I tend to avoid writing about pool chemical incidents. First they are so common place because the chemicals are so ubiquitous; so I would be spending an inordinate amount of time writing about these incidents. And secondly the news reporting on the incidents is almost uniformly poor that there is little to be gained by trying to analyze the incidents for lessons learned for the greater chemical handling and transportation industries.
But occasionally, the incident is bad enough and the reporting poor enough that I just can’t help myself. An incident Monday near Orlando, FL is just such a case. The local TV station reports that “a tanker was delivering chlorine to a pool supply facility when the driver accidentally pumped the chemical into a sulfuric acid tank, making ‘mustard gas’. Where to begin…
First off, while chlorine gas is used in some large swimming pools or water parks, most pool supply houses use sodium hypochlorite, essentially household bleach, just slightly more concentrated (if in liquid form). This is the favored form of chlorinating pools because it is much less dangerous than chlorine gas and does not require sophisticated handling equipment. Most home pool users are more likely to use the powdered from as it is even easier to handle safely than the liquid.
Sodium hypochlorite in either form is very reactive and has a tendency to give off chlorine gas when it does react. The most common accidents involve using it in conjunction with acids (also used in the pool treating business to adjust pH). If the acid and ‘bleach’ are added to the pool too closely together they react very quickly and the chlorine gas production is so fast that a large bubble is formed and it looks like the pool water is ‘exploding’.
The chlorine gas cloud (unless released in a confined space) is seldom really dangerous as the concentration if fairly low. It will certainly irritate the moist membranes of the eyes, nose and throat. The larger the cloud or the closer an individual is to the cloud the more severe this irritation will be. If the cloud is large enough, concentrated enough, or in a small enclosed area the chlorine gas can be lethal, but that is seldom the case.
Sulphuric Acid and Bleach
Sulfuric acid is a special case. The reaction between any acid and sodium hypochlorite is exothermic. With sulfuric acid, especially concentrated sulfuric acid the temperature rise can result in sulfuric acid fumes which are potentially more dangerous than the chlorine gas produced in the reaction. This is because the sulfuric acid is very corrosive and can cause chemical burns as well as severely irritate the lining of the eyes, nose and throat.
‘Mustard Gas’ is not produced. In fact, there is no such thing as ‘mustard gas’. Mustard agent is an oily liquid that produces blisters when it contacts the skin or other body surfaces. People that inhale mustard agent are actually inhaling droplets not a gas. Sulphur mustard is not produced from hypochlorite and sulfuric acid. It is produced by the reaction of hydrochloric acid and a chemical weapon precursor called thiodiglycol or thio-bis-ethanol.
What Probably Happened
Remember, I wasn’t there and only have the news reports and my experience as a chemist in an industrial environment to guide me, but here is what probably happened. A truck driver shows up to make a delivery of liquid sodium hypochlorite to a pool supply house. It’s raining and the driver is in a hurry and is probably not familiar with the establishment. He hooks up his hose to the sulfuric acid tank which was inadequately marked.
The hypochlorite reacts immediately with the sulfuric acid produces heat, steam, chlorine gas and sulfuric acid fumes. Fortunately the tank is adequately vented so that it does not catastrophically collapse due to over pressure. The cloud comes out of the top of the tank and immediately engulfs the driver. The cloud spreads off site and injures some passers-by. Fortunately the rain knocks down the worst of the cloud and no one, beyond perhaps the driver, is seriously injured.
Unnoticed in the story is the off-site acid contamination. If the local storm drains go to a sewage treatment plants (more and more common) then the plant has a temporary upset in its treatment process as the acid water kills off many of the bugs used to treat the sewage. If the drains lead directly to a local water way then there might have been a small fish kill associated with the incident, depending on the amount of rain that was falling. The greater the rainfall the smaller the fish kill (the old ‘dilution is the solution to pollution’ saying really is based in part on observable fact).
What Should Have Happened
A delivery driver should never be allowed to unload into a storage tank without at least someone from the facility pointing him at the proper tank. Tanks and off-loading lines should be clearly and unequivocally marked with the contents of the tank. When tanks of incompatible materials exist at the same facility they should be physically separated enough to make it extremely difficult for the reactive materials to put into the wrong tank.
A best practice is for the off-loading lines to be double locked. Two different people should provide the keys to unlock the lines before unloading can begin. Both people should independently verify the contents of the material to be off-loaded before the keys to the tanks are provided. It goes without saying that the same keys should not be able to unlock tanks of different materials.
The mixing of incompatible materials in the unloading process can be a very serious problem and is more common than chemical professionals would like to admit. In my opinion it would be the easiest form of attack on a chemical facility, especially since most delivery content checks are paperwork checks only. If the paperwork gets attached to the wrong trailer through deliberate action then the trailer will almost certainly get unloaded into the wrong tank.