[nimbus]

Hey guys,

Sorry to make my second post be a question but I'm pulling out my hair and my instructor hasn't emailed me back yet. I'm in a basic analytical course and we are determining the wt% of carbonate/bicarbonate for an unknown. I know how to find this, but I'm having trouble resolving my endpoints.

I took the first derivative around the endpoints. Half the curves are well rounded and I can easily tell the endpoint, but the other half look like stair cases, or have a valley in the middle. I'm not entirely sure what to do here, as I don't like guesstimating end point volumes! I tried taking the second derivative but that actually increases the noise.

Any ideas?

[Borek]

Can you post a plot?

[nimbus]

http://img714.imageshack.us/gal.php?g=std11stderiv.png

Here's the gallery. You can see that the standard curves are really easy to see the endpoint, but the unknowns are very fuzzy. Not sure if that is the nature of bicarb/carb mixtures or if I'm a terrible titrator, but I was very consistent in my meter recordings.

Sorry that the axes aren't labeled:
pH is Y, volume of HCL is X
for first derivative
it's dpH/dvol Y
average vol X

[saden99]

That's [sort of] the nature of these types of titrations.

I'm looking at some old graphs of a diprotic acid and a few polyprotic acids and I recall them being a pain to get good information from, but you shouldn't expect completely accurate information with one run. One of the points of an analytical course is to help you understand that the more data you have available, the better your results should be which leads to a more confident conclusion.


Was there anything else you had to account for in the experiment? Ionic strength, "corrected" pH, etc?

[Borek]

If I understand correctly, plots for the 1st derivative for standard are shown for full curve, while plots for unknown are shown zoomed, only for a very small area close to the endpoint. Are you sure your plots for STD - when zoomed - look much better? Full titration curves look very similar.