Swimming Pool Terms

Glossary of Swimming Pool Terms

Glossary of Swimming Pool Terms



A chemical compound that releases hydrogen ions in water solution. Chemicals such as muriatic acid or sodium bisulfate used to lower pH or alkalinity.

Acid Demand

A measure of the amount of acid required to reduce pH to a predetermined level. This can be accomplished by use of an acid titration procedure (Acid Demand Test)


Microscopic forms of plant life that enter the pool by rain, wind and dust storms. There are numerous varieties — some are free floating; others grow on walls and surfaces and come in different colors. Some are more resistant to chemical treatment than others. Though harmless to swimmers the presence of algae discolors the water and indicates improper sanitization. Black algae which grows on pool walls and floor can actually crack plaster finish.


Chemicals that prevent and control algae. Some prevent algae growth; others are designed to kill specific types of visible algae growth


The amount of bicarbonate, carbonate and hydroxide compounds present in water solution. A measure of the pH-buffering capacity of water.


A chemical compound of hydrogen and nitrogen that combines with free chlorine in pools to form chloramines, or combined chlorine.

Ammonia Nitrogen

Brought into pools by swimmers: perspiration, urine or waste. Reacts with chlorine to form chloramines. Causes eye irritation.


The Association of Pool and Spa Professionals. A trade organization of people and institutions in the swimming pool and spa industry.


Microorganisms present in all water supplies. Continuously entering the water via swimmers, dust, etc. Without proper sanitization pools are a perfect breeding ground for bacteria, many of which can cause disease or infection.

Balanced Water

The correct ratio of mineral content and pH level that prevents pool water from being corrosive or scale forming.


A chemical which raises pH when added to the water, like sodium carbonate or caustic soda.

Base Demand

A measure of the amount of alkali material required to raise pH to a predetermined level. This can be accomplished by use of a base filtration procedure (Base Demand Test).

Bather Load

The number of individuals using a pool or spa in a 24 hour period and the pool and spas principal source of bacterial and organic contamination.

Break Point Chlorination

The point in a rising chlorine residual at which the concentration of available chlorine becomes great enough to oxidize all organic matter and ammonia compounds in a pool completely. Chlorine added thereafter will be in an uncombined, or free state. Breakpoint is characterized by a sudden drop in total residual available chlorine. The magnitude of the drop depends upon the amount of combined chlorine present and other factors.


A chemical compound containing bromine, sodium or potassium bromine in solution; will produce free bromine if an oxidizer is introduced.


Chemical that resists pH change, e.g. sodium bicarbonate

Calcium Hardness

The calcium portion of the total hardness. About 65-75% of total hardness. Concentrations of calcium determine whether water is soft too little) or hard (too much). Higher hardness levels can cause cloudy water and scale. Lower levels can harm the pool and its equipment.

Caustic Soda

Also known as sodium hydroxide and is used to raise pH.


A group of chlorine ammonia compounds formed when chlorine combines with organic wastes in the water. Chloramines are not effective as sanitizers and are responsible for eye and skin irritation as well as strong chlorine odors (also known as Combined Chlorine).


Chemical sanitizer that kills bacteria and algae.

Chlorine, Combined

The reaction product of chlorine with ammonia or other pollutants, also known as chloramines.

Chlorine Demand

The amount of chlorine necessary to oxidize all organic matter present in pool water, chloramines, bacteria and algae.

Chlorine, Free

Chlorine available to kill bacteria or algae. The amount of chlorine available for sanitization after the chlorine demand has been met. Also known as chlorine residual.


The chemical 2,4,6, trihydroxy-striazine, also known as stabilizer or conditioner. It stops sunlight from dissipating chlorine strength.


Caused by unbalanced and aggressive water. Metal parts are eaten away, usually due to acidity or very soft water conditions.

Cyanuric Acid

Chemical used to prevent the decomposition of chlorine by ultraviolet (UV) light. .

Diatomaceous Earth

White powder composed of fossilized skeletons of one-celled organisms called diatoms. Porous, containing microscopic spaces. Used as a filter medium for swimming pools.


A chemical that will destroy infection-causing organisms.


The preferred regent used in test kits to measure and indicate free available chlorine. The presence of chlorine turns the indicator pink.

