Muriatic acid is commonly used in the pool industry, but what does muriatic acid do to a pool, and how effective is it at doing it?
This article will break down everything you need to know about muriatic acid, including why it’s often the best acid to use in a swimming pool, and how to use it safely, step by step.
Short answer: Muriatic acid is used to lower the pH level and total alkalinity in a pool, though it’s more often used to lower pH while raising the alkalinity back up through other means. It’s also sometimes used to acid wash a pool.
What is Muriatic Acid?
Muriatic acid, also known as hydrochloric acid, is naturally produced in the human body as stomach acid.
The type of muriatic used in pools, however, is manufactured by dissolving hydrogen chloride gas in water. This is also used for a number of commercial cleaning, production, and building processes.
Muriatic acid is sold in specialized plastic jugs and is typically available in either 15% or 31% strength variants.
Why Use Muriatic Acid in a Pool?
Muriatic acid is used to lower both the pH level and total alkalinity in a pool when they’re too high.
Regardless of which strength acid you’re using, for every 0.1 reduction in pH, you’ll see an average 1.6ppm reduction in total alkalinity.
Oftentimes, however, pool owners use muriatic acid primarily to reduce the pH level, knowing the total alkalinity can easily be raised back up in isolation using sodium bicarbonate (baking soda).
Finally, muriatic acid can sometimes be used to acid wash pools, though this only applies to plaster pools (concrete or gunite).
When is pH and Alkalinity Too High?
Your pH level is considered too high when it’s over a pH of 7.8, and total alkalinity is considered too high when it’s over 120 parts per million.
When a high pH level is sustained for too long, it makes your water cloudy, scales your equipment and surfaces, weakens chlorine’s sanitizing power, and can even be harmful to your skin and eyes at extreme levels.
High total alkalinity not only pushes your pH upwards (as alkalinity is a cause of high pH in itself), but it also makes it harder to reduce your pH since alkalinity acts as a buffer.
In both cases, muriatic acid will lower these levels back down into the appropriate ranges, assuming the correct dosage is used.
Muriatic Acid vs Alternative Acids
Other acids are sometimes used in place of muriatic acid to lower the pH level and alkalinity in a pool in the same way.
Specifically, these are:
- Dry acid, or sodium bisulfate.
- Sulfuric acid, or sulphuric acid (rarely used)
There are a couple of significant upsides that make muriatic acid the more attractive option for most pool owners, but also one downside that makes dry acid or sulfuric acid the better choice for some.
Let’s talk about them.
It Doesn’t Contain Sulfates
Every time you use dry acid or sulfuric acid to lower your pH level, you’re adding sulfates to your water.
A low sulfate level isn’t going to cause you too much trouble (most pools contain some sulfates), but a high concentration will eventually cause your water to attack concrete/plaster, damage the vinyl liner, and corrode the metal.
While it can take many months to reach dangerously high sulfate levels through the use of dry or sulfuric acid, it’s only a matter of time with repeat use as they cannot be removed from pool water without draining and refilling.
Muriatic acid doesn’t contain sulfates and therefore doesn’t pose any of these risks when using it in your pool.
It’s Considerably Cheaper
The cost of pool supplies is largely dependent on where you live.
Muriatic acid, however, is almost always cheaper than other forms of pool acid, especially if you buy in larger packs of 2 or 4.
Bought locally, you can typically expect 1 gallon of 31% strength muriatic acid to cost between $5 and $8
Using some pool math, we know it would require roughly 10lb of pre-dissolved dry acid to achieve the same pH reduction. This amount of dry acid will set you back anywhere from $25 to $50.
For example, using some pool math, we know going from a pH level of 7.9 to 7.5 requires roughly 12 oz (354ml) of 31% strength muriatic acid, or 16 oz (1 lb) of dry acid granules
Using average online prices:
- 354ml of muriatic acid equates to about $7 in cost (1 gallon at $20)
- 1 lb of dry acid equates to about $9 in cost (2 lbs at $18)
As you can see, muriatic comes out cheaper at smaller volumes. Price can sway in favor of dry acid at much higher volumes, however, since you can now buy 25 lb and 30 lb tubs for a heavily discounted price.
Sulfuric acid is usually comparable in price to muriatic acid.
It’s More Difficult to Handle
Unlike dry acid, which comes in granular form, muriatic acid comes in liquid form and is, therefore, a little more difficult to handle.
