SO2 testing

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Measuring Free SO2 is very useful for properly managing sulfite levels in wines and beer. Several methods of testing the level of free SO2 exist, each with its own advantages and disadvantages. The methods described below focus on measuring free SO2, but they can also be used to measure total SO2 with some modification.

Many other methods to measure SO2 exist, but they require advanced equipment that is not readily available to home brewers.[1]

Go to the sulfite page for more general information about sulfite and its uses.

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 phenolic compounds (i.e. tannins) or ascorbic acid.[1][2][3][4] With a small and moderately-priced setup, this test can be performed at home in less than 30 minutes.[5] The main drawback to using this method is the time and complexity involved in the testing process. It is also known as a modified Monier-Williams technique.

It is not known how accurate this method is for testing unboiled wort.

Chemistry Basics

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 provided with the kit.

Reaction Solution Prep (flask 2)

  • 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 (flask 1)

  • 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.[6] 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 pushing air at the ideal rate of 1.0-1.5 L/min.

A high amount of acetic acid (AKA vinegar, AKA Volatile Acidity [VA]) may falsely elevate the result.

Ripper Method

This is a relatively quick method to test free SO2 and requires less equipment and reagents compared to the aeration-oxidation method.[7] Unfortunately there are compounds in wine and beer that and cause inaccurate readings. Nevertheless, it is a very commonly used method even in small commercial wineries.

Chemistry Basics

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.

Ripper Instrument Options

The basic procedure is inexpensive and requires minimal materials. However, 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.[8][9] The Hanna Instruments 84500 is a similar analyzer.[10] 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.[2] The convenience of use also comes with a higher price tag.

Titrets® are inexpensive and consist of a self-contained Ripper method. They are extremely inaccurate in the presence of either ascorbic acid and/or tannins and possibly other phenolic compounds as well.[2][11][12]

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.[13] Accuvin test strips are less affected by ascorbic acid and tannins than Titrets®.[2] These have a relatively limited shelf life.

Less information is available about how these products compare to the other more accepted methods, but test strips are certainly not the most robust or reliable testing method.

Additional Resources

  • FermCalc 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 ) )
  • MoreWine links videos of the aeration-oxidation testing method kit from MoreBeer/MoreWine.
  • Cornell is developing alternate methods to directly measure molecular SO2.

References

  1. ↑ a b Guido, Luis. "Sulfites in beer: reviewing regulation, analysis and role." Sci. agric. (Piracicaba, Braz.) vol.73 no.2 Piracicaba Mar./Apr. 2016
  2. ↑ a b c d Pambianchi, Daniel. "Benchmarking of SO2 Analysis Instruments and Methods in Wine Applications." Techniques in Home Winemaking, 12 Jan. 2014.
  3. ↑ Peak, Bob and Vineyard, Nancy. "How To Use and Test Free SO2 in Wine." The Beverage People.
  4. ↑ Buechsenstein, J.W. and Ough, C.S. "SO2 Determination by Aeration-Oxidation: A Comparison with Ripper." Am. J. Enol. Vitic. 29, 161. 1978.
  5. ↑ "Home SO2 Test Kit Instructions." MoreWine.
  6. ↑ "Standardization of NaOH and Titration of an Unknown Weak Acid."
  7. ↑ Eisenman, Lum. "Measuring Free Sulfur Dioxide."
  8. ↑ "The Ripper Titration: Recent Improvements in Measuring SO2." Vinmetrica.
  9. ↑ Souders, Doyle. Email correspondence to Vinmetrica.
  10. ↑ "Sulfur Dioxide Mini Titrator for Wine" product page. Hanna Instruments.
  11. ↑ "Sulfite Test Kit - Measure Sulfite in Wine - Titrets® Titration Cells." Chemetrics.
  12. ↑ Mansfield, Anna K. "Sulfur Dioxide (SO2) 3 Ways." Cellar Dweller. Cornell University. Jan 2011.
  13. ↑ Sulfite testing strips. The Modern Brewhouse website. 2021. Accessed March 31, 2021.
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