Practical investigations
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I am planning to do the effect of pH on vitamin C in my A2 coursework. Is this an acid-base reaction? Does that mean I need to do a titration of vitamin C against as base with some use of indicators because vitamin C is an acid? Could you give me a run-down and some ideas of how to carry out this titration ? Thanks.
Igloo writes ....
The chemistry of ascorbic acid is complex. It is classified as an acid since the molecule can rearrange into what is known as an enol, a very weak acid which can subsequently lose protons to produce what is known as an enolate. This enolate structure rapidly degrades and is more susceptible to oxidation than the original molecule, which explains why ascorbic acid needs to be kept at a pH below 7 in order to stabilise it. An acid-bese titration is really not suitable here.
Since you want to study the effect of pH on the concentration, you need to add a sample of known volume and concentration into a buffer solution of known pH, and then leave for a fixed period of time. Finally the mixture can be titrated with a suitable reagent, such as DCPIP, NBS or iodine. All these methods are mentioned in various FAQs on the React website.
When using buffers at pH values higher than 7, you will need to bring the pH down below 7 again just before carrying out one of the titrations mentioned above. These only operate reliably under acidic or slightly acidic conditions.
Risk assessment
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updated: 21 January 2008
