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Determining the acidity constants of a diprotic acid (in my case, tartaric acid) by titration with NaOH and pH-meter is not possible if pKa2 - pKa1 < 2.

This is because there are no "pH jumps" at the equivalent points, so you cannot exploit the method that uses the "midpoints": enter image description here In fact, at pH=pKa1 the concentration of H2A is not equal to the concentration of HA- (the simplification performed in the equilibrium constant equation is not valid), as can be seen from the speciation diagram: enter image description here

This is how the titration curve of 1.5 g of anhydrous tartaric acid VS 1 M NaOH looks like. enter image description here

How could this be done?

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    $\begingroup$ If the experimental numerical data for the curves are available, try the numerical analysis of the 1st and second derivative of the curve before the endpoint, i.e. with higher resolution. As derivatives from numerical data are usually noisy, you may try kernel filtering by e.g. Savitzky-Golay filter available for MATLAB as sgolayfilt ( in compatible Octave too) $\endgroup$
    – Poutnik
    Jun 1 at 10:04

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