Whenever you are testing your pool water, the first thing that you need to adjust, before you add chlorine or anything else is your pH.  Maybe you don’t know why this is or have wondered if it is really that important because most people think that sanitizer (chlorine) is the only thing you need to worry about. This short article will let you know why proper pH levels are vital to a pool water’s overall health, and why you fix pH problems first.


Most pools are sanitized by chlorine; it has been around a long time, sanitizes quickly, and it relatively easy to find.  Salt Chlorine Generators turn the salt water into the same thing chlorine tablets turn into when they are put in water.

“It is often impractical to store and use poisonous chlorine gas for water treatment, so alternative methods of adding chlorine are used. These include hypochlorite solutions, which gradually release chlorine into the water, and compounds like sodium dichloro-s-triazinetrione (dihydrate or anhydrous), sometimes referred to as “dichlor”, and trichloro-s-triazinetrione, sometimes referred to as “trichlor”. These compounds are stable while solid and may be used in powdered, granular, or tablet form. When added in small amounts to pool water or industrial water systems, the chlorine atoms hydrolyze from the rest of the molecule forming hypochlorous acid (HOCl), which acts as a general biocide, killing germs, micro-organisms, algae, and so on.”

For a Salt Generator Pool, pool salt is “added to the pool water to a level of 2500 to 3500 ppm depending on the manufacturer and system. Low voltage DC electricity is applied to the inline cell and the unit produces chlorine gas, hydrogen gas, sodium ions, hydroxide ions. These mix to form the formation sodium hypochlorite instead of chlorine gas. The sodium hypochlorite turns into hypochlorous acid (HOCl – the quick kill form of chlorine) and hypochlorite ion (OCl-). The hydrogen gas releases into the atmosphere as it bubbles up into the pool water.”

Chlorine is affected by pH

When pH gets too high, the ability of chlorine (or bromine) to do its job (sanitize and clean your tub).

“Hypochlorous acid (HOCl) and hypochlorite (OCl-), which are together referred to as “free available chlorine”, coexist in an equilibrium relationship which is influenced by temperature and pH. Where water is acidic (low pH) the above equilibrium shifts to the left and results in a high percentage of the free available chlorine being in the form of HOCl. Where the water is basic (high pH), a high percentage of the free available chlorine is in the form OCl.

The efficiency of Hypochlorous acid in killing microorganism is about 40 to 80 times greater than hypochlorite.  The effectiveness of chlorination is highly dependent upon the pH of the water source, the number of bacteria, algae and other organic matter. Thus, water having a low pH will result in a high concentration of HOCl which is the more potent biocide. At pH 8, only about 22% of the chlorine will be in the active HOCl form, at pH 7, 73% will be in the HOCl form, and at pH 6, about 96% of the chlorine will be in the HOCl form (Nakayama and Bucks, 1986). If water pH is above 7.5, may be necessary to add acid to lower the water pH.”

The most important variable, that is also the most misunderstood, is pH (Hydrogen Potential/Power). What actually is pH? The scientific definition is the logarithm of the reciprocal of the hydrogen ion concentration. Or more simply stated, pH is a number between 0 and 14, denoting various degrees of acidity or alkalinity. Neutral water has a pH of 7. Water below 7 is acidic and becomes more acidic as it approached zero. Water above 7 is alkaline and alkalinity increases as it approaches 14. What is the pH of your water and how does it improve or impair the chlorine’s ability to be effective? When the pH of your water is 7 or below, chlorine will act primarily as a sanitizer. At this level, it is very effective at killing bacteria. At 7.4, chlorine will act equally as a sanitizer and oxidizer. Above 7.8, the chlorine will act principally as an oxidizer.
The pH of chlorine is 11.7. It would seem logical that adding chlorine into water having a neutral pH would make the water more alkaline and this is true in a majority of the cases. Keeping this in mind, it may be necessary to lower the pH of water with an acid in order to increase the efficacy of chlorine in the killing of bacteria if your water has a high pH to begin with. When chlorine is added to water it becomes hypochlorite ions (OCI) and hypochlorous acid (HOCI) in a quantity determined by the pH as indicated by the chart below:”
ConclusionAs you can see, when the pH gets higher or more alkaline, the efficiency of chlorine really diminishes.

In order to make sure your pool is properly sanitized, it is important to make sure the pH is in range.  The pH of the human eye is 7.2, so pH levels lower or much higher can cause eye irritation.  High pH also leads to scale build-up.

Rainwater is acidic, meaning lower pH, so if your pool isn’t covered during rain, the pH will probably be affected. After rain or a wind storm, it is always a good idea to check your pool water’s alkalinity and pH.

Low pH can ruin heat exchangers on your heater and lead to a costly repair.  The copper heat exchanger corrodes and adds copper to your water.  Copper from a heat exchanger can stain your swimming pool.

Other Water Chemistry information can be found on this blog.  Check out the article on Cyanuric Acid (Stabilizer) and read up on how it affects your chlorine’s ability to kill bacteria.  Or, if you need a general overview of pool water chemistry visit this link about pool water basics. If you are unsure about your pH, bring in a sample and we will test it for you for free.