How to Lower Alkalinity in a Hot Tub

Reviewed by Andrada Simion, Master of Science in Chemistry

  • Proper alkalinity is an important part of hot tub safety. Alkalinity helps keep pH levels stable. A healthy level in a hot tub should be neutral: 7.2-7.8 is the safest range, but 7.4-7.6 is ideal. Levels above this range is a sign that total alkalinity (TA) is too high. Test hot tub alkalinity regularly.
  • Alkalinity is measured in parts per million (ppm). Ideally, the alkalinity of a jacuzzi will fall between 80 and 120ppm. High levels increases the basicity of the solution. It occurs when there are high levels of contaminants. These can be excess chemicals, bacteria, sweat, or dirt.
  • Since alkali are bases, alkalinity decreases when acidity increases. Some acids are sold as pH decreasers because of their ability to lower alkalinity, which reduces the basicity of the solution. The two most common decreasers are sodium bisulfate or muriatic acid. These compounds react with bicarbonate and increase the concentration of hydrogen.
  • Sodium bisulfate is a dry acid that resembles table salt (it is a salt itself). One measurement of sodium bisulfate should lower alkalinity by 10ppm. The measurement itself depends on the volume of the jacuzzi. For example, a 1000 gallon spa requires 3.5 oz of sodium bisulfate to reduce alkalinity by 10ppm, but a five hundred gallon hot tub will only require 1.75 oz for the same reduction. Allow the pump to circulate for twenty minutes before turning it off. Wait an hour and then retest. Repeat as needed.
  • Muriatic acid is an aqueous solution of hydrogen chloride, so one must be careful with it. It tends to be cheaper than sodium bisulfate, and liquid acids tend to be more aggressive than dry ones. A 750 gallon spa only needs between .9 and 2.4 fl oz of muriatic acid to reset pH levels to 7.6. However, it cannot be poured straight into the spa. Take a small bucket and fill it with spa water. Gently add it into the bucket. Always pour the acid into the water, never the other way around. Mix it gently. Allow the pump to circulate the mixture for three to six hours before retesting.
  • Muriatic acid can be very dangerous; wear rubber gloves and safety goggles while using it. When using any acid, it is always safest to wait around 30 minutes after reaching the desired TA before entering the hot tub. Acids that destroy chemicals can be incredibly damaging to skin.
  • Since TA and pH are not interchangeable; sometimes alkalinity might need to be decreased even when pH is in the ideal range. Remember: pH refers to the acidity of the tub, while TA refers to the amount of substances that lessens changes in pH. Aeration along with acid treatment will reduce alkalinity without decreasing pH. Usually, this process is only needed when TA is moderately too high, as a spiked TA will cause pH to go up as well.
  • Determine how much acid you need based on the volume of the tub. Turn on the jets and air features, and keep them on through the entire process. Initially add only half the amount needed to reduce the alkalinity. For example, if 3 oz is needed to bring TA down to the desired level, only use 1.5 oz. Allow at least thirty minutes of aeration before retesting TA and pH. Repeat the process until reaching the ideal ranges.
  • Recently, vinegar has become a popular suggestion for balancing a jacuzzi because it contains acetic acid. However, this is a misconception. Vinegar is typically less than 10% acetic acid, and the acid itself is very weak. Vinegar can lower pH, and is safe to use for cleaning the spa, but will not will not help with alkalinity problems.
  • Jacuzzis require regular testing and maintenance. However, regularly testing and addressing any problems will keep them safe and enjoyable. TA is easy to address, and the spa will be safe to use shortly after making adjustments.