pH and oxidation

Jon,

Wine oxidation can change the pH, but it is a weak relationship.

SO2 + H2 O H+ + HSO3- 2H+ + SO3--

This above equation indicates that sulfur dioxide in water exists in three forms. (1) Some of the sulfur dioxide exists simply as dissolved gas (SO2) and this form is called molecular sulfur dioxide. (2) Some of the sulfur dioxide exists in the singly charged bisulfite ion form (HSO3- ) and (3) some in the doubly charged sulfite ion form (SO3-- ).

The two double arrows also show how chemical equilibrium is maintained. If a few SO3-- ions were to disappear (for whatever reason), the equation shows that a spontaneous reaction would take place from left to right. A few HSO3- ions would convert into SO3-- ions, and a few SO2 molecules would convert into HSO3- ions. The ratio of the SO2, HSO3- and SO3-- would then be restored.

Since the arrows point in both directions, this equation also shows the reaction can take place from right to left. In this case, if some SO2 disappeared, then some of the HSO3- ions would spontaneously convert into SO2 and some SO3-- ions would convert into HSO3- ions. Equilibrium would be maintained, and the ratio of the SO2, HSO3- and SO3- would again be restored.

This equation also shows that when sulfite (SO3--) disappears, hydrogen ions (H+) are produced and the pH of the wine is changed.

So, when wine oxidizes in the presence of sulfur dioxide, the production of hydrogen ions (as sulfite disappears) causes the pH to decrease (and the titratable acidity to increase). This effect is small but it can be significant in high pH wines. A reduction of 0.1 or so in pH is often observed when high pH wines are aged in barrels for a year or more.

Lum Del Mar, California, USA

Lum,

Thanks for the reply. I had a small batch of Sangiovese that I did not know the pH before it fermented. After resting in the bottle for a year, I then checked the pH to find it at 3.83 and SO2 at 40ppm.

So I was trying to determine if the wine partially oxidized due to inadequate SO2 levels or if the pH was a little high to begin with. After reading your post, I'm assuming the pH was probably high to begin with and the SO2 was too low.

Thanks again for the information. Jon