Titration of Vitamin C in Fruit Juice
- Fruit juice (orange, grapefruit, lemons, etc) and/or prepared juices of other fruits or vegetables
- Apple, tomato, carrot, orange soda, etc.
- Reaction mix in 10 mL repipet (see previous protocol)
- Standardized 0.01 N iodine in 500 mL flask with funnel
- Four 250 mL Erlenmeyer flasks
- 50 mL buret in buret stand on a white piece of scrap paper
- Fine strainer (if the juice is to be prepared from fruit) funnel
- 10 mL pipets or 5 mL displacement pipets with tips
1. Prepared juices may be used directly unless they have excessive pulp in them. For pulpy juices (such as citrus), squeeze approximately 40 mL of juice, filter through a fine strainer supported in a funnel into a 250 mL Erlenmeyer flask.
10 mL reaction mix
2. Prepare three 250 mL Erlenmeyer flasks by repipetting 15 mL of reaction mix into each. (A 10 mL repipet may be set on 7.5 mL, and add two “squirts.” (It takes more reaction mix than clear specimens to show the endpoint more clearly.)
3. Pipette 10.0 mL of the juice to be tested into each of the three prepared flasks.
titrating fruit juice finished titration
4. Titrate as in the Titration Protocol . Since the juice is colored, the endpoint will be less obvious than with clear solutions, but a distinct bluish darkening will be seen (often a greenish color).
NOTE: The titrated juice may lose its blue upon sitting, but if it is bluish for 5 – 10 seconds, ignore any loss of color after that.
5. Calculate the # mg of vitamin C in 100 mL of juice as follows:
mean mL iodine x conversion factor x 10 = mg vit. C/100 mL.
6. Since 1 mL of fruit juice is very close to 1 g, the concentration of vitamin C in 100 g of the fruit is essentially equal that in the juice. Enter your data into the class data sheet in the computer express as mg/100 g fruit). Be certain to get a copy of the class data sheet for your notebook. Compare these data with the “book value” for these fruits.