Managing sulfur dioxide (SO2) is one of the most critical skills in winemaking. It protects your hard work from oxidation and microbial spoilage, but getting the levels "just right" is a bit of a balancing act.



Here is a guide to help you master SO2 additions based on your wine's pH.

Understanding SO2: Free vs. Bound

When you add sulfite to wine, it doesn't all stay "active." It splits into two forms:

  • Bound SO2: This portion binds with elements like acetaldehyde, sugar, and oxygen. Once bound, it no longer protects your wine.


  • Free SO2: This is the "unbound" portion. This is the only part we care about for protection, as it provides the antioxidant and antimicrobial benefits your wine needs.

The Goal: Use the minimum amount of Free SO2 required to reach a beneficial "molecular" saturation level—roughly 0.5 ppm for reds and 0.8 ppm for whites—without adding so much that it affects the smell or taste.

The pH Connection


The amount of Free SO2 you need depends entirely on your wine’s pH.

  • Higher pH (less acidic): Requires more Free SO2 to be effective.

  • Lower pH (more acidic): Requires less Free SO2 to be effective.

Because SO2 is more "active" in acidic environments, knowing your pH is the only way to determine your target Free SO2 level.

Step 1: Test Your Current Levels

Before adding anything, you must know your starting point. Even if you haven't added sulfite yet, grapes often arrive from the vineyard with some SO2 already present.


Testing Options:

  • Titrets (W510): The simplest and most affordable option for home winemakers. While not perfectly accurate, they provide a solid reference point.

    • Pro-Tip: Test before adding SO2, then test again after. The difference between the two results tells you exactly how much of your addition remained "Free."

  • Manual A.O. Setup (MT130): A professional-style "Aeration-Oxidation" wet lab setup. It takes about 15 minutes per test but is highly accurate.

Step 2: Calculate the Addition

Once you have your current Free SO2 level and your target level (based on your pH chart), you can calculate your addition.

The Formula

To find the grams of Potassium Metabisulfite (KMBS) needed, use the following equation:


ppm of Free SO2 needed x 3.785 x Gallons of wine  
                                570 


Note on the math: The number "570" represents the fact that KMBS is roughly 57% SO2 by weight, adjusted to keep your ppm units simple.

Example Calculation

  • Current Free SO2: 12 ppm

  • Target Free SO2: 24 ppm (based on a Red wine with a pH of 3.5)

  • Amount to add: 12 ppm ($24 - 12 = 12$)

  • Volume: 5.5 gallons


The Math:

{12 x 3.785 x 5.5} / {570} = 0.438 grams of Metabisulfite


Quick Tips for Success

  • Don't Guess: It’s tempting to "eyeball" it, but SO2 is measured in tiny increments. A small digital scale that weighs to 0.01g or 0.1g is a winemaker's best friend.

  • The "Starter" Rule: If you are using Titrets and aren't sure of their accuracy, use the "Test-Add-Test" method mentioned above to see how your specific wine "binds" the SO2.

  • Storage: Keep your Metabisulfite in a cool, dry, tightly sealed container. If it loses its pungent "sharp" smell, it may have lost potency.