Titration of NaOH and C2H204 using phenolphthalein

[Apost8]

The other day, we performed a simple titration of NaOH and C2H204 using phenolphthalein as our pH indicator. I performed 3 trials and I calculated % error from .5 to 2%. What is the proper way of determining the end-point? How pink should the titrant be before you stop adding? This method doesn't seem to be an incredibly accurate way of determing the end-point of a reaction.

 

[The Bob]

The solution, before titration, is colourless. This means that when you hold it up to your white tile (you were using a white tile, right? ) you will see white. You should titrate the two until you see a very slight pink ting, check using your tile. Once you can see the solution is pink, using your tile, then the reaction has finished and this is the end-point.

It is a common way to do it at schools and colleges. The idea is that it allows a good evalution for pupils and allows errors that would allow the correct answer to be obtained.

I personally avoided titrating anything as much as possible. The accuracy, which is your problem, was never resolved for myself.

Cheers,

The Bob (2004 ©)

 

[Borek]

In short: stop at first traces of pink :)

 

[turbo]

Perhaps clarification is in order. As you titrate, the base will cause little clouds of pink that clear very quickly at first, then more and more slowly. At the point where you notice that the pink is clearing slowly, add the base one drop at a time until the pink does not entirely clear. That is your end-point.

 

[Apost8]

Thanks all for your advice. The % error I calculated for my experiment was within the margin given to us by our prof, so I guess I did it correctly. I guess I was just a little surprised at how much room for error there is. There were people in the lab who had to do 4 or 5 trials before they got their % error to within the established margin. Luckily I avoided that and went home early :)