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Vapor: The most dangerous state

From our childhood days in elementary school science class we have been taught that matter exists in three states: namely solids, liquids and gases. Recently an additional state known as plasma has been recognized but it is more theoretical than practical importance at present.

H2O can exist in any of the three common states as steam, ice or water and, when the conditions are right, it can be found in all three simultaneously. The set of conditions under which this phenomenon occurs is known as the triple point.

There are a few substances for which the pressure at the triple point is so high that under normal conditions these solids pass directly into the gaseous state from the solid. Examples of these are solid carbon dioxide (CO2) or “dry ice” and naphthalene (C10H8), commonly encountered as the active ingredient in traditional moth balls and water. Under certain conditions these materials can pass from the solid state into the gaseous state by means of a process known as sublimation.

If water, steam and ice share the same molecular formula, namely H2O, then how do they have three distinct states of matter? The answer lies in what is known as the Kinetic Molecular Theory (KMT), which basically states that all molecules existing at temperatures above absolute zero are in motion and must perforce possess kinetic energy or the energy of motion manifested as force. If force equals mass times acceleration (F = MA) and the mass is constant due to the molecular structure, then the only variable is acceleration which translates into motion and thence to velocity.

In the solid state the molecules merely vibrate within a rigid matrix or lattice. As the temperature is raised, the increase in energy causes the magnitude of this vibration to increase, ultimately to the point at which the molecules are able to break out of the matrix and begin to move freely within the mass of material. The solid has become a liquid and the temperature at which this change occurs is known as the melting point (or freezing point if the temperature is decreasing and the liquid becomes solid). Beyond this point, it becomes necessary to confine the material in a container such as a flask, drum or tank.

Every molecule within the body of the liquid is acted upon by forces from every direction and these cancel each other out. However, a certain number of molecules form the surface of the liquid, and these are acted upon by the attractive forces from within the liquid without having any opposite forces to counteract them. Hence the liquid tends to pull into itself those molecules in the surface. The result is the assumption of spherical shaped drops when a liquid is unrestrained since this geometric configuration provides the maximum possible volume within the smallest surface area.

In the case of a liquid contained in a vessel, such as a bowl, the unbalanced forces acting on the surface molecules create what is known as the surface tension membrane (STM). This is an actual membrane and, though it is fragile and it is self-sustaining, its existence can easily be confirmed by the following demonstration:

Water is placed in a tall glass container, filling it two or three inches. Next, a like amount of salad oil is added down the side of the container, so as not to disturb the water. The container is allowed to stand until any bubbles or globules of oil have disappeared. Sprinkle a small amount of coarse ground pepper onto the top of the salad oil. The pepper will slowly sink to the bottom of the oil layer until it comes to rest on the STM, the interface between the oil and the water, where it remains suspended. If one now takes a stirring rod and breaks up the STM, the pepper sinks until it reaches the bottom of the container. Meanwhile, the STM recreates itself and the demonstration can be repeated.

Molecules within a liquid are in motion and have velocity that can be measured and calculated. However, the result gives a value for the velocity of the average molecule. Some molecules move at a slower rate and some “speed demons” exceed it. They will move randomly in all directions until they collide with other molecules, the sides of the container or the STM. Some of these “speed demons” move so fast that their momentum causes them to break through the STM and become free floating particles of gas or vapor in the atmosphere near the liquid. As they leave the liquid, the fast-moving molecules carry their energy with them thereby reducing the energy content of the remaining liquid. When the energy content is reduced, the temperature is lowered and the number of molecules leaving the liquid is reduced, a phenomenon known as “auto refrigeration”.

People think of vapors as coming from liquids as steam coming from hot water or vapor coming from liquefied petroleum gas (LPG). People think of chlorine, carbon dioxide and oxygen as gases. In actuality, these are one and the same. Steam is water vapor or gaseous water.  Propane gas is really the vapor arising from the liquid when the pressure is reduced. Chlorine “gas” is the vapor arising from liquid chlorine, as are carbon dioxide or oxygen. For the purposes of this discussion the term “gas” includes those materials commonly thought of as “vapors.”

