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Simple Hot Tub Chemistry - Alkalinity & pH


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Simple Hot Tub Chemistry

  1. Hot Tub Chemistry Fundamentals

  2. Why you can't treat a portable hot tub like a Swimming Pool

  3. Total Alkalinity & pH

  4. Sanitize & Oxidize

  5. Step-by-Step Hot Tub Chemistry Checklist with dosage tables

  6. Simple Hot Tub Chemistry Selector

  7. Hot Tub Chemical Damage

Hot Tub Water Conservation

Hot Tub Electrical Safety

Start-up Chemistry

City of San Diego 2013 Water Quality Report:

Every water district must publish annual reports on water quality which is a great place to learn more about the local water suppply and what you really need to put in your hot tub.

You can find your water district and their water quality reports here:

Looking for quick, step-by-step instructions? See: Simple Hot Tub Chemistry Checklist

Treatment Plants
Alkalinity ppm
74 - 123
74 - 114
116 - 115
7.2 - 8.7
6.9 - 8.6
7.0 - 8.9

*Water supply pH will fluctuate throughout the season based on factors such as rainfall and the mix of water imported from other sources outside of San Diego County. Therefore, it's important to test every new batch of water before making any adjustments.

Water Balance

pH (Acid vs. Alkaline)

One of the first lessons on chemistry in just about any science class is measuring whether a solution is acidic or alkaline and then plotting the results on the pH scale. You may remeber wearing goggles and gloves to dip test strips into things like milk, tomato juice and tap water to determine whcih are acidic and which are alkaline.

On the pH scale, anything below 7.0 is acidic. Anything above 7.0 is alkaline.

Ideal hot tub pH range: 7.4 to 7.6

The water quality report above shows the water supply can vary from a low pH of 6.9 to a high pH of 8.9 so you'll probably have to make different adjustments to each new batch of water.

The small volume of water in a portable hot tub means pH can fluctuate dramatically during normal use. If the pH of your hot tub water is too low or too high it can irritate your eyes and nose and dry out your skin but, more importantly, pH affects how well chlorine and bromine can kill germs.

"The germ killing power of chlorine varies with pH level. As pH goes up, the ability of chlorine to kill germs goes down." -CDC Your Disinfection Team: Chlorine & pH

High pH reduces chlorine effectiveness

Adjusting and maintaining pH is important in keeping your hot tub sanitary.

At a pH of 8.0 chlorine effectiveness is reduced by 75%. At a pH of 6.5 chlorine is very effective but more unstable and may deplete faster. (Bromine effectiveness is also reduced at high pH but to a lesser degree than chlorine.)

As you can see from the chart above, the ideal range of 7.4 to 7.6 falls in the steepest part of the chlorine effectiveness curve which means small changes in pH can make big differences in chlorine effectiveness.

See Simple Hot Tub Chemistry Checklist for instructions on testing and adjusting pH.

Total Alkalinity

Since pH is a measure of whether your water is acidic or alkaline, it stands to reason total alkalinity is somehow related to pH.

If you think of pH as the balance or ratio between acid and alkaline material then total alkalinity is the quantity of alkaline material measured in parts per million.

Both samples have the same pH (acid/alkaline balance) but different total alkalinity

Ideal Total Alkalinity: 80 to 120 ppm

In practical terms: total alkalinity stabilizes pH so you can make minor adjustments.

Consider the total alkalinity graph above. If you add 1 oz of acid when total alkalinity is low, there isn't much alkaline material to absorb or buffer the acid so pH might change dramatically.

However, if you add 1 oz of acid when total alkalinity is high, there is a lot of alkaline material in the water to buffer the acid so the change in pH will be much smaller.

If you have ever fought with 'pH bounce' when very small adjustments slam pH from one end of the scale to the's because total alkalinity was too low.

If you have ever had 'pH lock' when pH won't budge no matter how much stuff you dump in the's because total alkalinity was too high.

