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.
