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 Your Pool's 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.
What Causes Pool pH to Become Unbalanced?
Many things can affect the pH balance in a swimming pool:
- source water
- water treatment chemicals
- airborne debris
- swimmer waste
Common Problems Caused by a LOW pH:
- etching of the pool surface
- metal corrosion
- chlorine loss
- wrinkles in vinyl liners
- eye and skin irritation
Common Problems Caused by a HIGH pH:
- scaling (which can lead to clogged filters, clogged heater elements, and reduced circulation)
- cloudy water
- chlorine inefficiency
- eye and skin irritation
How to Control Your Pool's 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 Should I Add to My Pool?
This depends on how high your pool's 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|
1 1/4 oz.
1 1/2 oz.
1 1/4 qts.
2 1/2 qts.
2 1/2 oz.
1 1/4 qts.
1 1/2 qts.
1 1/2 qts.
2 1/2 qts.
1 1/4 gal.
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
Question: To adjust my pool Ph, how long after adding muriatic acid should I wait to add chlorine?
Answer: After lowering the pH, allow the water to circulate for 30 minutes. Then the chlorine can be added.
Question: 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?
Answer: 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.
Question: I’ve added a half a gallon of muriatic acid to a 10000-gallon pool and my Ph and alkalinity are still very high, what do I do?
Answer: Since muriatic acid will lower BOTH pH and alkalinity, the answer is simple. Continue adding muriatic acid (1/4 gallon at a time) let circulate for a few hours and retest until the pH is at the correct level (7.4-7.6). If the alkalinty is still a little high at that point, it will come down eventually on it's own.
Question: What do I do after adding too much shock to my pool?
Answer: If you've added too much shock, then there's a risk of the chlorine level being dangerously high. To lower the chlorine level, you can approach this in two different ways. To lower it immediately, sodium thiosulfate can be used (sometimes called "After-shock" or simply known as chlorine reducer). The other option would be to wait several days and let the level come down on its own.
Question: What else can I use besides muriatic acid to lower the pH of my pool?
Answer: Sodium bi-sulfate can be used. This is dry acid, sometimes called "pH down"
Question: What if I don't have a test kit for ph level? I'm tending to my sister's pool while she's out of town - I put a full gallon of muriatic acid in the pool like she said - but she didn't mention waiting, or testing. Is there a way to tell without a kit?
Answer: Testing the pH is very important. There is no way of knowing the level without testing. The best advice I could give would be to at least purchase some cheap test strips.
Question: Can I add muriatic acid to a pool that has a liner over the cement? Will it affect the liner or melt it at all?
Answer: All pools need to have pH adjusted. Muriatic acid is commonly used, but also dry acid. If you are adjusting the pH in a vinyl-lined pool using muriatic acid, add slowly to the deepest part of the pool with the pump on and water circulating. This will NOT harm the vinyl liner although unbalanced chemicals over time will have damaging effects.
Question: What should the pH be for a 30,000-gallon inground pool?
Answer: The pool size or gallon amount makes no difference. The pH level should be 7.4 to 7.6 in any pool.
Question: How long do I have to wait to swim after adding muriatic acid?
Answer: If it's small pH adjustment then it is ok to swim after 1 hour of pool circulation. If a large quantity of acid is being added, wait 24 hours then re-test the pH to make sure the water is not too acidic.
Question: 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?
Answer: 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
Question: Do I pour muriatic acid in the skimmer or directly into the pool?
Answer: The acid should always be added directly to the pool, preferably near the deepest part of the pool with the pump running and water circulating. After adding the acid, allow at least 1 hour of circulation.
Question: Should I test the pool water's pH with the filter turned off?
Answer: It's better to test chemicals with the pump on and water circulating.
Question: How do I raise the alkalinity of pool water?
Answer: Sodium bi-carbonate (baking soda) will raise the alkalinity.
Question: You mentioned that a pH of 7.5 is most comfortable swimming. What is the optimal swimming Alkalinity?
Answer: Alkalinity is best between 80 - 120 ppm. I usually strive for 90 to 100 for the best chemical balance.
