How Much Muriatic Acid to Add to Adjust Swimming Pool pH

Updated on July 15, 2018
robhampton profile image

Rob is a licensed pool service industry professional with over 20 years of experience.

What Is pH?

pH is a measurement of the hydrogen ion concentration in water. The pH of pool water affects the comfort of swimmers as well as pool maintenance.

"pH" stands for "protens hydrogen" or "power of hydrogen." Water conducts electricity and is easily ionized, so it will always contain some level of hydrogen ions. The pH scale is exponential and is set up so that the smaller the pH number measured in the water (or any solution), the more acidic it is. The higher the pH, the more alkaline it is.

Why Does pH Need to Be Balanced?

Controlling the pH is important not only for comfortable swimming, but also for the long life of the materials that make up the pool.

Considering that the pH of teardrops from a human eye is about 7.5, having a pH as close to that number as possible will make swimming more comfortable. As a pool service tech I sometimes hear people complain that swimmers' eyes hurt because the chlorine level is too high. But in these cases, an unbalanced pH level, not a high chlorine level, is almost always the problem.

Many things can affect the pH balance in a swimming pool:

  • disinfectants
  • source water
  • evaporation
  • water treatment chemicals
  • airborne debris
  • swimmer waste
  • aeration

Common problems caused by LOW pH include:

  • etching of the pool surface
  • metal corrosion
  • staining
  • chlorine loss
  • wrinkles in vinyl liners
  • eye and skin irritation

Common problems caused by HIGH pH include:

  • scaling (which can lead to clogged filters, clogged heater elements, and reduced circulation)
  • cloudy water
  • chlorine inefficiency
  • eye and skin irritation

Controlling pH

To lower the pH, the most common liquid acid used is muriatic acid, also known as hydrochloric acid.

To raise the pH, soda ash (sodium carbonate) is normally used. Sodium bicarbonate (baking soda) can be used to raise pH if the alkalinity is low, but it’s more commonly used to raise alkalinity.

What Is Alkalinity?

Briefly, alkalinity refers to dissolved salts, like carbonates, that keep pH stable. If alkalinity is in the right range, pH and chlorine levels are easier to maintain.

To lower alkalinity, add muriatic acid. Muriatic acid lowers both pH and alkalinity.

To raise alkalinity, add sodium bicarbonate (baking soda). This is different from sodium carbonate (soda ash).

How Much Acid to Add

This depends on just how high the pH is and the size of the pool. If you are using test strips, it's usually hard to determine exactly how high the pH level is; you just know it's high. A better test kit, or a DPD drop test kit, will use phenol red to give you a better idea of your true pH reading. Some better test kits include an "acid demand" test to help calculate a more precise dosage.

If you have a pH reading of 7.8 or higher, and an average size (15,000 gallons) in-ground pool, you should add 1/4 gallon (a quart) of muriatic acid, and re-test after the water has circulated for an hour. If you have a smaller pool, or an above-ground pool, you would start with one cup of acid, and then re-test and add more if needed. If you have a spa, we are talking only capfuls at a time. Very little acid is needed to adjust pH in a spa.

For a more exact idea of how much acid to add, see the table below, or see this pool calculator that will give you an amount based on estimated pool volume and chemistry.

How Much Acid to Add For Your Starting pH and Pool Volume

pH
1000 gal
5000 gal
10,000 gal
15,000 gal
20,000 gal
25,000 gal
50,000 gal
7.6-7.8
1 1/4 oz.
6 oz.
12 oz.
18 oz.
24 oz.
1 qt.
2 qts.
7.8-8.0
1 1/2 oz.
8 oz.
16 oz.
24 oz.
1 qt.
1 1/4 qts.
2 1/2 qts.
8.0-8.4
2 1/2 oz.
12 oz.
24 oz.
1 1/4 qts.
1 1/2 qts.
2 qts.
1 gal.
Over 8.4
3 oz.
16 oz.
1 qt.
1 1/2 qts.
2 qts.
2 1/2 qts.
1 1/4 gal.
How much muriatic acid to add to your pool. Check the pH with a test kit or test strip and use the pool calculator (link above) to estimate your pool's volume in gallons.

