Method for the determination of SO2 1. Summery of method This method describes the determination of SO2 according to the modified Monier – Williams method. It involves the acidification of a sample, followed by the distillation of SO2 (with Nitrogen Gas) out of the sample in to a hydrogen peroxide solution. The SO2 and hydrogen peroxide reacts to form H2SO4 according to the following reaction: H2O2 + SO2 SO3 + H2O H2SO4 This weak acid is then titrated with Sodium hydroxide till a end point is reached. 2. Apparatus

2.1. SO2 Distillation apparatus figure 1. 2.2. Volumetric flask 1lt × 3. 2.3. Volumetric flask 500ml × 2. 2.4. Volumetric flask 100ml × 2. 2.5. Amber reagent bottle × 2. 2.6. Pipette volumetric 15ml. 2.7. Pipette volumetric 10ml. 2.8. Pipette volumetric 50ml. 2.9. Dropper. 2.10. Scale 2 decimal (0.00g) 2.11. Heating mantle / Bunsen burner. 2.12. N2 Cylinder with regulator and fittings for 5mm ID pipe. 2.13. Water connection, with constant water flow and fittings for 5mm ID pipe. 2.14. Dr Schillings Burette 50ml. 2.15. Magnetic stirrer (Optional) 2.16. Wash Bottle 500ml. 2.17. Beaker 250ml. 2.18. Funnel 35mm 2.19. Cylinder 100ml. 2.20. Tubing

3. Chemicals

3.1. 0.1N NaOH ampoule (code 1.09959). 3.2. 30% Hydrogen peroxide. 3.3. 85% Orthophosphoric acid. 3.4. Methanol AR 3.5. Distilled or deionized water. 3.6. Absolute Ethanol. 3.7. Methyl Red 3.8. Methylene Blue

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4. Stock Solutions

4.1. 0.1N NaOH solution. Use a Merck titrisol ampoule (code 1.09959) and make up to 1lt according to instructions.

4.2. 0.01N NaOH solution Pipette, using a one mark 50ml pipette, 100ml of the 0.1N NaOH stock solution in to a 1000ml volumetric flask. Fill to mark with distilled or deionized water.

4.3. Hydrogen Peroxide solution 0.6% Pipette using a one mark 10ml pipette, 20ml of 30% hydrogen peroxide in to a 1000ml volumetric flask. Fill to mark with distilled or deionized water.

4.4. Indicator solution Weigh ± 0.3g methyl red in to a 500ml volumetric flask, rinse funnel with ethanol. Fill flask to mark with ethanol and shake well. Mark clearly methyl red. Weigh ± 0.5g methylene blue into a 500ml volumetric flask, rinse funnel with ethanol. Fill flask to mark with ethanol and shake well. Mark clearly methylene blue. Transfer 50ml of methyl red to 100ml volumetric flask and fill the flask to mark with ethanol. Shake well. Transfer 50ml of methylene blue to 100ml volumetric flask and fill the flask to mark with ethanol. Shake well. Mix two indicators in amber reagent bottle. Mark as SO2 indicator.

5. Methodology

5.1. VERY IMPORTANT! Before preparing distillation flask (4) the distillation receiver must be prepared as follow: To the distillation receiver (7) 15ml 0.6% H2O2 (Hydrogen peroxide) solution is added and 60ml distilled or deionized water using the 100ml cylinder. Add to the mixture a few drops of indicator. The solution will turn blue. Add then a few drops of the 0.01N NaOH (sodium hydroxide) solution to the mixture until it turns light green.

5.2. The N2 wash bottle is prepared by adding 0.1N NaOH solution to it. 5.3. Add 15ml of 85% Orthophosphoric acid to the dropping funnel. (4.2) 5.4. Prepare the distillation flask (4) by placing 15g ± 1g of sample, 50ml methanol and 50ml

of distilled water into the flask. 5.5. Connect the distillation flask to the apparatus. Insert the N2 injector (4.1) and dropping

funnel (4.2) containing the phosphoric acid into the distillation flask (4). 5.6. Open the stop cock on the dropping funnel (4.2) so that the phosphoric acid flows into

the distillation flask (4). 5.7. Ensure the tap water to the system is opened and has a constant flow. 5.8. Open the N2 stream to the system (c.a. ±60 bubbles/min) 5.9. The liquid in the distillation flask is heated to boiling by a burner or heating mantle and

then held at a gentle boil. 5.10. The time of distillation, including heating up period should take approximately

15 min.

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5.11. When distillation is complete, remove the nitrogen injector (4.1) from the distillation flask (4), turn of the heat.

5.12. Remove the receiver flask (7), rinse the gas inlet tube (7.1) from the outside and the inside by removing the stopper (6.1). Let all the rinse water run in to the receiver flask. This ensures that all the SO2 is captured.

5.13. The solution is titrated with 0.01N NaOH immediately after until endpoint is reached (liquid turns to green color).

5.14. Calculation: 1ml 0.01N NaOH represents 0.32mg SO2 Mg/kg SO2 = A × 320 Where A = ml 0.01N NaOH titrated Weight of sample

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7.1

3. Wash Bottle 3.1 Wash bottle head 4. Distillation flask 4.1 N2 injector 4.2 Dropping funnel 5. Splash head 6. Reflux condenser 6.1 Stopper 7. Receiver flask 7.1 Gas inlet tube 8. Control bottle (optional) 8.1 Control bottle head

Figure 1

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