Chlorine Chemistry as a function of pH

[SunnyOptimism]

This graph clarified a lot of ideas I had about Cl chemistry in pool water -



Basically it shows what the make-up of your free chlorine is with respect to pH. Chlorine in water is a balance between hypochlorous acid (HOCl) and the hypochlorite anion (OCl^-). Or, in HS chemistry speak -

HOCl <--> H⁺ + OCl^-

Since hypochlorous acid is the more potent germicide than hypochlorite (household bleach is sodium hypochlorite), you want a lower pH environment in order to shift the balance of the equation towards the left side. When water becomes too alkaline (pH near 8.0), your free chlorine is dominated by the hypochlorite species and is therefore less effective.

With that said, one cannot just go to extremes in pH as other factors will dominate (low pH damages pool liners and equipment and high pH causes severe Ca scaling).

Just thought I'd share the above for those who get geeked-out about chemistry....

 

[jblizzle]

This is not complete accurate ... this is only if there is ZERO CYA in the water. Once you have CYA in the water, I think the curve shifts to the right, which makes the difference much smaller in the normal pool pH range (7-8)

There are many threads discussing this on the forum right now.
Like this one: http://www.troublefreepool.com/threads/79499-seems-like-a-ph-of-7-2-would-be-ideal

 

[JasonLion]

In nearly all of the cases that come up on the forum, that diagram is wrong. With CYA in the water the change in HOCl concentration with PH (over the range of interest) is close to 10%, small enough to completely ignore.

 

[chem geek]

The curve isn't shifted to the right, but rather is flattened due to Cyanuric Acid (CYA) buffering hypochorous acid (HOCl). This post has the traditional graphs with no CYA compared to the actual graphs with CYA (both in linear and log graphs). Basically, going from a pH of 7.5 to 8.0 has the HOCl drop by around 50% if there is no CYA in the water but with CYA it only drops around 15%.

 

[jblizzle]

Thanks for straightening me out

 

[SunnyOptimism]

Thanks for the replies especially the link to the thread showing the more detailed chemistry with addition of CYA stabilizer (i'll read through it more thoroughly tonight after the kids are asleep). I knew the chemistry was more complicated than the simple chlorine versus pH graph but what I was hoping to post was just the idea that chlorine exists in different forms, some more germicidal than others and the idea that there are specific reasons why you want particular ranges of chemicals in your pool. Not everyone is a chemistry expert so sometimes simpler is better...

But many thanks to you all.

 

[chem geek]

Yes, chlorine is in different forms, some more germicidal than others, and this is the main reason why the FC/CYA ratio is so important because it ensures that there is enough of the germicidal form to prevent algae growth.

The effect of pH, however, is significantly moderated by CYA that attempts to maintain a constant germicidal chlorine level in spite of varying pH. In practice, this works reasonably well in the pH range used in swimming pools (7.0 to 8.0). Since we normally target a pH of 7.5, the germidical chlorine only drops by 14% going to a pH of 8.0 and only rises by 36% going to a pH of 7.0 (only rises by 17% going to a pH of 7.2).

So we generally ignore the pH in our recommendations for the FC/CYA level especially since our recommended pH range (in Recommended Levels) is 7.5 to 7.8. This recommended pH range (along with the recommended TA) lessens the rate of pH rise in pools dosed with hypochlorite sources of chlorine and is also a better pH range for eye comfort.