OK, I'm no chemist, I admit it.

[chem geek]

In fact, soda ash does neutralize some of the Cyanuric Acid. The Cyanuric Acid is present in the water in several different forms, but the most common are as Cyanuric Acid (CYA) and as Cyanurate Ion (CYA-). When you add soda ash, some of the Cyanuric Acid gets converted to Cyanurate Ion [EDIT] I had that reversed when I first wrote it -- sorry about that [END-EDIT] and this is why CYA contributes to total alkalinity -- it is a pH buffer just like the carbonates (baking soda or bicarbonate) buffer system, but not as powerful since it's lower in quantity.

Remember that neutralizing an acid does not mean destroying it. CYA (and the carbonates such as carbonic acid) is a "weak" acid in that it does not completely dissociate and give up all of its hydrogen ion that would lower the pH. This is in contrast to "strong" acids such as Muriatic Acid, that pretty much completely dissociates into hydrogen ion and chloride ion. Nevertheless, when you neutralize the Muriatic Acid, you are still left with chloride ion so in a similar sense you don't "destroy" the acid when neutralizing it. Only the hydrogen ion portion of the acid gets neutralized -- the rest of the acid remains intact as an ion (a charged chemical species in water).

Richard