What is chlorine and why do we measure it?

If you’ve ever gone swimming in a pool, you’ve probably heard of chlorine, the chemical used to keep swimming water clean. If your eyes felt itchy after a swim with your eyes open underwater, you can blame chlorine for that too.

But what exactly is chlorine and why do we use it in our water?

What is chlorine?

Chlorine is a chemical element from the halogen family. Halogens produce salts like table salt, which consists of sodium chloride (NaCl). Yes, that’s chlorine you see there.

Image courtesy of theleveredge.com

You can produce chlorine by putting saltwater through a process called electrolysis. A current passed through metal plates in the salt water causes the NaCl to separate into sodium (Na) and chlorine (Cl).

Why do we use chlorine?

Even though most of us know the use of chlorine mainly in water applications, you can find it in industries such as pharmaceutical, textile, and paper. In all cases, chloride acts as a reagent for chemical reactions. It can bleach, oxidize, or disinfect.

Let’s come back to the pool example. Chloride dissolved into water forms hypochlorous acid, HOCl, and hypochlorite, OCl. These chemicals destroy harmful bacteria and microorganisms.

However, you must measure and monitor the chlorine level in the water. Too much would waste the chemical and increase costs, and too little wouldn’t completely disinfect your water.

What do we need to measure?

Before we talk about the ways to measure, let’s understand what we’re measuring. As mentioned earlier, chloride in a solution produces hypochlorous acid and hypochlorite. We call these two elements free chlorine.

The amount of each component will depend on the pH of the solution. With a pH value of around 7.5, hypochlorous acid and hypochlorite has a ratio of roughly 1:1. For lower pH values, you’ll have more of the acid form, HOCl, than OCl-. And for higher pH values, you’ll have more of the alkaline form than the acid form.

Image of chlorine measurement
Image courtesy of waterworld.com

Besides the free chlorine, you may also have other chlorine compounds, like chloramine (free chlorine + ammonia) and organically bound chlorine. We call these compounds combined chlorine. The amount of free chlorine plus combined chlorine form the total chlorine amount in the solution.

How do we measure chlorine?

Industrial sensors usually measure either free or total chlorine in the solution. For drinking water, you can also measure chlorine dioxide. Regardless of which you need to measure, the sensor will use the amperometric method that we discussed in the dissolved oxygen article. We’ll do a quick recap here for those who missed that one. Everyone else can skip to the next paragraph.

Image of  chlorine measurement sensor
Image courtesy of emerson.com

The amperometric method consists of two metallic electrodes covered by an electrolyte. A permeable membrane lets the medium diffuse through it. A voltage goes through the electrodes, reducing the chlorine and producing Cl- on the measuring cathode. On the silver anode, the silver oxidizes into silver chloride (AgCl).  This process produces a current relative to the concentration of chlorine in the solution.

What sensor would work best?

The sensor you choose will depend on your application. You’ll often find free chlorine sensors in pool water. Chlorine dioxide sensors show up a lot in drinking water, food, and process water applications. Last but not least, wastewater treatment plants commonly use total chlorine sensors to measure the water’s disinfection status. One of those should work for your process.

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