# 32 How Much Sulphite Is Needed?

Neither SO2 nor sorbate kills yeasts; they inhibit them, and can
prevent microbial activity, but only if cell counts are low. If you
have a mounting problem, they won't do a good job in controlling it.
The amount of sulphite needed depends on the pH of the wine -- the
lower the pH the less you need (at pH 3.2, you need 21ppm (21mg/L) free
SO2; at pH 3.5, you need 50ppm (50mg/L) free SO2.) This has to do with
A) the fact that the active form that inhibits bacteria forms better at
lower pH's and B) the lower the pH, the better the acidity in the wine
is in itself able to protect the wine. The following is the pH
dependant equilibrium. The forms depicted in the left are favoured by
higher pH's; the right by lower pH's.

```SO2 + H2O <---> HSO3- + H+ <---> SO3-- + H+
```

1 ppm = 1 mg/L, therefore for 5 imperial gallons of wine with a pH of
3.2, you need:

```5gal*4.5L/gal = 22.5L

21mg/L*22.5L = 472.5mg
```

Since this is free SO2, we need a conversion for potassium and sodium
metabisulphate, (K2S205 and Na2S205 respectively) which are 1.74 and
1.48 respectively. So we need 0.8g or 1.7g of each respectively -- a
little under an eigth of a teaspoon. Through the same process you need
a quarter teaspoon for 5 gallons of wine with pH 3.5. A campden tablet
has a mass of either 0.44 or 0.55 gram (depending on where you get your
tablets), or about 1/15th or 1/12th of a teaspoon respectively.

It's always important to remember that both of these products work
better with low pH's, so a non-standard wine (i.e. fruit wine) may
require really large amounts due to high pH.

There is unfortunately no handy way to actually kill all the yeast in

As a general guide, here is how much sulphite is needed as per the pH of

the following free sulphite levels are needed:

``` pH   Red     White
2.90    7ppm  11ppm
3.00    8     13
3.20   13     21
3.40   20     32
3.60   31     50
3.70   39     63
```

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