Chlorine or Chlorine Dioxide: What are the Differences?
The main difference between chlorine and chlorine dioxide is their chemical structure and properties. Chlorine is a simple element, while chlorine dioxide is a compound.

What is Chlorine?

Chlorine, with the symbol Cl, is an element of the periodic table that belongs to the halogen family. Fun fact, halogens are characterized by their ability to exist in several forms at the same temperature: solid, gaseous and liquid.


In its liquid form, chlorine is a solution of sodium hypochlorite in a basic environment. In reality, liquid chlorine is just bleach with varying concentrations. Solid chlorine refers to crystals of trichloroisocyanuric acid or calcium hypochlorite. In its gaseous form, chlorine is much more concentrated since it is generally not in solution.


Chlorine gas is produced commercially by electrolysis of a brine solution: sodium chloride is usually used as the brine. In the United States, the most common technique for creating chlorine is with the help of a diaphragm cell that creates chlorine gas and sodium hydroxide. On the other hand, the use of mercury cells is also very common since it allows the production of a much more concentrated caustic solution than with the diaphragm cell.


Chlorine is activated when it comes into contact with water. For example, when added to pure water, it reacts at the molecular level to create hypochlorite acid (H0Cl) and hydrochloric acid (HCl). These two acids will affect the alkaline concentration of the water and the pH of the water.


Common uses of Chlorine

Generally speaking, when referring to water treatment, chlorine and its derivatives are used for disinfection, microorganism control, ammonia removal, taste and odour control, colour reduction, organic matter destruction and a few other uses requiring an oxidizing agent.


The Benefits and Drawbacks of Chlorine


Chlorine has been proven to have many uses in water treatment. These include the reduction of most bacteria and viruses that can be found in water and protection against recontamination. In addition, its ease of use and relatively low cost make chlorine a versatile solution.


However, the protozoal protection offered by chlorine is relatively low and its effectiveness in turbid water is equally low. Finally, chlorine can leave an unpleasant odour and taste in the water.



What is Chlorine Dioxide?

Chlorine dioxide is a powerful oxidizing agent that is also a derivative of chlorine. There are several methods to produce ClO2, but the most common method is by the reaction of chlorine gas with sodium chloride.

Similar to ozone, chlorine dioxide operates through the free radical electrophile (electron attraction) process.


Common Uses of Chlorine Dioxide

Chlorine dioxide is very often used for the same purposes as chlorine. However, it has its own uses. For example, it is used in the bleaching of textile fibres, as a disinfectant to provide drinking water, as a bacterial control agent in the food industry or even in the oil fields to remove biofilms and control odours.


The Benefits and Drawbacks of Chlorine Dioxide

Chlorine dioxide has the distinction of being extremely volatile. This is an advantage when conditions require that products be "lost" in the water systems, but its precarious volatility can also be a hazard. Another advantage is that the way ClO2 works means that it does not emit trihalomethanes or haloacetic acids when treating water.


When properly dosed, chlorine dioxide is very effective against phenols without creating the taste problem of chlorinated phenols. In addition, unlike chlorine, pH fluctuations hardly affect the effectiveness of chlorine dioxide.


There are two main disadvantages to chlorine dioxide. Firstly, unlike chlorine, it is not as effective against complex organic molecules and ammonia. On the other hand, the fact that it does not react with these types of contaminants means that it does not produce chlorinated organic compounds that could be harmful to the environment.

Secondly, its volatility means that it must be manufactured on the site where it will be used since it is unstable. These characteristics make chlorine dioxide an agent with a high risk of explosion if improperly handled.


What makes these two Compounds so Different?


Although they both have the word "chlorine" in their names and are both electron-receiving oxidizing agents, chlorine and chlorine dioxide are very different from each other. They differ in that chlorine has the ability to receive 2 electrons while chlorine dioxide can receive as many as 5. To put this in perspective, on a mole by mole basis, the fact that chlorine dioxide can receive 3 extra electrons means that it is 2.6x more efficient than chlorine.


Finally, as stated above, chlorine dioxide works through the free radical electrophile mechanism while chlorine oxidizes by substitution or addition. Therefore, ClO2 is more similar to ozone than to chlorine in the way it works.Start writing here...

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Physical, Chemical, or Physico-chemical?
Water treatment technologies can be divided into multiple families. Among them, here is a definition of physical, chemical or physico-chemical treatments.