Chemical Forums

Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: ROSETTA on January 28, 2008, 07:35:20 PM

Title: pKa experimentally
Post by: ROSETTA on January 28, 2008, 07:35:20 PM

Hi, can anyone please tell me if the following is correct.
To determine the pKa (of acid) experimentally titrate with base and plot the number of ml of base against the recorded pH.
When the endpoint of the reaction is reached there will be a sharp vertical line on the titration curve..  This corresponds to the point at which all of the acid has been converted to conjugate base.  If this occurs at 30ml, the pKa is halfway to the endpoint, and therefore, is the pH at 15ml of base.  Is the pKa therefore the point on the curve, which corresponds to the pH at 15ml of base.

All help greatly appreciated
Thanks Rosie

Title: Re: pKa experimentally
Post by: enahs on January 28, 2008, 09:20:45 PM

Simplified. Yes.


(http://www.dartmouth.edu/~chemlab/chem3-5/acid1/graphics/chemistry/chem3.gif)

Title: Re: pKa experimentally
Post by: Borek on January 29, 2008, 03:42:31 AM

I would rather put it another way. pKa is a measure of acid strength. It happens, that pH at 50% titration equals pKa, but it is not pKa definition.

Title: Re: pKa experimentally
Post by: ROSETTA on January 29, 2008, 08:09:24 AM

thank you, thats great
Rosie