**2. Autoxidation and kinetics of tartaric acid**

In the study of wine oxidation, the original work by Fenton should be considered as it involves major components found in wine and wine ageing: tartaric acid, iron, and oxygen. It is well known that Fenton used hydrogen peroxide, with Fe(II), to drive the oxidation of tartaric acid. However, he also qualitatively describes the oxidation of ferrous tartrate with air [6], without the addition of hydrogen peroxide, which is now described as autoxidation. It is this reaction and its kinetics with air that sparked further exploration and discussion of pH dependences and autocatalytic kinetics [7, 8]. The study of tartaric acid, iron, and oxygen kinetics under wine-like conditions adds yet another dimension to our understanding of this famous reaction.

Wine-like conditions, in this study [9, 10], restrict the pH to an experimental range from 2.5 to 4.5, while constraining reactants: tartaric acid to 4 g/L (26.7 μM), Fe(II) to 5 mg/L (89.5 μM), and oxygen to 8.5 mg/L (265 mM). In addition, wine is generally stored in the dark or in darkened bottles to prevent photooxidation. Fenton indicates that the colorimetric response from this fundamental reaction does not appreciably happen without light [11], however with current spectrophotometric instruments, the reaction can be followed without the acceleration from light. The work presented here will also explore a special condition where Fe(II) is equimolar to oxygen, 265 μM, which leads a deeper understanding of the chemical reaction, component limitation and the underlying kinetics.

The time course measurements of dissolved oxygen consumption and Fe(III) formation (**Figure 1**) show the autocatalytic nature of tartaric acid oxidation with three distinct phases: initiation, propagation, and termination. The initiation phase clearly shows a kinetic pH dependence. The work by Smythe [8], proposed the kinetic importance of pH and the Fe(II)-tartrate complex. Tartaric acid, a dicarboxylic acid, exists as three species in this pH range: tartrate (R–), bitartrate (RH-), and tartaric acid (RH2). With varying pH, the tartrate species concentration changes [12–15], thus changing the free Fe(II), and more importantly Fe(II)-tartrate concentration which correlates with a kinetic role in the initiation. The free Fe(III) and Fe(III)-tartrate ligand(s) concentration also changes across this pH range which increases the intricacy of the reaction mechanics and the intermediate species.

The same time courses (**Figure 1**) show a distinct 1:1 molar relationship between oxygen consumption and Fe(III) formation. It would be expected that Fe(III) formation correlates with Fe(II) consumption. This relationship between iron and oxygen must be maintained when developing a kinetic mechanism for the reaction.

The propagation and termination phases vary with pH as it does with initiation. Increased pH reduces the initiation period, the rate of propagation and the extent to which oxygen is consumed. The termination phase will elucidate additional kinetic considerations when elevated Fe(II) concentrations (265 μM) at pH 2.5 and pH 3.0 (**Figures 2** and **3**) are evaluated. At these levels, the Fe(III) formation terminates as the oxygen concentration is depleted, however beyond this point Fe(III) is consumed, apparently returning to Fe(II). This would indicate a secondary reaction with Fe(III) and an intermediate, thus driving the conversion back to Fe(II).

#### **Figure 1.**

*Oxygen consumption and Fe(III) formation at pH 2.5 to 4.5. Initial conditions at 89.5 μM Fe(II) to initiate the autocatalytic reaction in air-saturated 26.7 mM tartaric acid at pH 2.5 ( ), 3.0 ( ), 3.5 ( ), 4.0 ( ), 4.5 ( ).*

#### **Figure 2.**

*Oxygen consumption at pH 2.5 to 4.5 with 256 μM Fe(II). Autocatalytic reaction in air-saturated 26.7 mM tartaric acid at pH 2.5 ( ), 3.0 ( ), 3.5 ( ), 4.0 ( ), 4.5 ( ). Reproduced from [10], with the permission of AIP Publishing.*

**Figure 3.**

*Fe(III) formation at pH 2.5 to 4.5 with 256 μM Fe(II). Autocatalytic reaction in air-saturated 26.7 mM tartaric acid at pH 2.5 ( ), 3.0 ( ), 3.5 ( ), 4.0 ( ), 4.5 ( ). Reproduced from [10], with the permission of AIP Publishing.*

#### **Figure 4.**

*Hydrogen peroxide formation. Autocatalytic reaction initiated with 265 μM Fe(II) in air-saturated 26.7 mM tartaric acid at pH 2.5 ( ), 3.0 ( ), 3.5 ( ). Reproduced from [10], with the permission of AIP Publishing.*

Hydrogen peroxide has been proposed as a reactant for tartaric acid oxidation [6] and an intermediate in wine oxidation [1, 16]. The simultaneous measurements of hydrogen peroxide at elevated Fe(II) concentrations (265 μM) also show a pH dependency (**Figure 4**). The lower pH levels have a measurable hydrogen peroxide formation with a peak concentration roughly developing as oxygen is depleted. This peak timing can be clearly seen in modeling fitting figures (**Figure 5**). It appears that a

*The Kinetics of Autoxidation in Wine DOI: http://dx.doi.org/10.5772/intechopen.103828*

#### **Figure 5.**

*Fitted consumption and formation curves with proposed mechanism. Dissolved oxygen ( ) and predicted ( ), Fe(III) ( ) and predicted ( ), and hydrogen peroxide ( ) and predicted ( ) time traces modeled with dissolved oxygen, Fe(III), and hydrogen peroxide time traces at 265 μM initial Fe(II) in air-saturated 26.7 mM tartaric acid at (a) pH 2.5, (b) pH 3.5 and (c) pH 4.5. Reproduced from [10], with the permission of AIP Publishing.*

more rapid propagation rate leads to a greater peak hydrogen peroxide concentration. In turn, this peak hydrogen peroxide concentration has a 1:2 molar ratio with the decomposition of Fe(III) (**Figures 3** and **5**). Such a decomposition has been previously explored [17], however oxygen does not appear to be regenerated from the decomposition of hydrogen peroxide in this reaction.
