Prelab Assignment – Please turn your answers in on GaView prior to lab
1. How will you prepare a .02% (m/v) NBS solution? Calculations should be in your notebook before arriving to lab.
2. Provide the structure of vitamin C in its reduced form (ascorbic acid), its ionized form (ascorbate ion- this is at equilibrium with reduced form when dissolved), and its oxidized form (dehydroascorbate).
3. Why do you think that the indicator solution contains acetic acid? How might its absence affect the titration?
4. How much vitamin C has been reported in the fruits that we will be using in today’s lab? Cite your reference. Be sure to list all units on your values. Do not report % of RDA per serving.
Estimating the Vitamin C Concentration in Fruit Juices: A Redox Titration
* Acquire laboratory skill in redox titration.
* Acquire laboratory skill in the use of a glass pipet.
* Standardize a solution to be used as a titrant.
* Conduct multiple titrations to determine an average concentration of Vitamin C present in a fruit juice sample.
In this lab, each pair of students will standardize an N-bromosuccinimide (NBS) solution with a known amount of Vitamin C and use this standard solution to determine the amount of Vitamin C present in a fruit juice sample.
Vitamins are essential micronutrients in our diet. We need them in small amounts to stay healthy. Although they were originally thought to be “vital amines,” we now know that there are two categories of vitamins: fat-soluble (A, D, E, K) and water-soluble (the B complex and C). Most vitamins act as cofactors with metabolic enzymes to maintain our life processes. Vitamin C, or ascorbic acid, is a water-soluble molecule that acts as an antioxidant, i.e. if oxidizing agents attack a cell, then the vitamin C acts as a target in place of another more important target such as a protein. Vitamin C also acts as a cofactor for an important enzyme that synthesizes collagen, the most abundant protein in vertebrate animals (such as humans). Collagen is a fibrous protein in connective tissue that literally holds us together. Vitamin C deficiency results in a disease called scurvy, which is characterized by skin lesions, bleeding gums, and tooth loss. Historically, vitamin C played an important role in the discovery of the “New World.” Sailors on long voyages with no available fresh fruit or vegetables commonly developed scurvy, which was described by Jacques Cartier during his exploration of North America in 1535. In 1768, James Cook introduced the addition of lime juice to the diets of British sailors, who then became known as “limeys.” By 1795, Lind advocated fresh citrus as a cure for scurvy. Dutch and Hungarian sailors also used sauerkraut as a source of vitamin C, and American sailors carried cranberries. The chemical structure of vitamin C was not discovered until 1932. More recently (1970’s) the two-time Nobel laureate Linus Pauling controversially advocated large doses of vitamin C as a prevention for the common cold. Vitamin C can be measured using a technique called titration. Because vitamin C is easily oxidized, we can use this chemical reactivity to detect it in solution. The reagent N-bromosuccinimide (NBS) is an excellent oxidizing agent and will oxidize all the reduced vitamin C (ascorbate ions) present to the oxidized form (dehydroascorbate). To determine when the titration is complete, an iodide-starch indicator is added to the reaction mixture. When all the Vitamin C has been oxidized, excess NBS will oxidize iodide anions (I-) to a triiodide (I3-) product which binds to the starch producing a dark blue starch-triiodide complex.
succN-Br+ Ascorbate ionsuccN-H + Br- + dehydroascorbate (NBS-oxidized) (reduced form)(reduced) (oxidized form)
In today’s experiment you...