Sulfur dioxide determination
For a description of the sulfur dioxide, please refer to the data sheet of the molecule. The determination of sulfur dioxide is important for two reasons:
- only if the free SO2 and even more the molecular SO2 are present in a certain concentration do they exhibit antiseptic and antioxidant action, by fixing the oxygen present in the wine, improving its taste and aroma;
- for hygienic-sanitary reasons, the regulation establishes the legal limits for total SO2.
Expression of the parameter
The sulfur dioxide value, whether total, free or combined, is expressed in mg / l. Article. 65 of the EEC Reg. N. 822/87 of 16 March 1987 states that, without prejudice to the more restrictive provisions that can be applied by the Member States for wines produced in their territories, the total SO2 content of wines at the time of their release for consumption cannot exceed 160 mg / l for red wines and 210 mg / l for white and rosé wines.
Sulfur dioxide determination
The Official EEC method (EEC Regulation No. 2676/90, Official Journal of the European Communities L 272 of 3/10/90, Annex 25: Sulfur dioxide) provides that sulfur dioxide is entrained by an air or nitrogen current and is fixed and oxidized, by bubbling, in a dilute and neutral solution of hydrogen peroxide. The sulfuric acid formed is dosed with a titrated sodium hydroxide solution. Free sulfur dioxide is extracted from wine by cold entrainment (10 ° C), while total sulfur dioxide is extracted from wine by hot entrainment (100 ° C).
The usual methods include, among the most precise and reproducible ones, the Ripper - Schmitt method, which provides for the volumetric determination of SO2 by iodometric titration. To know the content of free SO2, the titration is carried out directly on the wine; for total SO2, the determination is made after alkalization, in order to split the aldehyde-sulphurous compounds.
The analysis is based on the iodimetric oxidation of sulfur dioxide according to the reaction:
SO2 + I2 + 2H2O → H2SO4 + 2HI
As an indicator, the starch balance is used, which with the free iodine I2 turns blue because it forms starch iodide: in red wines the color change is highlighted by a clear change in the red hue of the wine; in white wines, the color change is highlighted by a bluish color. The titration is carried out at pH 1 for the free SO2 on the acidified wine; for total SO2, titration is carried out after freeing the SO2 combined with an alkaline hydrolysis and acidifying again.
Determination of free sulfur dioxide with the Ripper - Schmitt method
In an Erlenmeyer flask, exactly measured, pour 50 ml of wine taken from the freshly uncorked bottle; 25 ml of sulfuric acid are added. Then add a tip of a starch-weld spatula (indicator used in iodometric titrations) and titrate with a solution of I2 0.1 N until a blue color change.
Determination of total sulfur dioxide with the Ripper - Schmitt method
After taking 50 ml of wine and placing it in a flask, an alkalizing agent (25 ml of NaOH 1N) is added and the flask is immediately stopped. It is stirred and left to react for about 20 minutes; after which, 25 ml of sulfuric acid are added. After adding a tip of a starch solder spatula, it is titrated with a solution of I2 0.1 N, until a blue color change.
The result, for both the determinations just described, is expressed with the following formula:
A x 6.4 = mg / l of SO2
where A = ml of I2 0.1 N used at the end of the titration. The determination of combined sulfur dioxide takes place by simple difference between total and combined sulfur dioxide:
Total SO2 - free SO2 = mg / l of combined SO2
Sulfur dioxide (source: www.my-personaltrainer.it)
Davanzo P., “Appunti di enologia”, Sixth Edition (available online in pdf)