GEORGE L. GILBERT Denison University Granville. Ohio 43023
Oxidation-Reduction in Blood Analysis: Demonstrating the Reaction in a Breathalyzer
SUSM~TTED SY
John M. Anderson SUNV at Allrsd Alfred, NV 14802 CHECKEDBY
Ronald E. DiStefano Northampten Area County CommvnMy College Bethlehem. PA 18017 The objective of this demonstration is to illustrate an application of a fundamental concept in chemistry, oxidation-reduction. Quite often, human hlood alcohol analysis is performed by law enforcement agencies using an instrument commonly referred to as a Breatha1yzer.l The principle of operation is quite straight forward and involves the oxidation of the alcohol in expired breath by a solution of potassium dicbromate (K2Cr207).This can easily he demonstrated to any size class by carrying out the redox reaction in a quadrant Petri dish on an overhead projector. Preparation The standard solution of KzCr~07used in most Breathalyzers is 0.025% wlv in 50% vlv H2S04.This solution is too dilute to produce a detectable color when placed in a Petri dish and projected onto a screen. Instead, a 0.25-0.30% wlv
K2Cr204in 50% vlv H2S04 solution is used. This solution is easily prepared hy mixing approximately 0.25-0.30 g of K2Cr207in 100 mL of 50% vlv H2SOa. The acid is needed to reduce the volatilitv of the alcohol so that oxidation-reduction can occur. Just ~ r i o rto the demonstration, d a c e enough K d h 0 7 so~utiodto cover the bottom of three bf the quadrangin the Petri dish. In one of the quadrants, add enough ethanol until the color change is complete (orange -pale green). This will be used as one color reference. Procedure Label the projected solutions in the Petri dish and to one of the two remaining K2Cr207 solutions slowly add ethanol dropwise, stirring if necessary noting a color change. The remaining K2Cr207 solution is used as another color reference. Point out to the students that the color change in the solution is proportional to the amount of alcohol added. In the Breathalyzer the hlood alcohol concentration is proportional to the partial pressure of alcohol in expired breath, and the amount of alcohol being oxidized is proportional to the color change in solution. This is the basis on which the Breathalyzer operates. A discussion of the reaction should follow: I Breathalyzer Is a Trademark of Smith & Wesson Electronlcs Co.. Eaton, NJ 07724. Detailed Information on how the Breathalzyer spectrophotometer works is available upon request.
Volume 67 Number 3 March 1990
263
An aldehyde 1" alcohol
RCH20H
+ Cr2O;Z
-
H
+
R-h=O
orange
,I
crz0,z-
R-C,
Cr3+ pale green
+ OH
Cr3+ pale green
strate the Breathalyzer. If one is available, its operation reveals many other chemistry concepts. As the individual breathes into the Breathalyzer, the captured gas is first trapped in a cylinder at constant pressure, temperature, and volume. This is an application of the ideal gas law-assuring consistent amounts of gas for each sample. Dalton's law of partial pressure and the concept of mole fraction are also introduced with respect to the composition of expired breath. In addition, the principles of spectrophotometry can be demonst~ated.~ Acknowledgment
a carhoxylic acid Law enforcement agencies are usually willing to demon-
264
Journal of Chemical Education
I would like to thank Police Chief MarkL. Whitman of the City of Hornell Police Department, Hornell, NY, for his cooperation and assistance.
