FermCalc - Sulfite Calculator

# Introduction

The Sulfite Calculator determines the amount of potassium or sodium metabisulfite to add to a given volume of wine in order to achieve the desired concentration of sulfur dioxide (SO2) in the wine.  The target can be specified in terms of free SO2 or molecular SO2.

# Input Field Definitions

Target Type – The type of SO2 concentration target, either free SO2 or molecular SO2.

Sulfite Type – The type of metabisulfite to be added to the wine, either potassium metabisulfite or sodium metabisulfite.

Sulfite Form – The form of the metabisulfite being added to the wine, either campden tablets, powder/crystals, or stock solution.

Campden Tablet Sulfite Mass – The mass of the sodium or potassium metabisulfite in the campden tablets being used.  This is most commonly 440 mg or 550 mg.  This is not the total mass of the tablets because campden tablets also contain inert fillers and binders in addition to sulfites.

Stock Solution Concentration – The concentration of the stock solution, required only if stock solution is selected as the sulfite form.

Target Free SO2 – The target free SO2 concentration in the wine, required only if free SO2 was selected as the target type.

Target Molecular SO2 – The target molecular SO2 concentration in the wine, required only if molecular SO2 was selected as the target type.

Initial Free SO2 – The current free SO2 concentration in the wine.

Wine Volume – The volume of the wine being sulfited.

Wine pH – The pH of the wine being sulfited.  This value is used with the equilibrium constant to relate free SO2 concentration and molecular SO2 concentration.  It is required only if molecular SO2 was selected as the target type, or if an estimate of molecular SO2 is desired.  Range: 0 to 14

SO2 Equilibrium Constant – The equilibrium constant (pKa) of SO2.  This value is used with the pH to relate free SO2 concentration and molecular SO2 concentration.  It is required only if molecular SO2 was selected as the target type, or if an estimate of molecular SO2 is desired.  Range: 1.75 to 1.85

# Output Field Definitions

Resulting Molecular SO2 – The calculated molecular SO2 concentration, reported only if free SO2 is selected as the target type.

Resulting Free SO2 – The calculated free SO2 concentration, reported only if molecular SO2 is selected as the target type.

Campden Tablets Required – The calculated number of campden tablets required to achieve the target SO2 concentration, reported only if campden tablets was selected as the sulfite form.

Powder/Crystals Required – The calculated mass of metabisulfite powder/crystals required to achieve the target SO2 concentration, reported only if powder/crystals was selected as the sulfite form.

Stock Solution Required – The calculated volume of stock solution required to achieve the target SO2 concentration, reported only if stock solution was selected as the sulfite form.

# Calculation Details

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

Compound Formula Molecular Weight
Sulfur Dioxide SO2 64.06
Potassium Metabisulfite K2S2O5 222.3
Sodium Metabisulfite Na2S2O5 190.1

Since one mole of metabisulfite will yield two moles of free SO2, and the number of moles of a compound is equal to its mass divided by its molecular weight, we can write:

 ms/mws = 2mm/mwm (1)

where

ms = mass of free SO2 added, kg
mws = molecular weight of SO2, grams/mole
mm = mass of metabisulfite added, kg
mwm = molecular weight of the metabisulfite, grams/mole

The mass of SO2 added is equal to the volume multiplied by the increase in concentration, or:

 ms = v(ct - ci) (2)

where

v = volume of wine, liters
ct = target concentration of free SO2, kg/liter
ci = initial concentration of free SO2, kg/liter

Combining equations (1) and (2) and solving for the the mass of metabisulfite to add mm gives us:

 mm = v(ct - ci)mwm / (2mws) (3)

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 amount of free SO2 required for 1 unit of molecular SO2 is:

 f = 10(pH - pKa) + 1 (4)

where

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

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

 mm = v(fcm - ci)mwm / (2mws) (5)

where cm is the target molecular SO2 concentration. Commonly used values of the equilibrium constant pKa are 1.77 and 1.81. The default value in FermCalc is 1.81.

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.