Central Texas Wine Guild

SO2 is a gas with a nasty, pungent sulfur smell. It is soluble in water, and has potent antimicrobial properties. Additionally, it has important antioxidant effects (it reacts with acetaldehyde, and it inhibits some enzymes that lead to browning of wines, although at the pH associated with wine, SO2 doesn’t actually do much binding with oxygen directly). Appropriate additions of SO2 (correct quantity, correct timing) can inhibit the growth of spoilage organisms, help prevent oxidation, and actually reverse some of the problems with oxidation if already present.

Unfortunately for us, we have to be a bit careful with SO2—too much and our wine tastes like a match—the burning sulfur from a match produces sulfur dioxide.

SO2 in water (technically in an “aqueous solution”) exists in an equilibrium state with some other related molecules. These related molecules, for our purposes, are commonly referred to as “SO2” even though, strictly speaking, they are something different.

How do we get SO2 into our wine? We could add SO2 directly by bubbling SO2 gas into our wine (complicated and dangerous). Better, we could bubble SO2 gas into water (and then add the water), but this is also complicated and dangerous for the home winemaker. Alternatively, we could add a much more stable powdered form of SO2 to our wines (easy and much safer for home winemakers), or dissolve the powdered form in water, and then add that solution.

This last approach is far and away the best approach: use the powder, and either add a known amount of powder, or dissolve the powder in water to make a solution of known strength, and then add that solution to the wine. For us, the best form of SO2 would be potassium metabisulfite (sodium metabisulfite could be used, but most agree that it is best to avoid extra sodium in wines). Now, the metabisulfite itself isn’t really what we care about: we want SO2. Fortunately, when potassium
metabisulfite (KMS, or KMB, or KMBS) dissolves in water, it breaks down in a reaction with the water molecules, producing some SO2. Unfortunately, not all of the various SO2 products are free in the
solution; some (in wine or must) become bound to various organic molecules, and these basically are of no use to us. Bound SO2 has no significant antimicrobial or antioxidant activity. Further complicating things is the fact that only a portion of what is called “SO2” is actually the active molecule; this portion is called “molecular SO2” and the amount of it in our wine is pH dependent. The other forms of sulfur dioxide in wine are HSO3 and SO3. More on this in a moment.

pH as you will recall is a measurement of acidity; the LOWER the number, the MORE acid the wine. And pH is a logarithmic scale: each unit lower means that it is 10 times more acid. So, pH of 3 is 100 times more acid than is a pH of 5 (!). As such, even small pH differences (pH 3.2 vs 3.7) can and do have big consequences. It turns out that at lower pH, more of the SO2 compounds are in the molecular (i.e. active) SO2 form. And conversely, as pH rises, exponentially less of the molecular SO2 is present.

The pretty picture below shows how pH affects the amount of the molecular SO2 that we actually care about. On the horizontal axis is pH; more acid to the left, less acid to the right (stand up, sit down,
fight fight fight!). The vertical axis is the percent of the particular molecule in question in the solution. So, let’s look at pH 4. At this point, almost all of the sulfur dioxide compounds are in the form of HSO3. At the bottom we can see that a miniscule amount is available as molecular SO2. As pH fall even a little bit, we start getting increasing amounts (albeit still small) of molecular SO2. This is the range that we winemakers work in: if the pH is a bit too high, almost no amount of added KMS will produce significant amounts of molecular SO2, and if the pH is ideal, not much will have to be added to get the desired protection.

So, here is the summary of the various SO2 types:

• Molecular SO2: this is what we most care about. This refers to the actual molecule of sulfur dioxide, dissolved in water/wine, waiting to do something beneficial for us (kill a bacteria or yeast, or react with another molecule to minimize oxidation faults). Typical amounts of this that we want to have in our wine are 0.5 ppm (parts per million) for a red, and 0.8 ppm for a white.
• Total SO2: this is all forms of SO2 and SO2 related molecules (includes molecular SO2, HSO3, and SO3, and includes the free and bound portions). Knowing this number is relevant to the extent that it helps guide us when we want to know how much we need to add to get a desired result.
• Free SO2: This is the unbound portion of all of the various SO2 molecules. It and pH determine how much molecular SO2 our wine will have. But, because pH is important, it is possible to have a high free SO2 and yet have very little molecular SO2 in the wine (which, as discussed above, is all that we care about). And, vice versa.

Now, our Hanna Mini-Titrator will measure Total SO2 and Free SO2. If we know the free SO2, and we also measure the pH, we can determine from a table the molecular SO2. If our molecular SO2 is in the desired range, we are done!

This table is below. If our wine has a pH of 3.4, and your goal is to have 0.5 ppm of molecular SO2, you would need to have a Free SO2 of 32 ppm. Note that at a pH of 3.7 the amount of Free SO2 needed is DOUBLE.

If our molecular SO2 however is low, then we need to add more. How much more? That is where Jon’s article http://centraltexaswineguild.org/articles/making-so2-adjustments before comes in to play (now would be a good time to go and re-read it). Basically, we need to add enough potassium metabisulfite so that after much of it is bound (we can get an idea about that from the ratio of free to total in our measurement with our kit) we will have enough free so that at whatever pH our wine is at we will now have enough molecular SO2 around.

So, for our meeting, I would suggest that for starters, we measure the pH and “free SO2” in our wine. We will have the tables handy to look up and see how much molecular SO2 this is. If you are in range, you are done.

If you are low, then you have two choices:

• Make a guess about how much of the potassium metabisulfite that you add will become bound, and then add the amount needed to provide your wine with enough molecular SO2.
• Measure the total SO2 (again, using the kit), and make a more educated estimate of how much to add. For example, if the total is 40 and the free is 10, then 75% of the SO2 is bound. Thus, if you want to get the free to 30, you would need to add enough to increase the free by 20 (from 10 to 30); this would require addition of a total of 80 (since in this example 75% is bound).