pH Level, Temperature, Concentration - Factors in Chlorine Sterilizing

pH Level

When chlorine is dissolved in water it is rapidly hydrolysed to hydrochloric acid and hypochlorous acid, which in turn are partially ionised. The Chlorine Cl₂ ion hypochlorous acid (H OCl), and hypochlorite ions (OCl) from the hypochlorous acid exist in equilibrium. However their relative proportions are governed by concentration, pH level and temperature.

Similarly when bleaching powder or calcium hypochlorite 70% is added, hypochlorous acid is formed as follows:

Ca (O Cl₂) + 2H₂O > 2HO Cl + Ca(OH)₂

This also holds true for sodium hypochlorite and the chlorinated cyanurates.

The pH level of the water is the most important factor in maintaining good conditions in the pool.

It can be seen that as the pH rises the killing rate of bacteria decreases.

pH	% Free Chlorine as Hypochlorous Acid
pH 7.0	72%
pH 7.5	45%
pH 8.0	20%
pH 8.2	15%

pH level can be described as the measure of activity of the acidity or alkalinity in the water, but not the quantity.

Reserve Alkalinity, however, is a measure of the actual quantity of soluble alkali salts in the water and is normally expressed as the equivalent parts per million (ppm)of calcium carbonate.

For normal swimming pool operation the pH level should be kept within the limits of 7.5 - 8.2

The pH level of the water has a very important bearing on the ratio of mono, di and trichloramines, which exist together during break-point chlorination.

At too low a pH for example, pH7, an appreciable quantity of nitrogen trichloride (trichloramine) can exist side by side with free chlorine after point B on the curve and will not oxidise out of solution; the concentration increasing as the free chlorine level rises. As the pH rises, the amount of di and trichloramines formed during breakpoint become less. However at pH of 8.0 and over the monochloramines formed initially exist for a time with free chlorine and is then oxidised to nitrogen without the apparent intermediate formation of di and trichloramines, which cause eye stinging and the so-called odour of chlorine.

The efficiency of free chlorine as a sterilizing agent is practically all due to the hypochlorous acid, see chart below.