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I am carrying out an investigation into how Vitamin C degrades over certain temperatures. However, instead of using cabbage I am using lemon because the green colour produced by the cabbage makes it difficult to test for vitamin C using DCPIP, which is the indicator I intend to use. This means I will not have to add phosphoric acid as the ascorbic acid is already at low pH levels and so won't be oxidised. However, is there anything else I should take into account? For example does citric acid react with NBS?
And as I intend to heat the lemon up in a water bath (as I believe this is the most accurate way to maintain both time and temperature) should I peel the fruit or leave the skin on? How do I go about cooking the lemon, or is it the lemon juice I need to heat?
In addition, I’ve been told once I’ve investigated how an increase in heat affects vitamin C levels I should change the variable. Now obviously with a Vitamin C investigation the obvious variable would be to add some acid to see whether less Vitamin C was lost, however as the Vitamin C is already in citrus acid I wasn't sure what variable I could change? No oxalic acid will be necessary either, will it?
Any assistance on the matter will be greatly appreciated!
Igloo writes ...
In the first paragraph of your question you are talking about using DCPIP with a lemon rather that with cabbages because the green extract makes the end-point more difficult to detect, and then you finish up by mentioning NBS, so I am not certain which technique you have decided to use. I shall assume that you wish to carry out both methods. You are correct in assuming that the use of phosphoric acid is less important since the acids present in lemon juice help to stabilise the vitamin C, but you will still need to carry out titrations promptly, since vitamin oxidises – albeit slowly – even at low pH. As for NBS you can safely carry out titrations in the presence of citric acid, as it is a fairly specific oxidant and generally gives good and reliable results.
As regards your second paragraph, since your investigation is concerned with the degradation of vitamin C in lemons, I would place the peeled – but not skinned – lemon in a water bath at a known temperature for a fixed period of time. I suggest that you choose a fairly lengthy period of time, say an hour or so. This would give time for the heat to penetrate into the centre of the fruit.
Finally if you want to study the effect of another variable you could:
1 use different varieties of lemon
2 use lemons of varying ages
3 study the effect of cutting up the lemon or perhaps liquidising it.
Since your investigation is based on lemons it needs to involve realistic ideas, which is why, for example, the altering of pH, say, using aqueous ammonia as a neutralising agent, is a somewhat artificial procedure, since this would never happen in practice.
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 14 October 2007
