OK, so if a symmetrical graph is expected (as my graph shows), my question is how I can get the same pKa value from both method? The completion points of both graphs are the sample, corresponding to pH~5, and if I find the half volume of the first graph, a pKa around 7. And if I find the half volume of the second graph, a pKa around 3-4 is expected. So the pKa is different from the 2 methods.
So I am confused what these 2 pKa refer to. if I just want to know the pKa of oleic acid (weak acid), and only have sodium oleate (its conjugated base), what should I do?