Always test and adjust total alkalinity before adjusting pH.

The water quality report above shows total alkalinity is pretty close to ideal all season long. If total alkalinity gets too high in a hot tub, it can be corrected with chemistry but the small volume of water in a portable hot tub means it's often easier and faster to just drain the tub and start over.

See Simple Hot Tub Chemistry Checklist for instructions on testing and adjusting Total Alkalinity.

Avoid High pH in Hot Tubs

The pH vs. Chlorine curve shows the germ-killing power of chlorine is more effective between pH values of 6 and 7 in slightly acidic water.

You may also notice the curve between a pH of 6.0 and 7.0 isn't as steep which means small changes in pH have less impact on chlorine effectiveness.

The ideal pH range of 7.4 to 7.6 is partly based on swimming pool chemistry where swimmers will likely go underwater exposing their eyes, nose and mouths to the water. Swimming pool chemistry is usually based on the following priorities:

  1. Comfort of swimmers
  2. Effectiveness of disinfectant
  3. Protection of equipmen

If the pH in a swimming pool falls outside this narrow range swimmers may experience red, itchy eyes, dry, irritated skin and inflamed mucous membranes in their nose and sinus cavity.

During normal use, most people don't go underwater in a hot tub
so keeping the water in this narrow pH range isn't as critical

A pH above 7.0 also keeps the water slightly alkaline and helps protect pool structure such as concrete and plaster which can dissolve if pH is too acidic. It also protects metal parts like hand rails and ladders or heater components such as cast iron headers or copper heat exchangers from acidic corrosion..

A portable hot tub is made almost entirely of chemical resistant plastics
which are much more resistant to chemical damage caused by low pH.

If the pH in your hot tub is too high chlorine effectiveness can be reduced or blocked entirely.. You must fix it or the water may not be sanitary even when you add more chlorine.

Managing pH in a hot tub requires a small change in priorities:

  1. Effectiveness of disinfectant
  2. Comfort of swimmers
  3. Protection of equipment

If the pH in your hot tub is a little low, you don't have to worry about etching the plaster or concrete and chlorine kills germs even better in slightly acidic water.

That's not to say you can ignore acidic water in your hot tub but it's better to have slightly low pH and effective sanitation than high pH which blocks sanitation.

In the smaller volume and higher temperature of a portable hot tub
you ca't afford to block sanitizer so high pH must be avoided.

See Simple Hot Tub Chemistry Checklist for instructions on testing and adjusting pH.

Things that change pH

If you have already read Hot Tub Chemistry Fundamentals, you know the more stuff you pour into the water the more complicated water chemistry becomes and this can lead to undesireable reactions.

However, even with minimal chemistry pH in a hot tub can change based on factors such as how often you run the jets. Dissolved Carbon Dioxide in the water creates carbonic acid which tends to lower the pH of water.

The high temperatures and jet action of a hot tub causes dissolved Carbon Dioxide to gas off so pH has a tendency to climb even without the influence of other factors such as chemistry or bather waste.

Lowers pH
Raises pH
Bather waste (sweat, body oil, urine, etc.)
Jet action/Aeration
Sodium Bisulfate (pH-, pH Down, Spa Down, etc.) Sodium Bicarbonate (Spa Up, Alkalinity Up, Alkalinity Increaser)
Chlorine tablets (Trichlor, 3" tablets, 1" tablets, 1" sticks) Sodium Carbonate (pH Up, pH+, pH Increaser)
Bromine (tablets, granular) Sodium Hypochlorite (Liquid Chlorine, bleach)
Non-Chlorine Shock

Calcium Hypochlorite (Granular Chlorine)

Fresh water Fresh water


Since the pH of the water supply changes on a seasonal basis, adding fresh water may have different affects at different times of year.

Use this to your advantage by draining a few inches of old water and adding fresh water when the pH of the water supply will correct the pH in the tub.

©Affordable Hot Tub Repair, 2014