Letters to the Editor

Gas Phase Measurement

In the April 2007 issue of WE&T on pages 81 and 83, the Certification Quiz question 3 on parts per million in the gas phase is a good test of one’s knowledge of the meaning of ppm in the gas phase. Sometimes, I have seen ppm mistakenly interchanged with mg/L in the gas phase. While ppm and mg/L are very close to being equal in dilute water solutions, they are not interchangeable in gases at ambient conditions. The quiz answers are correct, but I suggest that published articles on gas phase concentrations clarify ppm to be ppm by volume by using the designation of ppmv for gas phase concentrations.

For gases at ambient conditions (an ideal gas), volume percent equals mole percent equals pressure percent. So molar, volume, and pressure percentages are the same but, ppm and mg/L are not the same. For gases, units of ppmw or mg/L would be widely different values than ppmv primarily because one liter of gas does not weigh 1000 grams like one liter of water.

Thomas D. Hendrey
Whitley Burchett and Associates, Inc.
Walnut Creek, Calif.