Dry Acid

A granular chemical used to lower pH and or total alkalinity. Safer to handle than liquid (muriatic) acid.

Free Chlorine

Also called available, usable chlorine. It is the most active form of chlorine that is free to kill bacteria and algae.

Hardness Water

Refers to the quantity of dissolved minerals, chiefly calcium and magnesium compounds, that may be deposited as scale in pipes, pools and heaters.

Langelier Index

A mathematically derived factor obtained from the values of calcium hardness, total alkalinity, and pH at a given temperature. A Langelier index of zero indicates perfect water balance (ie., neither corroding nor scaling).

Magnesium Hardness

A measure of the magnesium salts dissolved in water – not a factor in water balance.

Muriatic Acid

A dilute solution of hydrochloric acid, used to reduce pH and alkalinity. Also used to remove stain and scale.

Non Chlorine Shock

An oxygen based shocking compound. Non Chlorine shock is fast dissolving so it allows swimming just 15 minutes after use.


National Swimming Pool Foundation. A research, education and safety organization representing both the pool industry and the general public.

Organic Waste

Waste such as saliva, urine, perspiration and sun tan oils which swimmers introduce into the pool or spa. Most organic wastes will not filter out and must be removed by shocking/oxidizing.


Orthotolidine an organic test regent that turns yellow-green in the presence of chlorine, bromine or iodine.


The process of breaking down organic wastes into simpler elemental forms or bi products. Also used to separate combined chlorine and convert it into free chlorine.


The negative logarithm of the hydrogen-ion concentration of a water solution. A pH below 7.0 is considered acid. A pH above 7.0 is considered alkaline. Above 7.8, the water is too alkaline and could cause cloudiness and scale formation. Below 7.2, the water is to acidic and could cause corrosion, plaster etching and vinyl liner cracking. Improper pH also causes chlorine’s germ-killing power and causes swimmer discomfort.

pH of Saturation

The ideal pH for perfect water balance in relation to a particular total alkalinity level and a particular calcium hardness level, at a particular temperature. The pH where the Langelier Index equals zero.

Phenol Red

Chemical agent used for testing pH in the range of 6.8 – 8.4.


Abbreviation for parts per million.


A chemical which disinfects (kills bacteria), kills algae and oxidizes organic matter.

Saturation Index

A mathematical calculation, based on the interrelation of temperature, calcium hardness, total alkalinity and pH, that predicts if the pool water is corrosive, scale forming or natural.


Crust of calcium carbonate, the result of unbalanced pool water. Hard insoluble minerals deposited (usually calcium bicarbonate) which form on pool and spa surfaces and clog filters, heaters and pumps. Scale is caused by high calcium hardness and/or high pH. The regular use of stain prevention chemicals can prevent scale.


Also known as super chlorination or break point chlorination. Ridding a pool of organic waste through oxidization by the addition of significant quantities of a sanitizer.

Soda Ash

Chemical used to raise pH and total alkalinity (sodium carbonate)

Sodium Bicarbonate

Commonly used to increase alkalinity of water and stabilize pH.

Sodium Bisulfate

Chemical used to lower pH and total alkalinity (dry acid).

Sodium Hydroxide

Also known as caustic soda, A by-product chlorine generation and often used to raise pH.

Sodium Hypochlorite

A liquid that provides 12% to 15% available chlorine. One of the most commonly used products for chlorination of pools. Produces hypochlorous acid when added to pool water. Use care when handling.


Chemical used to prevent the decomposition of chlorine by ultraviolet (UV) light.

Super Chlorination

The practice of adding 8 – 10 times the normal chlorine dose to destroy algae or reach breakpoint for the reduction of chloramines.


A method of testing by adding a reagent of known strength to a water sample until a specific color change indicates the completion of the reaction.

Total Alkalinity

The total amount of carbonates, bicarbonates and hydroxides in the pool. Total Alkalinity affects and controls pH. If total alkalinity is to high, pH will be hard to adjust. If it’s to low, pH will be unstable, difficult to maintain within the desired range. The total alkalinity level should be 80 to 150 ppm, depending on sanitizer.

Total Dissolved Solids (TDS)

The accumulated total of all solids that might be dissolved in water.


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