Chlorine Questions on the Certification Exam

Besides math, no topic presents us with more certification exam questions than chlorine. Why? Because chlorine is still — by far — the most widely used disinfecting chemical for drinking water in the USA. And because disinfection is still — by far — the most important step in drinking water treatment.
Many of us chlorinate with sodium hypochlorite bleach, calcium hypochlorite granules or tablets, or on-site sodium hypochlorite generators. But the most common form of chlorine used in water treatment is … chlorine. Elemental chlorine essentially has an infinite shelf life, because it is an element — that basic form of matter that cannot be broken down into simpler forms. So it can be manufactured at remote locations and easily transported to your treatment plant without decaying along the way.
At room temperature, chlorine is a gas. That complicates transportation (and storage), since gases take up a much bigger volume than solids or liquids. In fact, for chlorine, the gas occupies a volume about 400 times greater than the liquid — at normal pressures. But what if we were to pressurize the gas? That wouldn’t make any difference for a solid or a liquid, but for a gas, it can greatly decrease the size of the storage and transportation equipment. If we apply enough pressure to a gas, we can even get it to condense into a liquid — and thus reduce the volume of chlorine gas by about a factor of 400. Think of this the next time you enter your chlorine storage area: what if you had 400 times as many containers as you have now? So pressurization is really helpful when we deal with gaseous chemicals.
A common test question asks what the first thing that should be done to address a liquid chlorine leak. The real answer is “RLH!” (Run Like Heck!) But the answer that the exam is looking for is to rotate the storage tank so that the leak is near the top. The liquid is more dense than the gas, so the bottom of the tank will be filled with liquid, and gas will form a pocket near the top — as shown in the illustration. By rotating the vessel to place the leak near the top, you have converted a liquid leak into a gas leak — and now things are 400 times better. Of course, at this point you still have an uncontrolled chlorine gas leak, so things aren’t great! But you have greatly diminished the leak risk.
Could we pressurize a liquid enough to turn it into a solid? Yes! If you don’t believe me, ask the team trying to put a cap over the crude oil leak in the Gulf of Mexico. The water around the leak is cold, but not frozen — probably around 40 degrees Fahrenheit. The leaking fluid contains a lot of water — not just oil — predominantly in the liquid phase. However, the great pressure at that depth (around 5000 feet, so if 2.31 feet = 1 psi, the pressure is over 2000 psi!) will cause much of the liquid water trapped within the capping vessel to change into the solid phase. The formation of ice within the capping vessel led to the failed first attempt to stop the leak — darn that pressure!
My schedule is pretty tight the rest of the week, so this may be my last post prior to the exam. Best of luck to those of you taking the exam on Saturday. Make me proud! After the exam, please send me an e-mail at to tell me about your exam experience — especially what surprised you the most about the exam. And don’t let the exam “pressure” get to you!

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