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How Does a Low pH Affect Chlorine Levels in a Pool?

Chlorine works better as a sanitizer when the pool pH is low, but acidic water creates a number of other problems.

Chlorine sanitizes well in acidic water, but the water itself is corrosive.

Chlorine levels and pH level are the two most important chemical balances to maintain in a swimming pool, and pH affects chlorine levels, especially when it's too high. When the pH level is too low, chlorine actually sanitizes more efficiently, but the pool water becomes corrosive. It can damage pipes, pool liners, metal components, the pool pump and floaties such as beach balls, that remain in the pool too long. Acidic water also causes skin and eye discomfort for anyone who uses the pool.

How Chlorine Works as a Sanitizer

Whether you treat your pool with sodium hypochlorite (liquid bleach), calcium hypochlorite, lithium hypochlorite or chlorinated isocyanurates, the compound releases hypochlorous acid (HOCl) when it dissolves. This acid reacts with contaminants, and as it does, the amount of free available chlorine (FAC) goes down. This doesn't mean the total amount of chlorine in the pool goes down -- it just forms nonreactive compounds with such contaminants as ammonia, nitrogen, and organic compounds from perspiration, urine and other waste. When you shock the pool, you introduce oxidizing compounds that free the chlorine from these compounds, thus increasing the level of FAC.

Because HOCl is an acid, it reacts with negative ions present in the water when the alkalinity and pH are high. This reduces the amount of FAC. When the pH is below the ideal value between 7.2 and 7.6 -- which means the water is acidic -- there is more free chlorine available to combine with contaminants.

It's a Bad Idea to Keep the pH Low

At first glance, it seems that if keeping the pH of the water low increases the FAC it must be a good thing. Unfortunately, this isn't true for swimmers or for the pool itself. People swimming in acidic water can experience one or more of the following problems:

  • Stinging ears
  • Dry, itching skin
  • Loss of natural oils from the hair and skin.

Acidic water can cause the following damage to the pool:

  • Corrosion of metal parts, such as ladders, railings and screws
  • Damage to the filtration and heating system, including the pump
  • Deterioration of stone, plaster, grout, concrete and tiles.

When acidic water corrodes metal surfaces, the metal particles remain in suspension in the water, but they eventually fall down as sediment and stain the bottom of the pool and the liner.

Ideal pH and Chlorine Levels

Pool professionals generally agree with the Centers for Disease Control that the ideal pH for a swimming pool is between 7.2 and 7.8. Since pH 7 is neutral, this means that the water should always be slightly alkaline. You can raise the pH with baking soda or soda ash, when necessary.

The CDC also recommends keeping the concentration of free chlorine around 1 part per million in swimming pools, and around 3 ppm in hot tubs. Most pool testing kits offer methods to make this determination. You should also test the total chlorine level, which is the amount of FAC plus the amount of combined chlorine. When the concentration of combined chlorine is more than half that of FAC, it's time to shock the pool. You calculate the concentration of combined chlorine by subtracting the concentration of FAC from the total chlorine concentration.

Avoid keeping the chlorine levels too high. This upsets the pool's chemical balance and creates a number of its own problems.