Ask Miss Science

Bleach and Ammonia

The question about soap reminded me of this one I've been wondering about. Bleach and ammonia are both common ingredients in household cleaners, and I know that they are toxic in combination. But what is the difference between them, and why would I choose one over the other to clean a bathtub or toilet or floor? Do they both kill bacteria? Viruses?

-- Posted by Nancy to Ask Miss Science at 7/28/2004 07:57:43 PM

Bleach and ammonia are both bases. That is, both solutions contain hydroxide ions (OH^-). There are other reasons we can call them bases, too, but I'll leave that somewhat advanced discussion out.

To a chemist, ammonia is the gas NH₃, but commonly "ammonia" means a solution of the gas NH₃ in water.

"Bleach" isn't usually used as a chemical term. The closest common chemical term is "bleaching agent," which means exactly what you'd expect it means. Common household bleach is a solution of chlorine gas in water.

Common solutions of ammonia contain hydroxide ions because the gas, NH₃, produces them when it dissolves. The chemical equation describing the result of dissolving ammonia (the gas) in water looks like this:

NH₃(aq) + H₂O(l) <----> NH₄⁺(aq) + OH^-(aq)


Notice the arrow in that equation. It means that these species are all in equilibrium with each other -- they are all present at once. In other words, when you dissolve NH₃ in water (H₂O), some of the NH₃ stays more or less like it was when it was a gas, except that it is now surrounded by water molecules. Some of the other NH₃'s, however, react with nearby water molecules, taking a hydrogen ion (H⁺) away from them and leaving a hydroxide ion (OH^-).


Bleach contains hydroxide ions because chlorine doesn't dissolve very well in water. You see, if you make the water very basic -- by adding lye (NaOH), for example -- the chlorine will dissolve pretty well. The equation looks like this:

Cl₂(g) +2 NaOH(aq) <----> NaClO(aq) + H₂O + NaCl

Notice the double arrow, again. Let's talk a little more about equilibrium.

Chemical equilibrium is in a delicate balance. If you do something to change the number of any of the species present, the rest of the species in the equilibrium will react to the change. They react in a way that tends to counteract whatever change you made.

For example, assume you do something that adds extra OH^- ions to the ammonia solution. Now, there are too many OH^- ions; so, the whole system will react to correct that. To correct the influx, some of the NH₄⁺ ions and OH^- ions will react backwards and form NH₃ and H₂O. In the bleach equation, adding extra hydroxide (OH^-) ions causes the forward reaction to occur, which keeps the chlorine gas (Cl₂) dissolved. So, bleach typically contains a great excess of hydroxide ions.

Some of you might see, based on this, one reason why bleach and ammonia should not be mixed. Adding bleach causes the ammonia's reaction to go backwards, pushing the reaction to the left and then pushing the ammonia (gas) out into the air. NH₃, by the way, is very unpleasant to breathe. In actuality, there are other reactions going on. Depending on the temperature and on the relative amounts of bleach and ammonia, you might get ammonia (the gas), chlorine gas or one of a class of foul compounds called chloramines.

So, now that it is certain that we should not mix the two, the next question is which to use. If your concern is germ killing, bleach is certainly the way to go. However, ammonia does a pretty decent job, too. If you would like further information on the subject, I suggest this website containing an EPA report on the effectiveness of various cleaners. On this site a number of common household cleaners are compared in terms of effectiveness of removing soil and eliminating microbes.

The information used in this post is widely available on the internet and in chemistry textbooks.

posted by Ask Miss Science @ 7:52 PM