Other Pool Chemicals

Many other chemicals are also required in pool in addition to chlorine.  This is because the pH, total alkalinity and hardness of the water also need to be regulated.

pH

The pH value affects the amount of HOCl formed (see table5. below) as     the disproportionation reaction of chlorine with water is an equilibrium dependent on pH.  The pH of the water therefore determines how effective the chlorine present is at disinfection.

pH 6.5 90% of chlorine will be hypochlorous acid
pH 7.5 50% of chlorine will be hypochlorous acid
pH 8.0 20% of chlorine will be hypochlorous acid


At pH 6.5 however the water would be acidic enough to corrode the metal of the pool circulation system and would be uncomfortable for swimmers.  The ideal pH for a pool is therefore 7.2 to 7.6 as the midpoint is the body's natural pH.  If the pH drifts from this range the pool operator would have to add more chlorine to ensure disinfection is adequate.

At high pH the water will cause stinging eyes and sore throats.  There is also a danger of scale forming on surfaces as at ~pH 8 calcium in the water combines with carbonates to form calcium carbonate, CaCO3, or limescale.  CaCO3 can also form tiny particles which float in the water and give it a cloudy or turbid appearance.

At low pH the acidic water can corrode metals leaving staining metal oxides on surfaces.  Sometimes the precipitated metals can float about in the water and colour some thing it comes into contact with, such as hair!

(pH scale used without permission from www.llv2.terc.edu)

 

Total Alkalinity

This is a measure of the amount of alkaline materials in the water; these are usually bicarbonates but at high pH carbonates and hydroxides can be present as well.  Alkalinity is related to pH as the amount of alkali (or hardness) present will determine how easily changes in pH can occur.  In other words, the alkaline species act as buffers.

If the alkalinity is too low (below 80ppm) rapid fluctuations in pH can occur while above 200ppm the water is too buffered making it difficult to correct thee pH if it drifts from the required value.  The alkalinity needs to be between these values so the pH can be adjusted if necessary but rapid fluctuations do not occur.

 

Water hardness

Water is said to be 'soft' if it has a hardness of less than 50ppm CaHCO3 and an alkalinity of under 30ppm as CaCl25.  The pH of soft water can be unstable but addition of calcium containing chlorinating agents will help to increase water hardness.

 

Chemicals

So some common chemicals also added to pool water include:

Cyanuric acid, or stabiliser is also used.  Low levels of this help prevent wastage of free chlorine through decomposition in sunlight as hypochlorite is very unstable:

                                  

Cyanuric acid associates with hypochlorite (OCl-) forming a stable compound which is not vulnerable to sunlight but will allow the chlorine to sanitise.  Too high a level of this however will impede the effectiveness of added hypochlorous acid as a disinfectant as too much OCl- will be tied up as di- or trichlorocyanuric acid:

                       

 

Alternatively, instead of using hypochlorous acid and stabliser, the chlorinated cyanuric acid compounds (known as dichlor and trichlor for short) can be used as the method of adding chlorine to the pool.  Stabiliser is no longer required as dichlor and trichlor dissociate into hypochlorite and cyanuric acid.