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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#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 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.
The ASBC<ref name="ASBC" /> equation for TA in units of lactic acid is as follows:
[[File:TA Formula.gif|center|500px|Titratable Acidity Example]]
We need a precise volume of the beer and the specific gravity of the beer. If the specific gravity cannot be measured easily for some reason, then estimating it will be fine for this equation since small differences in specific gravity don't greatly impact the results of the equation. In this example, we have 15 mL with a specific gravity of 1.015. We also need NaOH in liquid form. Typically, it is sold in 0.1M form. Now, the trickiest part of this is adding precise amounts of NaOH (say, 0.1-0.5 mL at time), to your 15 mL of beer. Every time you add NaOH, you must vigorously stir the sample so it is well-mixed. Then you can measure its pH. You continue this until you reach the desired pH baseline of 8.2.
:''Note: The baseline value of 8.2 pH is somewhat arbitrary, but it is the US (ASBC<refname="ASBC">[http://methods.asbcnet.org/summaries/beer-8.aspx ASBC Methods of Analysis. Total Acidity. Retrieved 11/8/2017.]</ref>) and Australian industry standard. A pH of 7 is a neutral pH and the pH of water, whereas ~8.2 is near the equivalence point for a lactic acid/sodium hydroxide reaction. A pH of 8.2 is also where a titration dye, phenolphthalein, changes color. A well-calibrated pH meter is easier to use than dye, not to mention its superior accuracy and precision, if used correctly (well-calibrated, probe well-maintained, etc.). A pH of 7 is the European industry standard for measuring TA in wine <ref>[http://www.awri.com.au/wp-content/uploads//TN14.pdf "TN14 - Interconversion of acidity units" Industry Development and Support. Australian Wine Research Institute. Retrieved 09/15/2016.]</ref>. See also [https://www.facebook.com/groups/MilkTheFunk/permalink/1877187898976082/ this MTF thread].''
At or around a pH of 8.2, we have reached our equivalence point for a titration of pure NaOH and pure lactic acid. We need to convert the moles of NaOH we added into moles of lactic acid, and then divide the equivalent grams of lactic acid by the original volume of beer. That gets us g/L, and our titratable acidity. For a numerical example, assume '''15mL''' beer with a gravity of '''1.015''', '''5mL''' 0.1M NaOH: