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Laboratory Test Procedures arrow Sulphur - Free & Bound arrow Sulphur - Free and Bound - Rankine

Sulphur - Free and Bound - Rankine

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"Free" Sulphur Dioxide

1.             Remove 0.3% Hydrogen peroxide from refrigerator, place 50ml of 0.3% H2O2 in a 100ml glass beaker.

2.             Place beaker in sink containing warm water. Stir solution with a thermometer to adjust temperature to 20°C.

3.             Use a 10ml volumetric pipette to transfer 10ml of 0.3% H2O2 to the 2 - necked, pear shaped flask.

4.             Add 3 drops of mixed indicator. The solution may then change to a purple colour. Therefore add 0.01N NaOH to adjust colour to an olive green. Alternatively, the solution may naturally change to a green colour. In this case no addition of 0.01N NaOH is required. The amount of sodium hydroxide used (if any) for colour adjustment is unimportant and does not have to be recorded. Connect the 2-necked flask to the top of the aspiration apparatus.  

5.             Adjust temperature of wine sample to 20°C.  Using a similar procedure as in steps 1 and 2. Use a 20ml volumetric pipette to transfer a sample of wine to the 50ml round bottom flask.

6.             Also add 10ml of 25% phosphoric acid using a 10ml volumetric pipette to the 50ml round bottom flask.

The 10ml of 25% phosphoric acid can be added to the 50ml round bottom flask by one of two methods.
(a) Adding it directly to the flask then connecting the flask to the swan neck adaptor or
(b) Connecting the 10ml pipette to the top of the air bleed tube and transferring the phosphoric to the flask. The plastic lock nut which secures the bleed tube to the swan neck adaptor must be loose (ie. not tight) during the transfer from the pipette into the top of the air bleed tube, otherwise the acid will over fill and run down the side of the air bleed tube.  Once transfer of the 25% phosphoric acid is complete, the lock nut must be tightened to finger tight and no further as the glass thread will be broken.  The air bleed tube must be sufficiently large in internal diameter to fit the tip of the pipette. As free SO2 is produced as soon as the phosphoric acid is added to the wine, addition through the bleed tube prevents the escape of SO2 from the round flask.

7.             Connect tubing from air pump assembly to the air bleed tube.

8.             Turn on the air pump and adjust airflow rate using the plastic valve on the air pump assembly. The airflow into the 2-neck flask should not be too fast or too slow (theoretically 1L/min).

PLEASE NOTE: The advantage of the air pump assembly is that the volume of air produced by the air pump is exactly the same each time the air pump is used. Hence once the correct airflow is determined the valve should not require any further adjustment. Conversely the water vacuum pump will draw varying amounts of air through the apparatus under vacuum due to the variations in water pressure each time the tap is opened. This volume can be determined by performing this test on a standard SO2 solution until 3 similar results are obtained.

9.             Pump or draw air by vacuum through the apparatus for 25min.

10.           Remove the top, pear shaped flask, with bubbler still attached.

11.           Add 0.01N NaOH to burette containing a 50mm glass funnel.

12.           Record the initial volume (litre) of 0.01N NaOH in the burette.

13.           Turn the plastic stopcock in the bottom of the burette to add the 0.01N NaOH drop by drop into the 2 necked flask until the appearance of the green colour originally produced in the flask in step 4. Record the final volume (titre) of 0.01N NaOH.

14.           Calculate the amount of free sulphur dioxide (SO2) in the sample as follows.   

                   Free SO2(ppm) = (Final Volume of 0.01N NaOH - Initial Volume of 0.01N NaOH x 16)

                     (Note: ppm = parts per million = milligrams/litre)

For Example:

Initial Volume of 0.01N NaOH             =  12.42ml

Final Volume of 0.01N NaOH              =  15.60ml

   Free SO2                               =  (Final Volume - Initial Volume) x 16

                                    =  (15.60 - 12.42) x 16

                                    =  3.18 x 16

                                    =  50.88 ppm

            Round off         = 51 ppm


"Bound" Sulphur Dioxide

1.             After determining the ‘free’ sulphur dioxide, replace the pear-shaped flask, including the contents and bubbler, to its position on the apparatus. Draw air (aspirate) through the sample for 25 minutes while applying heat to the bottom flask containing the sample.

2.             Remove the top flask, stop the heating and titrate contents with 0.01N sodium hydroxide from burette as before. Record the titre.

3.             To calculate the amount of sulphur dioxide in the sample, multiply the titre by 16. Record as "ppm sulphur dioxide - "bound".


To calculate the total sulphur dioxide content of the sample add the "free" and "bound" values.

It is recommended that the accuracy of the techniques be checked regularly, using a known standard of sulphur dioxide. In red wines, some sulphur dioxide will be bound to the colouring matter, yet it will be recorded as "free sulphur dioxide". Thus the correct figure will be slightly lower. This applies, of course, to the free levels in both methods.

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