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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: rob on April 02, 2006, 09:59:33 AM

Title: Accurancy of Potentiometric Titrations
Post by: rob on April 02, 2006, 09:59:33 AM
I have done a potentiometric titration in the lab for determining the dissociation constant of a weak acid which is benzoic acid, using NaOH as a titrate. I took several measurements of the pH of the solution after adding certain quantitiies of NaOH. I used the results obtained for calculating pKa by two methods: 1- plotting pH against V and determining pH midpoint which is equal to pKa and 2- ploting pH against log V/ V0-V (V0 = volume of NaOH needed to reach the endpoint) and find pKa from the intercept on the pH axis. [ I'm not sure though if pH axis should be the horizontal or the vertical one??] From both these methods I got a value of approx 4.1 while the literature value is 4.21.
Now I have to outline possible sources of error that lead to a different value from the literature one and also to comment on the realtive accurency of the two methods for determining the pKa.

I would like some help with these as I don't know how to comment on the sources of error as I have to mention factors that have a major contribution on the error and haven't got a clear idea of them. Also don't know how to comment the relative accurancy of the two methods because from both of them I got a similar value about 4.1 while the correct one should be 4.21. Please I would greately appreciate if you could help me with these. Thanks in advance.
Title: Re:Accurancy of Potentiometric Titrations
Post by: Borek on April 02, 2006, 12:51:55 PM
Both methods are equivalent, nothing strange that they gave identical result.


is just another version of H-H equation - note that right side equals zero for half titration. Try to understand how to rearrange HH to obtain this version.

It will not work in case of a very weak acid, but for benzoic is OK.

So, your error in both cases has the same theoretical source (neglectable) and identical experimental problems. Plus, you probably ignored ionic strength ( of the solution.