Using Caustic Soda to neutralize Cyanuric Acid

SorensenR · Feb 15, 2023

I have a small 3,000 gal pool and use chlorine tablets which of course contain cyanuric acid.

Since I use a 200 g tablet every 5-7 days, my pH constantly drops, but I have worked out that I can maintain a good pH of 7.5 by adding about 30 g of caution soda (NaOH) to neutralize the cyanuric acid.

Is it a good idea to use caustic soda or am I doing it all wrong?

Jimrahbe · Feb 15, 2023

S,

Does not sound right to me, but I am not a chemical guy..

Let's see what Matt has to say... Calling @JoyfulNoise

Thanks,

Jim R.

PoolStored · Feb 15, 2023

SorensenR said:
caution soda (NaCl)

Caustic Soda is (NaOH) = Sodium Hydroxide. It is also highly corrosive. It can and is used in larger commercial pool to raise pH.
It will raise the pH quite a bit with minimal TA rise.

SorensenR said:
to neutralize the cyanuric acid

If you are trying to get rid of the cyanuric acid, the only way to do that is to replace CYA high water with water without CYA.

If you are trying to counter the effects the tablets have (lowering pH), then Sodium Hydroxide can do that, but we would recommend 20 mule team borax. It will raise the pH quite a bit with minimal TA rise.

Dissolving sodium hydroxide is highly exothermic. So, if you pre-mix the solution will get quite hot when the sodium hydroxide is added.

Sodium hydroxide is quite hazardous and should only be handled by someone who knows how to do it safely.

In general, I would advise people to avoid dealing with it in favor of something safer.

One solution, if your pH is going too low, you can raise the TA and the pH will not drop. You can raise TA with simple Baking Soda. This may be a better route than using Caustic Soda. Higher TA causes pH to rise. With a higher TA, it should counterbalance the pucks.

JoyfulNoise · Feb 15, 2023

Caustic soda, or lye, is NaOH (sodium hydroxide). It will dissolve rapidly in water and raise the pH substantially as opposed to the chlorine tablets which dissolve slowly and lower pH slowly. It is a very dangerous chemical to work with as it can quickly cause chemical burns on your skin and the dust is a serious inhalation hazard. It’s not a chemical you should have around a home due to its hazardous nature.

To offset the effects of the chlorine tablets, simply raise your TA to a higher level using baking soda. The added alkalinity will help to moderate the pH drop substantially without resorting to the use of dangerous, caustic chemicals.

mknauss · Feb 15, 2023

@JoyfulNoise
Is there not a correlation that if you add X amount of Trichlor, you should add Y amount of baking soda?

Mdragger88 · Feb 15, 2023

Continuously using trichlor tablets will increase your cya and make it hard to maintain the cya/fc balance that is required to keep the water sanitary
FC/CYA Levels.
Better option is to only use the tablets until you reach around 30/40 ppm cya then switch to liquid chlorine (Unadulterated bleach) which won’t have the acidic effects of trichlor.

Guide for Seasonal/Temporary Pools

Smaller seasonal pools can see just as much enjoyment as their larger counterparts. In this guide we'll show you how to maintain your temporary pool.

www.google.com

Leave the caustic soda for the chemists & get some borax to use when necessary.

Use
PoolMath to calculate additions & effects.

mknauss · Feb 15, 2023

FYI - In Thailand, other members have said that liquid chlorine / bleach is not available. They get substantial rainfall so the use of Trichlor is their only viable alternative to chlorinate their pools.

JoyfulNoise · Feb 15, 2023

mknauss said:
Is there not a correlation that if you add X amount of Trichlor, you should add Y amount of baking soda?

Assuming a starting pH of 7.5 and a TA of 80ppm with 3ppm FC and 30ppm CYA -

In an 11,356 liter (3,000 gallon) pool, 200 grams of trichlor (roughly 0.88 of a standard US 3" tablet) will lower the pH from 7.5 to 7.17 and the TA will decrease by 5.5ppm. That is the equivalent of adding 141mL of muriatic acid. It will take roughly 109 grams of sodium bicarbonate to restore the TA back to 80ppm.

