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Sulfur Dioxide (SO2) Measurement Methods

for Wine Analysis: An Assay Comparison

 

In water sulfur dioxide is referred to as sulfurous acid.  We shall just refer to is by it's chemical formula.  A number of analytical methods exist for measuring SO2, due to its widespread and historic use in the wine and the food industries in general. Common assay methods include the Ripper method, the aeration oxidation (AO) method, the potentiometric titration method and photometric methods.

    THE COLORIMETRIC RIPPER METHOD

The Ripper method for SO2 uses an iodine standard to titrate the SO2 in a sample. Free SO2 is determined directly while total SO2 can be ascertained by treating the sample with sodium hydroxide (before titrating) to release bound SO2.  The free and total SO2 analysis used in the Ripper test is based on the REDOX reaction:

 H2SO3 + I2 H2SO4 + 2 HI

A starch indicator is added to the wine sample and it is acidified with H2SO4. The sample is then rapidly titrated with an iodine solution. The completion of the reaction is noted when excess iodine is complexed. This is determined by a blue-black color end point in the presence of a previously added starch indicator.  Obviously, red wine must be diluted so that the color of the endpoint can be seen.  The results then have to be readjusted for the dilution factor.

But, the Ripper method is slightly inaccurate. The method suffers from the fact that the iodine reacts with phenols in wine, resulting in increased consumption of iodine and subsequently false-high SO2 estimation. The correct assessment of free SO2 using this method is susceptible to the fact that bound SO2 interferes with the measurement.  During both the Ripper and AO methods, the reduction of free SO2 results in a low bisulphite level and consequently, some bisulphite is released from the bound SO2 to re-attain equilibrium. In red wines, the release of SO2 from bound SO2-anthocyanin can significantly result in a false high measurement of free SO2. How rapidly the bound SO2 compounds dissociate to release free SO2 depends on the dissociation rates for SO2 binding with those respective compounds.   If the titration is performed automatically, the time needed is reduced and consequently these reactions will be minimized. It can also be particularly difficult using this method with reds, since the dark color of red wines makes it difficult to identify the end point of the titration.   

Additionally, the potential volatilization of SO2 during titration and the reduction of the iodine titrant by non-sulphite compounds such as phenols or pigments can affect the result. Other interferences include botrytis and ascorbic acid (results are false-high due to the competitive oxidation of ascorbic acid and SO2 by the iodine titrant).

    THE POTENTIOMETRIC METHOD (Modified Ripper)

Using a Platinum ORP electrode and a pump to dispense the titrant, automatic titrators have many advantages.  First, no dilution is required (and the extra calculations) since color plays no role in the assay.  Second, it is a very fast procedure (about two minutes) as the addition is totally automatic.  Thus some of the shortcomings of manual titrations are eliminated.  Third, the stirring speed is regulated and is repeatable.  Fourth, the titrant quantification is more precise, thus making the method inherently more accurate.  Fifth, a computer controls the titration and displays both a digital result and a graphical titration curve.  The results on some models can be ported to an attached computer or printer.  And finally, the method is much simpler to run which leads to a lower technician error rate.

    THE AERATION OXIDATION METHOD (AO)

This method is a variant of the “Monier-Williams” procedure for the analysis of total sulfur dioxide in wine.  It had been used by winemakers prior to the introduction of the Ripper Method.  By bubbling air thru wine, the SO2 can also be quantitatively determined.  Free sulfur dioxide is volatized and captured using a distillation procedure:

  1. A controlled flow of air or nitrogen is swept through a wine sample for a timed period.  Volatile sulfur dioxide mixed with the carrier gas is then trapped in a dilute solution of hydrogen peroxide.

  2. A classic titration of the sulfuric acid formed is performed with 0.01N NaOH.

  3. The reaction is:

                            H2O2  +  SO2    SO3-  +  H2→  H2SO4  

     

While some of the Ripper inaccuracies are eliminated, other factors are introduced.  One factor among others is the incomplete recovery of SO2. It is a much more time consuming procedure as well.  The glassware and other equipment required create another set of problems in doing repetitive tests.  Each test requires a recovery step followed by a titration with an accurate pH meter (instead of a color indicator like: methylene blue/methyl red/ethanol).  Doing multiple assays becomes problematic because of the recovery, titration and calculation times required.  The glassware set up is requires cleaning so breakage may also be an unforeseen danger. 

    PHOTOMETRIC METHODS

a) Absorbance

A number of spectrometric instruments are also available for wine analysis.  They are commonly used for Total Acidity and Phenols but not for SO2. These instruments rely on passing a specified wavelength of light thru a cuvet containing wine which has been treated with a packet of chemicals.  These instruments are more limited in types of wines that they will successfully analyze (i.e.- white only).  The method is not as accurate as the methods noted above.  Some of the inherent problems relate to the production of a constant wavelength, glassware shape, glassware cleanliness and interfering colors.  Not all parameters can be measured with this method.

b) Reflectance

By treating wine with a number of chemicals on a reactive test strip, a change in the reflected light intensity at a particular wavelength will be proportional to the concentration of a particular parameter.  This method is good if the directions are followed carefully and a series of dilutions are made because the range of the tests can be limited.  Some other sample manipulation must also be done with red wines to try to remove the interference due to it's color.  The strips do have a shelf life and have an accuracy of plus or minus 10%.  It has also been shown that this method has variations from strip to strip with the same analyte.  To overcome this, it is recommended that the test be repeated a number of times with the same wine to create an average result. This instrument can be used for trending but should not be relied upon for definitive measurements.  The testing time and calculations do become a drawback.  This is compounded with red wines.

    SUMMARY AND CONCLUSIONS


Despite the inaccuracies, the Ripper method remains the most common method used for free SO2  determination in winemaking (including commercial winery labs) due to its simplicity.  And, now that automatic potentiometric titrators are available and inexpensive, more and more wineries are making the switch in the technology of wine analysis for two main reasons:  significant increase in accuracy & a faster and even more simple procedure.

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Last modified: 29-Oct-2008