Vitamin C. I will investigate whether orange juice contains the most Vitamin C and how I can speed up and slow down the oxidation process of Ascorbic Acid.

The amount of ascorbic acid can be determined by a redox titration with a standardised solution of iodine. The iodine is reduced by the ascorbic acid to form iodide. As shown in the other half of this redox equation.

1. -1

Oxidation States

From AS Level, I know that all elements oxidation state is 0. Whatever the charge after the element that is their charge. (In this case iodide’s charge is -1). As we can see from the above equation, elements that are oxidised increase their oxidation state and elements that are reduced decrease their oxidation state that is why the iodine is reduced to iodide. The colourless iodide will be the end point of my Iodine titration.

Aim: To investigate the varying vitamin C concentrations in fruit juices

Hypothesis: Orange Juice contains the most vitamin c

I will investigate whether orange juice contains the most Vitamin C and how I can speed up and slow down the oxidation process of Ascorbic Acid.

Equipment:

            .         Safety goggles/Lab coat

·         Two Fruit juices (100 ml)

·         Vitamin C standard solution (1.0 g/ml)

·         Iodine solution

.         DCPIP solution (1.0g/ml)

·         Ethanoic Acid 5 %

         Distilled water

·         50-ml burette

·         Retort Stand and Burette Clamp

·         Pipette (25ml,10ml,50ml)

·         Conical flasks(250 ml)

·         100-mL Beakers

     Hazards

When using DCPIP I need to be careful not to inhale the dust when opening the bottle and to wear safety goggles so it doesn’t go in my eyes. Once I have diluted the DCPIP the risks are drastically reduced however I still need to wear goggles and a lab coat.

Ascorbic Acid should be treated as an irritant and therefore I should wear goggles and a lab coat when handling it.

I am using 5% Ethanoic acid however, it is not above 4 moles and therefore should be treated as an irritant just as DCPIP and Ascorbic Acid are.

My solutions of Iodine will not be greater than 1M so they should be treated as an irritant not harmful and corrosive. I will always wear goggles and lab coats.

When handling glassware I should be careful that none of it breaks, if it does I should ensure I do not touch the broken glass and seek help.

Method

Making up my Standard Solutions

Ascorbic Acid- 

• Use the same scales throughout to ensure, if there is an error that it is the same error all the time.
• Weigh out the mass of solid Ascorbic Acid (0.1g) by putting the solid in away from the balance so I do not spill any onto the balance, to make sure that I do not have less Ascorbic acid than I think I do.
• Dissolve into 120 cm3 of 5% Ethanoic Acid, ensuring I add rinsings so that all of the Ascorbic acid is dissolved.
• Add 380 cm3 of distilled water to make it up to 500 cm3.When I get close to the 500 cm3 required I will use a dropper to get it perfect and make sure that the bottom of the meniscus is on 500 cm3.

DCPIP

• Weigh out the mass of solid DCPIP (0.25g) the same way as above to ensure accuracy of weighing.
• Dissolve in 500 cm3 of distilled water.

Titration- DCPIP

• Use the same scales throughout to ensure, if there is an error that it is the same error all the time.
• Prime the Burette and rinse with Ascorbic Acid solution to prevent contamination. Fill the burette up to 0.00ml, and take the funnel out to prevent dripping. This will mean that you actually used more than you read off your burette.
• Titrate it against 25cm3 of DCPIP solution that has been measured out using one 25 ml Pipette to minimize errors.
• Do a rough titration so I know roughly where the end point is.
• Do 5 titrations to ensure accuracy and validity of results, repeating any anomalies.
• Read off the titre at the bottom of the meniscus.
• Repeat above procedures but for tropical juice and orange juice instead of Ascorbic Acid.

Titration-Iodine Solution

• Prime the Burette and rinse with Ascorbic Acid solution to prevent contamination. Fill the burette up to 0.00ml, and take the funnel out to prevent dripping. This will mean that you actually used more than you read off your burette.
• Titrate it against 25cm3 of Iodine solution that has been measured out using one 25ml Pipette to minimize errors.
• Do a rough titration.
• When doing your 5 real titrations, add starch to make determining the end point clearer. This is added near the end because if it is added at the start it can hold the ascorbic acid in a complex and the titration will not work properly.
• Read off the titre at the bottom of the meniscus. 
• Repeat above procedures but for tropical juice and orange juice instead of Ascorbic Acid.

The iodine solution, by reading the meniscus I can tell that this titre was 38.50 cm3

I found my Iodine titrations hard to do because they often formed big complexes with the Ascorbic Acid, preventing me to see the end point, so I will use DCPIP from now on.

Investigating how to speed up and slow down oxidation

After researching I found out that iron and copper ions can have an effect on the oxidizing ability of Ascorbic Acid.

Copper Sulphate

I added Copper Sulphate with Ascorbic Acid to see if that speeded up or slowed down its oxidation.

Iron

DCPIP    

Ascorbic Acid  

 DCPIP                

DCPIP          

Hazcards