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Why care about titratable acidity? pH quantifies the number of free hydrogen ions (or hydrogen ion equivalents) in liquid. Your palate does not measure pH directly. Your palate interprets a multi-variable substrate called beer, which also contains "weak acids" such as lactic acid that contain bonded hydrogen ions that are not measured by pH. 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. As a general rule of thumb, the lower the pH, the higher the TA, however, TA and pH are not directly correlated. TA has been proposed as a more accurate representation of perceived acidity because we taste both free hydrogen ions and those that are bound to organic acids <ref>[http://www.craftbrewersconference.com/wp-content/uploads/2015_presentations/W1320_Kara_Taylor.pdf Kara Taylor. CBC 2015 Presentation. 2015. Retrieved 11/14/2017.]</ref><ref>[http://www.cider.org.uk/phandacid.htm The Wittenham Hill Cider Portal. "Relationship between pH and Titratable acid in Cider Apple Juices". Retrieved 11/14/2017.]</ref>. There is some disagreement on whether TA is a more accurate measurement of perceived acidity than pH; see [[Titratable_Acidity#Limitations_of_TA|Limitations of TA]] for details.
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, which is where the "0.9" number comes from in the equation below). 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#Example|example below]], we express the TA value in terms of lactic acid. 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 35.0 measured in units of tartaric acid is equal to a TA 36.6009 0015 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.
The ASBC<ref name="ASBC" /> equation for TA in units of lactic acid (the 0.9 constant represents TA expressed in units of lactic acid) is as follows: