A Bit About Sodium Hypochlorite Chemistry

In this post we’ll take a look at one of the major chlorine based disinfectants that we are most likely to deal with and how it reacts when we add it to water.  This will involve a bit of chemistry, but don’t be afraid.  It won’t hurt, I promise!

Sodium hypochlorite is a very commonly used disinfectant in drinking water systems.  The chemical formula for sodium hypochlorite is NaOCl, which means it has one sodium atom (Na); one oxygen atom (O); and one chlorine atom (Cl) all bound together.  This chemical is commonly purchased as a concentrated solution that contains about 12% sodium hypochlorite by weight.  So if you took 1 gallon of the stuff, which weighs about 10 pounds, then 1.2 pounds would be sodium hypochlorite, another 2 ounces would be sodium hydroxide (NaOH), and the rest would be water.

In a typical situation which we will use as an example, this concentrated chemical would be diluted.  It would be fed at a controlled rate into water being pumped from a well before it goes into the distribution system.  In this situation, the sodium hypochlorite reacts with water and forms two new chemicals: Hypochlorous acid (HOCl) and sodium hydroxide (NaOH).  The equation looks like this:

NaOCl + H₂O → HOCl + NaOH

Sodium hydroxide is a base, meaning it will raise the pH of the water.  This is the opposite of an acid, which lowers pH.  pH, without getting too technical, is just a scale to measure how acidic or basic something is.  Hypochlorous acid in water partially comes apart, or dissociates, into ions, which are just atoms or molecules that have an electrical charge.  In this case, it dissociates into a hydrogen ion (H⁺) and a hypochlorite ion (OCl⁻), like this:

Hypochlorite anion

HOCl → OCl⁻ + H⁺

The hypochlorite anion (a negative ion) that results is what we are after.  Because of its negative charge and the oxygen atom hanging off one end, this ion is very reactive and can cause changes to many other molecules it comes into contact with.  It is known as a strong oxidizer, which just means it can force changes to other atoms or molecules by either adding an oxygen atom to them, or by stealing electrons from them.  So what does that mean in more practical terms?  Let’s look at the function of the hypochlorite ion as a disinfectant – how does it actually kill bacteria?  It might surprise you to know that no one actually knows for sure.  There are lots of theories and the reality is probably a combination of some or all of them, but for all of our scientific prowess we still aren’t exactly sure.  Some of the ways in which it has been proposed that it works is by punching holes in the bacterial cell wall and membrane, causing too much water to flow in and other cell contents to flow out; slicing up the cell’s DNA; or inhibiting glucose metabolism, causing near instant starvation.  Sounds brutal!

Sodium hypochlorite is one way of using the oxidizing capabilities of chlorine based chemicals to, among other things, help keep our drinking water free of microbiological organisms.  Next time, we’ll take a look at chloramines and see how they perform a similar function.