1. CHAPTER 6
Gases and Vapors

2. Learning Objectives
Describe the various properties of the chemicals that are most important to the practice of industrial hygiene.
Calculate the concentration of a chemical in the air when provided with the chemical’s basic parameters.
Describe the basic behaviors of gases and vapors in scientific and mathematic terms.
Identify various methods of collecting and analyzing gas and vapor concentrations in the air.
Identify the handheld and portable detectors used for direct measurement of gases and vapors in the environment.

3. Normal Temperature and Pressure (NTP)
Used for gas and vapor calculations
25ºC and 760 mmHg
At NTP, 1 mole of any gas will occupy L
Example: If mercury (Hg) has a molecular weight of 201 g/mol, how many grams are in 2 moles of Hg?
Grams = moles x molecular weight
Grams = 2 moles x 201 g/mol = 402 grams

4. Concentration of Gases and Vapors in Air
ppm = parts of contaminant per million parts of air
For a gas or vapor of a certain mass at NTP, the concentration in air can be calculated using:
Concentration calculated by volume comparisons:
ppm= ( mg m 3 )(24.45 L) (molecular weight)
ppm= Vcontaminant Vair × 10 6

5. Example Calculation
If you spill 60 mg of naphthalene (MW = 128) in a room that measures 3 meters (m) x 4 m x 4 m, what will be the concentration in ppm after all the liquid has evaporated?
Volume of room = 3 m x 4 m x 4 m = 48 m3
Ppm = [(60 mg) x (48 m3)] ÷ 128 g/mol
Ppm = 2880 mg/m3 ÷ 128 g/mol
Ppm = 22.5 ppm

6. Example Calculation
What is the vapor concentration if 1 gram-mole of acetone is spilled in a room that is 8 feet (ft) x 11 ft x 15 ft?
GIVEN: At NTP, 1 mole of any vapor occupies L
GIVEN: 1 cubic foot (ft3) = L
Volume of room = 8 ft x 11 ft x 15 ft = 1,320 ft3
Ppm = (volume of contaminant / volume of air) x 106
Ppm = [24.45 L ÷ (1,320 ft3 x L/ft3)] x 106
Ppm = x 106
Ppm = 654 ppm

7. Vapor Pressure
The amount of pressure that a liquid exerts on the inside of a closed container above the surface of the liquid
At equal temperatures, chemicals with higher vapor pressures tend to evaporate more quickly than chemicals with lower vapor pressures.
In general, chemicals will evaporate more quickly at higher temperatures.

8. Vapor Density The measure of how heavy the vapor is in air
Chemicals with higher vapor densities tend to settle near the floor.
Chemicals with lower vapor densities tend to rise to the ceiling.
Vapors with high densities can displace oxygen from the workers’ breathing zones, which can lead to physical/simple asphyxiation.

9. Specific Gravity (SG)
The mass of a substance compared to the mass of an equal volume of water
Water has an SG of 1.0 gram per milliliter (ml) or cm3 (cc).
A chemical with an SG of < 1 g/ml will float on the surface of the water.
A chemical with an SG of > 1 g/ml will sink to the bottom of the container and form a layer beneath the water.

10. Example Calculation
If acetone has an SG of 0.79, how many ml are in 1.0 g of acetone?
SG = mass of material (g) ÷ volume of material (ml)
0.79 = 1.0 g ÷ (x) ml
0.79x = 1.0 g
x = 1.3 ml

11. Boiling Point & Flash Point
Chemicals with boiling points at NTP evaporate quickly and are generally considered gases
Boiling points are affected by atmospheric pressure
BP tends to decrease with lower pressure and higher altitude
FLASH POINT
The lowest temperature at which a material gives off enough vapors to form an ignitable mixture

12. Fire Point & Auto-Ignition Temperature
The temperature at which the ignitable mixture will continue to burn
AUTO-IGNITION TEMPERATURE
The point at which a material will ignite without an external source of ignition

13. Molecular weight of chemicals
Indicates how the chemical will move and interact with air and other gases
Chemicals with HIGH molecular weights are LARGER and move more slowly than chemicals with low molecular weights

14. Brownian Motion
A measure of how much a gas or vapor molecule moves in the air.
The continuous ricocheting of molecules bouncing off one another which allows them to spread out in space.

15. Diffusion
The ability of gases and vapors tend to spread out and move from locations of high density to those of low density.
HIGH  Low
This movement causes them to become equally distributed in a space.

