Method for SO2 Measurement in Ambient Air (Modified West & Gaeke Method)

Principle:

SO₂ in ambient air is collected in a solution of Tetrachloromercurate (TCM) with the formation of disulfitomercurate compelx. The disulfito mercurate is treated with formaldehyde and acid bleached  pararosaniline to give  a violet colour of pararosaniline methyl sulfonic acid.  The intensity of the colour is measured on a spectrophotometer at 560nm wavelength.

Range & Sensitivity:

      SO₂ in the range of 25-1050 µg/m³ can be measured by this method. The lower detection limit can be increased by taking an increased volume of air. The analytical range of the method is 0.03-1.26 µg/ml .

Interferences:

1. NO₂ Concentration in the range of 2 ppm induces a fading colour. NO₂ interference is removed by adding sulfamic acid.
2. Ozone is eliminated by aging the sample
3. Upto 30 µg Mn, 10 µg Cr (III) & 22 µg Vanadium in 10ml absorbing solutions eliminated by adding EDTA.  

Range = 25 to 1050 ug/m3

(if a 30 litre air  passed through 25ml absorbing reagent  at Flow rate of 0.5 lpm; duration 60 min)

Detection Limit = 0.03 to 1.26 µg/ml •

• Low concentration can also be monitored by sampling larger volume of air at specified flow rate for decided sampling duration.
• For changed conditions a slop test is must
• Absorption efficiency of SO₂ need to be calculated by connecting two impingers in series.
• Lowest measurable concentration for specific condition.

Interferences- Minimization or Elimination

NOx   =  Add 1ml of 0.06% Sulfamic Acid

Ozone  =  Delay the analysis for 1hr after sampling.

Trace metals  =  1drop of 0.01% EDTA in absorbing   reagent prior to sampling.

Fe (III) = 6 µg/ml these concentration if built up in

Mn(II) = 1 µg/ml solution do not give positive

Cr(III) = 1 µg/ml interferences while analysis. •10 µg copper (II), 22 µg of vanadium in 10ml of solution are also not interfere in analysis There is no interference of NH₃, Sulphides and Aldehydes. 

Preparation of Reagent

A.For Sampling

  Absorbing Reagent (TCM): Dissolve 10.86g mercuric chloride, 0.066g EDTA and 6.0g potassium chloride in 1 liter distilled water

  CAUTION:- Highly poisonous if spilled on skin, flush off with water immediately.

B.   For Analysis

  1. Sulphamic Acid (0.6%): Dissolve 0.6g sulphamic acid in 100ml distilled water.

  2. Formaldehyde Solution (0.2%): Take 5ml formaldehyde solution (36-38%) and dilute by adding 1 litre distilled water.

  3. Working Para- rosaniline Solution (PRA):

  a) Dissolve 0.5gm of specially purified para -rosaniline in 100ml of distilled water and keep for 48hrs (Stock Solution)

  b) Mix 10ml of stock PRA (from 3a) with 15ml Conc. HCl and make up to 250 ml with distilled water.

Note: Prepare PRA solution 2 days before analysis.  

C. .For Preparation of Calibration Curve

  1. Iodine Solution (0.01N): Dissolve 1.27g iodine and 4g potassium iodide in water. Stir and make upto 1 litre with distilled water.

  2. Starch Indicator Solution: Make paste by dissolving 0.4g soluble starch and 0.002g mercuric iodide with a little water and make up to 200ml with boiled water.

