Why Add Acid to Hot Tubs?
Acid is added to hot tubs along with other chemicals to keep the water clean and to adjust the pH level of the water. Depending on factors like the "hardness" of your water, the amount of dissolved minerals and current pH, acid is used to lower the pH and total alkalinity, and soda ash or sodium bicarbonate is used to raise the pH.
What Type of Acid?
The most common type of acid used in pools is hydrochloric, or Muriatic acid. This type of acid is quite strong, so most fiberglass hot tub manufacturers caution against the use of muriatic acid in favor of sodium bisulfate, or powdered dry acid. Dry acid is safe and easy to use and is available in small amounts, which is ideal for spas and hot tubs.
Testing the Water
To keep a handle on the balance of chemicals in your hot tub, buy a water testing kit from your local pool store to test the pH of your water. Look for a testing kit that includes multiple tests: pH, total alkalinity, free chlorine and combined chlorine. Following the kit's instructions, add a small amount of pool water and a few drops from the test kit to find out the chemical balance of your spa.
How Much Acid is Needed?
Depending on the total alkalinity of the water used to fill the hot tub, you will need to adjust the pH and alkalinity of the spa water using acid. The ideal pH of spa water should be kept between 7.2 and 7.6. Start by using your test kit to find out the pH of your hot tub's water. If the pH is too high, add dry acid to lower the pH to an acceptable range. It is recommended to add no more than 1 oz. of dry acid to every 500 gallons of water at one time. Allow it to sit for one hour, then measure the pH and add more dry acid if needed.
Safety
When working with strong acids, it is important use caution and limit the chances of accidental exposure and contamination. Dry acid is safer to use than liquid Muriatic acid, but safety is still a concern. Be sure to open and mix dry acid in a well ventilated area, use gloves and eye protection if possible and immediately rinse accidental skin and eye exposures with large quantities of cold water.