Chlorine, pH and T/A

[PoolGate]

Quick question, does/can an imbalance in pH and total alkalinity (T/A) make any difference in the effectiveness of chlorine?

 

[mgtfp]

In a pool without CYA the concentration of the "active" form of chlorine (HOCl) is greatly dependent on pH. Below pH 7 most of the free chlorine is HOCl, above pH 8 it's mostly OCl^-, which is not as good a sanitizer/oxidizer. At pH 7.5 it's about 50/50.

Once you add CYA, it's completely different. Most of what your FAS-DPD test shows as FC is now chlorine bound to CYA, which doesn't have any sanitation power at all, but FC is kept in reserve there. That's why FC needs to be increased with CYA to have enough HOCl available. But the great advantage (apart from having FC protected from UV) is that the dependency of the HOCl concentration from pH is now much, much flatter.

With CYA in the water, you don't have to worry about FC getting less efficient at higher pH, as long as you don't go way to high. With pH in the 7s, you can ignore the influence of pH on the FC-potency.

There is no influence by TA on FC-potency, apart from TA driving pH up which would only be a problem in a non-CYA pool (in regards to FC, there are of course other issues with high pH that are still relevant in CYA-pools).

 

[PoolGate]

Thanks! To clarify, a low pH will not effect chlorine but a high pH can?

 

[mgtfp]

Yes. The lower the pH, the more effective the chlorine. But within the TFP recommended pH range, the effect is only in pools without CYA really significant.

There are graphs in this post:

Post in thread 'Pool Water Chemistry'

With CYA you need to look at the last plot on alogarithmic scale.