16. Ideal Gas Law
When the amount of the chemical is known and it is entirely evaporated, use the following equation to calculate the volume that the chemical will occupy at a given temperature and pressure:
PV = nRT

17. Generalized Gas Law
Can be used to quickly determine the pressure, volume, or temperature of a system:
P 1 V 1 T 1 = P 2 V 2 T 2

18. Charles’ Law
If the pressure in two different conditions remains the same and only the volume and temperature change, then the generalized gas law can be reduced to Charles’ Law with the following equation:
V 1 T 1 = P 2 T 2

19. Boyle’s Law
If the volume and pressure in two different conditions change, but the temperature remains the same, then Boyle’s Law can be used to calculate the new values:
P 1 V 1 = P 2 V 2

20. Example Calculation
If a worker spills 12 ml of acetone and it completely evaporates into a room that is 3 m x 4 m x 3 m, what is the airborne concentration of acetone in the room?
GIVEN: the SG of acetone is 0.79 g/ml
Volume of room = 3 m x 4 m x 3 m = 36 m3
SG = mass of material (g) ÷ volume of material (ml)
0.79 = x g ÷ 1 ml
Therefore, 1 ml of acetone = 0.79 g at NTP
So, if 12 ml are spilled…
12 ml x 0.79 g = 9.48 g of acetone in the room
BUT “concentration” requires you to evaluate the amount / space…
Therefore, 9.48 grams ÷ 36 m3 = 0.26 g/m3
To get mg/m3, simply multiply by 1,000 (1 g = 1000 mg) = 260 mg/m3

21. Four Reasons to Collect Air Samples
To obtain an accurate representation of air concentrations
To identify leaks or releases
To evaluate the effectiveness of controls
To assess exposures during particular work processes or activities

22. LOD & LOQ LIMIT OF DETECTION (LOD) LIMIT OF QUANTITATION (LOQ)
The point at which the measurement of an agent first becomes possible
LIMIT OF QUANTITATION (LOQ)
The concentration at which quantitative results can be measured with a high degree of confidence

23. Example Calculation
If an air sample in a room is collected at a rate of 1.2 liters per minute (LPM) for 4-hours, and there is 0.3 g of turpentine in the room, what is the actual concentration of turpentine?
GIVEN: SGturpentine = 0.87
Time = 4 hours = 4 hours x 60 min/hour = 240 minutes
Sample volume collected = 1.2 L/min x 240 min = 288 L
Concentration = 0.3 g ÷288 L
Concentration = g/L x (1,000 L/m3) = 1.0 g/m3
Concentration = 1.0 g/m3 x (1,000 mg/g) = 1,000 mg/m3

24. Sampling and Analytical Error (SAE)
SAE= (pump error) 2 + (lab error) 2
Sampling can be subject to errors
Flow of pump
Laboratory analytical errors
Total of errors in a collected sample is called the SAE
SAE is typically ~ 10%

25. Accuracy and Precision

26. Determining Sampling Duration
Total quantity of air sampled (liters)
Pump flow rate (liters per minute)
Sampling time duration (minutes)

27. Grab Sampling
Source: SKC Inc.

28. Detector Tubes & Colorimetric Indicator Tubes
Source: SKC Inc.

29. Air Sampling Pumps

30. Real-time Detection Instruments
Combustible-gas/multiple-gas monitors
Thermochemical detectors
Electrochemical detectors

31. Real-time Detection Instruments (Cont.)
Coulometric detectors = measure conductivity of ions in proportion to the gas concentration of the sample agent
Ionization detectors = use energy source in device to ionize the sample agent and correlate the agent’s concentration in the air
Schematic of a flame ionization detector. (A) GC column exit; (B) detector oven; (C) hydrogen fuel enters; (D) oxidant enters; (E) positive bias voltage; (F) flame; (G) collector plates; (H) signal transmitter; (J) exhaust port.
MiniRAE 3000 PID Source: SKC Inc.

32. Spectrochemical Direct-Reading Instruments
These use infrared (IR), ultraviolet (UV), or visible light photometers to measure the transmittance of light through the sample chamber.

33. Summary
A significant portion of an IH’s career is spent analyzing worker exposures to gases and vapors
IH technicians and professionals must be familiar with the processes and instruments used for calculating the concentration of gases and vapors in the air