The Role of pH in Regulating Organic Chemistry: Part 2

Many of the most common organic chemical reactions in wine production are influenced or even regulated by pH. A sound understanding of this relationship can open a range of possibilities and techniques which may not at first seem intuitive.

pH and Redox Reactions

Redox reactions involve electron transfers, where one species gains electrons and another, or more than one species, loses them*. These are very commonplace in wine chemistry.

In these reactions, the transfer of electrons means that the oxidation state of the atoms are changed. Reduction is where a species gains electrons, thus its oxidation state is reduced; and oxidation is where a species loses electrons, meaning its oxidation state is increased.

For example, if hydrogen gas is burned in air, molecular hydrogen (H2) is oxidized by molecular oxygen (O2) to form water (H2O). Electrons transfer from the hydrogen (which is oxidized) to the oxygen (which is reduced).
As you can see, oxidation and reduction occur simultaneously and cannot happen independently of one another, similar to the acid–base reaction.

However, the oxidation part and the reduction part are each referred to as “half reactions”, as the two half reactions always occur together to form the whole.

Often, H+ ions will be needed to electrically balance the half reactions occurring in an aqueous solution. Therefore, pH can be a factor in how readily these reactions take place as pH is related to the concentration of H+ in the solution. This, in simple terms, can change the driving force of reactions and thus the redox potential.

Redox potential is a measure of the tendency of a chemical species to acquire electrons or lose electrons [to an electrode] and can be measured in volts (V), or millivolts. In aqueous solutions, redox potential is a measure of the tendency of the solution to either gain or lose electrons when a new species is introduced. This is a particularly useful measure when making additions, such as sulphur.

Vintech Pacific will shortly introduce a new range of technologies from Agrovin, which allow for the instant and accurate measurement of redox potential of juice, must or wine, in tank. Access to this information will give winemakers the ability to much more accurately understand the processes occurring, before and while they occur and to base their winemaking decisions on measurable data.

*This is not an entirely accurate description, but sufficient for the purposes of this article. There are many reactions, such as those involving covalent bonds, that are classed as “redox” even though no electron transfer occurs.