Could have been caused by the ph probe which is

could have been caused by the pH probe which is notorious for being finicky and inaccurate, or it
may have been caused by a misreading of the volume of NaOH added or the fact that the
equivalence point had to be at one of the points where the pH was measured and may have
actually occurred between two of the readings. These errors were very relevant when calculating
the Ka value algebraically, but less relevant when calculating Ka graphically.
It is valid to use graphical analysis to determine the Ka of phosphoric acid because it is a
weak acid titrated with a strong base and therefore a sharp increase in the slope occurs at a
particular pH, making it fairly easy to find the equivalence point. The validity of this method is
supported by the results which were not exactly the true value of the Ka but were of the same
magnitude.

The cola sample must be degased to get rid of the CO
2
which would also react with the
NaOH to make an weak acid. The addition of another acid makes it harder to calculate the value
of Ka accurately and creates a lot of unnecessary work.
References
Bottomley, L., & Bottomley, L. (2014). Paper Chromatography: Separation of a Mixture Using
Physical Equilibria. In Chemical Principles I & II Laboratory Manual (pp. 161-165).
Plymouth: Hayden-McNeil Publishing.