OTHER CONCENTRATION UNITS Yves Alarie, Ph.D Professor Emeritus U niversity of Pittsburgh,USA

A.PARTIAL PRESSURE Fraction (Volume/Volume Composition) of the total pressure of a mixture exerted by a component of the mixture.

For example, at sea level, 1 atmosphere ( 760 mm Hg, 760 torrs, bars, 1.03 kg/cm 2, 14.7 p.s.i.a. or kPa and yes you were under the impression that SI units were going to simplify your life, fat chance! ), if we know the volume composition of each component in the mixture, we can calculate the partial pressure of each. Assuming the following % composition for air:

N 2 =78.6%=597 mm Hg Partial Pressure O 2 =20.8%=159 mm Hg Partial Pressure CO 2 =0.04%=0.3 mm Hg Partial Pressure H 2 O =0.5%=3.8 mm Hg Partial Pressure TOTAL 100%=760 mm Hg Total Pressure The volume composition can be in % or ppm, see above, since both are volume/volume unit. Therefore, 0.1% or 1,000 ppm = 0.76 mm Hg.

B.pV = nRT If the concentration is given as moles/Liter (n/V), the partial pressure p (in mm Hg) can be obtained using the value of the gas constant R as 62 and the temperature (T) in Kelvin. Seldom used in toxicology but certainly appropriate.

C.HENRY’S LAW AND SOLUBILITY COEFFICIENT The amount (moles or grams) of a gas or vapor dissolved in a liquid is directly proportional to the partial pressure of the gas in the gas (air) phase above the liquid. At equilibrium the partial pressure (in mm Hg) in both phases is equal, but the amount, in moles or grams, will vary between the two phases according to each gas. This ratio, S, is called the solubility coefficient and is given for a particular temperature.

S = Concentration in moles (or grams) of a gas in a liquid/concentration (same units) of a gas in air phase.