What effect does a polyprotic acid have on the titration curve and why?
The titration curve will have more reflection points, one for each hydrogen ion as it is titrated (unless theionization constants are so close to each other that the reflection points run together or are not cleanly separated.)
thank you!
I think I goofed slightly. I should have called those reflection points "inflection" points. Sorry about that.
A polyprotic acid is an acid that can donate multiple protons (hydrogen ions) per molecule. It has more than one acidic hydrogen atom. The effect of a polyprotic acid on the titration curve can be understood by considering its stepwise ionization.
During titration, the pH of the solution is measured and plotted against the volume of titrant added. The pH depends on the concentration of hydronium ions (H3O+) in the solution. In the case of a polyprotic acid, each deprotonation step results in the formation of a different species with a unique equilibrium constant (Ka).
The first deprotonation of a polyprotic acid is usually the strongest, and it has the highest Ka value. As a result, the first equivalence point on the titration curve is usually well-defined and corresponds to the stoichiometric point where all the acidic hydrogen atoms have reacted with the titrant. At this point, the pH rapidly changes, indicating the presence of excess titrant.
However, subsequent deprotonation steps of a polyprotic acid have lower Ka values, which means they are weaker and do not completely ionize. As a result, the titration curve for a polyprotic acid shows additional smaller equivalence points corresponding to the deprotonation of each acidic hydrogen atom. These equivalence points are less pronounced and may be difficult to distinguish on the curve.
The reason for this behavior is that the second (and subsequent) deprotonation becomes increasingly more difficult as the pH of the solution increases due to the loss of acidic hydrogen ions. The concentration of the species being deprotonated decreases, resulting in a weaker acid and a lower equilibrium constant.
In summary, a polyprotic acid affects the titration curve by exhibiting multiple equivalence points due to successive deprotonation steps of decreasing strength. These additional equivalence points reflect the presence of multiple acidic hydrogens and their ionization behavior.