SO2 testing

Aeration-Oxidation Method

This method is the gold standard for testing free sulfite since it is both highly accurate and precise, and it is not affected by the presence of tannins (or other phenolic compounds) or ascorbic acid. With a small and moderately-priced setup, this test can be performed at home in less than 30 minutes. The main drawback to using this method is the time and complexity involved in the testing process.

The chemistry:
Acid is added to a measured sample, which shifts all of the sulfite into molecular SO₂. The molecular SO₂ (in gaseous state) is moved into a second vessel using an air pump. There it reacts with hydrogen peroxide (H₂O₂) to form sulfuric acid (H₂SO₄). An indicator solution is used in the second vessel. The amount of sulfuric acid is then measured with titration with sodium hydroxide (NaOH) and then the amount of original free sulfite can be calculated.

Testing Procedure

This procedure uses the SO2 testing kit available from MoreBeer/MoreWine and these instructions are the same, just less wordy. Set up the instruments according to the instructions.

Reaction Solution Prep

• Add 10mL of 3% H₂O₂.
• Add 6 drops of indicator solution.
• Add 40mL of distilled water.
• If the solution is already green: Add 1 drop at a time of 0.01 N HCl until gray/pink.
• Add 1 drop at a time of 0.01 N NaOH until green.
• Affix stopper.

Wine Sample Prep

• Pipette 20mL of the sample to be tested. Accuracy is important in this step!
• Add 10mL of 25% phosphoric acid.
• Quickly affix stopper.

Testing

• Turn on air pump.
• Adjust regulator for 1 L/min.
• Allow to run for 10-15 minutes.
• Draw up 10mL of 0.01 N NaOH.
• Add 1 drop at a time until green (the same color it was at the end of the Reaction Solution Prep step). Accuracy is important in this step!

Calculations

[Free SO₂ ppm] = [mL NaOH used] × 16

Note that this calculates the free sulfite as SO₂ ppm (mg/L). This is how it is reported in the world of wine making since this is how it is actually measured via testing (as described here). In the beer world, sulfite is often expressed as the concentration (mg/L) of sodium metabisulfite added. To convert SO₂ to an equivalent concentration of sodium metabisulfite, multiple the SO₂ ppm by 1.48.

Ways to Maximize Accuracy

You may use a high resolution burette (AKA buret), typically with a stand, instead of a syringe for much greater accuracy measuring the NaOH.

The accuracy is dependent on the freshness of the testing solutions, particularly the NaOH. NaOH reacts with CO2 in the air over time. Keep it refrigerated. With an additional titration, you may standardize your NaOH solution to ensure the greatest test accuracy; otherwise it should be replaced periodically. The hydrogen peroxide also loses potency over time and should be kept refrigerated and replaced periodically. Make sure the solutions do not get cross-contaminated.

An air flow meter may be used to make sure the pump is running at 1 L/min, however this may not have much impact on the result if you can get close enough.

Ripper Method

This is a relatively quick method to test free SO₂ and requires less equipment and reagents compared to the aeration-oxidation method. Unfortunately there are several common compounds in wine and beer that and cause inaccurate readings. Nevertheless, it is a very commonly used method even in small commercial wineries.

The chemistry:
Acid is added to a measured sample, which shifts all of the sulfite into molecular SO₂. A titration is then performed with iodine, which reacts with the sulfite and water to form sulfuric acid. Starch is used as an indicator. A modified Ripper method incorporates a second test whereby the SO₂ is removed via acidification and then oxidation with H₂O₂. The result of this test is used to remove the error in the test introduced by phenolic compounds in the sample.

The starch indicator isn't always easy to interpret, especially if the sample is dark, such as a red wine or beer/wort with dark malts.

The Vinmetrica SC-100 and SC-300 are both capable of measuring free SO₂ quickly and reliably and they do not utilize a starch indicator. However, even when the modified Ripper method is utilized by such an advanced electronic instrument with electrochemical sensors, it still suffers significant inaccuracies resulting from the presence of both tannins and ascorbic acid. The convenience of use comes with a higher price tag as well.

Titrets® are inexpensive and basically consistent of a self-contained Ripper method. They are extremely inaccurate in the presence of either ascorbic acid and/or tannins and probably phenolic compounds as well.

Test Strips

SO₂ testing strips are available from a few different manufacturers. They are quick and have a somewhat low entry cost, but they can be difficult to interpret and suffer from variable accuracy. Accuvin test strips are less affected by ascorbic acid and tannins than Titrets®. These have a relatively limited shelf life.

Less info is available about how these products compare to the other more accepted methods, but it is obviously not as robust of a testing method.

Additional Resources

This calculator is helpful for adjusting sulfite level in wines. Click the "sulfites" button to open the sulfite calculator.

This calculator is helpful if you just need to know the concentration of molecular SO₂. The equation used is [Molecular SO₂] = [Free SO₂] / ( 1 + 10^{( pH - 1.81 )} )

This page links videos of the aeration-oxidation testing method kit from MoreBeer/MoreWine.