[jcwright]

Hi, this is my first post on chemicalforums.com, I am a geologist who is currently on a chemistry conversion course to become a high school teacher.

A part of one of our assignments we have had to design and undertake an experiment suitable for 16-18 year olds undertaking a-level chemistry in the UK. I chose (much to my detriment) an acid base titration to calculate the quantity of vitamin c (ascorbic acid) in 500 mg supermarket own brand tablets. (I should have opted for the REDOX method)

Firstly NaOH/HCl back titration was undertaken to ascertain the molarity of the NaOH, which worked fine!

The second part involved dissolving a crushed vitamin c tablet in distilled water (so far so good), a cloudy solution was produced and it was interpreted that the cloudiness was from the binding agents in the tablet. The powdered tablets weighed approx 1.14g, of this it is interpreted that 99% of the 500 mg vitamin C was removed from the weighing boat and dissolved

The NaOH of approx 0.25 M was slowly titrated in to the vitamin C tablet solution and after approx 5 cm the solution turned pink.  After agitation using a magnetic stirrer the solution then cleared and more NaOH was added.  However every time the end point was reached, further changes back to a clear solution occurred (this occurred over a 5 hour time period).  In the end no definite end point could be found as with agitation the solution cleared.

Does anyone know what is going on with this experiment?
Why does the solution continue to clear?  Is it because of the binding agents locking away some of the ascorbic acid?

Hope you can help.

J

[Borek]

Have you tried with other indicator?

Not that I have any idea what may have happened. However, I can be interested in what you find

Try to check Journal of Chemical Education archives, I suppose such titration have been already tried.

[Borek]

Interesting. I have searched JCE index and it seems nobody published any paper on alkalimetric detemination of ascorbic acid, but there are plenty of papers on iodometric titration.

[jcwright]

Thanks for you help, I could not see anything either

[vandenbergeben]

Ive read this post because i was having some problems with titrations and i was reading my chemistry book and it stated that fenolftaleine is pretty unstable in a base and it will decolor in time. I've got no clue how fast this happens, but it seems like that might be the problem.

[Borek]

Unlikely - when I was taking pictures of indicators (see http://www.titrations.info/acid-base-titration-indicators-preparation) phenolphtahlein solution had stable color for many hours.