Phosphate problem?

[chem geek]

I have graphs here in The China Shop. However, the fact that the active chlorine concentration without CYA drops so dramatically at higher pH is a bit of a moot point since its absolute concentration is so much higher to begin with because there is no CYA to moderate it.

As Evan wrote, the effect of pH on active chlorine is significantly reduced when CYA is present. With an FC of 3 ppm and no CYA, going from a pH of 7.5 to 8.0 goes from an active chlorine (hypochlorous acid) concentration of around 1.5 to 0.7 or roughly a 50% drop, but with 30 ppm CYA in the water it goes from 0.042 to 0.036 or a drop of around 14%. CYA is like an active chlorine buffer just like the carbonates (and borates) are a pH buffer.

As for a lower TA having the pH be more stable when using hypochlorite sources of chlorine, the simplest way to understand this is that the carbonates portion of Total Alkalinity (TA), which is mostly a measure of bicarbonate, has TWO effects: 1) it is a pH buffer so higher TA is more resistant to changing pH from outside influences and 2) it is a SOURCE of rising pH itself where higher TA results in faster carbon dioxide outgassing that causes the pH to rise. This latter effect outweighs the former so as ironic as it may seem lowering the TA leads to greater pH stability when the reason for that rising pH is the TA in the first place (if the reason is primarily from something else, such as plaster curing, then a higher TA might be helpful up to a point).

Richard