Pool Chemistry explanation? (Warning, nerd sniping)

[NEBackYard]

In August I am having a pool put in, and I'm interested in trying to learn the fundamentals behind pool chemistry. I've been reading online about weak acids, buffers, and ions, and it sounds vaguely familiar from my college chemistry classes 10 years ago, but I'm hoping you smart pool people can correct me/fill in some gaps / explain it a little more for me. I've tried to lay out my understanding of the concepts in terms of "To accomplish A, we do B, but we need to account for C. To account for C we need to do D etc." Hopefully the formatting is somewhat understandable. I will edit this initial forum post as people make clarifications/corrections.

Hopefully someone enjoys responding to this. I've tried reading other articles on this topic, but they seem to be too dumb (CYA protects chlorine like sunblock) or too smart (academic papers I can't quite digest)

1. To swim, add water to a pool
   (If we stopped at 1, algae and bacteria from the environment would grow in the pool)
   
   
2. To kill the algae and bacteria, introduce an excess of Hypochlorous acid (HOCl) and ClO-
   (If we stopped at 2, any remaining HOCl) would be reacted away with water and sun's UV (H2O+HOCl+Sunlight = O2 +HCl)
   1. Add Salt (NaCl) to water, which breaks down into Na+ and CL- ions
   2. Run salt water through SWCG which generates Hydrogen (H2), Sodium ions (Na+), and hydroxide ions (OH-), and Chlorine (Cl2)
      1. H2 bubbles out into atmosphere as a gas
      2. Na+ remains in the water
      3. OH- remain in the water
      4. Cl2 combines with water to form a balance of Cl2 and HOCl
      5. HOCl combines with water to form a balance of HOCl and ClO- and H+
         
3. To protect the remaining HOCl acid from the sun's UV rays, convert the HOCl into sodium dichloro-s-triazinetrione, which does not break down from UV
   (If we stopped at 4, eventually all of the HOCl and ClO- would be reacted away)
   1. Add Cyauranic acid (C3H3N3O3), forming a balance of HOCl, C3H3N3O3 and sodium dichloro-s-triazinetrione.
   2. As the sun converts HOCl to O2 and HCl, or HOCl gets used to destroy algae/bacteria, the sodium dichloro-s-triazinetrione reverts back to HOCl to maintain its desired balance
4. To keep an excess of HOCl and ClO- present, continually run the SWCG

Questions
Q1: Where does alkalinity come into play? I think I read that you add Sodium Bicarbonate to form another buffer? I think this would come in after 4? If we stopped at 4, the pH would shift in such a way that the balance of Cl2 and HOCl, or dichloro-s-triazinetrione and HOCl would be off, and the HOCl would be "trapped" in the non-disinfecting compounds and not useful?

 

[Texas Splash]

You've posted a mouthful, and honestly it's a bit more complicated than it needs to be. So let's break it down even simpler:
1 - Balance the FC to the "current" CYA to prevent algae and keep the water sanitary. We balance according to the FC/CYA Levels.
2 - With a vinyl pool, CH is typically "N/A" unless you have something like a heater with a minimum requirement.
3 - PH and TA help to keep the water from being too aggressive which can lead to corrosion and irritating the the swimmer. Too high and there's a propensity for scale. The TA serves as a basic "buffer" to the pH, helping to keep it somewhat stable as in most cases, the pH tends to rise - hence the muriatic acid to lower it. The TA has a very broad acceptable range, anywhere from as low as 50 to much higher, but the higher the TA (i.e. over 80) the more likely pH will try to rise quicker.

 

[Texas Splash]

Pools needs cyanuric acid (CYA) to give the FC a place to adhere to and protect from the sun's UV. The amount of CYA, anywhere from 30 to 50, or in some cases higher, is a direct result of the pool's location in the country (heat and intensity of the sun) and if it has an SWG. CYA is presented in various forms. Sometimes as a byproduct to chlorine tabs or powders (trichlor/dichlor), but also in its purest form of granular stabilizer or liquid conditioner. A higher CYA level requires the FC to be balanced higher according to the FC/CYA Levels.

