Free Sulfur Dioxide: Aeration Oxidation Test (Tried & True)

By: Tom Payette, Winemaking Consultant  

A precise test for a winery to measure the free Sulfur Dioxide in juices, musts, wines and at bottling is critical.  Selecting the proper test can be challenging due to cost and expertise.  Make certain to investigate the options of the many available tests to determine which one gives reliable results when working with your wines.  Having the precise number is imperative to take proper action if needed.  One of the tests, aeration oxidation, is discussed below.  It may seem antiquated but many well established cellars still adhere to this testing system.

  The equipment to run the aeration oxidation test is not inexpensive, the chemistry somewhat daunting, and the glass apparatus has the appearance of being overly fragile and complicated.  The units are expensive however, since we are seeking to measure a chemical in parts per million. A tool to do just that is bound to be costly.  The mechanics and chemistry are not complicated.  Without drawing large chemical equations a summary follows:  Free SO2 is drawn off of a sample of wine after a shift in the pH brings the sample to a very low ph range allowing all the free SO2 to be released and measured.  Once in the gaseous state, the gas is pulled through a hydrogen peroxide reaction flask with an indictor in the flask’s solution.  The hydrogen peroxide (H2O2) reacts with the SO2 to form H2SO4 (a very low concentration sulfuric acid) and H2O (water).  This in turn lowers the ph of the Hydrogen Peroxide collection flask and then the lab technician can titrate this sample with a weak base.  The weak base is measured volumetrically to determine how much of the base it took to neutralize the acid sample and then mathematically use that to formulate the Sulfur Dioxide content of the wine being tested.

Tools and Chemicals

•   25% Phosphoric acid

•   30% Hydrogen Peroxide

•   0.01 Normal Sodium Hydroxide

•   0.01 Normal Potassium Acid Phthalate or Hydrochloric acid.

•   Aeration oxidation indicator dye

•   10 milliliter burette – class A volumetric

•   20 milliliter pipette – class A volumetric

•   25 milliliter delivery cup squeeze bottle set at 10 milliliters delivery –  approximate.

•   100 milliliter graduated cylinder

•   Burette stand and a burette clamp

•   Vacuum source (adjustable) – only a light vacuum source is needed.

•   Aeration oxidation apparatus

•   Distilled water

Mixing and Standardizing Chemicals

•   Purchase or pre-mix your sodium hydroxide to 0.01 Normal.

•   Standardize the 0.01 Normal Sodium Hydroxide against the 0.01 Normal Potassium Acid Phthalate or 0.01 HCL.  Do this daily before each day’s work with this unit to be certain your results will be accurate.  This is a critical part of this test because it must be exact!

•   Purchase or premix the 25% Phosphoric acid.  Pre-mixing may be done by adding 1 portion of 85% phosphoric acid to 2 portions of distilled water. Example:  333.33 milliliters phosphoric acid to 666.33 milliliters water to make one liter.  Transfer a portion of this mixture to the preset squeeze bottle, set at 10 milliliter delivery, so it is ready for use.

•   Purchase the dye indictor because making it is much too daunting.

•   Purchase 30% Hydrogen Peroxide and mix 3.0 milliliters into 97.0 milliliters of distilled water (this should be ample to cover very high free SO2’s).  Add about 2 drops of 0.01 Normal Sodium Hydroxide to this fresh stock mix to shift the pH slightly to the basic side.  This will help the indictor solution to give a vivid “seawater” green at the beginning of each test and after the titration is complete.  Do not use store purchased hydrogen peroxide from a local pharmacy.

Procedure

1.  Make sure your apparatus is clean, set up properly and in good working condition.   After cleaning, rinse all of the lab ware with distilled water to remove any minerals and residues.  Allow to dry.

2.  Make sure the chemicals are readily available and mixed to the proper concentrations.

3.  Fill the 10 ml volumetric burette with 0.01 N. NaOH and make sure all air bubbles are out of the stopcock and delivery tip area.  Inspect.

4.Standardize the 0.01 N NaOH if not already done above.  This is crucial for good measurement.

5.  Rinse the reaction flask with a small amount (5 milliliters) of the premixed 0.3% (or greater) hydrogen peroxide (H2O2).

6.  To the Hydrogen peroxide reaction flask add the amount of 0.3% H2O2 (approximately 10 mils) and 5-6 drops of dye indicator.

7.  Attach the reaction flask into the clamp holder and place the glass bubbler portion back into the reaction flask.

8.  Select a fresh representative sample of the wine to be tested that was already collected and placed in the lab.  Using the 20 milliliter volumetric pipette transfer exactly 20 milliliters of wine into the lower boiling flask.

9.  Be ready to place the wine sample boiling flask onto the remaining glassware to complete the set up.

10.      Add 10 milliliters of the 25% phosphoric acid to the wine sample in the lower boiling flask.  [Be careful with this phosphoric acid since it may splash onto you or into unwanted areas of the apparatus.  Perhaps hold it away from the Aeration-Oxidation set up and over a sink.

11.      Immediately place the rubber stopper onto the wine sample boiling flask making the assembly complete and a “closed system”.

12.      Now turn on the vacuum source to vacuum air through the complete set up.  You should see a “boiling” action in both the wine sample boiling flask and the reaction flask.  This should be vigorous but not so vigorous that effluent is being transferred from one reaction vessel to the next – only air should be moving through the tubing.

