Practical investigations
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When typing up my project write-up I came across a problem and was wondering if you would be able to help me. My project is on determining the ascorbic acid content of a lemon by using DCPIP. However I carried out an experiment to find out whether citric acid affects the levels of DCPIP. When asked why I did this experiment I could not give an answer. My teacher told me it had something to do with stabilising a reaction? Please help quick.
Igloo writes ...
Ascorbic acid is stabilised at pH levels below 7, which is why it is often found in nature in acidic conditions. For example, in lemon juice, the acid present is largely citric acid.
If you are carrying out experiments to find the concentration of vitamin C in a solution of citric acid, you can carry out titrations with a solution of DCPIP. In these you are assuming that vitamin C is the only substance reacting with DCPIP.
But what if citric acid itself reacts with DCPIP? This is why it is conventional to carry out what is known as a control experiment, i.e. a titration with citric acid alone, so that the titre obtained can be subtracted from all the other titration readings.
This is probably what your teacher means by the “citric acid affecting the levels of DCPIP”. In practice it is found that this control titre is close to zero, so students often think that the control titration has been a waste of time. Even so, when making up solutions of vitamin C in acidic solvents it is important to demonstrate that the solvent does not also interact with the DCPIP.
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: 01 May 2007
