Question

When determining the concentration of phosphoric acid in Coca Cola, was it an appropriate step to use the volume between equivalence point 1 and 2 instead of just using the volume to reach equivalence point 1?

Answer 1

Yes it is an appropriate step to use the volume between equivalence point 1 and 2 instead of just using the volume to reach equivalence point 1. Remember that at the first equivalence point one proton has been titrated, while at the second equivalence point, two protons have reacted. If your results show that V_eq2 > 2 V_eq1, then not only H₃PO₄ but also H₂PO₄⁻ was in present in the drink. Calculate the concentrations of both. K_a2, this is the point when half the “acid” (H₂PO₄^-) has been converted to “base” (HPO₄^2-). This is also known as the “half equivalence point.”

Identify on your graph the volume of OH- added which is ½ way between equivalence point 1 and equivalence point 2. The observed pH at that volume = pK_a2. The 3rd endpoint for H₃PO₄ cannot be detected using a pH meter.

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