Determination of Vitamin C

1
Determination of Vitamin C
2
Experimental Objectives

• Determine the mass of vitamin C in a given tablet
• Determine the mass of vitamin C in a serving of a
  commercial juice product

3
Procedure Overview

• Vitamin C (Ascorbic Acid C6H8O6) titrated with
  N-bromosuccinimide
• Vitamin C is oxidized
• Two carbons each lose 1 electron (2 electron
  loss)
• N-bromosuccinimide is the oxidizing agent
• One Br is reduced from 1 to -1 (2 electron gain)
• Stoichiometry is 11

4
Procedure Overview cont.

• Endpoint
• Excess N-bromosuccinimide oxidizes iodide to
  iodine (acidic solution required)
• KI source of iodide
• Acetic acid source of acidic solution
• Iodine reacts with starch to form a blue complex
  (endpoint)
• Starch solution - indicator

5
Prepare Vitamin C Solution

• Crush provided tablet using a mortar and pestle
• Transfer ALL of the crushed tablet to a beaker
  using DI water from the wash bottle to rinse.
• Dissolve in DI water in beaker.(some insoluble
  binders will not dissolve)
• Transfer ALL of the solution to a 100 mL
  volumetric flask. For complete transfer rinse
  beaker with small quantity of DI water and
  transfer to flask.
• Dilute solution in flask to the mark
• Mix well
• All of the vitamin C is now dissolved in 100.00
  mL of solution

6
Fill Buret with N-bromosuccinimide

• Obtain 200 mL of N-bromosuccinimide Record
  molarity
• Rinse buret with N-bromosuccinimide
• Fill buret
• Remove air bubbles
• Record the initial buret reading to two decimal
  places (This need NOT be exactly at 0.00)

7
Prepare Aliquot of Vitamin C solution

• Aliquot a known fraction of a whole
• Pipet 5.00 mL aliquot of vitamin C solution into
  an Erlenmeyer flask
• (Note This aliquot contains 5/100 or 1/20th of
  the vitamin C from the tablet)
• Add KI, acetic acid, and starch to act as the
  indictor

8
Pipet using proper technique

• Pipet from a clean dry beaker
• Rinse the pipet with solution to be pipetted
• Avoid drawing solid into pipet
• Do not blow-out the pipet
• td to deliver
• tc to contain

9
Titrate Vitamin C solution with
N-bromosuccinimide

• Set-up Erlenmeyer flask
• Add clean stir bar
• Place on magnetic stir plate
• Adjust speed of stir bar to minimize splash
• Add N-bromosuccinimide from buret
• Rinse walls of flask as you near the endpoint
• Endpoint is the first drop to cause a lasting
  color change to blue
• Perform a quick and dirty titration to
  determine approximate endpoint.
• Complete three quantitative trials

10
Prepare Sample of Commercial Product

• Obtain 120 mL of assigned product
• Pipet 25.00 mL of assigned product into an
  Erlenmeyer flask (Use proper technique)
• Dilute with 50 mL of DI water
• Add KI, acetic acid, and starch to act as the
  indictor

11
Titrate Commercial Product with
N-bromosuccinimide

• Set-up Erlenmeyer flask
• Add clean stir bar
• Place on magnetic stir plate
• Adjust speed of stir bar to minimize splash
• Add N-bromosuccinimide from buret
• Rinse walls of flask as you near the endpoint
• Endpoint is the first drop to cause a color
  change
• Actual endpoint color will vary with commercial
  product
• Perform a quick and dirty titration to
  determine approximate endpoint and endpoint
  color.
• Complete three quantitative trials

12
Clean-up

• Dispose of all liquid waste in liquid waste
  container.
• Do not overflow the container
• Do not create unnecessary mess
• Return the mortar and pestle to the bins under
  the sink. They will be used by the next section
  of lab students. Do not hide them in your
  drawer!! ?

13
Analyze Data Vitamin C Tablet

• Relate volume of N-bromosuccinimide to moles of
  N-bromosuccinimide
• (volume solution, L)(molarity of solution)
  moles solute
• Relate moles of N-bromosuccinimide to moles
  vitamin C present in aliquot
• 11 Stoichiometry
• 1 mole vitamin C / 1 mole N-bromosuccinimide

14
Analyze Data Vitamin C Tablet cont.

• Convert moles of Vitamin C in aliquot to mg of
  Vitamin C in aliquot (Vitamin C - C6H8O6)
• C present in aliquot to mg Relate mg of vitamin
  of vitamin C in solution (from tablet)

15
Analyze Data Commerical Product

• Relate volume of N-bromosuccinimide to moles of
  N-bromosuccinimide
• (volume solution, L)(molarity solution) moles
  of solute
• Relate moles of N-bromosuccinimide to moles
  vitamin C present in 25 mL sample
• 11 Stoichiometry
• 1 mole vitamin C / 1 mole N-bromosuccinimide

16
Analyze Data Commerical Product

• Convert moles of Vitamin C in sample to mg of
  Vitamin C in sample (Vitamin C - C6H8O6)
• Relate mg of vitamin C present in 25.00 mL to mg
  of vitamin C in a serving of your commerical
  product
• Example Tang