pH rise and fall with liquid chlorine

Jump to latest Follow Reply
JPMorgan

JPMorgan

Gold Supporter
May 22, 2018
847
Elmhurst, IL
Pool Size
60000
Surface
Plaster
Chlorine
Liquid Chlorine
How long does it take the pH to fall back to "normal" level after adding liquid chlorine? I think it would make sense to turn off our controller for a period of time, so that it does not pick up the pH rise and start pumping acid into the pool when it is not really needed.
 
Once the chlorine is consumed by the UV and or organics, the pH will return to what it was prior to adding the chlorine, excluding any pH rise due to outgassing of CO2.
 
I think what you really want to know is whether the daily addition bumps the ph up enough such that the sensor thinks acid is needed.

When i was using liquid a few years ago, I don’t think my daily additions of 3-4ppm impacted the ph enough to notice it, even on a temporary basis before it reversed itself. Poolmath doesn’t include an impact of liquid chlorine on ph.

Anyone @JamesW care to estimate the potential temporary ph bump?
 
Ideally, the FC should be introduced slowly and continuously such that the FC level never changes.

If you introduce chlorine at the exact same rate as it is lost, there should be near zero effect on the pH.
 
JamesW said:
Ideally, the FC should be introduced slowly and continuously such that the FC level never changes.

If you introduce chlorine at the exact same rate as it is lost, there should be near zero effect on the pH.
Click to expand...
Yes. But people using liquid are generally adding it once daily, not continuously. Theoretically that impacts ph temporarily until the chlorine breaks down and ph returns to the “pre-spike” level. The question is whether the temporary ph impact of the daily chlorine addition might be enough to trigger a sensor to automatically add acid (or whether it is even enough to be measurable).

In my personal experience it was small enough impact to not be measurable, but my observations haven’t exactly been scientifically recorded
 
If the FC goes up, the pH goes up and the feeder will get triggered if the pH exceeds the setpoint.

They are already trying to maintain the pH as high as the regulations allow.

With a feeder system, try to introduce chlorine continuously to avoid pH rise.
 
For example - at a pH of 7.5, CYA 50ppm, TA 80ppm, the addition of one gallon of bleach to 10,000 gallons of pool water will increase pH by +0.13.

If you decrease the TA to 60ppm, the pH change is +0.17 and if you change the TA to 120ppm, the pH increase drops to +0.09.

None of those changes would be particularly visible on a phenol red pH test.

Pool Math use to call out the pH change when one added chlorine but because of programming limitations, it really was not accurate and it would confuse people who were trying to make FC adjustments to their water.

PH - Further Reading

www.troublefreepool.com www.troublefreepool.com
 
2NaOCl + H2O --> OCL- + HOCl + 2Na+ + OH-

For an end pH of 7.5, half of the hypochlorite becomes hypochlorous acid, which creates a hydroxide ion, which raises the pH.

OCL- + HOCl --> O2 + H+ + 2Cl-

On consumption, a Hydrogen ion is created, which reduces the pH.

2NaOCl + H2O --> O2 + 2Cl-+ 2Na+ + H+ + OH-

2NaOCl --> O2 + 2Cl-+ 2Na+
 
Last edited:
Raising the FC by 1 ppm with 12.5% Sodium Hypochlorite should increase the pH by about 0.020.

Raising the FC by 10 ppm with 12.5% Sodium Hypochlorite should increase the pH by about 0.20.

However, this depends on the starting pH, ending pH, TA etc.

So, the effect is not an exact number.

If the pH is at 7.6 and the set point is 7.6, then it might only take 1 ppm of FC increase to trigger the acid feeder if the sensor decides that the pH has been exceeded.

You can also try to suppress the acid feeder by limiting where it turns on and off.
 
Last edited:
Right. So for a typical daily fc addition of 3-4ppm, the temporary ph impact is negligible, and will reverse itself over the course of the fc being consumed.

To the OP, don’t worry about turning your probe off as the daily ph swing isn’t meaningful.
 

Enjoying this content?

Support TFP with a donation.

Give Support
I think that they have a feeder, so it is not like they are dumping in chlorine only once or twice per day.

It should be going in almost continuously rather than being added in large amount infrequently.

It might be on ORP control, but it can be set to run at a specific feed rate.

Maybe go to a SWG rather than adding sodium hypochlorite.
 
Sorry I didn't chime in sooner. Thanks for the input. Yes... we do have a chlorine feeder that adds the liquid chlorine slowly and steadily, but I have noticed that the level of FC in the pool can swing somewhat depending on conditions. I try to maintain around 3 ppm, but even with the automatic feeder FC sometimes drops to around 1 ppm (even though ORP is in range). So, the other day I decided to manually add 2 ppm FC to bring the level up. When I do this manual addition of chlorine on occasion, I was wondering if I should shut off the controller temporarily and for how long in order to prevent acid being fed to the pool unnecessarily (since it will drop back down again eventually).
 
JamesW said:
It might be on ORP control, but it can be set to run at a specific feed rate.
Click to expand...
Are you saying that there is an alternative to relying on ORP control to feed the liquid chlorine.... that there is a way to set a specific feed rate, i.e. 2 gallons per day? The pool company that installed the equipment didn't say anything about this. This might be a better way to maintain FC in the pool at a steady level. We have WiFi-enabled controllers, so I can adjust ORP (and pH) levels from my phone. If we can set a specific feed rate for chlorine, can that also be adjusted remotely? Thanks for any additional information on this.
 
JamesW said:
The problem is that if they are at 7.6 and the pH increases to 7.7, the acid feeder kicks on until the pH goes to 7.5.
Click to expand...
Yes.... this is exactly what I observed when I added 2 ppm of chlorine to the pool manually the other day. So.... would it make sense to turn off the feeder for a period of time when I do this, so that the acid feeder does not kick on? If so, how long would it take for the pH increase to drop back to the 7.6 pre-set level?
 
I would try to have the equipment feed slowly and consistently rather than rely on ORP as long as you can monitor the level so that it does not get out of control.

If you have to manually dose, maybe shut off the acid feeder for an hour or two to let the pH come back down.

The best method is to feed as continuously as possible to keep the fc at the same exact level.
 
JamesW said:
I would try to have the equipment feed slowly and consistently rather than rely on ORP as long as you can monitor the level so that it does not get out of control.
Click to expand...
Yes.... I think this makes sense. We go through about 1.5 to 2 gallons of 12.5% chlorine a day in our 60,000 gallon pool. But.... how do you switch from ORP control to a steady feed? The equipment is new and no one has shown me how to do that.
 
JamesW said:
If you have to manually dose, maybe shut off the acid feeder for an hour or two to let the pH come back down.
Click to expand...
Perfect! Just the information I was looking for. I do like to raise the FC periodically by adding it manually, so I will shut off the acid feeder when I do this. Does it make a difference if I shut off the acid feeder or just turn off the controller for a couple hours?
 
You must log in or register to reply here.
Thread Status
Hello , This thread has been inactive for over 60 days. New postings here are unlikely to be seen or responded to by other members. For better visibility, consider Starting A New Thread.
Top Bottom
Back
Jump to latest Follow Reply