It is very nice to enjoy a refreshing and relaxing bath in the pool on a hot summer day. We should know that pool water is like a living organism and in order to enjoy a nice bath we should be actively involved in its care and monitoring of some parameters.


The addition of chlorine or chlorine compounds to water is intended to disinfect it. Chlorine has a strong germicidal effect and has prevailed among other disinfection methods, (such as ozone, ultraviolet, bromine or iodine disinfection) for two main reasons. It is significantly cheaper as a chemical compound than other substances while the presence of residual chlorine ensures continuous disinfectant action in cases of subsequent contamination of water.

Forms of chlorine

Chlorine exists in gaseous form, in liquid form and in solid form. Chlorine gas is extremely toxic and poisonous and its use is not allowed except for special cases and only by specialized personnel. The liquid form exists as liquid bleach (with 16% availability in chlorine) and as household bleach (with 5% availability in chlorine). The solid forms use calcium hypochlorite, lithium hypochlorite and chlorinated isocyanuric acid derivatives, which depending on the chlorine atoms in their molecule are marketed under the names Di-chlor and Tri-chlor.

Reactions of chlorine compounds with water

All the aforementioned compounds of chlorine but also chlorine itself, when in contact with water react with it and give an acid, hypochlorous acid (HClO). This is the active form of disinfection. The microbicidal action of hypochlorous acid is due primarily to the ease with which it approaches microorganisms and then to the invasion of their metabolism.

What is the "required" chlorine

When chlorine is added to water, it does not immediately begin its disinfectant action, but initially acts as an oxidizing agent, reacting with various compounds present in the water. The amount of chlorine consumed for this purpose is called the required chlorine.

What is "blocked chlorine"

When the pool water contains ammonia and other nitrogenous compounds that have been introduced into the water by bathers, then chlorine will react with these compounds to form chloramines. Chloramines have an unpleasant odor and cause irritation (stinging and redness) of the eyes and skin. People believe that these effects are due to excessive chlorination of water. They are actually due to chloramines.

How chloramines are destroyed

One way to destroy the chloramines which, in addition to the above, also cloud the pool water is to add an extra amount of chlorine, ie to do hyperchlorination (shock).

What is "free" or "residual chlorine"

The chlorine that remains free in the pool water and which is offered for disinfection and destruction of pathogenic microorganisms is called free or residual chlorine. This chlorine can be found either in the form of hypochlorous acid (HCL) or in the form of hypochlorite ions (OCL -). Which form will prevail and in what percentage, is something that depends on the (pe-ha) of the pool water. But our free chlorine has to face another enemy, especially when it comes to outdoor swimming pools, as it decomposes in bright light and high temperatures.

 According to the international literature, the acceptable limits of chlorine in pool water are 1-3ppm.

 While the pH limits are 7.4-7.6 and the alkalinity should be between 80-120 ppm.

Advantages of Tri-chlor tablets

  • Available in chlorine concentrations up to 90% making it preferred for large pools.
  • During the day it shows reduced chlorine loss.
  • Reduces the growth of algae in the water by forming an invisible protective layer on the surface that reflects sunlight.
  • It dissolves slowly in water, allowing continuous control of the available chlorine.

According to the international literature the acceptable limits of cyanuric acid are less than 40 pp