Practical investigations
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I am investigating the different concentrations of vitamin C in fresh orange juice and fresh grapefruit juice and I am wondering what the acetic acid (ethanoic) is used for, and why?
What is the DCPIP tablet used for?
How long do the solutions have to be prepared before titrating?
Igloo writes ...
The ethanoic acid is used to ensure that the pH remains below 7 and that the concentration of vitamin C remains constant prior to carrying out an analysis. Vitamin C tends to decompose in neutral and alkaline conditions.
DCPIP is an oxidising agent and is rapidly reduced by vitamin C at room temperature. It is often supplied in tablet form, and will need to be made up into solution. The reaction between DCPIP and vitamin C forms the basis of a common method of analysing vitamin C in vegetables and fruit. Since DCPIP forms a strongly blue-coloured solution, the titration is self-indicating, and the end-point occurs when DCPIP – from a burette – forms a permanent pink/blue coloured solution when run into a solution of vitamin C, e.g. your fruit juice.
DCPIP solutions should be used as soon as is convenient after being made up, and they should be standardised with a solution of vitamin C of known concentration before you carry out titrations with your fruit juices.
You will find more details in our FAQs on the web site. Use “vitamin C” as your search phrase. We have also written a tutorial on vitamin C Investigations. You will find this at:
http://www.chemistry-react.org/go/Tutorial/Tutorial_22827.html
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: 22 July 2007
