8
edits
Changes
m
no edit summary
pH measurements are important in medicine, biology, chemistry, agriculture, forestry, food science, environmental science, oceanography, civil engineering, chemical engineering, nutrition, water treatment & water purification, and many other applications <ref name="bates"></ref>.
pH is best tested in sour beers using a [[PH pH Meter]] and is most useful for biological parameters. Microbial growth, vitality, and death are evaluated based on pH rather than TA. This means pH should be used when testing sanitizer, [[Wort Souring]], starter cultures, etc.
===TA===
Titration is an attempt to quantify an unknown substance with a known one. Titratable acidity asks how much of a given base (in our case sodium hydroxide, NaOH) neutralizes the acid(s) (lactic, phosphoric, etc.) in a volume of liquid. The units of TA can be quoted in g/L, or in other words, so many grams (of a specific acid) in so much substrate (beer) brings the pH of that substrate to a predetermined pH (for instance, a pH of 7 or 8.2).
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.
* Pipettes and glassware, with precision down to 0.1 mL. Alternatively, you can use a precision scale to dose the base into the beer, if you know the density of both liquids (preferred method).
We need a precise volume of the beer. In this case, we have 15 mL. 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 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>.''
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, 5mL 0.1M NaOH: