[mspharm1]

Sooo, acid-base equlibria concerning titrating weak acids with strong bases or weak bases with strong acid are easy to calculate and one can easily understand the concepts. But what about titrating a weak acid with a weak base, i heard its a bit more complicated...i can't find any literature on that,...how to find the pH of the equivalence point and so on...

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=pH-amphiprotic-salt

http://www.chembuddy.com/?left=pH-calculation&right=pH-salt-simplified

[ARGOS++]

Dear Mspharm1;

Sorry!, - IMHO it makes very little sense to select a situation, that may present you more problems then solutions.

The Titrand you are not able to select, because you are interested in its content.
But it is very nearly “all times” possible to select a corresponding strong “Reaction Partner” for the Titration.
So by all heavens why should you select a weak, which may give you only problems to identify the end point of the titration?

The equivalence point is, "of course", not moved/attracted, but/and much harder to detect, because you will end mostly with an Indicator problem.

[Edited!]
Notice the corrections, and my next Posting #3!

Good Luck!
                    ARGOS⁺⁺

[ARGOS++]

Dear Mspharm1;

Pardon!  for my Statement about the Equivalence Point!

Of course! that for a weak Titrand there is strong dependence on the kind of the weak Titration Partner!
(I misguided myself by remembering the first Picture in the Attachment, where they defined the Point D as the Equivalence Point.)

But in the Situation of weak-weak we must use the second Picture, which shows us a strong dependency for concentration!
Point A and B are Equivalence Points for different concentrations of different weak Titration Partners!

Sorry for my mistake!


Good Luck!
                    ARGOS⁺⁺

[Borek]

Copare both curves. The longer jump near inflection point, the easier the end-point detection. Color change of indicator takes around 2 pH units. Assuming indicator that changes pH at exactly the endpoint of your titration, you will be around 1 pH unit from the endpoint when you will be sure color has changed. Judging from the plot, it means around 5% intrinsic error. At the same time - when using strong base - error still exists, but is around 0.1% (endpoint pH - 8.71, after additional 0.1% of titrant is added pH is 9.67 - so you are 1 pH unit from the endpoint and color change is clearly visible).

This is for 0.1M solutions.

Download my BATE and play with titration curves (free 30 days trial, so at no cost) to see how it goes.

[mspharm1]

Yes i know all of that things, titrating an amphiprotic salt,buffers with amphiprotic species, and that the range of most indicators is +-1 pH, and i know that it is easier to calculate the curve when titrating a weak acid with a strong base or a weak base with a strong acid (very systematic approach). I meant to ask you something different. How do i use the Ka and Kb in the case of the weak-weak titration to calculate the curve. For instance titrating acetic acid with CO3(2-), i read somewhere that the equivalence point for both 0,1M species is 6,5 somewhere around there,is that because maybe Ka(acetic) > Kb(CO3(2-)). Is there some systematic approach to treating this kind of equillibria (sorry cause i'm asking you guys to spoon fed me with info but i really can't find anything on this subject).
Another thing also bothers me,if i may ask, it's not totally conected with this subject but nontheless. But when one has a titration curve already ploted, one can find the end (or equivalence) point using the second derivative, but what is the f(x) of the first function exactly.. i know that  the y it is ph=-log/H+/ and on the x the V of the titrant...but what is then the whole function, cause there are so many different titration curves (polyprotic acids,bases..). Sorry if my english is not so good (i'm not from the Anglosaxon countries).. I hope that you understand my questions.
Thanks for the answers!

[Borek]

Take a look here:

http://www.chembuddy.com/?left=pH-calculation&right=titration-curves-calculation

or even start from here:

http://www.chembuddy.com/?left=pH-calculation&right=toc

but what is then the whole function, cause there are so many different titration curves (polyprotic acids,bases..)

There is no function like the one you are looking for.

And don't worry about your English, you are not the only Slav here