I'm not sure where the OP keeps the TA of this pool but, if trichlor is the only available source of chlorine, then the TA should be maintained with baking soda at a value of no less than 80ppm. The TA range should be held between 80-100ppm to offset the acidity of the trichlor use. Trichlor dissolves slowly so as long as the pH and TA are measured regularly, there really shouldn't be any concern for the pH to crash. Using caustic soda will only cause wild swings in the pH and is mostly unnecessary as long as the pH and TA are properly monitored.

JamesW · Feb 15, 2023

SorensenR said:
I have a small 3,000 gal pool and use chlorine tablets which of course contain cyanuric acid.

What is the current CYA?

As the CYA increases, the percentage of TA that is CYA increases and the carbonate alkalinity goes down.

You need to make sure that the CYA does not exceed 80 ppm.

0.47 grams of NaOH is equivalent to adding 1 gram of baking soda to raise the TA.

If the pH is going down, the TA is too low.

If the TA is correct, the pH will not drop from using tabs.

What is the pool surface?

What is the CSI?

PoolMath

www.troublefreepool.com

PoolMath

Discover the calculator behind the perfect pool, PoolMath. Dealing with pools has allowed TFP to become the original authors of the best calculator on the net.

www.troublefreepool.com

SorensenR · Feb 16, 2023

Thanks all for the quick replies and good suggestions.
I just bought the house and changed all the water 3 months ago.
3,000 gal of fresh water + 1 kg of baking soda was all I added. TA was around 70 after the fill.
I have since used about 20 chlorine tablets x 200 g and since each tablet increase my cyanuric acid by about 8 ppm, I am currently around 160 ppm.
I had to 600 g of caustic soda to get pH back to 7.5.
Bleach is available in Thailand, so is 90% Chlorine in powder form.
I like the idea of stop using the chlorine tablets once cyanuric acid is around 50-80 ppm and switch to bleach, but I had the impression that unstabilized chlorine like bleach burn off in 20 minutes or less.
It is a vacation house, so when emply for a week or 2 the tablets work well, except of course for the cyanuric acid build up.

Comments are welcome, but 2 quick question.
1. Can I stop using tablets and switch to bleach or 90% powder chlorine? If so how much and how often should I add?
2. Was my 3,000 gal + 1 kg baking soda starting point about correct or should I have added more?

I am aware that the only way to reduce cyanuric acid is to change the water, but I am on a remote island 🏝, and it is my plan to change the water ever rainy season, which is November. Until then, pool changing volume of water is simply not available.

Mdragger88 · Feb 16, 2023

If there were no cya in the water then that would be true about using liquid chlorine/bleach but with 30+ppm cya already in the water the fc burning off rapidly isn’t a problem.
If you have bleach available you should switch to that now & chlorinate appropriately to keep the water sanitary
Which means a minimum fc of at least 12ppm lest nasties will grow.
.... "Min FC" is 7.5% of the CYA level
.... "Target FC" is 11.5% of the CYA level
.... "Shock FC" is 40% of the CYA level

Chlorine/CYA Chart

EDIT: You will find a simplified and standardized version of this chart at Pool School, Chlorine / CYA Chart. Chlorine/CYA Chart by Chemgeek FC = Free Chlorine in parts-per-million (ppm) CYA = Cyanuric Acid in parts-per-million (ppm) CYA ........... Min FC3 ..... Target FC ...... Yel/Mstrd...

www.troublefreepool.com

If at all possible you should exchange some water to get the cya down now. November is really far away.
FC levels above 10ppm render the phenol red ph test invalid & can bleach out test strips so you can see how maintaining this level of cya & it’s necessary corresponding fc can be tumultuous.
How are you currently testing the water ?

About the powdered chlorine…
It’s likely either trichlor, dichlor or cal hypo.
Trichlor & dichlor add cya along with the fc,
Cal hypo adds calcium along with the fc,
With these the fc gets consumed daily but the cya or calcium stay in the water & build up until it is replaced.
If you’re away alot, a salt water chlorine generator may be the best solution for the daily chlorination if they are available there.

How to Chlorinate Your Pool

Before chlorinating your pool, you should know these simple tips. Learn the best way to add chlorine to your pool.

www.troublefreepool.com

JamesW · Feb 16, 2023

SorensenR said:
I have since used about 20 chlorine tablets x 200 g and since each tablet increase my cyanuric acid by about 8 ppm, I am currently around 160 ppm.

Do you have a test kit?

