Question

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Experiment 7: Titration II volume of deionized water so that(20 mL of solution contains 40 mg of vitamin C. For example, if you have a 500-mg tablet, crush it up and dissolve it entirely in 250 mL of deionized water. Then use 20 mL of that solution for standardization. Is the quantity 20 mL important? What would happen if you used 250 mL? Place the buret in the clamp on the ring stand. Using a funnel, carefully fill the buret with iodine solution (note: iodine will stain!). Why are you putting iodine solution in the buret, and titrating the vitamin C sample, instead of putting the vitamin C sample in the burer, and ritrating a sample of iodine? What do you think would hannen if you did the titration this way around? Be sure that the tip of the buret is filled with solution and contains no air spaces. Drain the iodine to the zero mark on the buret. Oops! Your lab partner drained more iodine solution that necessary. How might this affect your results? What should you do? Transfer 20 mL of the vitamin C solution to the 250-ml Erlenmeyer flask. Add 20 drops (-1 mL) of 0.2 Macetic acid, and 10 drops of 2% starch solution - this step is critical! Why is this step critical? What is the chemical importance of this step? Swirl to mix well. Place the flask under the buret and begin adding iodine solution. Try to swirl the flask continuously under a slow stream of iodine solution from the buret. When you begin to see the formation of a purple swirl, slow down! Note the volume reading on the buret. Add iodine dropwise until the solution remains purple (or blue-black). Don't overshoot the endpoint!

Answer 1

Vitamin C (Ascorbic acid ) is determined by titration with a standard solution of Iodine (I₂) , Iodimetry.

• 20 ml. solution is standard for titration, you will 250 ml it will to titration errors. End point determination will impacted by this. If we use 250 ml. as analyte than will require larger volume of standard iodine solution (titrant) in Burette , it will require refilling of burette and will increase to errors. End point is always at higher value than equivalence point, using 250 ml will increase this excess amount and we will be reporting over-estimated results.While titrating handling larger volume without committing errors will be a tough task, stirring , ensuring completion of reaction after each addition will be affected ; and will add to titration errors.
• Titration is carried out to find out the exact volume of one solutions (analyte) that reacts with a known volume of another(Titrant).The known volume is usually taken using a pipette, which is poured in titration flask. Usually standard solution of known concentration is taken in buret and solution of unkown strength is taken in flask, once added this can be used for multiple titration. Since, we use repeat samples of unknown (Vit, C) for precision, once filled in buret Standard solution can be used many times. Titration reaction must occur rapidly. In this reaction when all of vitamin C is oxidized, iodine/ triiodide will react with starch to form a blue-black complex, marking the end point.

If we carry titration other way round , it will increase labour put in titration; since it is a redox tiration equilibrium will be different in opposite titration, our end point will be different . Now ,color change will be blue to colorless,indicating completion of a reaction in which I₃^- is consumed. We will have more titration error , obeserving end point from colorless to colored is easier and faster than colored to colorless.

• Over-draining will lead to titration error, you will be over-estimating the amount of Vit,-C present in sample. You can add known amount of analyte (unknown sample) depending on excess addition , and titrate for end point; change your calculation accordingly for new volume of unknown sample.
• the End point of the titration, is determined by: the blue colour (due to formation of starch – iodine complex) appearance. So, addition of starch/acetic acid is critical step for determining end/equivalence point making completion of reaction.

Starch, forms a dark blue complex with I₂ / I₃^- and used to signal presence of excess I₃^-  
– (color change: colorless to blue).   Acetic acid added to reduce viscosity, firmness and cohesiveness of starch, thus improving end point detection.