The Chemistry of Metabisulfite and Sulfur Dioxide in Wine.

TRIGGER WARNING:  I assume that the reader knows a couple of things about chemistry.  If you are little rusty in your chemistry knowledge, here are some quick tips:

• Sulfur, oxygen, and hydrogen atoms are given the symbols S, O, and H.
• Compounds are groups of atoms that are bonded together.  The formula of a compound gives the type and number of all the atoms it consists of.  For example, the formula "H₂O" means that the compound has two hydrogen atoms and one oxygen atom.  Charges of ions are given as superscripts at the end of a formula  (e.g. SO₃^2-, which means one sulfur and three oxygen atoms with a negative two charge).
• In molecular structures, we show bonds between atoms with lines.  One line means a "single bond", two lines mean a "double bond", etc...
• To write a chemical reaction, we write the formulas of reactants, draw an arrow, and then write the formulas of the products.

OK, with that out of the way....

Potassium metabisulfite is ubiquitous in wine making.  It is the active component in campden tablets and can also be purchased in crystalline form at your local wine making supply store.  It is added to wine in order to produce sulfur dioxide, which is very effective at killing off unwanted microbes that can spoil the wine.  So, how does it work?

Metabisulfite is a complex anion (negatively charged ion) that consists of two sulfur and five oxygen atoms.  It has a -2 charge.  Generally, metbisulfite is sold as a potassium salt.  In potassium metabisulfite, the negative charge is balanced with two potassium ions (+1 charge each).  Sodium can also balance the negative charge.  However, sodium metabisulfite is not as common as the potassium form.  In terms of generating sulfur dioxide, it does not matter whether you use sodium metabisulfite or potassium metabisulfite - they both react the same way.

To see how metabisulfite is transformed chemically into sulfur dioxide, let's start by looking at the molecular structures of three compounds:

1.  Sulfur dioxide, SO₂.  The structure is relatively simple.  A sulfur atom is bonded to two oxygen atoms in a bent structure.

2.  Sulfite (SO₃^2-) and hydrogen sulfite (HSO₃^-).  These two chemical species are the principal forms of sulfite that exist whenever a sulfite compound is dissolved in water.  They are simply acid-base forms that interconvert with the addition or subtraction of hydrogen ions (H⁺).  That means that the relative amounts of the two depend on the hydrogen ion concentration, which is commonly measured as the pH.  At the pH of wine, which is, say, between 3 and 4, sulfite (SO₃^2-) only exists in minute amounts.  The most abundant form is hydrogen sulfite (HSO₃^-).

3.  Metabisulfite (S₂O₅^2-).  In the structure, it kind of looks like there is an SO₂ molecule attached to an SO₃^2- ion, with a bond between the two sulfur atoms.

When metabisulfite is added to water, it reacts immediately with water to generate hydrogen sulfite:

From a chemical point of view, you can think of the oxygen atom in H₂O "attacking" one of the S atoms (the one bonded to two oxygen atoms) to form a new sulfur-oxygen bond.  This breaks the bond between the two sulfur atoms and results in two hydrogen sulfite ions.  This may not be the exact mechanism (I have not looked it up), but it's a nice way to think about it.

Let's say you are making wine with a kit and the secondary fermentation is complete.  You just added the packet of potassium metabisulfite to your wine and you start stirring.  The reaction above just took place, and you now have a lot of HSO₃^- dissolved in your wine.

HSO₃^- reacts with H⁺ ions in the wine (remember, your wine is somewhat acidic) to form dissolved, "molecular" SO₂, which is written as SO₂^.nH₂O.  This means that SO₂ is "hydrated", or associated with water molecules, in solution.

This reaction is written as a chemical equilibrium, which means that at any instant, some of the compounds on the left are reacting to make compounds on the right, and vice versa.  This is why there are two reaction arrows.  After some time, the reaction comes to equilibrium, which means that the concentration of each compound stops changing.  At equilibrium, the relative concentrations of reactants and products are defined by an equilibrium constant, K, that is unique to the reaction.  For this reaction, K is written as:

Concentrations are indicated by the square bracket around each compound.  If you were to measure the equilibrium concentrations of SO₂^.nH₂O, HSO₃^-, and H⁺, entered the values in the expression above, and did the calculation, the answer would be 62.5.

If any of the concentrations change, then the concentrations of the other compounds must change over time so that the system comes back to equilibrium (and the expression again equals 62.5).  So, let's say you add an acid blend to your wine to change the flavour.  By adding acid, you have increased [H⁺], and the expression will no longer equal 62.5.  In order to get back to 62.5, the equilibrium must shift:  [SO₂^.nH₂O] will increase slightly; and [HSO₃^-] will decrease.  This is an important practical result.  The more acidic the wine, the more SO₂ will be produced from HSO₃^-.  Wines that are more acidic require less added metabisulfite.

At room temperature, SO₂ exists as a gas (not a liquid or solid).  Consequently, some of the SO₂ will be released as a gas.  This is one reason why, after adding metabisulfite to wine, you might observe bubbles coming out of the wine as you stir it.  Stirring can also release dissolved CO₂ that was produced during fermentation.

What happens to the dissolved, molecular SO₂ in the wine?  Several things:

• some of the SO₂ will become chemically bound to various wine components, like complex sugars.
• some escapes as a gas
• some stays free and dissolved, in equilibrium with HSO₃^-.

The goal is to have enough dissolved, molecular SO₂ in the wine to keep the microbes at bay.  Typically, 0.5 ppm SO₂ is needed for red wines, and 0.8 ppm is needed for white wines.  Commercial winemakers use analytical chemistry techniques to measure the SO₂ content in their wines.  This lets them make sure they meet regulations for the SO₂ content of wines that are sold on the market.  My wife, who is also a chemist, once had a summer job at a winery near Melbourne where she performed SO₂ assays, among other tasks.

The average home winemaker does not need to worry about measuring the SO₂ in their wines.  We can use a simple rule of thumb:  one campden tablet per gallon of wine will give you approximately the right SO₂ concentration.

For all the expert and amateur winemakers out there, I would love to know your thoughts on how much SO₂ is really needed in wines.  For example, in the book The Way to Make Wine, the author suggests adding campden tablets every time you rack your wine.  This strikes me as overkill.  On the other hand, some winemakers use none at all.  What is your experience?

Let me know if you find this useful, or, even better, if you have questions.  I will do my best to answer.

Merry Christmas!

Bibliography

Chemistry of the Elements, Greenwood, N.N. and Earnshaw, A., Pergamon Press, New York, 1984.

The Way to Make Wine, Warrick, S., University of California Press, Berkeley, 2010.