Practical investigations
Read our general notes on Risk Assessment
Vitamin C investigations
Summary of ideas with links to FAQs
Vitamin C investigations are becoming increasingly popular and we now have a large number of FAQs on the site. This tutorial draws together some of the ideas involved and gives some useful web-links.
A few of you have chosen to investigate the vitamin C content of different brands of tablets, but the majority are basing your investigations on the vitamin C concentrations in fruit and vegetables. Some of you have investigated a range of plant sources. Others have chosen a particular fruit or vegetable and compared the vitamin C content of particular varieties, or have taken samples from different parts of the plant material.
See also Sorted FAQs on vitamin C investigations
1 Analysis
There are several ways in which vitamin C concentration can be determined. In all cases a suitable solution will need to be prepared beforehand. In the case of fruit and vegetables, samples will need to be liquidised and filtered through muslin, probably using phosphoric acid. The function of the phosphoric acid is dealt with in the following FAQ:
The vitamin C calculation (Food Science chapter 2) to find quantity doesn't say why phosphoric acid is used and what a blank titre is, and why it is included? Please explain. Thank you.
Here are more FAQs about the extraction of the Vitamin C:
• I am doing an investigation into the effects of cooking on fruit and vegetables. I can make the solution up from the fruit by adding 100 cm3 of distilled water to 100 g of fruit but how will I be able to find the concentration of vitamin C if I have diluted it in the first place?
Is there any other method I could make up the solution?
• I've been doing research on Vitamin C titrations and I've often read that adding oxalic acid to sample before analysis is a good way to "stabilise" the ascorbic acid. My question is: what exactly does oxalic do to stabilise the ascorbic acid and prevent oxidation?
There are several analytical techniques which can be used. Here is a question about the relative merits of these techniques:
What are the advantages and disadvantages of using the different techniques (NBS, DCPIP and colorimetry) to detect vitamin C content? Thanks.
A Titration with DCPIP (2,6-dichlorophenolindophenol)
The following FAQ web link deals with the theory behind this titration:
What is the chemical formula of DCPIP, and what is the redox equation for its reaction with vitamin C?
and the following links refer to the experimental technique:
• I am having real trouble finding a method on investigating the effect of cooking on the vitamin C content in cabbage. Could you give me quick run-down on the procedure? I want to do it using DCPIP and already know how to standardize the solution.
• How should I standardise dcpip for an experiment to find the vitamin C content of orange juices?
• I am currently carrying out an investigation to find the amount of vitamin C in foods. When I was trying to standardise my DCPIP the solution went from dark blue to colourless and not to pink. Is there something I am supposed to add? Please help!
• I have questions on the Vit C content of cabbage experiment.
1. What is the dye factor for? Is it some kind of comparison?
2. Is the enzyme ascorbic acid oxidase present all the time in cabbage. Doesn't it just destroy all the Vit C?
• Hi, my investigation is on the effect of vitamin C concentration over a range of temperatures in orange juice. First of all how is vitamin C destroyed in the heating process and what does it become? Also the technician in my school told me to put the orange juice in the burette because if you put it into the beaker it will oxidise because it is exposed to air. But I can't see any anywhere that tells you to do this? Why do you put the DCPIP in the burette?
B Titration with NBS (N-bromosuccinimide)
You will find a helpful FAQ at:
I was wondering if you could give me some information on the reaction between ascorbic acid and NBS solution. I have recently done a practical on it and am writing it up. I am in desperate need of more sources.
C Titration with iodine
Several of you have adopted this approach, but report that the method gives rather patchy results, and have tended to use it as a secondary technique.
D Colorimetry
An FAQ which refers to the colorimetric analysis of vitamin C will be found at:
I'm investigating methods of determining Vitamin C in juice drinks. I've researched titrations using DCPIP and NBS but I need another method for comparison. I think I could use a colorimeter (like industry) but can't find any references. Can you point me in the right direction? Thank you.
[2] Effect of temperature and other factors on Vitamin C concentrations
FAQs which deal with the redox processes involved when vitamin C ‘decomposes’ are at:
• Would it possible to provide me with any information about the reaction between ascorbic acid and the enzyme ascorbic acid oxidase? The more detailed the better, but mainly referring to the redox reaction of Vitamin C.
• It was suggested by my teacher to use pieces of fruit, boil them for different periods of time and then plot a graph to show the relationship between the amount of time boiling and the vitamin C content. The next step would be to add either lemon juice or HCl (??) to the fruit that was to be boiled as apparantly these protect the vitamin C and stop it from decomposing so in theory, boiling should not have such a great effect. I cannot, however, find where it is sugested that lemon juice/HCl protect the vitamin C? Also, do you believe it is a reasonable investigation to conduct?
and
For my A2 investigation I need to find some detailed theory on how temperature affects vitamin C. I can't find the theory anywhere. I’m just told that temperature does affect it. Is it possible to tell me how at all? I’m particularly interested with how it affects the vitamin C contained in fruit juices. Thank you.
Here is some information as to why the boiling of vegetables reduces the vitamin C concentrations.
I am currently about to start my investigation. My hypothesis is that the longer broccoli is boiled, the less vitamin C it will contain. I was wondering as to why exactly this is so and what happens to the vitamin C due to the length of time of boiling?
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updated: 12 March 2008
