CYA - FC - pH Interaction Observations/Question

[rgf4095]

Preface: I understand that UV light lowers the ORP of FC due to the interaction of CYA and Chlorine. I've read most of chem geek's threads on this topic, and I have observed this behavior directly.
Question: How does this UV-CYA-FC interaction affect pH? What I have observed is that, during the course of the day, pH will drop following the same curve as the ORP during the day and rise again during the night. Hopefully someone here can shed some light on this.
Background: I have a functioning, relatively stable ORP/pH control system implemented on my pool. To achieve this has taken the better part of 5 years, but I have managed to work out 99% of the challenges introduced by having an outdoor pool in the extreme environmental conditions of the Arizona desert. (I'd be happy to share this story, but perhaps on another thread). I first noted the pH changes when I was running the control system 24hrs/day. I noticed that the acid usage always increased during night hours and my TA usage was disproportionately high. I assumed (wrongfully) that it was simply a reaction to the introduction of a strong base (Sodium Hypochlorite) during the daytime. Later, when I realized that running the control system 100% of the time was not needed, I also noted that my acid and TA usage went way down with no change in the pH stability. I started to look carefully into this by tracking the diurnal curves of both ORP and pH (with no Chlorine or Acid additions) and was able to confirm this behavior.
Thank you in advance for any help/insight you may have!
Robert

 

[JoyfulNoise]

Is your chlorine being generated by an SWG or injected from a liquid storage tank?

Can you post some screenshots of your diurnal curves for pH/ORP?

 

[rgf4095]

Hi Matt - thanks for you reply. I use liquid chlorine. Historical note: The system was a SWG type many years ago.
I'm going to need a day or two to get a clean set of curves. I will post it then.

 

[rgf4095]

Ok - finally got the requested curves. Had to wait for the right combinations of weather conditions. These curves were generated under the following conditions:
1. Autofill disabled. (Note: Incoming city (Pheonix) water has a pH of 7.2-7.4 and a TA of 90 - 100 ppm.)
2. Chlorine injection disabled.
3. Daylight conditions - no clouds, UV index of 3 @ ~3pm
4. pH setting at 7.6
5. no wind during this period.
6. air temperature variation: low 50F, high 70F.
7. pool temp about 60F.
8. TA =60, CYA =30, FC =3, TDS = ~1500.


The screenlogic graphs are very primitive. The ORP resolution is about 5mV, and the pH resolution is 0.1.
The pH curve looks like a sawtooth because the resolution is so large. For example, if the pH reading is between 7.50 and 7.59, the graph is recorded as 7.5. Same for 7.4 and 7.6.
The significance of the pH curve is that it shows during no sunlight conditions, the pH rises (hence the need to feed acid, as shown during the hours of between 4 and 7am (Dec12) and midnight and 7am (Dec 12-13). No acid feed is needed during sunrise to sunset as the pH falls below the pH set limit of 7.6. This was during a daytime UV index of 3. The lowest pH reading was between 2-3 pm, corresponding roughly with the peak UV index for the day. During the summer months where the UV index is 11+, this effect is far more pronounced.
Please note that the ORP also follows the day/night cycle similar to the pH. This ORP behavior is well know to the TFP community.
Also please note that I have tried to eliminate as many variables affecting pH as possible.
It is my speculation that the interaction of sanitizer with the stabilizer and UV is also affecting the pH in a manner similar to ORP. I just don't have the knowledge/experience to definitively explain this behavior.
Thanks!
Robert

 

[JoyfulNoise]

Robert,

Thanks for the graphs. That helps me understand the extent of what you're seeing. Just let me say upfront that those pH swings are fairly minor and your pool water is well within any recommended range.

As for the deeper question of UV light and chlorine, the answer is - it's very complicated! But there are some general trends that can be explained. At first blush, UV light is nothing more than another "oxidizer" that is capable of affecting the chemical bonds between atoms. In a very simplistic sense, the following reactions would occur -

2 OCl^- + UV_A/B ---> 2 Cl^- + O₂

and

2 HOCl + UV_C ---> 2 H⁺ + 2 Cl^- + O₂

Pool water generally has a mix of hypochlorous acid (HOCl) and hypochlorite anion (OCl^-) with a 50/50 split at a pH of roughly 7.5. The hypochlorite anion is much more susceptible to UV photolysis than the hypochlorous acid because there is far more UVA and UVB light striking the pool water than UVC. Hypochlorous acid is degraded by UVC light. As you can see, the photolysis of hypochlorous acid will create an acidic proton (H⁺) that will lower pH.

However, that is not the whole story. UV light tends to generate a lot of free radicals in water. Free radicals are atoms and molecules that don't have a higher oxidation state but are more energetic than their normal state because the outermost electron has absorbed more energy and is now in a higher energy band around the nucleus. Radicals are much more potent oxidizers than their normal counterparts but are extremely short lived and undergo both a continuous chain reaction that generates them as well as extinction reactions that convert them back to a lower oxidation state.