Whether a substance exists as a gas (vapor) or a liquid (and occasionally a solid) depends upon the ambient conditions. Two parameters determine whether we have a gas (vapor) or a liquid, temperature and pressure. Charles’ Law tells us that as the absolute (Kelvin) temperature of a gas rises, the volume increases proportionately, assuming constant pressure. Boyle’s Law states that pressure increases so the volume decreases — this time assuming constant temperature. Since in the practical world both temperature and pressure are usually variables, both of these laws are, for computational purposes, combined into what is known as the Universal Gas Law, which multiplies the original volume (V1)  by a temperature factor (T2 / T1) and then by a pressure factor (P1 / P2) to arrive at the final volume of the gas. The working formula for this law is: V2 = V1 X  T2 / T1 X P1 / P2.  All tempe-ratures must be in Kelvin (oC+273) while pressures must be stated in millimeters of mercury (mmHg), atmospheres or bars.

According to the formula, as the absolute temperature decreases, the volume of the gas or vapor equals zero. In other words, the matter simply ceases to exist. Obviously this doesn’t happen. Instead, the gas or vapor undergoes a phase change and becomes a liquid.

For each gas or vapor there is a unique temperature above which the material cannot exist in the liquid phase no matter how much pressure is applied to the system. This is known as the critical temperature and the pressure required to liquefy the gas at this temperature is known as the critical pressure. In short, a substance cannot exist as a liquid above this point.

All of the properties of gases discussed here have a major bearing on the safety of those responding to an incident involving significant quantities of gaseous material as well as the outcome of their response effort. Because of this, there are a number of things that should be kept in mind when responding.

1. Gases may be involved in any incident due to a chemical reaction between a commodity and the environment or other involved materials. A case in point: the lading of a car carrying calcium carbide (CaC2) appears to be inert but when brought in contact with water it gives off large quantities of acetylene (C2H2), which is a highly flammable gas with an almost unlimited flammable range. Acids such as hydrochloric (HCL) may react with roadway material to give off CO2 and/or other gases such as hydrogen sulfide (H2S) or hydrogen cyanide (HCN), both of which are highly toxic. Therefore, all incident sites must be checked constantly for the presence of unsuspected gases or vapors. One cannot depend on their nose. A properly fitted SCBA protects against toxic vapors but blocks the sense of smell and does not protect against flammables. Another source of gaseous atmospheric contaminants is the products of incomplete combustion. Any fire is sure to involve some type of carbonaceous material. If the temperature of the fire is high enough and there is no shortage of oxygen, combustion of carbon will be complete giving rise to carbon dioxide, CO2. (C + O2 ‡CO2). However, suppression efforts decrease the combustion temperature and possibly cause a shortage of oxygen. In this case, the combustion is incomplete, resulting in the production of carbon monoxide (CO) according to the following equation: (2C + O2 !2CO). This is the old water gas reaction which has been used to produce fuel for internal combustion engines. The presence of nitrogen or sulfur in the combustion mixture raises the possibility of the generation of ammonia (NH3) hydrogen cyanide (HCN), phosgene (COCI2) and/or hydrogen sulfide (H2S), all of which are toxic. If there is plenty of oxygen and the temperature of combustion is high enough, all of these will be consumed by the fire. Therefore, there may be times when allowing a fire to burn itself out is a valid option.     

2. Gases can, and will, move. A cloud of gas may be invisible but it is there and it can cause damage. Constant monitoring of the atmosphere is a must, as is accurate knowledge of the weather conditions on site, not at the weather service 30 miles away. Wind shifts must be considered as to their effect on the conditions at the work site. 

3. Gases have mass; some are heaver than air and some are lighter. Natural gas, which is made up chiefly of methane (CH4) is slightly lighter than air and will therefore rise from ground level. LPG or propane (C3H8) is slightly heavier than air and tends to sink into low places and remains close to the release point. This makes a big difference in case of a gas leak in proximity to a storm sewer, underpass or other low-lying appurtenance. Gases can cause death by displacing the atmosphere with its supply of oxygen. For this reason even a relatively harmless gas such as CO2 or nitrogen (N2) can become deadly if an unsuspecting worker walks into a cloud of it and cannot retreat. The gas will not kill him but the lack of oxygen certainly can.

