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Titratable acid reserve

As medical toxicologists, we often get consulted on exposures to caustic agents. Caustic agents typically are at the extreme ranges of pH: eg, approaching 0 and 14 for acidic and alkali substances, respectively.

An important concept is the titratable acid reserve (TAR). Not all clinically significant caustic substances have extremes of pH. For example, glacial acetic acid is about a 17 Molar solution of acetic acid and water, which has pH of 2.4. However, an aqueous solution of hydrochloric acid with pH of 2.4 would only be a 0.004 Molar solution. So, despite having the same pH, glacial acetic acid would have 4250 times as many Moles of acid as the aforemention solution of HCl.

Hoffman, et al defined TAR as a the amount in mL of either 0.1M HCl or 0.1M NaOH required to bring 100 mL of solution to a pH of 8.0.

The general idea with TAR is that the more acid or base a particular xenobiotic takes to normalize a solution to a pH of 8.0, the more acid/base  living tissue exposed to that xenobiotic will have to neutralize. So, TAR is likely a better indicator of the caustic potential when compared to pH. The previously described solutions (glacial acetic acid and HCl both at pH 2.4) would likely have drastically different local tissue and systemic effects if ingested by a patient.  

In the 1989 Hoffman, et al paper, dissected dog esophaguses were exposed to a variety of caustic xenobiotics. Histologic damage was then assessed and referenced to each xenobiotic’s TAR. Although there is some overlap in TAR values 24-40, you can extrapolate that TAR less than 5 did not have any histologic damage; where as TAR greater than 40 universally had some demonstrable histologic damage. 

Examples of caustic agents with exceptionally high TAR include:

  • Phenol
  • Glacial Acetic Acid
  • Crystalline drain cleaner
  • 4% ammonia surface cleaner
  • Battery acid (eg, from lead based automotive battery)
  • Liquid toilet bowl cleaner

Examples of caustic agents with notably low TAR include: 

  • Over the counter bleach
  • Laundry and dish detergents 

So, utilizing TAR is likely a better way to guide clinical decision making regarding the decision to recommend admission for GI consult and possible endoscopy versus PO challenge and likely medical clearance. 

Further research could include update analysis of TAR in current and commonly available products available to the public. The resulting database of TAR’s could help further guide medical toxicologists decision making. 


Hoffman RS, Howland MA, Kamerow HN, Goldfrank LR. Comparison of titratable acid/alkaline reserve and pH in potentially caustic household products. J Toxicol Clin Toxicol. 1989;27(4-5):241-246.

Goldfranks Toxicologic Emergencies 11e