Determination of Vitamin C (Ascorbic Acid) Contents in Vegetable Samples by UV-Spectrophotometry and Redox Titration Methods and Estimation the Effect of Time, Cooking and Frozen on Ascorbic Acid Contents

Abstract

The concentrations of Vitamin C, also known as L-ascorbic acid, in twenty two different types of fresh vegetables were determined by spectrophotometric and Back Redox Titration methods. The effects of some conditions such as time, cooking and frozen on the Vitamin C contents were studied. The samples were collected from different markets in Benghazi city from different places. Each sample or a part of it was weighed, squeezed in a juicer machine and centrifuged, the volume and the weight of the supernatant liquid were taken. The supernatant liquid was homogenized with 5% metaphosphoric acid-10% acetic acid solution. The ascorbic acid was oxidized to dehydroascorbic acid by bromine water in presence of acetic acid and the excess of bromine was removed by a few drops of 10% thiourea . A coupling was done with 2,4-dinitrophenyl hydrazine by putting the solution in a water bath at 37°C temperature for three hours, it was cooled in an ice bath and was treated with chilled 85% H₂SO₄ .  A red colour complex was produced and the absorbance was measured spectrophotometrically at 521 nm. Beer’s law was obeyed. Another method for the determination of ascorbic acid was done by an iodimetric back-titration in which an excess measured amount of iodine was generated in the sample and then the unreacted iodine was titrated with a standard solution of sodium thiosulfate. The estimation of ascorbic acid contents was done for fresh, after 4 and 7 days, cooking and frozen samples.