Alcohol Facts by National Draeger
EXHIBIT B-6
DEFINITION
The weight of any gas that dissolves in a definite volume of liquid is directly proportional to the vapor pressure that the gas exerts above the liquid. This proportion, is different for different temperatures.
MEANING
If one shakes a closed bottle containing an alcohol solution and air, the amount of alcohol in the air will build up to a certain point and stop. This point is what chemists call equilibrium. At equilibrium for any given temperature and pressure, there will be a definite ratio between the amount of alcohol in the air and in the water. This ratio will be the same for any given temperature and atmosphere pressure; the higher the temperature, the more alcohol will be in the air. This ratio can be expressed as follows.
Thus, if one knows the value of the constant (K) and one of the other values, the third value can easily be calculated.
APPLICATION TO BREATH TESTING
Alcohol in alveolar air comes into equilibrium with alcohol in the blood. Thus, if the constant is known and the amount of alcohol in the breath can be measured, the amount of alcohol in the blood can be determined. The temperature at which the breath leaves the mouth is 34 centigrade. Therefore, the ratio between the concentration of alcohol in blood and alveolar air with which it is in equilibrium must be determined at 34 centigrade. This ratio was found to be approximately 2100:1. Thus, 1 milliliter of blood will contain the same weight of alcohol as 2100 milliliters of the alveolar air with which it has come to equilibrium.
