Analytical Procedures - SO₂ in Acid
February 12, 2002

Introduction Equipment Reagents Procedure Calculations

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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