Do You Know Your Hot Tub Chemistry?

Hot tubs have fundamentally different chemistry than pools, and that’s mostly because of volume: A small pool will typically have more than 100 times the volume of a hot tub.

This much smaller volume means that hot tubs are much more impacted by bathers. Even a few bathers using a hot tub for 10 to 15 minutes can have an effect on the chemistry.

Furthermore, the high temperatures of hot tubs allow chemical reactions to occur very quickly. This leads to rapid chemistry shifts that are not seen in cool pools.

The Basics: pH and Alkalinity

The basic score for acidity is pH: a scale measuring hydrogen ions, or acidity. A high pH means very few ions, and thus very alkaline water. A low pH means many ions, creating very acidic water.

The scale is exponential: a score of five is ten times as acidic as a score of six; a score of none is ten times as alkaline as a score of eight.

The pH affects your hot tub immensely. Chlorine sanitizers cannot function at pH values above 7.8. Higher pH’s than eight cause scaling and cloudy water, while pH’s lower than 7.2 causes skin irritation and corrosion.

Alkalinity is a related, but different, measurement. Alkalinity is the percentage of alkaline chemicals that could neutralize acid.

Alkalinity acts as a buffer: if the pH gets too low, alkalinity will react to keep it high. Alkalinity helps control pH and keeps it in predictable bounds.

The Advanced Stuff: Why Hot Tubs Change

Most hot tub chemical changes occur because of the actions of the sanitizer.

Chlorine is highly volatile: it works by attacking any organic material not protected by an acid barrier (living skin is covered by a thin layer of acidic sweat and oil, while dead skin flakes and bacteria are torn apart by chlorine).

Chlorine destroys organics by oxidizing them: forcing them to take on oxygen ions from the water. This frees up more hydrogen ions, messing with the pH.

Alkaline chemicals react to the change by neutralizing the acid, reducing the amount of available buffer. This causes rapid swings in pH and alkalinity. This becomes less predictable over time, as the chlorine itself breaks down.

Bromine is more predictable, but still causes chemical changes by oxidizing organics.

Dealing with Chemistry

With chlorine and bromine, the best course of action is a schedule of daily adjustments to pH and alkalinity. Ideally, filters should also be cleaned often to prevent hidden buildups of organic material from messing with your chemistry when you’re not looking.

A more proactive solution is to invest in a less volatile sanitizing system, like ozone. Ozone systems still work by oxidation, but have fewer by-products than chlorine and bromine, and fewer overall impacts on hot tub water.

While your hot tub cover doesn’t change your hot tub’s chemistry, it does prevent unwanted bacteria from entering the tub while not in use.

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