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Analytical Procedures - SO2 in
Acid
February 12, 2002
Introduction
The analysis of sulphur dioxide (SO2) in sulphuric acid is
generally done to determine the level of SO2 in the product acid. The
presence of high concentrations of SO2 into the acid means that the product
acid stripper or crossflow stripper isn’t working properly.
The analysis of SO2 is performed by oxidizing it with an excess of
iodine and titrating the excess with sodium thiosulphate.
Equipment
- Usual laboratory equipment
- 600 ml Erlenmeyer flask
Reagents
- Iodine 0.1 N solution. (Dissolve 25 g of potassium
iodide (KI) in as little water as possible. Weigh accurately 12.69 g of resublimed
iodine and dissolve it completely in a saturated potassium iodide solution. When the
solution of iodide is completed, dilute the solution accurately to one liter. A
large quantity can be made up one time, if desired. Iodine solution should be kept
in a well stoppered, dark coloured bottle in a cool place. It will be standardised
against the thiosulphate solution).
- Starch solution. (Place 5 g of starch in a mortar, add
little water and mix very well with pestle. Pour in a liter of boiling water and rest over
night. Separate the insoluble and add 1.3 g of salicylic acid and 4 g of zinc chloride, as
preservative).
- Sodium thiosulphate 0.1 N solution. (Dissolve 24.82 g of
Na2S2O3 . 5 H2O grade reagent in water and
make up to 1 liter. Rest for 20 days in a well stoppered bottle. Before using
check the factor with potassium dichromate 0.1 N solution)
Procedure
Place ~400 ml of water in a 600 ml Erlenmeyer flask.
Add 10 ml of 0.1 N iodine solution, and, gently, 50 cc of sulphuric acid (weighed).
Shake well with a glass rod and titre the excess of iodine with 0.1 N thiosulphate
solution. When the yellow colour of the solution is beginning to disappear, add a
few millilitres of starch solution and continue titring to colourless.
Calculations
SO2 ppm = ( V1 x f1
- V2 x f2 ) x 3.2 x 103 / W
where:
V1 = ml of 0.1
mol/L iodine solution used
V2 = ml of 0.1 mol/L
thiosulphate solution used
W = grams of sample weighed
f1 = corrective
coefficient of 0.1 mol/L iodine solution
f2 = corrective
coefficient of 0.1 mol/L thiosulphate solution
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