Muriatic Acid versus granular acid

Problems relating to pH and total alkalinity.
Increase ph, increase TA. Reduce pH, reduce TA.
pH chemistry advice and techniques for the pool.
DARTH DOUGY
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Muriatic Acid versus granular acid

Postby DARTH DOUGY » Thu 12 Feb, 2009 15:31

My question is in two parts:

(1) Is Muriatic acid in liquid form better to lower TA than granular products? I read an article that states that the Muriatic acid liquid is what is supposed to be used to lower TA more efficiently. I have been using a granaular dry acid product from Leslie's, and I never get anywhere with it -- my TA is staying at 150.

(2) Does anyone know a good supplier to order Muriatic acid?

Article below:

Why you should not use muriatic acid to lower the pH of your pool.

Many customers come to us with the problem of their pool water pH constantly rising on them. While there are many reasons for the pH to continuously rise, one of the most common can be due to the use of muriatic acid to lower the pH. It is important that you understand what muriatic acid does when added to your pool water and why this can result in the pH constantly rising on you.

First, you need to understand what Total Alkalinity is and how it affects the pH of your pool water. By definition Total Alkalinity is a measurement of water's ability for the pH to change. In other words, the higher the Total Alkalinity, the harder it is for the pH to change. It is important that you maintain the Total Alkalinity between 80-120 PPM. (This can vary somewhat depending on local and source water.)


Muriatic acid is a dilute form of hydrochloric acid which is a very strong acid. Even diluted, muriatic acid is still a very strong acid. When you add muriatic acid to your pool (even if you have diluted it more in a bucket of water), not only do you lower the pH of your water but you also lower the Total Alkalinity of the water. With the Total Alkalinity lowered, it is now easier for the pH to rise again (this can be due to many reasons), so if you add more muriatic acid to lower the pH, you also lower the Total Alkalinity even more, thereby making it even easier for the pH to go up again. As you can see, it is a vicious circle you are in.


How do you lower the pH without lowering the Total Alkalinity? You should use a dry acid (pH lower, pH minus, etc.) mixed into a bucket of water first, then poured around the pool. Dry acid is a much milder acid than muriatic acid and when diluted in water will have almost no effect on the Total Alkalinity of the water while lowering the pH. This will allow you to maintain the Total Alkalinity level, which will make it more difficult for the pH to change, thereby reducing your pH fluctuation.


Why does the dry acid not reduce the Total Alkalinity like muriatic acid does? Because for the Total Alkalinity to go down, you must get the pH below 5.5 in the water. Adding muriatic acid (because it is so strong) lowers the pH of the water. When you pour it to below 5.5, the chemical reaction that lowers Total Alkalinity now takes place. Adding a dry acid (a much milder acid) especially when diluted in a bucket of water does not lower the pH in that area to below 5.5. Therefore the chemical reaction that lowers Total Alkalinity does not take place.


Now, when your pool professional tells you that it is cheaper in most cases to use a dry acid than to use muriatic acid, you will understand.


Note:If your source water (water you add to pool) has a very high Total Alkalinity, you may want to use muriatic acid to lower both the pH and the Total Alkalinity at the same time.


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Re: Muriatic Acid versus granular acid

Postby Guest » Thu 12 Feb, 2009 16:15

DARTH DOUGY wrote:Because for the Total Alkalinity to go down, you must get the pH below 5.5 in the water.


I don't think that's accurate - in a nutshell any (strong) acid added to water will lower both pH and alkalinity. If the starting alkalinity is high then it will take more acid to drop the pH. But no matter what you'll still be consuming alkalinity in the process.

You can purchase muriatic acid at the hardware or pool store.
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Muriatic Acid versus granular acid

Postby chem geek » Thu 12 Feb, 2009 20:11

Yes, this is total B.S. Muriatic Acid (31.45% Hydrochloric Acid) and dry acid (93.2% Sodium Bisulfate) are both strong acids and have IDENTICAL effects on pool water when using comparable amounts. 1 cup of Muriatic Acid is the same as 10.8 ounces weight or about 7.2 ounces volume of dry acid.

