Re: Why is bleach an acid?

To answer this question, I need to review a little bit about acid and base chemistry. Let me start to answer your question by reviewing what an acid-base reaction looks like. By the simplest definition (Arrhenius Theory), and acid reacts like:
HA --> H+ + A-
(often, this is reaction is written to show the water as well: HA + H2O --> A- + H3O+)

and a base reacts like:
BOH --> B+ + OH- or BOH + H3O+ --> B- + 2 H2O

An important bit of terminology is that the 'stuff' on the other side of the arrow is called the 'conjugate' (meaning 'opposite') of the left side (in other words, A- is called HA's conjugate base, and B+ is called BOH's conjugate base). If the acid or base spontaneously reacts as above 100%, it is referred to as a "strong" acid or base. A strong acid is effectively pH = 0, and a strong base is effectively pH = 14 in water (you can actually have even stronger acids or bases, but something called the "leveling effect" limits pH's in water to between 0-14.)

Some substances do not completely react - these are called "weak" acids and bases. As an example, only 10% of a weak acid may react as above, giving:
0.9 units HA + 0.1 units H+ (or if you prefer, H3O+) + 0.1 units A-

The degree to which the acid dissociates into H+ and OH- can be communicated by the 'acid ionization constant', Ka, where
       ([H3O+]*[A-])
Ka = -------------
           [HA]
(the brackets [] mean 'concentration', which is the number of moles of the molecule present divided by the volume of solution it is in.)

Increasing values of Ka indicate more strength, i.e. more of the acid added to the solution dissociates into H+ and A-. This Ka value is an innate property of the acid molecule, and this formula applies to any solution containing HA, H+, and A-.

Now that we have defined some important terms, I will try to answer your actual question. Hypochlorous acid has the formula ClOH. In water, it is a weak acid, undergoing the reaction
ClOH + H2O --> ClO- + H3O+

The Ka value of this reaction is 2.9x10^-8, from which you can calculate that 0.017% of the ClOH dissociated into ClO- and H3O+. So, yes, Hypochlorous acid is an acid albeit a pretty weak one. However, ClOH is too unstable to simply buy and store. If you wanted to treat your pool or make your own bleach, you would actually purchase sodium hypochlorite, which is the same molecule except with the hydrogen atom replaced with a sodium atom. This "salt" is stable. However, let's think about what happens when it is added to water. The salt dissociates into Na+ and OCl-. OCl- is the conjugate base of hypochlorous acid. OCl- reacts with the H3O+ that is naturally present in water, meaning some of the OCl- remains as the dissociated salt (Na+ + OCl-), while the rest reacts to form hypochlorous acid (ClOH). The proportions would be the same as above, meaning only 0.017% of the OCl- would remain in this form – the other 99.983% would react with H3O+ to make ClOH and water. So, what you normally would buy to make bleach is actually a base, though once in water, you have both the acid and base present. While it seems strange at first to think of an acid and a base coexisting, this is actually pretty common in nature (in fact, a similar reaction occurring in your blood is critical in keeping your body's internal pH just right!)

Hope this helped!

Kevin Davies

Some helpful references for further reading: http://www.shodor.org/UNChem/basi c/ab/
http://www.chemtutor.com/acid.htm
http://en.wikipedia.org /wiki/Sodium_hypochlorite