Why are pH and Titratable Acidity [TA] not "proportional"?

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The question asks why it is that wines or musts with different TAs can have the same pH and vice versa.

In a must or wine there are both free and bound, hydrogen particles.

pH is a measurement of how many free positively charged hydrogens are around.

When measuring TA, you add sodium hydroxide to the must. What happens is, you deal with the free hydrogens, those responsible for the pH. But even as you do and as you continue to add more sodium hydroxide, you actually start to unhook bound hydrogens and make them free.

Only when you have added sufficient sodium hydroxide to unhook all accessible hydrogens is your measurement of TA complete.

Since the proportions of free and bound hydrogens varies greatly according to grape varietal, ripeness, growing conditions and so on, so does the relationship between pH and TA.

Charles Plant 2001