How to Lower pH and Alkalinity with Acid

February 12, 2021
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Overview:

Using acid lowers both the pH and alkalinity in water. It lowers pH at the expense of alkalinity because you need more dissolved carbon dioxide in water to lower pH. Since acid does not contain CO2, it converts carbonate (CO3--) and bicarbonate (HCO3-) alkalinity into carbonic acid (H2CO3) by adding Hydrogen ions (H+). Carbonic acid is aqueous CO2 (CO2 (aq) + H2O H2CO3 (aq)), which is what brings the pH down. Learn more here.

You can also lower pH directly by simply injecting CO2 into the water. Doing this will bypass the reduction of alkalinity and can actually lead to a gradual rise of alkalinity as the pH naturally comes back up (Henry's Law). Carbonic acid converts back into bicarbonate alkalinity.

This procedure outlines how to safely add acid to water to lower pH and alkalinity.

Shortcut: select type of acid:

Related Article: How to properly add acid to a swimming pool


How to Add Liquid Acid

Adding liquids like muriatic and sulfuric acid is a much more straightforward and self-explanatory procedure, but they are also more dangerous. Acid is dangerous to handle, so ALWAYS use safety gear! Glasses and gloves are important because the acid can burn your skin immediately on contact. You can use the Orenda app to figure out the exact dose of muriatic acid you need in fluid ounces (or mL if you set the app to the Metric system). Note that the Orenda app does not offer dosage for sulfuric acid, so you will need to do that calculation independently.

pre-dilute acid, muriatic acid dilution, how to add acid to a swimming pool, how to lower pH in a pool, how to lower alkalinityYou will need:

  • A reliable test kit
  • A 5-gallon plastic bucket
  • Safety equipment - glasses and gloves
  • The liquid acid of choice
  • A plastic measuring cup for liquids

Step 1

Test your pool water and your fill water for pH and alkalinity. Record results, and if you want, you can put them on the left side of the Orenda app calculator. Set your desired chemistry on the right side, and tap Get Dosage. Bring the liquid acid of choice to the pool with the other items listed above.

Step 2

Dip the bucket into the pool until it is at least half full of water. Measure and pour the prescribed amount of acid into the bucket. The objective is to achieve at least a 10:1 dilution ratio. So for half a 5-gallon bucket (2.5 gallons), you should not exceed one quart of acid (32 fl.oz.) of acid. If you need more than the bucket can hold, repeat the process as needed, or bring an extra bucket.

Step 3

Acid is heavier than water and will sink to the bottom, which is why dilution is so essential; pre-dilution reduces the average density of the acid. Pour the diluted acid around the perimeter of the deep end of the pool. Rinse the bucket with pool water to clean it out when you're done. NEVER pour acid directly into a skimmer or gutter!

PRO TIP:

After pouring acid, close the bottle and dip the bottle in the pool to ensure that the outside of the bottle does not have acid on it. Sometimes acid can dribble out onto the outside of the acid bottle, which can leave marks wherever you set the bottle down (like the concrete deck or your truck!). Rinse it quickly to ensure this does not happen to you.

How to Add Dry Acid

Sodium Bisulfate

The procedure for adding granular acid is pretty much the same as adding calcium chloride or sodium bicarb to a pool. Measure the dry chemical, pre-dissolve in a bucket, and pour around the perimeter of the pool (never into the skimmer directly).

NEVER mix different types of dry chemicals together! This is especially true for chlorines and acids. Never mix any type of acid with chlorine of any kind, because the fumes can be toxic and even lethal. And always add a dry chemical to water, never water to the dry chemical.

You will need:

  • A reliable test kit
  • A 5-gallon plastic bucket
  • Safety equipment - glasses and gloves
  • Something to stir with - like a wooden paint-stirring stick
  • Dry acid (sodium bisulfate)
  • A plastic measuring cup or scoop

Step 1

Test your pool water and fill water for pH and alkalinity (and CYA if you are trying to add CYA). Record results. The Orenda app will tell you exactly how much CYA you will need but does not offer dry acid dosing results, which means you will need to figure out the dosage on your own. Bring the granular acid of choice to the pool with the other items listed above.

Step 2

Dip the bucket into the pool until it is about 3/4ths full of water. Slowly pour the measured amount of dry acid into the bucket. Do not exceed 5 pounds of acid per bucket. Generally, you will not need so much anyway.

Step 3

Stir until completely dissolved. Pouring undissolved acid into the pool can cause problems. Acid is heavier than water and will sink to the bottom. If not fully dissolved, it can damage the surface of the pool. Always pre-dissolve.

Step 4

Slowly pour the completely dissolved solution into the pool. Consider pouring about half the bucket, then let fresh pool water into the bucket to re-dilute, and stir again, and slowly pour again. Use your best judgment. Do NOT pour the solution directly into the skimmer. Add it around the perimeter of the pool, starting in the deep end.


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All information provided is intended for educational purposes and is not implied to replace consultation with a qualified pool professional. It is recommended that all information from this or any other source is to be performed assuming individuals performing these functions will consult local state and federal requirements before you act upon it in any way. While this site attempts to provide information that may be relevant to you, no guarantees are made that some relevant information will not be missed. We recommend you consult a local pool professional before acting. 

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