How to Determine Free SO2 at Bottling
By Tom Banach

When you hear SO2 management, your eyebrows might rise.  But if you keep it simple, the results of managing your wine SO2 can keep your wines protected and at their finest for years to come. I have put together the following information so that you can easily maintain your sulfur dioxide. It is easier than you think.

Things you need to know

• Potassium metabisulfite is 57% sulfur dioxide. ¼ teaspoon weighs 1.25 grams.
• Campden tablets are 48% sulfur dioxide. 1 tablet contains .44 grams of potassium metabisulfite.
• One gallon is equal to 3.785 liters.
• There are 1000 mg (milligrams) in one gram.

Equipment you need:
A pH meter and Chemetrics Titrets (the Titrettor is optional). Easy to follow instructions come with each pack.

At bottling the desired amount of free SO2 should be determined by the color and pH of the wine

• If the pH is in the range 2.9 - 3.1, white wines should contain 10-16 ppm and red wines 7-10 ppm.
• If the pH is in the range 3.2 - 3.4, white wines should contain 21-32 ppm and red wines 13-20 ppm.
• If the pH is in the range 3.5 - 3.7, white wines should contain 40-63 ppm and red wines 25-39 ppm.

“At bottling the desired amount of free SO2 should be determined by the color and pH of the wine.”

Let’s say we have five gallons of dry red wine with a pH of 3.5 that we are about to bottle. We check the starting amount of SO2 using a titrette, and it says we currently have 10 ppm. Checking the chart above, we see that we want to get the wine within a range of 25-39 ppm, so we decide to shoot for around 30 ppm free SO2. The difference is 20 ppm. But since this wine is dry, we will add an extra 5 ppm to make the total SO2 adjustment 25 ppm.

Now for the math:
5 (number of gallons) times 3.785 (number of liters in a gallon) = 18.925 liters.
18.925 liters times 25 (ppm needed to be added) = 473.125 ppm SO2. We will get back to this number in a moment.
Now we want to find the weight of the SO2. For this example we will use potassium metabisulfite instead of Campden tablets.
1000 mg times .57(the amount of SO2 per gram of potassium metabisulfite) = 570mg.

To finish this calculation, divide 570 into 473.125 = 0.83 grams of potassium metabisulfite that needs to be added to the wine.

1/4 teaspoon of potassium metabisulfite weighs 1.25 grams; therefore 1/8 teaspoon weighs .625 grams.  We need .83 grams, so the final total amount will be about 1/6 teaspoon potassium metabisulfite. Or we could add two Campden tablets, since each Campden table contains .44 grams and two of them would be .88 grams, close enough.

Remember, when adding either of these to your wine, dissolve first in warm water and stir into wine.

This article reprises Tom’s presentation at the general club meeting on January 10. Resources for his demonstration and this article were obtained from Alison Crowe, Domenic Carisetti and Chemetrics.