Why is ascorbic acid unstable in water

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Redox behavior of vitamin C.

Ascorbic acid is a powerful reducing agent. The redox potential is (4).

This property is decisive for the chemical behavior of the substance. The redox behavior:

  • is the basis for the effectiveness of vitamin C in the organism
  • justifies the use of ascorbic acid as an antioxidant in food (E300)
  • forms the basis of most analytical evidence.

Ascorbic acid is easily and reversibly oxidized to dehydroascorbic acid in aqueous solution. The reaction takes place on the hydroxyl groups of the enediol structure in stages via the formation of an unstable free radical, semidehydroascorbic acid.

Dehydroascorbic acid is subject to spontaneous, irreversible hydrolysis in aqueous solution. This reaction leads to the opening of the ring structure with the formation of 2,3-diketogulonic acid. However, the physiological effect as a vitamin is lost.

The rapid oxidation of the dissolved ascorbic acid, which ultimately leads to the destruction of the vitamin, is the cause of the instability of the substance.

Since protons are involved in the redox reaction, the position of the equilibrium is strongly dependent on the value. Ascorbic acid is only relatively stable in acidic solution. Traces of heavy metal ions (such as Cu2+, Fe3+, Ag+) act catalytically and accelerate the oxidation extraordinarily. Other influencing variables are light, temperature and the amount of dissolved oxygen.

Conclusions

  1. In food processing, limiting the loss of vitamin C through appropriate treatment (short storage time, low heating, etc.) is of great importance.
  2. For the analysis of vitamin C, the stabilization of the ascorbic acid or the subsequent products (see above) is an essential part of the sample preparation and determines the selection of the detection methods.