Introduction

The Sulphite Additions panel calculates the amount of Potassium or Sodium Metabisulphite to add to a wine or must to achieve a target SO₂ concentration.

FermCalc offers two target type options for this calculation:

• Free SO2 Level - Specify the total free SO₂ level and calculate the required sulphite addition as well as the resulting total molecular SO₂ level.
• Molecular SO2 Level - Specify the total molecular SO₂ and calculate the required sulphite addition as well as the resulting total free SO₂ level.

Making a Calculation

To make a calculation, follow these steps:

1. Select Calculation > Sulphite Additions from the menu, or select the Sulphites tab at the top of the main window.
2. Select either Free SO2 Level or Molecular SO2 Level from the Target Type menu.
3. Select either Potassium Metabisulphite or Sodium Metabisulphite from the Sulphite Type menu.
4. Select either Campden Tablets, Crystals, or Solution from the Sulphite Form menu. If Solution is selected, enter the strength of the solution.
5. Select the appropriate units for the input and output fields using the drop-down menus in the right-hand column.
6. Enter the Wine Volume, Wine pH, and Initial Free SO2 Level of the wine being treated in the appropriate fields.
7. If you've selected Free SO2 Level in the Target Type box, enter the Target Free SO2 Level. FermCalc will then calculate the required sulphite addition and the resulting molecular SO₂ level.
8. If you've selected Molecular SO2 Level in the Target Type box, enter the Target Molecular SO2 Level. FermCalc will calculate the required sulphite addition and the resulting free SO₂ level.

Calculation Details

Assuming that all of the added SO₂ remains free and none becomes bound, each molecule of Sodium or Potassium Metabisulphite yields two molecules of free SO₂. So, to calculate the amount of Metabisulphite to add, we need to know the molecular weights of both SO₂ and the Metabisulphite.

Compound	Formula	Molecular Weight
Sulphur Dioxide	SO2	64.06
Potassium Metabisulphite	K2S2O5	222.3
Sodium Metabisulphite	Na2S2O5	190.1

The formula for the amount of Metabisulphite to add to yield a target concentration is:

mm = v(ct - ci)mwm / (2mws)

(1)

where

m_m = mass of Metabisulphite required (kg)
v = volume of wine (liters)
c_t = target concentration of free SO₂ (kg/liter)
c_i = initial concentration of free SO₂ (kg/liter)
mw_m = molecular weight of the Metabisulphite
mw_s = molecular weight of the SO₂

The portion of the free SO₂ that protects wine from oxidation and spoilage is the molecular SO₂. The amount of molecular SO₂ in the wine depends on the amount of free SO₂ and the pH of the wine. The formula for units of free SO₂ required for 1 unit of molecular SO₂ is:

f = 10(pH - 1.77) + 1

(2)

where

f = units of free SO₂ required for 1 unit of molecular SO₂
pH = pH of the wine

The mass of Metabisulphite required to yield a target molecular SO₂ concentration is:

mm = v(fcm - ci)mwm / (2mws)

(3)

where c_m is the target molecular SO₂ concentration.

If the target free SO₂ level is less than the initial value, output fields are highlighted in red and an error message is displayed.

For some excellent information on the use of SO₂ in winemaking see this article by Charles Plant, or this one by Daniel Pambianchi.