While granules can be easily poured and dissolved in a bucket without much risk to the handler, liquid acid is more prone to accidental spillages and inhalation directly from the bottle.
Muriatic acid also gives off more potent fumes that can cause swelling and repository issues if inhaled, though this can be mitigated by taking note of wind direction and pouring at arm’s length.
Regardless, muriatic acid isn’t nearly as dangerous as most people believe, and you can always start with the lower strength version until you’re more comfortable with handling it.
How to Use Muriatic Acid to Lower Your pH and Alkalinity
Follow these steps to learn how to lower your pH level using muriatic acid:
1. Test, Test, Test.
Before you even think about adding muriatic acid to your pool, you need to test the water to know where you currently stand.
So grab your pool water test kit or testing strips and do your thing.
Once you have a baseline of where all of your levels are, you can determine what strength of acid to use, and in what quantity.
2. Nail Down the Muriatic Acid Amount
To save you from using too much acid or not enough, we’re going to calculate the precise amount of muriatic acid to add.
For this part, you will need:
- Your starting pH and total alkalinity
- Your pool’s water capacity (see here)
Once you have those numbers, plug them into this pool chemistry calculator to get an exact dosage amount (noted in the pH section).
Note: In general, per 10,000 gallons, 10 oz of 15% strength muriatic acid will lower pH by around 0.1 and 1.6ppm, or roughly 5 oz for 31% strength. These numbers will shift slightly depending on your starting pH and alkalinity.
3. Put Your Safety Gear On
When dealing with concentrated acid, at the very least you should be wearing rubber gloves and protective eyewear.
While spilling muriatic acid on your skin may seem like the worst outcome, it’s actually the fumes that pose a more serious health risk. For that reason, consider wearing a nose plug or mask to avoid inhalation.
It’s also worth skipping on your favorite t-shirt as any spillages will likely burn a hole in your clothing, though it won’t burn through immediately.
4. Pour the Muriatic Acid into Your Pool
Now it’s time to add muriatic acid to your pool water.
It’s best to pour the jug of muriatic acid directly into your pool without dissolving it first. Pre-dissolving adds an additional step to the process where spillages can happen, and it simply isn’t necessary.
Here’s how to do it:
- Take your jug of muriatic acid in one hand and your measuring cup in the other hand.
- Position yourself at the deep end of your pool, somewhere in front of a flowing return jet (your pump should be running).
- Extend both of your arms over the pool, as far away from you as possible to avoid breathing the fumes.
- Slowly pour the acid into your measuring cup before tipping it into the pool, counting as you go to ensure the correct dosage.
- If any muriatic acid gets onto your hands, immediately wash them off in the pool. You can also do the same with the jug and measuring cup after use.
5. Run Another Test
Assuming you keep your pump running, it should only take around 15 minutes for the muriatic acid to take effect.
When testing the water a second time, be sure to use the same method you used in step 1 for consistency. For example, if you used a test strip earlier, be sure to use another test strip from the same company.
The new test results should reveal a much lower pH and total alkalinity reading compared to the initial test.
Remember, the ideal levels are:
- pH level between 7.2 and 7.8
- Total alkalinity between 80 and 120ppm
If your pH or alkalinity is still slightly too high, you probably miscalculated the dosage and will need to add more acid to bring them back into range.
If you lowered your pH and alkalinity too much, you can use soda ash to bring both pH and alkalinity back up. Soda ash is a direct counter to muriatic acid.
6. Add Baking Soda to Fix Alkalinity (Optional)
Oftentimes, muriatic acid is only used to fix a high pH level, even though it also reduces total alkalinity.
If that’s true in your case, you’ll need some baking soda.
Baking soda raises total alkalinity back up with virtually no impact on the pH level (the effect on pH is so small it’s not even worth talking about).
You can refer back to this calculator to see how much baking soda to add to your pool, or you can use the averages below.
- 5 oz of baking soda raises alkalinity by roughly 2ppm
- 14 oz of baking soda raises alkalinity by roughly 5ppm
- 27 oz of baking soda raises alkalinity by roughly 10ppm
It’s a Multi-Purpose Chemical
To summarize: “What does muriatic acid do to a pool?”
Well, it’s commonly used to reduce the pH and/or total alkalinity of the water to bring it back into balance.
It can also be used on plaster pool finishes during an acid wash; a process that essentially removes a layer of plaster to get rid of deep stains and other unsightly markings.