.

·       Iodine solution -0.01N: Prepare approximately 0.01N iodine solution by diluting 50ml of stock solution (0.1N) to 500ml with distilled water.

·       Starch Indicator solution: Triturate 0.4g of soluble starch and 0.002g of mercuric iodide preservative with a little water. Add the paste slowly to 200ml boiling water. Continue boiling until the solution is clear; cool and transfer to a glass stoppered bottle.

·       Stock Sodium Thiosulphate Solution: -0.1N:

      Prepare a stock solution by taking 25g of sodium thiosulphate pentahydrate in a beaker, add0.1g sodium carbonate, and dissolve using boiled and cooled distilled water making a solution up to a final volume of 1litre.allow the solution to stand 1 day before standardizing.

             To standardize, accurately weigh, to the nearest 0.1mg, 1.5g of primary potassium iodate dried at 180ºC, dissolve, and dilute to 500ml in a volumetric flask.

             Take 50 ml of iodate solution by pipette into a 500 ml iodine flask, add 2 g of potassium iodate and 10 ml (1:10) hydrochloric acid and stopper the flask. After 5 min titrate with stock thiosulphate solution to a pale yellow color. Add 5 ml starch indicator solution and continue the titration until the blue color disappears. Calculate the normality of the stock solution.

Normality of Thiosulphate Solution

The normality of this solution, N, is calculated as follows:

where

               N   =         M X2.80

                                     V

V= volume in ml of thiosulphate used, and

M= mass in g of potassium iodate.

·    Sodium Thiosulphate Titrant-0.01N

Dilute 100 ml of the stock thiosulphate solution to 1 litre with freshly boiled distilled water.

 

• Standardized Sulphate Solution for Preparation of working Sulphate –TCM         

            Solution

Dissolve 0.30 g of sodium metabisulphite (NaHSO₃) or 0.40 g of sodium sulphite (Na₂SO₃) in 500 ml of freshly boiled and cooled distilled water. Sulphite solution is unstable; it is therefore important to use water of the highest purity to minimize this instability. This solution contains the equivalent of 320-400 µg/ml of SO₂. The actual concentration of the solution is determined by adding excess iodine and back-titrating with standard sodium thiosulphate solution. To back-titrate, pipette out 50 ml of the 0.01N iodine solution into each of two 500-ml iodine flasks A (blank) and B (sample). To flask A (blank) add 25 ml of sulphite water and into flask B (sample) measure 25 ml of sulphite solution. Stopper the flasks and allow to react for 5 min. By means of a burette containing standardized 0.01 N thiosulphate, titrate each flask in turn, to a pale yellow colour. Then add 5 ml of starch solution and continue the titration until the blue colour disaapears.

 

• Working Sulphite- TCM Solution

Measure 2 ml of the standard sulphite solution (7.10) into 100-ml volumetric flask by pipette and bring to mark with 0.04 M TCM (see 7.2). Calculate the concentration of sulphur dioxide in the working solution in µg of sulphur dioxide per milliliter. This solution is stable for 30 days if kept in the refrigerator at 5⁰C. Otherwise, prepare fresh, when needed.

Calculation

                    C = (V₁ –V₂) x N x 32000 x 0.02

                                            25

       where

            C  = concentration of SO₂ solution in µg/ml in ml;

            V₁ = volume in ml of thiosulphate used for blank;

            V₂ = volume in ml of thiosulphate used for sample;

             N = normality of thiosulphate;

      32000 = milli equivalent weight SO₂, µg;

           25  =  volume of standard sulphite solution, ml; and

         0.02 = dilution factor.

 

·       Purified Para Rosaniline Stock Solution -0.2 percent concentration

The para rosaniline dye shall a wavelength of maximum absorbance at 540 nm when assayed in a buffered solution solution of 0.1 M sodium acetate-acetic acid; the absorbance of the reagent blank which is temperature-sensitive to the extent of 0.015 absorbance unit/⁰C, shall not exceed 0.170 absorbance unit at 22⁰C with a 1 cm optical path length, when the blank is prepared according to the prescribed analytical procedure and to the specified concentration of the dye; the calibration curve, shall have a slope of 0.030±0.002 absorbance unit/µg SO₂ at this path length when the dye is pure and the sulphite solution is properly standardized.