 

[NEBackYard]

You've posted a mouthful, and honestly it's a bit more complicated than it needs to be. So let's break it down even simpler:
1 - Balance the FC to the "current" CYA to prevent algae and keep the water sanitary. We balance according to the FC/CYA Levels.
2 - With a vinyl pool, CH is typically "N/A" unless you have something like a heater with a minimum requirement.
3 - PH and TA help to keep the water from being too aggressive which can lead to corrosion and irritating the the swimmer. Too high and there's a propensity for scale. The TA serves as a basic "buffer" to the pH, helping to keep it somewhat stable as in most cases, the pH tends to rise - hence the muriatic acid to lower it. The TA has a very broad acceptable range, anywhere from as low as 50 to much higher, but the higher the TA (i.e. over 80) the more likely pH will try to rise quicker.

@Texas Splash , I agree the steps you laid out are the "how" of pool chemistry, I am trying to understand the "why"

 

[Newdude]

Check the TFP Wiki in the left sidebar and also towards the bottom of any forum page. It's all nicely alphabetical and you can go through more advanced discussions about each parameter and how they correlate if so.

 

[ajw22]

To raise the pH of the pool to make it comfortable for swimming, add muiratic / hydrochloric acid (HCl)

1. HCl combines with OH- to form water and H+

Muriatic acid lowers pH, not raises it.

pH really does not effect the comfort of the swimming. You can swim in a wide range of pH and never feel the difference.

 

[NEBackYard]

@ajw22 thanks, I corrected the initial post

 

[JoyfulNoise]

1. Read this --> Pool Water Chemistry
2. Rethink your chemistry because some of it is wrong
3. Build your pool
4. Buy a proper test kit --> Test Kits Compared
5. Add water and balance according to TFP recommended levels
6. Swim and enjoy your pool ... it's not a chemistry set but you're welcome to treat it like one if that brings you joy ..

 

[reggiehammond]

@Texas Splash , I agree the steps you laid out are the "how" of pool chemistry, I am trying to understand the "why"

@chemgeek posted a lot of threads over the years re: the how and why of pool chemistry. Go find those threads and you'll have everything you need.

 

[NEBackYard]

@JoyfulNoise can you ID the parts that are incorrect? I'm happy to google and search on the forum more, but I need to know where to start.

@reggiehammond thanks! I got a little scared by the first post in chemgeek's stickied thread, but the later posts are a little more digestable at my level. Thanks!

 

[JamesW]

Pool Care Basics

Overnight Chlorine Loss Test

SLAM Process

PoolMath

FC/CYA Levels

Test Kits Compared

Become a TFP Supporter

 

[JoyfulNoise]

Mostly your assumptions that chlorine will significantly drive the pH one way or another. It doesn't. Liquid chlorine is only slightly alkaline when added to the pool because of the excess lye used in the manufacturing process. SWG's are pH neutral. What you have to understand about those chlorination processes is that when chlorine is added to water, it might generate a hydroxide anion or give off a hydrogen ion when it hydrolyzes, but the exact opposite reaction happens when the active chlorine compound reacts with something, either disinfection or oxidation. So, in total, chlorine additions don't drive significant long term pH changes. When you read through the advanced chemistry that Chem Geek posted, you'll get a sense of that.

Be very careful about what you read on the internet regarding pool chemistry. Most of it is wildly wrong and posted by people that have little or no scientific or engineering training. They grab onto chemical reactions that they read about on Wikipedia or ChemLibre and just assume that's what it is. It's a wee bit more complicated than that

 

[NEBackYard]

I removed the references to the pH getting moved by addition of Cl2, and and also removed adding muiratic/hydrochloric acid

 

[Newdude]

I removed the references to the pH getting moved by addition of Cl2, and and also removed adding muiratic/hydrochloric acid

Please don't. Incorrect info which gets corrected is how everybody learns down the road. Threads get read an untold # of times after the fact by folks with your same questions finding it in a search.