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The Thermodynamics of Drunk Driving

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Robert Q. Thompson

Department of Chemistry, Oberlin College, 130 W. Lorain St., Oberlin, OH 44074

J. Chem. Educ., 1997, 74 (5), p 532

DOI: 10.1021/ed074p532

Publication Date (Web): May 1, 1997

Section:

History, Education, and Documentation

Abstract

Chemical and instrumental tests for driving under the influence of alcohol (DUI) measure the concentration of ethanol in the breath (BrAC), while state DUI laws are described in terms of blood alcohol concentration (BAC). Consequently, accurate and fair conversion from BrAC to BAC is crucial to the judicial process. Theoretical treatment of the water-air-ethanol equilibrium system and the related blood-breath-ethanol system, based on principles from general chemistry and biology, yields an equation relating the ratio of BAC to BrAC to the absolute temperature of the breath, the fraction of water in the blood, and the enthalpy and entropy of vaporization of ethanol from aqueous solution. The model equation predicts an average value for the ratio of 2350+100, not significantly different from reported experimental values. An exponential temperature dependence is predicted and has been confirmed experimentally as well. Biological, chemical, and instrumental variables are described along with their contributions to the overall uncertainty in the value of BrAC/BAC. While the forensic science community uses, and debates, a fixed ratio of 2100, the theoretical model suggests that a value of 1880 should be used to reduce the fraction of false positives to <1%.

Keywords (Audience):

First-Year Undergraduate / General

Keywords (Domain):

Public Understanding / Outreach

Keywords (Subject):

Thermodynamics

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This article has been cited by 4 ACS Journal articles (4 most recent appear below).

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  Clarifies information presented in a recent articles describing an experiment involving breath–alcohol analysis using an FTIR spectrometer.
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The Flawed Nature of the Calibration Factor in Breath-Alcohol Analysis
Dominick A. Labianca
Journal of Chemical Education2002 79 (10), 1237
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Uncertainty in the Results of Breath-Alcohol Analyses
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Journal of Chemical Education1999 76 (4), 508
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  Arguments are presented to support the position that many DUI arrestees can be in the absorptive state. Under that condition, breath-alcohol analysis can be particularly discriminatory to test subjects, and the author provides data consistent with this ...
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