·       Preparation of Stock Solution

Dissolve 0.5 g of para rosaniline chloride in 100 ml distilled water. Keep it for 2 days and filter the solution. The filtrate solution is stable for 3 months, if stored in the refrigerator.

·       Working Para Rosaniline Solution

Add 15 ml concentrated hydrochloric acid to 10 ml stock para rosaniline solution and dilute to 250 ml with distilled water in a 250 ml volumetric flask. It may be stored at room temperature in an amber coloured bottle for one to two weeks, if stored in a refrigerator.

·       Calibration Curve-Procedure with Sulphite Solution

Measure by graduated pipette amounts of the working sodium sulphite tetrachloro mercurate (TCM) solution (such as 0, 0.5, 1, 2, 3 and 4 ml) into a series of 25-ml volumetric flasks.

·       Add sufficient TCM solution to each flask to bring the volume to approximately 10 ml. Then add the remaining reagents as described in 8.2. For maximum precision use a constant temperature bath. The temperature of calibration shall be maintained within ± 1⁰C and within the range of 20 to 30⁰C. The temperature of calibration and that of analysis shall be maintained within two degrees.

·       Plot the absorbance against the total concentration in micrograms sulphur dioxide for the corresponding solution.

·       The total micrograms sulphur dioxide in solution equals the concentration of the standard in micrograms sulphur dioxide per milliliter times the milliliter of sulphite solution added (µg SO₂ = µg ml SO₃ per ml x ml added).

·        A linear relationship should be obtained, and the y-intercept should be within 0.03 absorbance unit of the zero standard absorbance. For maximum precision determine the line of least squares.

·       Determine the slope of the line of best fit, calculate its reciprocal, and denote as B, the calibration factor.

·       This calibration factor can be used for calculating results provided there are no redical changes in temperature or pH. At least one control sample containing known concentrations is recommended to ensure the reliability of this factor.

·        Determination

For each set of determinations prepare a reagent blank by adding 10 ml of unexposed TCM solution to a 25-ml volumetric flask. Prepare a control solution by measuring 2 ml of working sulphite –TCM solution into a 25-ml volumetric flask by pipette. To each flask containing sample or control solution or reagent blank, add 1 ml of 0.6 percent sulphamic acid and allow to react for 10 min to destroy the nitrite resulting from oxides of nitrogen. Add 2 ml of 0.2 percent formaldehyde solution and 5 ml of para rosaniline solution. Start a laboratory timer that has been set for 30 min. Bring all flasks to volume with freshly boiled and ooled distilled water and mix thoroughly. Within 30 to 60 min, determine the absorbances of the sample, reagent blank, and the control solution at 560 nm using cells with a 1cm path length.

Use distilled water (not the reagent blank) as the optical reference. This is important because of the colour sensitivity of the reagent blank to temperature changes which may be induced in the cell compartment of a spectrophotometer. Do not allow the coloured solution to stand in the absorbance cells, because a film of dye may be deposited. Clean cells with the help of alcohol and pipe-cleaner after use. If the temperature of the determinations does not differ by more than 2⁰C from the calibration temperature, the reagent blank should be within 0.03 absorbance unit of the y-intercept of the calibration curve. If the reagent blank differs by more than 0.03 absorbance unit from that found in the calibration curve, prepare a new curve.

·       Absorbance Range

If the absorbance of the sample solution lies between 1.0 and 2.0 the sample solution can be diluted to 1:1 with a portion of the reagent blank and read within a few minutes. Solutions with higher absorbance can be diluted up to six fold with the reagent blank in order to obtain on-scale readings within 10 percent of the true absorbance value.

·       Calculations:

Sample Absorbance (SA):

Dilution Factor (D):

Calibration Factor (B):

Volume of Air Sampled @STP (V1):

Conc. Of SO2 (C): (SA X 1000 X B X D)/V1

Conc. Of SO2 (ppm): C X 3.82 X10^-4 ppm