Pool Shock | Pool Chemicals Explained

In this article we provide an outline of the main chemicals used in maintaining a swimming pool.  Much of the information is also relevant for spa and hot tubs and will help you in understanding the best pool chemicals for your needs.  The products described below are widely available.  Check prices online before you make the decision to buy pool chemicals as the prices may be cheaper.  Buying wholesale is also a good way to get hold of cheap pool chemicals, but make sure you store them correctly and be aware that some have a limited shelf life.

Why do we need pool chemicals?

Any pool owner will tell you that pool maintenance requires adjustments to pool chemistry for a number of reason:

• Disease carrying pathogens such as bacteria, viruses and other micro-organisms can grow in un-sanitized water.
• Unbalanced pool chemicals can damage various parts of the pool including the liner and filtration equipment.
• Improperly balanced pool water can irritate the skin, nose  and eyes.
• Water that does not contain the correct pool chemicals can become cloudy and unpleasant smelling

What swimming pool chemicals do you need?

1. A primary sanitizer

To ensure that your pool is hygienic and safe for swimmers you need to introduce a disinfecting agent.  The two main sanitizers used in pools are chlorine and bromine.  There are also saltwater pools but the name is a little misleading as they are not chlorine free.  They simply use a process of in-situ chlorine generation via a process of electrolysis where dissolved salt is used as a store for the chlorine -  instead of adding chlorine directly.

Chlorine is the most popular sanitization method for swimming pools and the product is available in a number of different forms:

• calcium hypochlirite, typically sold in granular form or as tablets
• sodium hypoichlorite, a liquid, often used in pools with surfaces that are sensitive to bleach.
• sodium dichlor, which does not require the addition of cyanuric acid (see section on other chemicals below) and is normally supplied in granular form.
• lithium hypochlorite, usually available as a free-flowing powder.
• Trichlor, mostly sold in tablet form and typically has a high percentage of available chlorine.

Whatever type of chlorine you use, it will react in the water to form hypochorous acid and other chemicals that kill bacteria and other pathogenic organisms through an oxidization reaction.

If bromine is used as the primary sanitizer, it also works through an oxidation process.  Advocates claim that bromine is less harsh and skin and eye irritations are less common that with chlorine.  Adding bromine to the pool is a little more complicated as the product dissolves more slowly than chlorine.  But the process of disinfection is similar.  Bromine reacts with the pool water to form hypobromous acid which acts as the main disinfection agent.   An automatic feeder is generally needed and the overall cost of sanitization can be more expensive.

2) Pool shock treatment

In addition to ongoing sanitization, pool water needs to be “shocked” on a regular basis.  Both chlorine and chlorine free pool shock are available and both involve super-oxidizing the pool water to bring it to a point of breakpoint oxidation (often referred to as breakpoint chlorination in chlorine pools) eradicating contaminants and restoring the disinfectant power of the primary sanitizer.  

3) Stabilizer

In chlorine sanitized pools a stabilization agent is sometimes needed to extend the usefulness of chlorine.  The most commonly used is cyanuric acid and this reacts with the chlorine to form a more stable compound that does not degrade in sunlight.

4) Alkalinity and pH balancing pool chemicals

If the pool water is either too acidic or too alkaline if can cause undesirable side effects.  Acidic water can cause corrosion and damage to metal equipment as well as irritation to swimmers.  Water that is too alkaline can result in water that appears murky and can cause scaling on pool surfaces.  In addition, both chlorine and bromine only work effectively within a narrow band of pH – between 7.2 and 7.8.

The most common chemicals used to reduce high water pH are muriatic acid (ideally in liquid form) and sodium bisufate granules.  But if the total alkalinity of the pool is out of suggested levels, then a pH reducer will also be needed.  If the pH of the pool water is too low this is normally due to low alkalinity which can be adjusted with sodium bicarbonate first before trying to increase the pH.  If the pH does need raising adding a base or alkali should do the trick.

5) Algaecides

Algae can be carried into pool water by the wind and rain.  The three most common types seen in pools are a green floating algae that grows rapidly and can be seen floating on pool surfaces or clinging to the walls, yellow algae which collects as a powdery deposit and black algae which forms in layers and clings strongly to pool surfaces.  Black algae is the hardest to get rid of. Even the best pool vacuum cleaners will struggle to dislodge its long, penetrating roots.

If you keep your pool properly sanitized you should avoid algal blooms. But if they do occur an algaecide may be needed.  The cheapest and most widely used algaecide is quaternary ammonium salts or “quats”.  A stronger form of algaecide is known by the shorthand “polyquats“.  Copper salts and colloidal silver are also sometimes used.

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