SOP: Determination of Total Sulphur Dioxide by Hydrogen Peroxide Distillation (Method I)

GIFSQ — Global Institute for Food Safety & Quality

Principle

The sample is acidified and the evolved SO₂ is swept with CO₂ or nitrogen into cold hydrogen peroxide, where the sulphurous acid is oxidised to H₂SO₄. The sulphuric acid formed is titrated with standard NaOH.

Apparatus required

• All-glass distillation apparatus with gas inlet tube, 500-ml round-bottomed flask, condenser (400 mm jacket length), 250-ml conical flask and trap, with 24/40 standard taper or 18/9 spherical joints
• 500-watt heater
• 5-ml microburette (for samples very high in SO₂ use a 50-ml burette)
• pH meter

Reagents

a. 3% Hydrogen peroxide solution: dilute 100 ml of 30% hydrogen peroxide to about 700 ml in a 1000-ml graduated cylinder, mixing by pouring back and forth; titrate a 100-ml portion of the diluted solution in a 250-ml beaker with 0.1 N NaOH to pH 4.1 using a pH meter (do not return this portion); calculate the NaOH required to neutralize the main solution, add it, stir and check the pH. To standardize: pipette 10 ml into a 100-ml volumetric flask and make up to volume; pipette 5 ml of this diluted solution into a 500-ml flask, add about 300 ml of water and 10 ml of 6 N H₂SO₄, and titrate with 0.1 N potassium permanganate to a permanent pink colour. 1 ml of 0.1 N KMnO₄ = 0.0017 g of H₂O₂.

b. 0.1% Bromophenol blue solution: dissolve 0.1 g in 100 ml of water.

c. 0.1% Phenolphthalein indicator: dissolve 0.1 g in 50 ml of ethyl alcohol and dilute to 100 ml with water.

d. Sodium carbonate, saturated solution: dissolve enough sodium carbonate (CP) to prepare a saturated solution; add several drops of phenolphthalein; discard when it becomes decolourized.

e. Concentrated HCl.

f. 0.05 N NaOH solution (0.1 N for samples very high in SO₂).

Procedure

Circulate cold water through the condenser

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Add from a graduated cylinder 20 ml of 3% hydrogen peroxide to the conical flask and 5 ml to the trap; assemble and connect to the condenser

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Weigh 50 g of blended or well-mixed portion and rinse into the 500-ml flask through the gas inlet tube, using 300 ml of water

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Replace the gas inlet tube immediately, making sure all connections are well greased and tight; remove it, slowly add 20 ml of concentrated HCl, and replace it

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Check that bubbles of CO₂ (passed through a gas-washing bottle filled with sodium carbonate solution) or nitrogen (99.9% pure) enter the receiving flasks at 15 to 20 bubbles per minute; if not, check the joints for leaks

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Connect the 500-watt heater on high; in about 5 min, when the solution starts to boil, adjust to a slow boil — dried fruits or vegetables require boiling for 1 hr; 30 min is enough for all other products

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After the required boiling time, wash the hydrogen peroxide solution from the trap into the conical flask and rinse the trap with water

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Add 3 drops of bromophenol blue indicator and titrate with 0.05 N NaOH solution to a pale sky-blue end point using the 5-ml microburette

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Run a blank titration on 20 ml of hydrogen peroxide and correct the results accordingly

Calculation

1 ml of 0.1 N NaOH = 3.2 mg of SO₂

SO₂ = (Titre × Normality of NaOH × 32 × 1000) ÷ Wt of sample

Where,

• Titre = the volume of NaOH solution used, corrected for the blank (ml)
• Normality of NaOH = the normality of the standard NaOH solution used
• Wt of sample = the weight of the sample taken (g)

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.

Shipton — Food Pres. Quart., 14, 54 (1954); Ruck, J. A., Chemical Methods for Analysis of Fruit and Vegetable Products, Canada Department of Agriculture, Publication No. 1154 — as cited in the book.