Titration of Vitamin C

Titration is a tool used in the laboratory for quantitative chemical analysis of a chemical. It helps chemists to determine the concentration of a particular compound. Since the measurement of volume plays a key role in titration it is also referred to as volumetric analysis. The basics of food titration involve determining the concentrations of different compounds in foods such as vitamins, minerals and trace elements. It plays a major role in both the food and the pharmaceutical industry. All those labels on food packets and in dietary supplements contain the precise quantity of the different compounds contained within. These precise amounts are determined in the laboratory using the process of titration.

Titration uses a reagent, also known as a titrator or titrant, of a standard concentration. This titrant is used to react with the solution being analyzed, also known as the analyte. Since the volume and concentration of the titrant are known, the concentration in the analyte can be determined after the reaction. Let us consider how to determine the concentration of vitamin C in a solution. One way of determining vitamin C concentration is performing a redox titration using iodine.

Vitamin C is also known as ascorbic acid and is an essential vitamin required by the body for its proper functioning. Additionally, it is a powerful antioxidant and helps protect the body from the damaging effects of free radicals. During a redox titration, iodine is added, drop by drop, to a solution of vitamin C. This causes a chemical reaction, in the process oxidizing ascorbic acid into dehydroascorbic acid and reducing the iodine to iodide ions. Iodine will continue to be reduced into iodide ions as long as ascorbic acid is still present in the solution.

Once all the ascorbic acid has been oxidized, the excess iodine reacts with a starch solution which was premixed with the ascorbic acid solution. The reaction between iodine and starch produces a typical blue-black color due to the formation of a starch-iodine complex. This change in color is the end of the experiment. The volume of iodine required to completely oxidize the ascorbic acid and bring about the change in color can now be used to determine the concentration of ascorbic acid in the solution.

This method can be used to determine vitamin C concentrations in vitamin C supplements, fresh or packaged fruit juices and other foods such as vegetables and fruits. The method of redox titration is easier to perform than alternate methods of titration using potassium iodate but is not as reliable as the latter. This stems from the fact that potassium iodate solution is a lot more stable than iodine and makes for a much more reliable standard.

In a redox titration with potassium iodate, an additional reagent, potassium iodide is needed. When iodate ions (IO3ˉ) react with iodide ions (Iˉ) in the presence of an acid such as vitamin C, an oxidation-reduction reaction takes place. The iodate ions are reduced to iodine and the iodide ions are oxidized to form iodine. It is this iodine, formed by the oxidation and reduction of iodide and iodate ions respectively, that oxidizes the ascorbic acid. Once all the ascorbic acid has been oxidized, the iodine formed reacts with the starch solution to form the blue-black color that indicates the end of the experiment.

Since this method used the oxidation-reduction reaction of iodide and iodate ions to produce iodine, which then reacts immediately with the ascorbic acid and starch, the stability of iodine is not a factor. This is also why this method is accepted as being the more accurate one.

  1. http://biology.clc.uc.edu/fankhauser/labs/anatomy_&_physiology/a&p203/titrations/vitc_protocol/vitc_protocol.html
  2. http://www.outreach.canterbury.ac.nz/chemistry/vitamin_C_iodine.shtml