DETERMINATION OF HYDROGEN SULPHIDE IN AMBIENT AIR
1.1 SCOPE
This method is applicable to the measurement of Hydrogen Sulphide in Ambient Air.
2.0 PRINCIPLE
The determination of Hydrogen sulphide by colorimetric method is based on the reaction which take place under suitable conditions between N,N-dimethyly –p-phenylenediamine sulphate, ferric sulphate and sulphide ions, resulting in the formation of methylene blue.
3.1 Respirable Dust Sampler.
3.2 UV-VIS Spectrophotometer
3.3 Impinger
3.4 Calibrated Rota meter
4.0 REAGENTS
4.1 Pure distilled water
4.2 Absorbing solution – Dissolve 4.3 gm of Cadmium sulphate in water. Add 0.3 gm NaOH dissolved in a small amount of water and dilute to one liter. Mix well before use.
4.3 N,N- Dimethyl –p- phenylene –diamine-sulphate solution- Add 50 ml of concentrated sulphuric acid to 30 ml of distilled water and cool . Add 12 gm of N,N-dimethyl –p-phenylene diamine on 27.2 gm of N, N-dimethyl – p- phenylene –diamine sulphate. Stir the blank solution till it s completely dissolved. Store it as stock solution in refrigerator. For the purpose of preparing standards, dilute 25 ml of the stock solution to one liter with dilute sulphuric acid (1:1).
4.4 Ferricc sulphate solution- Add excess of ferric sulphate in 100ml of water to obtain a saturated
Solution
4.5 Sulphamic acid solution – Dissolve 1.2 gm of Sodium sulphide of analytical garde ( N2S.9H2O),
In one liter of free – refrigerated distilled water and stopper it. Since the solution is very
Unstable, it shall be prepared just before use. It shall be stored in refrigerator. Standardize this
Solution against std. Sodium Thio sulphate( 0.025N) as follows.
Take 100 ml of distilled water in 250 ml conical flask and add 20 ml of std. iodine solution
(0.025N). Add 25 ml hydrochloric acid (0.1N) and titrate with Thiosulphate solution using
starch solution indicator. Note the titrate reading as A.
Take 100 ml of distilled water in 250 ml conical flask and add 20 ml of standard iodine solution (0.025N). Add 25 ml of HCL ( 0.1N) and 20 ml of sodium sulphide solution and repeat the titration with the thiosulphate solution. Note the titrant reading as B.
Calculate the strength of Sodium sulphide solution in terms of Hydrogen sulphide as .
( A- B) X Normality of thiosulpahe X 17 X 103
H2S, µg/ml = —————————————————————-
20
4.6 For the purpose of preparing std. for comparison, dilute a portion of the Sodium sulphide
Solution to that it contains equivalent of 10 µg/ml of Hydrogen sulphide.
4.7 Preparation of standards
Prepare the different concentration of Hydrogen sulphide solution as follows.
4.7.1 Arrange 11 tubes (50 ml capacity) in line and put serial numbers of them.
4.7.2 Add 25 ml of absorbing solution to each tube.
4.7.3 As indicated below, add to each of the 10 test tubes an amount of dilute sodium sulphide
solution necessary to give a concentration series of 1 µg to 25 µg in increasing order.
Tube No. 1 2 3 4 5 6 7 8 9 10
Blank
Amount in
ml of std.
sulphide solution 0 0.1 0.2 0.3 0.5 0.7 1.0 1.3 1.7 2.0 2.5
4.7.4 Add I ml of sulphamic acid , 0.6 ml 0f N, N-dimethyle-p-phenelene –diamine sulpahte solution and 0.05ml of ferric sulphate solution to each of the test tubes in that order, shaking well after each addition
4.7.5 Make up the volumes to 20 ml in each of the tubes with distilled water and mix thoroughly.
4.8 Preparation of calibration curve.
Allow 30 min for the development of full color in the tubes. Measure the color intensity of the solution in a spectrophotometer at 670 nm on transmission scale. Use the reagent blank as control. Draw the calibration curve of percent transmission versus micro-gramms of H2S
5.0 PROCEDURE
5.1 Sample Collection
5.1.1 Assemble sampling train. Place 35 ml capacity Impinger.
5.1.2 Procedure describe for short term and for long term sampling.
5.1.3 Short term sampling ( 30 min to 1 hour).- Place 20 ml of the absorbing solution in an impinger. Collect the sample at the flow rate of 1 lpm for 30 min or at 0.5 lpm for 1 hour using rotameter. Sealed the absorbing reagent from sunlight during ad after the sampling by covering the impinge with aluminium foil to prevent deterioration. Determine the volume of air sampled by multiplying the flow rate by the time in minutes and record the atmospheric pressure and temperature. Remove and stopper the impinger.
5.1.4 Twenty four hour sampling- Place 20 ml of the absorbing solution in an impinge and collect the sample at the flow rate of 0.2lpm for 24 hours. Make sure that no entrainment of solution results with the impinge. During collection and storage, protect from direct sunlight.
6.0 ANALYSIS
6.1 General; Both the samples and each of the reagents to be used for analysis shall be deoxygenated by passing a current of pure nitrogen through them for at least 5 min just before use
6.2 Sample preparation: if the precipitate is observed in the sample, remove it by centrifugation.
6.3 Add 1 ml of sulphamic acid solution Add 0.6 ml of dilute N, N-dimethyl-p-phenylene diamine sulphate solution and 0.05ml of ferric sulphate solution to the sample Mix well by shaking solution and 0.05 ml of ferric sulphate to the sample . Mix wee by shaking the impinge after the addition o f each reagent. Makeup of the evaporation losses by adding the absorbing solution to 20 ml mark of the imoinger. Allow the mixture to dvelop full colour by letting it stand for 30 min. Also prepare a reagent blank flowing the above procedure.
6.4 Measure the colour intensity of the sample in a spectrophotometer at 670nm . Compute from calibration curve the concentration in micrograms of Hydrogen sulphide in the sample.
7.0 CALCULATION
Micrograms of Hydrogen sulphide in the sample
Hydrogen Sulphide , µg/m3= —————————————————————- X 10 3
Liters of air sampled
8.0 REFERENCE
Indian standard Methods for Measurement of Air Pollution IS 5182 Part VII- 1973
(Reaffirmed 2003).
somasundaram82 