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Can’T I Just Lower My Ph By Adding Some Muriatic Acid?


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I am fighting an algae problem. I am keeping my water chemistry in check. My local pool store is telling me that a lower PH will allow the chlorine to be at a higher level, is that true? Here is what they are telling me.

Active Chlorine pH

97% 6.0

91% 6.5

76% 7.0

66% 7.2

50% 7.5

I do not plan on swimming any time soon and it is going to rain all weekend. Can’t I just lower my PH by adding some muriatic acid?

Won't this help me fight the algae better?

Thanks in advance.

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First of all, that table does not apply when there is Cyanuric Acid (CYA) in the water since CYA acts as an active chlorine (hypochlorous acid) buffer (see this post for more details). Second, the active chlorine level is MUCH more affected by the CYA level since it is roughly proportional to the FC/CYA ratio.

Now, that said, adding a lot of a hypochlorite source of chlorine (e.g. chlorinating liquid, bleach, Cal-Hypo, lithium hypochlorite) does raise the pH initially so lowering the pH somewhat first is OK, but not as low as you indicated. If you want to add Muriatic Acid to lower your pH to, say, 7.2 because you will be adding a lot of chlorine, then that's perfectly fine. However, it's more important that you add enough FC for your CYA level to be able to kill algae faster than it can grow. If you don't maintain the FC to be at least 7.5% of the CYA level in a manually dosed pool, then algae can grow. As for shocking to get rid of existing algae, raise your FC level to around 40% of the CYA level. If your CYA level is very high (say, 80+ ppm), then you should probably do a partial drain/refill to lower it since that will need to be done eventually anyway and you'll need less chlorine to kill the algae at a lower CYA level.

I noticed you also posted here and got similar answers.

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  • 3 weeks later...

However, it's more important that you add enough FC for your CYA level to be able to kill algae faster than it can grow.

wait a minute, unless i'm completely misreading this, it sounds like you are saying that your CYA level is doing the killing. cya is not a sanitizer. it's a uv shield. that's it. the FC is doing the killing, the cya is allowing the fc to be able to do it...

did i misread?

and no to the original poster, a lower ph has no correlation with the 'chlorine being able to be at a higher level.' the ph has to do with the balance of the water, making it fit for humans to be able to be in it. the chlorine is just there to kill kill kill, keeping the water free of hazardous bacteria. you might want to have your local shop clarify, because the only thing i can think of that they might be saying is, like chem geek said, taking a proactive step by first lowering the ph, then adding (liquid) chlorine to the water will get you back to the ph you want, instead of adding the liquid first, then adding acid to get it back down to where you want.

but that's not really what you need anyway. you just need a slightly higher level of sanitizer (chlorine of some sort) then you need some vigorous brushing to get all the levels of algae, then you need good filtration to filter out the dead algae that you just killed and brushed. none of that involves acid... since you don't plan on swimming any time soon, just get your chlorine level up to a 5 or so and keep it up while you brush daily. some algaecide will help too, but if you don't have it, don't go buy it.

i'm not a chemistry pro, but i take care of a ton of pools.

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wait a minute, unless i'm completely misreading this, it sounds like you are saying that your CYA level is doing the killing. cya is not a sanitizer. it's a uv shield. that's it. the FC is doing the killing, the cya is allowing the fc to be able to do it...

did i misread?

I believe you did, What chem geek is referring to is is the relationship between CYA and the activity of the chlorine in the pool. The higher the CYA level the higher you need to keep the FC to maintain the same ACTIVE amount of chlorine in the pool since the higher the CYA level the more chlorine bound up as chlorinated isocyanurates (which test as FC but do not have the sanitizing or algae killing ability). This effect is what is seen when we have an "ovestabilized pool", sometimes wrongly called "chlorine lock". The simple cure is to run the FC higher to compensate but in the long run CYA MUST be brought down to a workable range, This chart by Ben Powell of Pool Solutions might explain it better. He was the first one to publicize this effect that was first discussed in the 70's by John A. Wojtowicz when he was with Olin (who was one of the original manufacturers of trichlor). However, this info was potentially damaging to sales of this "new'" pool sanitizer at the time so it was not really publicized.

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For more technical details and references to numerous peer-reviewed scientific papers in respected journals, see the "Chlorine/CYA Relationship" section in the Certified Pool Operator (CPO) training -- What is not taught post. The relationship between chlorine and CYA has been known definitively since at least 1974.

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