Since your CYA is so high, the CYA makes up about 65 ppm of the TA.

So, a TA of 80 is actually about 15 ppm carbonate alkalinity.

What is the pool surface?

What is the CSI?

PoolMath

Discover the calculator behind the perfect pool, PoolMath. Dealing with pools has allowed TFP to become the original authors of the best calculator on the net.

www.troublefreepool.com

Mdragger88 · Feb 16, 2023

Here’s some info on obtaining test kits outside of the USA

Pool Test Kits - Further Reading

www.troublefreepool.com

SorensenR · Feb 17, 2023

You guys are awesome and this forum is great

.
I will love to switch to a salt water chlorinated pool, but my pool is rubber lined and have zero external plumbing, just an in-pool filter.
I am not sure what CSI stands for, so sorry can't answer that James.
We have a small rain season in May, so I will change as much wayer as possible, to get the cyanuric acid level down.
Does the following sound like a good plan forward?
Keep in mind I am using rain water, so zero TA or hardness as a starting point.
1. Fresh fill of 3,000 gal water + 3 table spoons of copper sulfate + 5 lbs of baking soda.
2. Use 6 chlorine tablets over the next 6 weeks. That should bring cyanuric acid to 50 ppm, then switch to bleach.
The big question is then, how much bleach to add and how often?
The local bleach is about 5% sodium hypochlorite.
The pool is in direct sunlight all day, if that is a factor.

SorensenR · Feb 17, 2023

Just want to add, I do have a Taylor K2006 test kit, so I can test pretty much anything.

Newdude · Feb 17, 2023

I just want to add that's bloody STUNNING.

5% bleach is fine, especially with a small pool, 4 ppm is 3.5 cups and you won't even need that much daily most of the season. Download PoolMath and it will tell you what and how much to add for any parameter.

SorensenR · Feb 17, 2023

Thanks for your kind words. I guess what my pool is lacking in size it is trying to compensate with the view

.
240 ml of 5% bleach will add 1 ppm of chlorine.
I could get a $60 dosing pump, like the below, and connect it with a digital timer.
Provided I first get the TA and cyanuric acid in range, does a daily dose of 500 ml bleach sound about right?
If I understand it correct, I would then most likely have to add small doses of HCl acid to keep the pH down, but that would be easy.

PoolStored · Feb 17, 2023

SorensenR said:
I have since used about 20 chlorine tablets x 200 g and since each tablet increase my cyanuric acid by about 8 ppm, I am currently around 160 ppm.

Use your K-2006 to test CYA. Use the dilution method:
Add pool water to bottom Line.
Add tap water to top Line.
Shake.
Pour out half so mixture is to bottom line.
Add reagent to top line.
Shake.
Test outside with back to sun and tube at waist level.
Pour back and forth a few times to see if you get the same result.
Double the result.

If you are in fact 160 CYA, you can't add enough bleach to reach an FC level to sanitize the water. FC/CYA Levels

SorensenR said:
That should bring cyanuric acid to 50 ppm, then switch to bleach.

SorensenR said:
The big question is then, how much bleach to add and how often?

You need to test your FC daily. Then you need to add enough to get your FC back to target range for your new CYA. I would recommend targeting the higher end of the range. For CYA 50, target is 6-8. Until you know your pool, I would target 9-10. Again this read...Link-->FC/CYA Levels

Once you understand, generally how much Bleach you need to add daily to get to your target, then you can use the pump to automate.

Do you have a smart phone? Download PoolMath. It will help you with how much 5% to add to a 3000g pool to get to your target from your FC test result!

PoolStored · Feb 17, 2023

If you don't have a smartphone, you can use this page to calculate how much bleach to add to increase your FC from your test result to your target:

PoolMath

Discover the calculator behind the perfect pool, PoolMath. Dealing with pools has allowed TFP to become the original authors of the best calculator on the net.

www.troublefreepool.com

PoolStored · Feb 17, 2023

PoolStored said:
Once you understand, generally how much Bleach you need to add daily to get to your target, then you can use the pump to automate.

You are lucky (????) that you are close to the equator, as your daily chlorine demand will be fairly stable. In my latitude, it changes drastically, and I have to change chlorine additions every 2-4 weeks.

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Using Caustic Soda to neutralize Cyanuric Acid

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