Laboratory work 4 Determination of the content of vitamin c as a biologically active additive in drinks from various manufactures

Laboratory work 4
Determination of the content of vitamin C as a biologically active additive in drinks from various manufactures
QUALITATIVE DETERMINATION OF ASCORBIC ACID
Ascorbic acid (vitamin C) 2,3-dihydro-α-gulonic acid lactone.
The method is based on a redox reaction between ascorbic acid and 2,6-dichlorophenolindophenol (Tillmans' stain).
2,6-dichlorophenolindophenol shows two types of reaction (figure). One type is due to a change in the pH of the medium, as in conventional acidometric indicators; in this case, there is a transition from an intense blue color in an alkaline environment to a pale red color in an acidic environment. The color transition occurs between pH 4 and 5, in this range indicator is purple. The second type of reaction is the OB-transition from a dark blue oxidized state to a colorless one. This reaction is used to determine ascorbic acid. Acid extracts from plants are titrated with an indicator solution (known titer) until a pink color occurs, due to an excess of the indicator in an acidic medium. Excess dye in an acidic environment gives a pink color.

ascorbic acid