The reason for rising pH has to do with the Total Alkalinity (TA) being too high since a higher TA means the pool is more over-carbonated. The outgassing of carbon dioxide makes the pH rise. Having more aeration or a lower pH has this happen faster.

You can learn much more about managing a pool properly at the Pool School.

The article refers to what is often called the "slug" or "acid column" method that supposes that a small area at low pH will lower the TA. This is fully debunked here . To lower the TA, you simply lower the pH using acid, then aerate the water and when the pH rises you add more acid to lower it. The amount of acid it takes to lower the TA is completely independent on how you do it. 1 cup of Muriatic Acid in 10,000 gallons will lower the TA by 3.1 ppm. To lower the TA by 10 ppm it takes 3.2 cups. The procedure for lowering TA is also described in this post.

As noted in the post above, you can find Muriatic Acid in hardware stores and most big box stores. You sometimes find it in half-strength (15%) where you need to use about twice as much for the same effect.

Where did you find this "article"? [EDIT] Never mind -- I found the source here . [END-EDIT]

[EDIT] I E-mailed the company with the aforementioned webpage and they have now removed it so this incorrect information will no longer be spread. [END-EDIT]

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Muriatic Acid versus granular acid

Postby Larry » Mon 09 Mar, 2009 13:13

I much prefer dry acid because it is far easier to handle and doesn't pose the same hazards as the liquid.

Liquid muriatic acid:
  • is corrosive when spilt
  • releases fumes that can choke, and destroys lung tissue (among other things)
  • is easy to splash causing damage to people or poolsides.
Dry acid (sodium bisulfate):
  • is easy to handle
  • won't even burn dry hands
  • doesn't smell, run or cause discomfort
  • is easy to apply
  • is easy to sweep up if spilled

Just my $0.02

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Muriatic Acid versus granular acid

Postby chem geek » Mon 09 Mar, 2009 14:51

Larry,

You are right about the nastiness of Muriatic Acid, but you can get half-strength versions that don't fume nearly as much. It's still nasty...just not as much. :shock:

Dry acid does increase the level of sulfates in the water (dry acid is sodium bisulfate). I don't know at what level, if any, that becomes a problem. I have read that magnesium sulfate has a particularly high salt recrystallization pressure so this might be a concern for splash-out on some kinds of stone surfaces, but I don't know for sure.

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Muriatic Acid versus granular acid

Postby Larry » Mon 09 Mar, 2009 16:08

Hi Richard

The area we are active in has awful groundwater, with average values being TA > 300ppm and pH > 8.5

A typical home pool in our care has 30k - 40k gallons and through the high maintenance season (May through September) we typically add at least 450 - 550 lbs (200 - 250kg) of sodium bisulfate in the first year. Admittedly we are running sand filters so weekly or fortnightly backwashing is the norm and we have fresh water replenishment as a result.

In the pools we maintain we pay special attention to CYA levels and so we use a combination of trichlor granules and sodium hypo. We have never had a problem as a result of sulfate accumulation (as far as I know) and we have no salt buildup around the pool.

Pools nearby regularly suffer from algae blooms, scaling of the pool and surrounds, and generally dull water. This is a result of the high TA/ pH and the raised LSI. Try as I might, I am unable to convince so many home owners to use this quantity of pH- and end up having to supply them with endless amounts of hypo and algaecides.

The winter months bring extended rainfall which not only dilutes the pool water, but keeps the pH under control. At times the pH can fall as low as 7.0 by the end of the spring rains, but the TA and pH always go up again with the start of the warmer weather.

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Muriatic Acid versus granular acid

Postby chem geek » Mon 09 Mar, 2009 16:54

Larry,

I'm a bit surprised that you aren't able to get a more stable pH using a combination of Trichlor pucks or granules (which are very acidic) with sodium hypochlorite (chlorinating liquid). If you were using only Trichlor, then normally the pH wouldn't be rising and you'd never be adding acid, but would need to use pH Up (sodium carbonate; washing soda) and you'd usually have your TA at around 120-140 ppm. It's possible that with a lot of aeration features and a higher TA, that the pH can be stable using Trichlor alone and you'd only add Alkalinity Up (sodium bicarbonate; baking soda), but that's pretty rare.

