Acid Determination of Vitamin C

Topics: PH indicator, Base, Acid Pages: 5 (1241 words) Published: November 2, 2010
Acid Determination of Vitamin C
Tablets and Salt Solutions
Chem 1290-016

The purpose of this lab was to use acid-base titrations to find the mass percent of ascorbic acid in a Vitamin C tablet. A known concentration of sodium hydroxide was used and the value found was compared to the stated amount of ascorbic acid listed on the vitamin container. Also in this lab the acidity of different salt solutions were tested using different indicators including litmus paper. Procedure: (titration)

1. Weigh a vitamin C tablet, record it’s mass, then use a mortar and pestle to crush the tablet. 2. Weight half the tablet then transfer quantitatively to an Erlenmeyer flask. Repeat with other half. 3. Set up a buret with a support stand and label a beaker as “waste”. 4. Rinse the buret with distilled water then NaOH.

5. Obtain 60 mL NaOH and record exact concentration. Record initial volume. 6. Add 75 mL distilled water to the flask then add 3 drops phenolphthalein to dissolved sample. 7. Allow NaOH to go into the flask 1 mL at a time. When closer to then end point add one drop. When the solution changes color for 20 seconds, the titration is done. Record the finial buret reading. 8. Pour solution down the drain with water. Add more NaOH to buret if needed. Repeat with a second indicator. 9. When done, clean the buret and glass ware.

Procedure (acidity of aqueous solution)
1. Obtain a well plate. Add 10 drop to the first four wells of each row with Vitamin C, acetic acid, ammonium hydroxide, ammonium chloride, sodium acetate trihydrate, and sodium carbonate. 2. Test each solution with red and blue litmus paper using a glass stirring rod. 3. Add one drop phenolphthalein to each solution and record observations. Repeat with methyl orange, bromcresol green and thymol blue. 4. Clean the well plate.

Concentration NaOH (lot MO) – 0.09860 M
Amount of Ascorbic Acid per tablet (from container label) – 500 mg 1st Titration (phenolphthalein)
Mass of Vitamin C Tablet - 0.6424 g
Mass of Crush Tablet – 0.2042 g Mass Transferred – 0.1936 g Initial Burette Reading – 8.02 mL Finial Burette Reading – 24.92 mL Amount NaOH transferred – 16.9 mL

(24.92 mL – 8.02 mL) = 16.9 mL
Mol of NaOH (base) transferred – 0.00167 mol
16.9 mL NaOH x ( 1 L / 1000 mL ) x (0.0986 mol / L ) = 0.00167 mol NaOH Mol ascorbic acid ( C6H806 ) – 0.001099 mol
0.1936 g C6H806 x ( 1 mol / 176.12 g )C6H806 = 0.001099 mol C6H806 2nd Titration (methyl orange)
Mass of Vitamin C Tablet - 0.6054 g
Mass of Crush Tablet – 0.3491 g Mass Transferred – 0.3456 g Initial Burette Reading – 9.12 mLFinial Burette Reading – 16.92 mL Amount NaOH transferred – 7.8 mL

Demo Observations:
Solution| Red Paper| Blue Paper| Phenolphthalein| Thymol Blue| Methyl Orange| Bromcresol Green| Vitamin C| No change| Red| No change| Orange| Magenta| Yellow| 6 M H2SO4| Darker| Red| No change| Purple| Light magenta| Yellow| 3 M HNO3| Darker| Dark red| No change| Dark blue| Red| Yellow| Conc. NaOH| Blue| Dark blue| Dark pink/purple| Blue| Yellow| Blue|

Observations of Salt Solutions and Indicators:
Solution| Red Paper| Blue Paper| Phenolphthalein| Methyl Orange| Bromcresol Green| Thymol Blue| Vitamin C Tablet| No change| Pink| Cloudy white| Orange| Pale yellow| Pale golden yellow| Acetic Acid| No change| Pink| No change| Orange| Pale yellow| Pale golden yellow| Ammonium Hydroxide| Dark blue| No change| Dark pink| Golden yellow| Bright light blue| Dark blue| Ammonium Chloride| Purple| Purple| Cloudy white| Golden yellow| Light turquoise| Olive| Sodium Acetate Trihydrate| Blue| Dark blue| Dark pink| Golden yellow| Bright light...
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