4. The possibility of a gas being present makes it mandatory that monitoring activities be initiated upon arrival at the incident site and carried out continuously until the response and final cleanup is complete. This monitoring must cover the entire site — not just the point of entry. Know which gas present, we must know what gas is present and, if possible, how much. The monitoring must be specific. If an instrument that is specific for a particular gas is not available, find a retired chemist and ask him for a “quick, cheap and dirty” on site test. An example of this is the bottle of lead acetate that sewer workers carried in bygone days. Before entering a manhole, they would soak a piece of paper towel in the lead acetate solution and lower it into the manhole on a string equipped with a clothespin. When they pulled it back up they looked for the tell-tale black stain on the paper. If it was there, they knew they were dealing with hydrogen sulfide (H2S) and strict precautions were mandatory. A rag soaked with ammonia water emits a white cloud of ammonium chloride (NH4C1) in the presence of hydrochloric acid (HCL) and the reverse is also true in the presence of ammonia (NH3) There are literally dozens of such tests that were quite common in the pre-electronic days and they are still useful in cases where an expensive and maintenance intensive instrument is not “cost effective” or is unavailable.

5. Gases are temperature sensitive. As a gas leaves the containment system through a leak or rupture, it carries energy with it. The result is a cooling of the system through auto-refrigeration. This effect may be great enough to reduce the flow of vapor and facilitate plugging of the leak.  Lower temperatures such as those encountered in northern latitudes during the winter months will reduce the pressure of the gas within the containment vessel. This author once encountered a student from the Alaska Railroad who recounted an incident where LPG had been transferred by means of a trash pump. His fellow students thought he was “blowing smoke” until he revealed that the temperature at the time of the incident was fifty degrees below zero (-50 Fahrenheit). At that temperature, one could quite easily carry LPG in a bucket.

As gases cool their density increases. They become heaver.  As a result they may hug the ground until they absorb enough heat to rise. Again, constant and continuous monitoring is crucial to the safety of all concerned. The converse is also true, and gases normally heavier than air become lighter if they are heated in the course of the incident.

6. Finally one must not forget aerosols, those clouds of finely divided particles of liquid that are suspended in the atmosphere and act much like true gases. We see these in consumer products such as spray paints, household deodorants, insect sprays and hair spray. The nefarious mustard gas (C4H8CI2S) used in World War I was, in reality, an aerosol. As a weapon it was very effective. The liquid adhered to the skin and continued to corrode rather than dissipate as would a true gas. The after effects of exposure were horrendous.

Aerosols can be inadvertently produced when materials are forced through a small opening such as a small leak in a containment vessel, and the possibility of their presence must not be overlooked. Aerosols tend to clog analytical instruments and render then insensitive unless adequate but inert filtration is provided for the sampling stream. This need for filtration carries with it the risk that the filtering agent may become contaminated with the aerosol and produce false positives. To prevent this, one should introduce a clean sample of air (perhaps from an SCBA) into the instrument after obtaining a positive reading. If the instrument is still showing a positive reading, the filter should be changed and the test repeated. It is also possible that the tubes, sensors and other components within the instrument could become contaminated. Should this happen, it requires complete disassembly, cleaning and purging of the instrument, an expensive and time consuming procedure normally requiring return of the instrument to a qualified service center or the manufacturer. To prevent such an instrument failure, the filter should be located as near to the intake of the sampling line as possible. In the event of contamination, only a short length of hose needs to be replaced. The line between vapors and aerosols can become blurred. An example is the carburetor on a gasoline engine. Is the fuel fed to the cylinders a true gas or an aerosol? In actuality it can be both, and this makes the selection of a filtering agent difficult. The use of an agent designed for the particular instrument in question and provided by the manufacturer is highly recommended.

In short, gases, vapors and aerosols can be dangerous. They can “sneak up” on the unwary responder when they are least expected and cause great harm. The best defense is a good offense and eternal vigilance is the price of safety.          C   

 

 
 

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