A2 Chemistry
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Is there a way in which ascorbic acid can be stopped from being oxidised (other than varying the temperature)? Am I correct in saying that bubbling oxygen through the acid would increase the rate of oxidation and therefore decrease the concentration of ascorbic acid? When titrating ascorbic acid and DCPIP my teacher told me to put the DCPIP in the conical flask and acid in the burette. What was the reason for this?
251109
Igloo writes ...
You can slow down the rate of oxidation by removing (or destroying) the ascorbic acid oxidase, the enzyme which causes the oxidation to occur rapidly in the presence of oxygen. You could ensure that no air (oxygen) comes into contact with the solution containing the ascorbic acid, and another way would be to reduce the pH by acidifying. You are correct in saying that bubbling oxygen through an ascorbic acid solution will accelerate the rate of oxidation, but the rate is in any case very slow if there is none of the enzyme present, e.g. in a prepared solution of ascorbic acid made up from the pure solid.
It is conventional to put the DCPIP solution in the burette. The advantages are:
(1) In this particular titration it is easier to detect the colour change from colourless to permanent red than vice versa.
(2) Since you are likely to be testing various solutions of ascorbic acid (eg from different natural sources), it is far more convenient to keep changing ascorbic acid solutions in conical flasks than it is to keep filling up a burette with the different samples.
However, it cannot be denied that when a coloured solution such as DCPIP is in the burette it is more difficult to read the volumes off the scale. This is why we normally read the top of the meniscus for coloured solutions rather than the bottom. After all, it is a difference in volumes that is being measured, e.g. from 0.0 to 20.0 cm3 rather than a particular single volume, so reading the top of the meniscus for all readings works well enough. Incidentally, when potassium manganate(VII) titrations are being carried out, this intensely purple coloured solution is also put in the burette during normal usage, so I cannot imagine why your teacher is recommending that you should depart from convention in this case.
In the end, of course, it doesn’t matter which solution goes where. It’s all a matter of practicality, and this is why we normally put the DCPIP in the burette!
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updated: 25 November 2009
