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Titratable acidity does not target a specific acid in the liquid you are measuring. Beer is composed of lactic acid, but also phosphoric acid, acetic acid, etc. While the latter are in minute quantities, they still affect the end result. For our purposes (and convention), we assume 100% lactic acid in the sample for our titration.
Why care about titratable acidity? pH quantifies the number of hydrogen ions (or hydrogen ion equivalents) in liquid. Your palette palate does not measure pH directly. Your palette palate interprets a multi-variable substrate called beer. Titratable acidity attempts to put another quantifiable handle on your beer akin to pH; the measurement better captures how “acidic” the beer may taste to you. Again, there are other acids than lactic in the beer, leading to differences in flavor between beers of the same TA.
Titratable acidity can be expressed in terms of different acids. In wine, TA is generally expressed in terms of tartaric acid (molecular weight of 150.09). In sour beer, TA is expressed in terms of lactic acid (molecular weight 90.08). To express TA in terms of a specific acid, the molecular weight of the specified acid is used in the TA calculation. In the [[Titratable_Acidity_2#Example|example below]], we lactic acid to express the TA value. See [http://www.awri.com.au/wp-content/uploads//TN14.pdf appendix 1 in this paper] on how to convert the titratable acidity value for different acids. Note that this is NOT a measurement of how much lactic acid or tartaric acid there is, it is an expression of measurement like how feet and meters are two different expressions of measurement for the same thing (distance). For example, a TA of 3.0 measured in units of tartaric acid is equal to a TA 3.6009 measured in units of lactic acid. Therefore, an argument can be made that TA measurements should always be specified as to which acid was used in the calculation.