13.      Time the operation of the units vacuum for 10 minutes – note the reaction flask should have turned purple in color in the first minute – if any Free Sulfur Dioxide is present.  Continue to monitor the test during the timed 10 minute interval making adjustments to the vacuum source, if necessary.

14.      Turn off the vacuum source after the desired 10 minutes.

15.      Disconnect the reaction flask (the one that turned purple) – from the rest of the lab set up assembly.  This will be done by removing the bubbler assembly from the unit and rinsing off any remaining solution on the bubbler tip with a small amount of distilled water.

16.      Turn to your pre-prepared and standardized 0.01 Normal Sodium Hydroxide burette and record the amount of solution in that burette.

17.      Titrate the solution in the reaction flask from the bright purple it has become back to the seawater green it was before starting the test. 

      (Note: Make sure to capture this endpoint since overshooting it will result in an inaccurate calculation of the sulfur dioxide on the high side)

18.      After the seawater green has been achieved, record the number of milliliters in the burette and write this number down on a piece of paper or in the lab book.

19.      Subtract the number of milliliters used to titrate the solution in the reaction flask and record this number to be used in the calculation portion of the test.  Obtain the exact volume of NaOH, to the tenth, used to neutralize the reaction flask content.  Example: 2.3 mils.

Calculation:

  Here is the formula to calculate the free sulfur dioxide:

Free SO2    =    Milliliters of NaOH * Normality of NaOH * 32000

                                          20 milliliters of wine sample tested

The Knowns Above Are:

  The milliliters of sodium hydroxide used to titrate the reaction flask effluent from purple to green as calculated from the reading on the burette.

  The normality of the sodium hydroxide used above since it was standardized recently.

  The given 32000 number

  The sample size of 20 milliliters.

  From this formula we now have enough information to calculate the free sulfur dioxide in the sample of wine.  If the Normality of the Sodium hydroxide is exactly 0.01N one may see results as follows:

  Let’s assume 2.3 milliliters of Sodium hydroxide was used to titrate the reaction flask effluent back to the seawater green color.  So 2.3 times 0.01 times 32000 divided by twenty yields 36.8 parts per million free sulfur dioxide present in the sample tested.  Part per million is also equal to milligrams per liter.

Summary

  This test is really very simple once one gets over the intimidation of the unit and chemistry.  It is recommended to follow these directions in the lab for 8 to 10 test runs and then to run several dozen others to commit the process to memory.  After running this process approximately one hundred times – read this procedure over again and much of the process will become crystal clear.  When learning this test, try to find a time when few interruptions will be expected so focusing on the task at hand will be easy.

Other Helpful Tips

  Once the phosphoric acid is added to the wine sample – all of the free SO2 is released – it is important to capture all of this SO2 in the Hydrogen peroxide solution – hence the need for connecting the wine sample boiling flask as rapidly as possible to the complete set up.  Make sure the Pasteur pipette is positioned to extend to the bottom of the boiling flask, well below the liquids surface, for best results.

  Set your aeration oxidation unit up as the manufacture has intended for the unit to be assembled.

  While performing the test make sure there is not an exogenous source of Sulfur Dioxide in the area of the testing – such as weighing metabisulfite and creating a dust or pungent aroma.  This may throw off the test leading to false readings on the high side.

  Endpoint: Be sure to catch the end point during titration at exactly the precise time the color changes being sure to swirl the reaction flask during titration.

  Send samples to an outside lab for a free sulfur dioxide test to see if the results achieved “in house” are in range with the certified labs.  Once accuracy is achieved, one may wean themselves off the outside lab but periodically send a random sample to an outside lab to pick up on any discrepancies.

  Wine samples tested should be at or below 68 degrees F and preferably at a cool 55 degrees F cellar temperature.

  Wines abnormally high in volatile acidity and carbon dioxide may give false high reading with this test.

  Water system pressure tanks, with many rural water systems, may cause a fluctuation in the vacuum source while oscillating between the pressure switch settings.  Be ready to compensate for this in the lab, adjusting the vacuum if an aspirator is used.

  The author does not recommend using this set up for total SO2 because it is time consuming, hard on the apparatus’s rubber parts and gives the unit a burned dirty look.  For total SO2 the author recommends the Ripper Method, provided ascorbic acid has not been used in the wine or any components of the blend.   An outside lab may be best for this test in terms of total SO2 as many wines rarely reach very high total SO2’s with today’s winemaking.

Reminders:

•   Make sure all chemicals and reagents are standardized and strengths known.

•   Make sure only gases transfer in the hoses – not liquids.

•   Keep a watchful eye on the vacuum to ensure it is proper and even.

•   Be patient and have quiet time to learn this procedure.

•   Sulfur dioxide is important.  An accurate measurement of the free sulfur dioxide in your wine, in PPM, is critical to your winemaking choices.

Other Suggestions to Consider:

1.  Run several tests on the same wine to see if the number can be reproduced. (?)

2.  Have an outside lab run some tests on the same wine.  Are your numbers in line?

3.  If using sorbates in wine – make sure you have ample SO2 in them at bottling.

4.  Run this test, at a minimum, monthly and more so as adjustments are needed and toward bottling.

5.  Consult a winemaker to determine an appropriate storage free SO2 of your wine and the correct bottling measurement.

  Tom Payette, Winemaking Consultant, has over 30 years’ experience with winery start-ups and assisting wineries already established in the industry.

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