Question: What do I do when my pool pH is good and the alkalinity is high?
Answer: The alkalinity in pool water should be no higher than 120. Because alkalinity tends to be a buffer for the pH, it's rare that I see high alkalinity and low to normal pH although it CAN happen. The alkalinity can be lowered using muriatic acid with the problem being that it also lowers the pH at the same time. The answer is to use both muriatic acid along with sodium CARBONATE also known as soda ash (Do not confuse with sodium BI-carbonate which raises alkalinity) Depending on how high the alkalinity is, add the desired amount of muriatic acid and let circulate for several hours. Once the alkalinity is within the desired range, you will use pH increaser (soda ash) to bring the pH back up without raising the alkalinity. Keep in mind that it doesn't take much soda ash to increase pH.
Question: My pool has 164,853 gallons. How much HCL do I need to use?
Answer: 164,853 is a very large pool. Please make sure this is the correct volume. This also can not be answered accurately without knowing the other chemical parameters, especially the pH as you are referring to muriatic or hydrochloric acid (HCL) Please try this chemical calculator link listed below:
Question: Do you need to dilute muriatic acid before adding it to a pool?
Answer: No, as long as there is circulation in the pool (pump running). Add acid and allow it to dilute on its own and circulate. No need to "pre-dilute" when adding to the pool.
Question: What am I doing wrong? I have a 20k gallon pool and have put in 2 quarts of muriatic acid because the pH and Ta are too high. 24 hours later, there is no change in my test numbers.
Answer: Let's start with the test kit you are using. Is it a liquid drop test? Are they test strips? Make sure the test kit you are using is working. With a liquid drop test kits, the test reagents have a shelf life and can give false readings. Same with test strips. Are they old? Have they been kept in a dry, cool place and sealed? Now..assuming the test kit is fine, and your pH and total Alkalinity readings are still high, the solution is simple. You need to continue adding muriatic acid (which drops both pHs an alkalinity) until the levels begin to come down. With the pump circulating, add one quart at a time. Circulate for 30 minutes and test again. Do this until desired reading of 7.4 - 7.6 is reached. Once the pH is down, you may need to make some minor adjustments to alkalinity. Also, try this chemical dosage calculator:
Question: Is sulfuric acid and Muriatic acid the same?
Answer: No, although sulfuric acid can be used to lower pH, it's less commonly used as it has a high concentration of sulfates. Over time, combined with the calcium in the water, will cause damage to the pool surface. Muriatic acid had a strength of 31% where as sulfuric acid has a strength of 38%. Sulfuric acid is much more corrosive than muriatic acid.
Question: How can I lower the alkalinity level of my pool?
Answer: Muriatic acid will lower the alkalinity. Keep in mind that it will lower the pH at the same time. If you have high alkalinity, usually the pH is also high although rarely I have seen high alkalinity and low pH. In this case, lower the alkalinity with muriatic acid and then bring the pH back up using sodium carbonate (soda ash, not to be confused with sodium bicarbonate which is used to raise alkalinity) Soda ash increases pH only without affecting alkalinity.
Question: How much sodium thiosulfate do I use to lower my chlorine level that is at 10 of my 35000-gallon pool?
Answer: To bring the chlorine reading from 10ppm to 4pmm add 1.31 pounds of sodium thiosulfate to 35,000 gallons of water.
Question: I have a 30,000-gallon pool. The T/A is 120, and the pH is 8.0. I put in two quarts of muriatic acid, and I pump at 3,000 rpm for 15 minutes and then hold at 1,500. How long do I have to wait until I can swim in the pool?
Answer: With two quarts of acid added to 30,000 gallons of water, it will be safe for you to swim in one hour.
Question: How do I lower the chlorine level of my 35000-gallon pool?
Answer: A chemical called "sodium thiosulfate" can be used, which is most commonly known as chlorine reducer. Depending on the desired reduction of chlorine, the dosage will vary. It can be purchased online or at any pool supply store. Thiosulfate is very effective in chlorine reduction and it doesn't require much to lower the chlorine level. Use sparingly and according to directions based on pool water volume
Question: I need to add 10oz of acid to my pool. How long must I wait to swim in it again?