If you are using enough sodium hypochlorite to see a pH rise, then that means you've got your TA too high. If you are limiting your use of Trichlor, you can try lowering the TA to 100 ppm or 80 ppm to find where the pH becomes more stable and balanced -- in your situation adding acid, just let the TA drop as it should be doing if you are adding acid regularly. If necessary, you can target a higher pH (say 7.7 instead of 7.5) or have a higher CH to keep the saturation index near zero, though I suspect that if your TA is high in your groundwater, that the CH is high as well so you really do want a much lower TA. If you want even more pH buffering, you can use 50 ppm Borates and that also acts as a mild algaecide as well.

Basically, a higher TA is only necessary if you are using acidic sources of chlorine such as Trichlor exclusively. If you are using other sources of chlorine or some mix of different sources and find that the pH is rising, then lower the TA. The only exception to this would be if the source of rising pH isn't from carbon dioxide outgassing but rather from something like new plaster curing in which case acid addition is clearly needed and a higher TA is helpful.

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Muriatic Acid versus granular acid

Postby Larry » Tue 10 Mar, 2009 01:28

Richard

We have tried using only trichlor. We use granules as the tablets are difficult to implement for that size pool. We use trichlor until the CYA reaches 80ppm then switch to an unstabilized product. The acidity of trichlor has a negligible effect on the TA/ pH.

It takes all that pH- to keep the pH down and drop the TA to around 80ppm, where the pH begins to be stable. We NEVER use pH+ or TA+ (sodium carbonate/ bicarbonate) as the water never needs it. We keep the water at a pH of 7.2 - 7.4 .

The CH is not high either, usually 180ppm - 250ppm. We try to avoid pushing the CH up too much because the slightest neglect of the water in the fall then causes scaling, and then we start with acid shocks and brushing to get things right again.

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Muriatic Acid versus granular acid

Postby chem geek » Tue 10 Mar, 2009 02:08

OK I get it. You basically are just lowering the TA slowly over time by continual use of acid to keep the pH down. At least you eventually see the pH getting more stable as the TA gets to 80 ppm. The main problem initially with the higher TA is the risk of scaling and the difficulty of keeping the pH down. The TA lowering procedure can accelerate that, but it still takes the same amount of cumulative acid either way. It's a pay now vs. pay over time tradeoff. 3 cups of dry acid in 10,000 gallons lowers the TA by 10.5 ppm.

For every 10 ppm Free Chlorine (FC) from Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm. It also decreases Total Alkalinity (TA) by 7.1 ppm (after the chlorine is used/consumed). Since your TA is so high initially, the acidity effect on the pH isn't that noticeable and the backwash and refill makes up for some of the TA loss so it doesn't seem to change either (until you add regular acid).

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Muriatic Acid versus granular acid

Postby Larry » Tue 10 Mar, 2009 02:46

By trial and error we have found the following to work best for us in a newly filled pool:
  1. Day 1: 35lbs (15kg) sodium bisulfate
  2. Day 2: 20lbs (10kg) sodium bisulfate
  3. Days 3 - 10: 7lbs per day (3kg) sodium bisulfate
  4. Month 1-2: 4.5lbs per day (2kg) sodium bisulfate
  5. Thereafter : 2lbs per day (1kg) sodium bisulfate
The first year is the most demanding. The following years we open the season with one 55lb (25kg) bag of sodium bisulfate for the initial shock (over 3-4 days). Then our acid demand is the same as the previous season; i.e. 2lbs a day.

Factors that cause our raised pH and TA levels are:
  • fill water
  • overflow pools that aerate the water as it falls into the overflow channel
  • high aeration as the overflow channel empties into the compensation tank (usually at least a 3-foot (1m) drop)
  • wind and vegetation factors

Larry

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