How to Add Acid

When you add acid, pour it into the deepest part of the pool, while the pump is running and circulating the water. Wait at least an hour before using the pool; if you added a lot of acid, wait 24 hours and test the pH before using the pool.

Questions & Answers

  • My above ground pool has a pH of 8.4. It holds around 16500 gallons of water. I have used a gallon of muriatic acid and shocked the pool. It is still green, and the pH is still at 8.4. What should I do?

    If the pH is still 8.4 after adding a gallon of acid, there is a more severe chemistry issue. Also be sure that the test kit being used is giving accurate readings. I would suggest brushing the pool and adding a floccing agent such as "Drop-Out" or "Drop-Down" which will sink all algae to the bottom of the pool. It will need to be then vacuumed to waste (this is sometimes difficult to accomplish with above ground pools). Some water will be lost during this process. After this is done, the water chemistry can then be re-balanced. If the pump/filtration system is not set up to vacuum to waste or "backwash," other options would be to keep the pH around 7.2, elevate the chlorine and filter out the algae cleaning the filter frequently. If all else fails, drain the pool and re-fill. Try this chemical dosage calculator to adjust chemicals.

    https://www.poolcalculator.com

  • My pool ph was 7.8 when tested at the local pool place. It called for 15 pounds of pH down. They sold me pH up, and it was added. It instantly turned pool water milky. They apologized and gave me one gallon of muriatic acid to add. It is still milky. How long does it take to work? Should I add another gallon?

    This is a devastating issue. A pH of 7.8 is not very high. Without knowing the pool volume (gallons), I can not accurately answer this question regarding dosage amounts. "pH up" is soda ash, or sodium carbonate. If you've added 15 lbs of this to your pool, the pH level is dangerously high. This is more than enough "pH up" to raise the pH quite a bit in even an Olympic sized pool. You have a chemistry disaster. I am also very confused why a pool place would tell you to add 15 lbs of pH down when the level is at 7.8. I would highly question their expertise, or lack thereof. The only advise I can give would be to use a test kit. Add muriatic acid as needed to bring the pH down to the correct level (7.4 - 7.6) and testing in between doses. The other option would be to try to hold the pool place responsible for this disaster and try to get them to correct the problem. I hope this helps

  • What else can I use besides muriatic acid to lower the pH of my pool?

    Sodium bi-sulfate can be used. This is dry acid, sometimes called "pH down"

© 2012 Rob Hampton

Comments

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    • profile image

      Beth 

      6 weeks ago

      My above ground pool around the bottom ring is dark color - looks like rust. It feels grainy. Chlorine is 2/4, pH is 8.4, alkalinity is 40, hardness is 150 & cyanuric acid is 50. How can I get rid of the ring?

    • profile image

      Joshua Lentz 

      2 months ago

      Thank you this was very helpful

    • robhampton profile imageAUTHOR

      Rob Hampton 

      3 months ago from Tampa Bay, Florida

      ocpool...Thank you for your comment. This article addresses specifically pH. High chlorine can also be an issue. If you have started a pool company, it might be a good idea to brush up on pool chemistry knowledge.

    • profile image

      ocpool 

      3 months ago

      Hi we just started our own pool company and I had no idea pH is usually the problem and not high chlorine levels. This will really come in handy, thanks a lot!

    • robhampton profile imageAUTHOR

      Rob Hampton 

      4 years ago from Tampa Bay, Florida

      Chris, just make sure the pump is running when you add the muriatic acid, you can swim in 45 minutes. It doesn't take long for it to dilute.

    • profile image

      chris 

      4 years ago

      how long after you add the acid that you can swim

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