  3. Sodium Thiosulphate (0.01N):-

  a) Primary Standard Potassium Iodate: Dissolve 1.5g Potassium Iodate (dried at 180°C) in 500ml distilled water.

  b) Stock Sodium Thiosulphate (0.1N): Dissolve 25g Sodium Thiosulphate and 0.1g Sodium Carbonate in 1 litre boiled and cooled distilled water. (Allow the solution to stand for one day before standardization)

Standardization of Sodium Thiosulphate :

i) Pipette 50ml of Iodate solution in 250ml Iodine flask.

ii) Add 2g Potassium Iodide and 10ml Hcl (1:10).

iii) Put stopper on the flask and allow to react for 5 min.

iv) Titrate with stock sodium thiosulphate solution to a pale yellow color.

v) Add 5 ml starch indicator. It shall give blue color.

vi) Continue Titrate upto disappearance of blue  color.

vii) Calculate the Normality as follows:

Where

V= Volume of Thiosulfate consumed (ml)

W= Weight of Potassium Iodate (g)

Sodium Thiosulphate (0.01N): Dilute Sufficient amount of standardized stock sodium thiosulphate solution to 250 ml with freshly boiled, cooled, distilled water.

4. Working  Sulphite – TCM Solution:

Stock Sulphite  Solution: Dissolve 0.3 gm Sod. Metabisulphite in 500ml recently boiled, cooled distilled  water. 

Strength of Sulphite Solution

  C= (V1 – V2) x N x K / V

Where

• C = Concentration of SO₂ (microgram/ml)
• V1 = Volume of Thiosulphate for Blank (ml)
• V2 = Volume of Thiosulphate for Sample (ml)
• N = Normality of Sodium Thiosulphate Solution
• K = 32000  m. eq.wt. of SO2

Working Sulphite – TCM Solution (2µg/ml):   Dilute sufficient amount of stock sulphite solution to 250 ml with TCM.

Preparation of Calibration Curve (SO₂) 

1. Pipette 0.0 (blank) 1.0, 3.0, 5.0, 7.0, 9.0 ml to a series of 25 ml  volumetric flasks.
2. Add sufficient TCM solution to each tube to bring the volume upto 10 ml.
3. Add 1 ml of sulphamic acid
4. Allow to react for 10min to destroy the nitrite resulting from oxides of nitrogen.
5. Add 2ml formaldehyde solution
6. Add 2ml working pararosanilne solution
7. Make up-to 25ml with distilled water.
8. Mix Thoroughly
9. Measure the absorbance at 560nm after 30 min but before 60 min against reagent.
10. Plot the absorbance versus concentration at different strengths.

Analysis

1. Mix Thoroughly the sampling bottle  by shaking it  several  times.
2. Take two 25 ml volumetric flask (Blank and Sampler)
3. Pippete 10 ml  unexposed TCM solution in Blank and  10ml exposed TCM in sample)
4. Add 1 ml of Sulphamic acid.
5. Allow to stand for 10 minutes.
6. Add 2 ml formaldehyde solution
7. Add 2 ml working pararosailine solution.
8. Make upto 25 ml with distilled water.
   • Note:- Mix thoroughly after addition of each reagent.
9. Measure the absorbance at 560 nm against the blank reagent by using spectrophotometer.

Major Precautions

Sampling and Analysis

• 0.5 – 2.0 litre/min through midjet impinger containing 20ml reagent.
• Sampling period need to decided such so that 3-30 µg SO₂ is accumulated in the solution while sampling for best reliable results.
• If sample is to be analysed after a day or more store sample at 5°C in a refrigerator. Ensure that sample is kept at low at temperature even during transit/Transportation.
• Sodium sulfite solution need to prepared fresh as it is unstable.
• Always use KCI salt in place of NaCI as it is available in more purer form
• If heavy metals expected add 0.07 gm EDTA disodium salt in absorbing reagent.
• Formaldehyde solution (HCHO) need to be prepared daily for analysis.
• Sulfamic Acid need to protected from air and it is stable for 1 week only.
• Use pure sigma make p-rosaniline
• Calibration curve preparation and sample analysis need to be done at similar temperature and it shall not differ more than few degrees.
• The Intercept with vertical axis of the line best fitting the points usually within 0.02 absorbance unit of the blank.
• Prepare calibration curve also by adding sulfamic acid.
• Delay analysis by 20 minute to over come impact of O₃.
• Temperature effect on blank 0.017 absorbance/°C any change in temperature upward/downward cause increase and decrease in absorbance value.
• Best results are obtained in constant temp bath.