For example, when a sanitizing chlorine ion (Cl⁺) from the active chlorine compounds in water absorbs a UV photon, it can become a chlorine radical known as a reactive chlorine species, RCS. These RCS's can then initiate all kinds of hydrogen bond transfers in various organic compounds that can create a self-sustaining chain reaction. This might look like the following set of reactions -

•Cl + R-H ----> •R + H⁺ + Cl^-
•R + HOCl ----> R-OH + •Cl

In other words, the chlorine radical (•Cl) reacts with an organic compound (R-H) that has a hydrogen bond. The chlorine radical breaks the hydrogen-carbon bond, and kicks the hydrogen out as an acidic proton. The organic molecule itself becomes a radical and then reacts with hypochlorous acid. The radical organic compound then becomes hydroxylated (R-OH) and the chlorine absorbs the energy and becomes a radical again. Obviously this self-sustaining reaction can't last forever and it is usually extinguished when the chlorine radical reacts with something that cannot itself become a radical. But the reaction illustrates a very important point -

Most UV/Chlorine reactions are acidic in nature and most chlorine oxidation reactions will produce an acidic proton. All of these reactions are pH dependent as well as concentration and UV wavelength dependent. So there is no one reaction to point to as dominant. This is also why adding liquid chlorine or generating chlorine with an SWG tends to be pH neutral over time - the rise in pH caused by adding chlorine in those forms is later offset by the many acidic reactions that occur.

As for your specific situation, you have seen that dosing with liquid chlorine does raise pH from both the chlorine hydrolysis as well as the additional small amount of lye contained in the LC source. But, when UV light is present and the chlorine is quite reactive, you are seeing some of those acidic reactions in the pH drop.

Sorry that there is no simple answer here but UV interactions can be quite complex in pool water. Depending on where you maintain your TA, you might be able to smooth out the variations more but, in reality, your pH swings are insignificant in terms of bather comfort. Acid demand is always going to be a moving target. As well, I assume you are not using much, if any, cyanuric acid stabilizer. CYA can have a profound effect on the reaction kinetics of chlorine but, as you know, even small amounts of CYA can cause havoc with an ORP control system. I think your system is operating as best as can be expected from a residential pool.

 

[rgf4095]

Thank you Matt!
I understand the pH swing illustrated in the graph is very minor. However it is very pronounced during intense UV days as I have observed as much as a 0.15 pH unit swing over 24 hrs. But as you say, it is well within normal limits.
I think the most important lesson(s) for me was that:
1. running the control system 24hrs a day was unnecessary (duh)
2. when checking pH and adjusting pH, do it at the same time of day.
This whole exercise was initiated because of high acid usage and TA consumption. Once I stopped trying to control the pH at night, both TA consumption and acid usage dropped dramatically.
This led to some interesting problems with the intellichem system. I do run the pool filter pump at night in combination with my solar panels to help cool the pool in summer months. But clearly I don't want to control the pH during this time. So I had to add a solenoid valve control programmed by the Easytouch system to cutoff flow to the intellichem probe cell when I need to cool the pool at night. The intellichem still complains that there is "no Flow" when the filter pump is operating, but this is better than turning off the intellichem and then having to wait several hours for the ORP reading to recover.

 

[JoyfulNoise]

Thank you Matt!
I understand the pH swing illustrated in the graph is very minor. However it is very pronounced during intense UV days as I have observed as much as a 0.15 pH unit swing over 24 hrs. But as you say, it is well within normal limits.
I think the most important lesson(s) for me was that:
1. running the control system 24hrs a day was unnecessary (duh)
2. when checking pH and adjusting pH, do it at the same time of day.
This whole exercise was initiated because of high acid usage and TA consumption. Once I stopped trying to control the pH at night, both TA consumption and acid usage dropped dramatically.
This led to some interesting problems with the intellichem system. I do run the pool filter pump at night in combination with my solar panels to help cool the pool in summer months. But clearly I don't want to control the pH during this time. So I had to add a solenoid valve control programmed by the Easytouch system to cutoff flow to the intellichem probe cell when I need to cool the pool at night. The intellichem still complains that there is "no Flow" when the filter pump is operating, but this is better than turning off the intellichem and then having to wait several hours for the ORP reading to recover.

Yeah, the "canned" systems from both Hayward and Pentair have lots of limitations and hardware-limits built into them to ensure that the average customer (aka, no one here on TFP) doesn't nuke their pool with too much acid or chlorine. For the advanced user, those systems can be really frustrating.

@MyAZPool and a few others on the forum have endeavored to create their own automation systems with chemical controls. If you haven't seen it already, this post has some very nice details -

Alternative Pool Automation and Sensor/Chemical Control and Integration

Industrial Automation 8-Layer Stackable Card for Raspberry Pi Pool Automation and Sensor/Chemical Control and Integration (nodejs-poolController / nodejs-poolController-dashPanel / relayEquipmentManager) - Table of Contents - - Preface 1. Background, Purpose and Scope 2. Credits 3...

www.troublefreepool.com

ORP can be very tricky to get right and a lot of blame for it's failure is on the industry for touting it as a turn-key solution which it most certainly is not. People don't understand the fundamentals of it nor how to properly implement it and then they get frustrated by it. Pool service technicians know next-to-nothing about them and so they are of no help whatsoever. As long as a pool owner understands how to use ORP control and all of the caveats that go with it, then it can be successful. TFP doesn't really promote it much because, for the average pool owner, it's way beyond anything they truly need to operate a pool effectively. For the pool "super user" and all the automation-nerds out there, it can be a fun project as long as you understand the risks involved.

Looks like you've got a good handle on your system so I'm sure it will all work out fine for you.