The Importance of pH to Free Chlorine Analysis

When adding Sodium Hypochlorite (Hypo1O, Anolyte, Calcium Hypochlorite, etc...) to water, the hypochlorite will take on two "forms"; Hypochlorous Acid (HOCl-) and (OCI+), these are known as "Free Chlorine". The concentration of each of these two forms is determined by the pH of the water it is added to.

Of these two forms, HOCl- is the form that has the most power as a disinfectant, OCI+ is much less powerful. As pH determines how much of each of these forms Will be present, pH is an important factor in making sure the "free chlorine" is an effective disinfectant.

The analysis of Free Chlorine using test strips or the pink DPD solution methods cannot tell the difference between these two forms of free chlorine, it measures both HOCl- and OCI+. For this reason it important to measure pH and apply a simple factor to calculate amount of "active free chlorine" that is working as a disinfectant.

The relationship between pH and the balance of the forms of free chlorine is shown below in the graph and table provided.

For ease of use, a simple factor can be applied to the free chlorine test result to calculate the "active free chlorine" present.

At a pH of:

  • 7.0 = 75% or 3/4 of the test result is "active free chlorine"

  • 7.5 = 50% or 1/2 of the test result is "active free chlorine"

  • 8.0 = 25% or 1/4 of the test result is "active free chlorine"

  • 8.5 = 12.5% or 1/8 of the test result is "active free chlorine"

As an example, a DPD test reading returned a free chlorine result of 1OOppm, the waters pH Was noted as 8 using the above factors, the percentage of "active free chlorine" (HOCI-) present at this pH is 25%, so 1OOppm 25% = 25ppm of "active free chlorine".

To improve the disinfection capability of the free chlorine, the pH of the water can be lowered using an (sulphuric, hydrochloric, citric acid, etc...) prior to adding the chlorine product. Never add acid to a chlorine product!

If the pH of the water was lowered to 7 the same amount of free chlorine (1OOppm) would raise the of "active free chlorine" to 75ppm, increasing the disinfection power by 3.

To achieve the same effect by adding more hypochlorite alone would have required more than 3 times the amount of hypochlorite.

For further information or assistance on this or alternative methods of disinfection, please contact your Water Treatment Services Representative.

#chemistry #chlorine #disinfection #coolingwatertreatment #thinkchemistry

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