Determination of sulphide by colorimetric method
AIM:
To determine the amount of sulphide present in a given sample by UV spectrophotometer colorimetric method.
PRINCIPLE:
Methylene blue is a blue colour dye containing sulphur atom formed from hydrogen sulphide react with ferric chloride and N,N dimethyl-1,4-phenylene diamine oxalate.the intensity of the blue colour is directly proportional to the concentration of sulphide ions.
APPARATUS REQUIRED:
1)Standard volumetric flasks.
2)UV spectrophotometer.
3)Pipettes.
4)Weighing balance.
5)Beakers.
6)Glass rods.
CHEMICALS REQUIRED:
1)Sulphuric acid.
2)N,N dimethyl-1,4-phenylene diamine oxalate.
3)Ferric chloride.
4)Diammonium hydrogen phosphate.
5)Reagent grade water(Distilled water or Millipore water).
REAGENTS AND PREPARATIONS:
1)Ferric chloride solution:
Dissolve 100g of Ferric chloride salt in 40ml distilled water.
2)Amine Sulphuric acid solution:
Stock solution: take 27g of N,N dimethyl-1,4-phenylene diamine oxalate in a 100ml standard volumetric flask containing 50ml concentrated sulphuric acid and 20ml distilled water.make up to the mark.
Take 25ml stock solution and mix with 925ml of 1:1 sulphuric acid.
3)Diammonium hydrogen phosphate solution:
Dissolve 500g of diammonium hydrogen phosphate in distilled water and make up to one liter.
PROCEDURE:
Take required number of 10ml standard volumetric flask.add 7ml sample.contineously add 0.5ml of amine sulphuric acid reagent and 0.15ml Ferric chloride.after 5 minutes add 1.6ml of diammonium hydrogen phosphate.make up to the mark with reagent grade water.if sulphide is present,the solution become blue.the appearance of blue colour is directly proportional to the concentration of sulphide ions.after 10 to 15 minutes,read the absorbance in uv spectrophotometer at 664nm.
CALCULATION:
S2- = A1 / A2 × S.C × 10 / 7
Where,
A1 = Sample absorbance of sulphide.
A2 = Standard absorbance of sulphide.
S.C = Standard concentration of sulphide.
10 = Constant (final make up volume).
7 = Constant (volume of sample taken).
EXAMPLE CALCULATION:
S2- = ( 0.033 / 0.030 ) × 0.03 × 10 / 7 = 0.04mg/l.
Determination of sulphide by titration method.
AIM:
To determine the amount of sulphide present in a given sample by UV spectrophotometer colorimetric method.
PRINCIPLE:
The hydrogen sulphide gas(very toxic) dissociate as hydronium ions and sulphide ions in aqueous medium.
H2S ------> 2H+ + S2-
When iodine is added to the sample,the sulphide ions react with iodine and reduce it as below
I2 + S2- --------> S + 2I-
When starch is added to that solution,the iodide ions and remaining iodine combine to form tri-iodide ion which react with starch and forms a dard blue complex.
I2 + I ------> I3-
I3- + Starch ------> dark blue complex.
The remaining iodine is reduced as sodium iodide by sodium thiosulphate.and sodium thiosulphate forms tetrathionate.
2Na2S2O3 + I2 -------> Na2S4O8 + 2NaI
APPARATUS REQUIRED:
1)Burette(50ml capacity).
2)Standard volumetric pipettes.
3)Standard volumetric flasks.
4)Iodine flasks.
5)Beakers.
6)Weighing balance.
7)Spatula
8)Glass rods.
CHEMICALS REQUIRED:
1)Hydrochloric acid.
2)Sodium thiosulphate.
3)Starch.
4)Salicyclic acid.
5)Reagent grade water.
REAGENTS AND PREPARATIONS:
1)6N HCL solution:
From 35% Conc.HCL, take 89.85ml and make up to one liter with distilled water.
2)Starch indicator:
Dissolve 2g of starch and 0.2g of salicylic acid in 100ml hot distilled water.
3)0.025N Sodium thiosulphate solution:
Dissolve 6.25g of sodium thiosulphate in reagent grade water and make up to 1000ml in standard volumetric flask.
4)0.025N Standard iodine solution:
Dissolve 25g of potassium iodide and 3.2g of iodine in distilled water and make up to 1000ml in standard volumetric flask.
PROCEDURE:
Pipette out 10ml iodine solution into a clean 500ml iodine flask.add 10ml distilled water.contineously add 2ml 6N hydrochloric acid.pour 200ml sample and add starch indicator.start titrate with 0.025N sodium thiosulphate solution taken in the burette.the end point is colour change from blue to colourless.
CALCULATION:
S2- = (Iv × In) - (Sv × Sn) × 16 × 1000 / S.v
S2- = Sulphide ions
Iv = Volume of iodine solution.
In = Normality of iodine solution.
Sv = Burette reading of thio solution.
Sn = Normality of thio solution.
16 = atomic number of sulfur.
1000 = For milligram conversion.
S.v = Sample volume.
EXAMPLE CALCULATION:
S2- = (10×0.025) - (5.0 - 0.025) × 16 × 1000 / 200 = 10mg/l.
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