PH and FC impact ?

[Riddler0520]

In addition to managing PH levels to control calcium scaling and eyes stinging/equipment damage is there a relationship to available Chlorine to fight bacteria ?

I came across this chart - fact or myth ???

At the target PH of say 7.5, this says you have 50% of available chlorine to fight bacteria ? And if you drift up to 8.0 or higher you lose a lot of available chlorine

any truth ?

 

[zea3]

Chlorine will be less effective at high pH, however you do not want to let pH drop below 7.0 I don't know if the numbers they give are an accurate representation of how much pH level effects chlorine efficacy. Where was the chart published?
@JoyfulNoise @JamesW

 

[Donldson]

I cannot speak for the specific numbers of the chart, but it is a real thing in the absence of CYA. When CYA is added to the water it hugely negates this effect and in the appropriate pH range of a swimming pool the chlorine will have roughly the same effectiveness. One of those "myths" which aren't really false but not entirely truthful when it comes to managing most pools.

 

[JamesW]

With no cyanuric acid in the water, the pH does determine the percentage of hypochlorite vs. hypochlorous acid.

The pKa of hypochlorous acid is about 7.53.

1÷(1+10^(7.53 – pH)) = hypochlorite percentage.

For example:

pH...............HOCl...........OCl-
7.0................77%.............23%
7.2................68%.............32%
7.4................57%.............43%
7.53.............50%.............50%
7.6................46%.............54%
7.8................35%.............65%

Hypochlorous acid and hypochlorite both oxidize and sanitize, but hypochlorous acid is better at both.

With cyanuric acid in the water, the equation changes because most of the chlorine is bound to cyanuric acid, but the pH does still effect the ratio. The effect is not as great, but it still matters somewhat.

The FC/CYA ratio is sufficient to keep the water in good condition up to a pH of about 8.0.

Even if the chlorine is OCl-, it still works and it continuously changes from HOCl to OCl-.

I would not worry about the percentage as long as the pH is in the recommended range of 7.2 to 7.8 and the FC/CYA is also in the recommended range.

In most cases, for most people, keeping the pH in the 7.5 to 7.8 range is the best overall choice.

In any case, I would not try to keep the pH below 7.4.

 

[mgtfp]

any truth ?

 

[Riddler0520]

With no cyanuric acid in the water, the pH does determine the percentage of hypochlorite vs. hypochlorous acid.

The pKa of hypochlorous acid is about 7.53.

1÷(1+10^(7.53 – pH)) = hypochlorite percentage.

For example:

pH...............HOCl...........OCl-
7.0................77%.............23%
7.2................68%.............32%
7.4................57%.............43%
7.53.............50%.............50%
7.6................46%.............54%
7.8................35%.............65%

Hypochlorous acid and hypochlorite both oxidize and sanitize, but hypochlorous acid is better at both.

With cyanuric acid in the water, the equation changes because most of the chlorine is bound to cyanuric acid, but the pH does still effect the ratio. The effect is not as great, but it still matters somewhat.

The FC/CYA ratio is sufficient to keep the water in good condition up to a pH of about 8.0.

Even if the chlorine is OCl-, it still works and it continuously changes from HOCl to OCl-.

I would not worry about the percentage as long as the pH is in the recommended range of 7.2 to 7.8 and the FC/CYA is also in the recommended range.

In most cases, for most people, keeping the pH in the 7.5 to 7.8 range is the best overall choice.

In any case, I would not try to keep the pH below 7.4.

Helpful and thank you

I have a new pool with lots of aeration which was good when lowering my TA which I’ve done from 120 now down to 60-70 (prob a bit to low)

my PH inches up to 8-8.2 and I drop a gallon of acid in and it drops PH to 7.2 but then pulls down TA. Think I’ll start doing only 1/4 a gallon and retest next day to see if I can drop down to either the 7.5 or 7.8 range which I think will stabilize TA more and not pull that down to far ?

is that the right approach ?