Determination of Alcohol in Breath
Richard S. Treptow Chicogo State University Chicogo, Winois 60628
for Law Enforcement
Law enforcement agencies in increasing numbers rely upon instrumentation to substantiate charees of alcoholic ikoxication. One such instrument is the dhromate-photometric breath analyzer, which can be used entirely in the field without a support laboratory.1.z Its operation makes use of several principles familar to the student of eeneral chemistw. A discussion of the breath analvzer ilh a t e s how seemingly abstract and unrelated c1;emical urinciules can be brourrht toeether in the form of a device having relevance to modem life. Legal definitions of alcoholic influence are generally expressed in terms of alcohol content in blood and typically are set a t about 0.15%. Use of the subject's breath to estimate blood alcohol is made possible by the fact that air from deep within the lungs is in equilibrium with pulmonary blood and by application of Henry's Law to the system. As shown in the figure the breath analyzer consists of a volumetric breath coilecting cylinder, a reaction ampoule, and a photometric detector. The breath collecting cylinder contains a piston which rises freely as air exhaled by the test subject is blown thmugh a tube (A) into an inlet hole in the cylinder bottom (B). When the piston reaches a premeasured height it becomes supported by an electromagnet and at the same time vent holes in the cylinder wall become exposed. As the subject continues to exhale the vent mechanism ensures that the final air collected is from deep within the lungs. It has a pressure of 1 atm and a 50-ml volume. The collected breath sample is next slowly expelled from the cylinder and bubbled into the reaction ampoule (C) containing 0.025% potassium dichromate and a catalytic amount of silver nitrate in 50% sulfuric acid. The reaction ampoule is thermostated at 50°C; after 90 s the reaction is complete. Oxidations by chromium (VI) are stepwise and tend to be complex. The overall result can be summarized 3CH,CH@H 2K,Cr,O, 8H,SO, 3CH,COOH 2Cr,fSO,), 2K,SO, 11H,O In the process chromium changes from red-orange to blue. The reaction amuoule is desiened - to also serve as an ootical cell of the detector. At the center of the photometric detector is a light source mounted on a carriage which moves freely in the horizontal direction. When centered the light is equidistant between the reaction ampoule and a standard am-
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poule (D).The standard ampoule is a sealed reference identical to the reaction ampoule except no alcohol is added. Separate beams of light pass first through each ampoule, go next through blue filters (E), and fmally impinge upon a pair of photovoltaic cells (F). The pbotovoltaic cells are connected to a galvanometer (G) which registers any difference in their electric currents. Before the alcohol reaction both ampoules are equal transmitters of blue light. With the light equidistant between them the photovoltaic cells generate identical electric currents and the galvanometer reads zero. After reduction of the chromium (VI) transmission of blue light by the reaction ampoule is increased. Zeroing the galvanometer now reauires movine the lieht closer to the standard ampoule. The alcohol &termination is based on the distance the light is required to move.3 Calibration is accomplished using known quantities of alcohol vapor. In actual practice the instrument scale is constructed to read percent blood alcohol directly. The effective Henry's Law factor used to accomplish this is that 2100 ml of breath from deep within the lungs contains the same amount of alcohol as 1ml of blood. For class use various modifications and elaborations of this procedure can be adopted. Collection of a breath sample of known volume and pressure is easily accomplished by water displacement. The photometric determination can be performed on any suitable instrument. T o simulate the breath of an intoxicated subject the vapor above a dilute alcohol solution can be used. The tech-' nique is readily extended to the determination of the pmof of alcoholic beverages or to the determination of other easily oxidized compounds.
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1 Information on the design of the breath analyzer furnished by OfficerC. Wingard, Cincinnati Police Department. 2Alternative procedures for alcohol determination by dichomate are reviewed in Snell, F. D., and Snell, C. T., "Colorimetric Methods of Analysis," 3rd Ed., D. Van Nostrand Co., Princeton, N.J., 1953, Vol. 111, pp. 46-50 and in Joslyn, Maynard A., (Editor), "Methods in Food Analysis," Academic Press, New York, 1970, pp. 454-8. 3 Whenever the instrument is in a zeroed position a simple relatianshi~exists between the position of the light and the concentration'of blue light absorbing reagent in eaih ampoule. Specifically, the concentration difference in the two ampoules is proportional to 2 log (d,ld,), where d r and d, are the distances trom the light to the reaction and standard photovolatic cells, respectively.
Dichromate-phatometricbr?athanalyzer.
Volume 51. Number 10, October 1974 / 651
