Swimming Pool Chlorine

Free Chlorine

Every sanitizer has two key functions, to sanitize (kill bacteria and all living organisms) and

also oxidize (destroy contaminants and waste). The most popular pool and spa sanitizer is

chlorine. Chlorine is also classified as a disinfectant, meaning that it is capable of killing

bacteria, algae and other organic material instantly. All chlorine does the same thing when it

is added to the water, regardless of the type of chlorine added. It forms free available

chlorine. Free chlorine is the form of chlorine that kills bacteria, algae and disease-causing

organisms. It is the attack dog that guards your pool against microbiotic intruders. (In

general, you wouldn’t want a dog in the pool, but this is an exception.)

You must maintain free chlorine at a sufficient level to disinfect potential contaminants on

contact. The more chlorine in the water, the more it can sanitize and oxidize the water.

(Remember that sanitizing and oxidizing are the processes that chlorine uses to keep the

water clear and clean.) However, if the free chlorine level gets too high, it can make the

water uncomfortable for swimmers. The trick is to keep the free chlorine level in the ideal

range. In a swimming pool, keep free chlorine at a minimum of 1 ppm (parts per million) and

a maximum of 10 ppm, with an ideal concentration of 1 to 3 ppm.

In spas the level needs to be maintained at a slightly higher level due to the smaller volume

and higher temperature. The minimum level should be 2 ppm in a spa, again no higher than

10 ppm, and ideally 3 to 5 ppm



We use pH as an index to express how acidic or basic a solution is. (The scientific definition

of pH is “the negative logarithm of the hydrogen ion concentration”.) A pH greater than 7.0

is basic, and a pH lower than 7.0 is acidic. In pools and spas, it is important to maintain the

water in the slightly basic range of 7.2 to 7.8. The National Spa and Pool Institute (NSPI),

the industry association in the United States, has set a standard of 7.2 to 7.6 as the ideal



If pH Is Low:

The water can corrode surfaces, metal equipment or fixtures.

Swimmers and bathers can experience discomfort from burning eyes and itchy


The chlorine may dissipate more quickly.

The water may cause pitting and etching of plaster surfaces.


If pH Is High:

Calcium and metals tend to come out of solution (the opposite of dissolving) at high

pH levels, creating the potential for staining and scale formation. The calcium and

metals will actually create deposits and discoloration on pool walls and equipment.

Swimmers and bathers can experience discomfort from burning eyes and itchy


High pH can contribute to cloudy water



Total alkalinity is the measure of the amount of alkaline buffers (primarily carbonates and

bicarbonates) in your water. These alkaline substances buffer the water against sudden

changes in pH. Total alkalinity is considered the key to water balance. It is the first

parameter you should balance when making routine adjustments to your water.

If you neglect to check the total alkalinity in your pool or spa, you may have trouble

balancing the pH. You may also notice that pH fluctuates suddenly despite your best efforts

to keep it in the ideal range. If the alkalinity is too low, anything introduced to the water will

have an immediate impact on pH. Abrupt shifts in pH can cause scaling or corrosion of

metal equipment and fixtures as well as other problems. When the total alkalinity is high,

the pH has a tendency to drift upward, causing scale to form.

Maintaining an ideal level of alkalinity will protect your pool or spa and its equipment from

the harmful effects of sudden pH fluctuations. Think of the alkalinity as training wheels: it

keeps the pH in balance without allowing it to tip too far to either side. Of course the pH can

still drift upward or downward, but that change will happen gradually as long as the

alkalinity falls within the ideal range. The ideal range of total alkalinity for pools and spas is

between 80 and 120 ppm (mg/L).

When the total alkalinity is too low, add sodium bicarbonate. If the total alkalinity is too high,

you can lower it by using muriatic acid or sodium bisulfate.


Total Chlorine

Chlorine in pool and spa water may be present in two forms. It is free chlorine that does

the hard work of killing bacteria and oxidizing contaminants. (When you add a chlorine

compound like Cal-Hypo or trichlor to your pool, you are actually adding free chlorine.)

When the free chlorine combines with these contaminants, such as oils, swimmer waste

and other organic compounds, it becomes combined chlorine, or chloramines. In pool and

spa water, this form of chlorine has very little sanitizing ability, and no oxidizing ability. You

can think of combined chlorine as a spent bullet.

Total chlorine is just the sum of both combined chlorine and free chlorine. In other words,

(total chlorine) = (free chlorine) plus (combined chlorine)

Knowing your total chlorine and free chlorine levels allow you to calculate combined

chlorine (combined chlorine = total chlorine minus free chlorine). If the total chlorine level is

higher than free chlorine, it is obvious that combined chlorine is present. In that case you

need to shock or superchlorinate your pool or spa. To shock the pool, you add a free

chlorine compound in an extra large dose. The high dosage of free chlorine will actually

oxidize (destroy, burn off) the combined chlorine.



Water hardness occurs as an indirect side effect of various chemical compounds. Calcium

and magnesium are the two primary minerals that make up hardness in water. Like

alkalinity and pH, hardness affects the tendency of the water to be corrosive or scaleforming.

(Scale is a deposit that forms on pool walls and equipment when the mineral

content of the water is too high.) By maintaining the ideal ranges for hardness and

alkalinity, you can keep scale formation to a minimum.

Low hardness levels require immediate attention! They can be very dangerous to your

system. Water that is not properly saturated with hardness—water in which the

hardness level is too low—will be very corrosive, causing significant damage to

metal pipes and fixtures as well as plaster. You must be sure to balance hardness

before adding any sanitizer to the water. Otherwise, the water will become even more

aggressive (corrosive); it can cause severe damage in a short period of time.

When the hardness level is low, increase the hardness immediately to limit the damage of

corrosive water. You can increase the hardness level by adding a chemical like calcium

chloride. When the hardness level is too high, excessive scale formation occurs, and the

water may become cloudy or discolored. Elevated pH and warmer temperatures will

increase scale build-up too. If the hardness level is too high, you can partially drain and refill

with fresh water.

The ideal level of hardness for your pool or spa water is from 200 – 400 ppm (mg/L). You

should test hardness when adding fresh water, and re-test until you have balanced the

water hardness properly. After that, test hardness a minimum of once per month throughout

the season. If you use calcium hypochlorite as a sanitizer, you need to test more frequently

to ensure that the level has not exceeded the upper limit.

Swimming Pool Chlorine