Answer: If your pool is anything larger than 8 or 10,000 gallons, with using only 10 oz. of acid, you only need to wait long enough for it to circulate and dilute. 20 minutes should be sufficient
Question: I have 62,000 gallons of water in my pool. How much acid is required for that many gallons of water?
Answer: I can not answer this accurately without knowing what the current pH level is. An example would be for a 62,000-gallon pool, if the pH is 7.8 with an alkalinity level of 90, you would like the pH level to be 7.4 (ideally), so you would add approximately 88 oz., or 2.6 quarts of muriatic acid. A pool chemical dosage calculator can be used. Try this one..
© 2012 Rob Hampton
Umesh Chandra Bhatt from Kharghar, Navi Mumbai, India on July 26, 2020:
Excellent article. Good reading. Thanks.
Rob Hampton (author) from Port Richey, Florida on June 13, 2019:
Hi David, you need to dilute the water..Drain some, add fresh.
David on June 13, 2019:
I put too much metal out in my pool by Doheny's my pool is only 10,000 gallons what should I do???
Sean on June 10, 2019:
I have a waterfall feature about 5' in length, 3" deep and 4' high (that's just the waterfall portion)..Where the water falls into is about 3.5' wide, 1' deep with a 1' depth (it's basically a rectangle tub where the water falls into it and is pumped back over the waterfall).--nothing swims in it. It flows over natural stone and i'm wondering (because i used to have a pool) if i can just use muratic acid and chlorine to keep the water clean or i can just use muratic acid to keep it clean from algae, etc.? I used those 2 chemicals for 4 years while I had a pool, monitored the pH balance and NEVER had a problem. Can i easily do the same for this and if so, about how much inital chlorine and/or muratic acid would I need to use for about 4-5 volume of water?
Arturo Abascal on May 09, 2019:
I just resurfaced the Pool a month and a half ago. It’s a 25000 gallon pool underground. I used city water to fill the Pool. My water is clear, I run the Pool 8 hours a Day. It’s a salt water pool. My ph has been around 8.00 and for the last few weeks I have been dumping Muriatic acid on a weekly basis to bring the ph down. I have been using the amounts recommended by pinch a penny. The Pool supplier tells me that it will take some time till the ph stabilizes it’s a new pool. Other factors like alkalinity and chlorine are correct, but I wonder how long will I continue using Muriatic acid and if the assumption from pinch a penny is correct. My salt level reads 3,000.
I thank you for your reply
Rob Hampton (author) from Port Richey, Florida on April 09, 2019:
Laura, Citric acid is not used to drop pH. I also can not recommend pH to be any lower than 7.4, Unless you are doing some sort of stain removal.
Laura on April 09, 2019:
Starting ph is 7.2 in 2500 gallons of water, how much citric acid is used to drop ph to 5
MARK on January 13, 2019:
I just added half a litter off acid hydrochoric into a friends pool as they have been away for 3weeks how long until i can swim in it, it was still clear before i add the acid
Rob Hampton (author) from Port Richey, Florida on December 24, 2018:
Hi Patsy, thanks for reading! So the most common alkalinity increaser is sodium bicarbonate (baking soda) and yes, it can be used to raise pH as well. If you have high alkalinity and low pH, soda ash (sodium carbonate) is best. If both alkalinity AND pH is low, than sodium bicarbonate is ideal.
Patsy Pakeerah on December 24, 2018:
If my ph in my pool is too low could i add some total alkalinity to balance it?
jarle stensrod on November 24, 2018:
we use paint instead of tiles in our pool but now the water is so cloudy 'whats the cause of it and how to treat it.thank you...
Beth on August 29, 2018:
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?
Joshua Lentz on August 12, 2018:
Thank you this was very helpful
Rob Hampton (author) from Port Richey, Florida on July 05, 2018:
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.
ocpool on July 04, 2018:
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!
Rob Hampton (author) from Port Richey, Florida on May 26, 2014:
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.
chris on May 26, 2014:
how long after you add the acid that you can swim