Potable water chlorination & pH changes

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Hi all,

Does chlorination via hypochlorite (food grade) or gaseous chlorine or via chlorine dioxide contribute a significant change in potable water pH before and after dosing?

Much appreciated

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10 Answers

  1. Hi Anvar,

    I suggest you carry out a complete water analysis of the water prior to dosing with Hypochlorite or Chlorine to determine if it contains carbonates and other salts that can affect the pH before you proceed with either Sodium Hypochlorite and Chlorine. 

    Also, performing a simple titration with the water and any of the disinfectants, checking the pH, will help you to know your quantity requirements as well as any pH adjustment measure (alkaline or acidic pH adjuster) to bring the treated water to the required pH value. Remember, analysis of the treated water is also important to ensure that all other parameters are within acceptable limits.

    Regards.

  2. Hi Anvar

    I agree with all replies from the academic perspective, however, considering the low dose requirements for potable free/residual chlorine levels and available electrolytes in the water itself, it will buffer the fluctuation in the PH and the variation will be insignificant

  3. Hi Anvar,

    Please see the below reaction below to answer your question. Dosing Cl2 gas and ClO2 will decrease the pH and NaOCl will increase the pH. And also, sodium hydroxide is a stabilizer for NaoCl, which increase the pH even more. 

    Cl2 + H2O→ HOCl + H+ + Cl–

    2ClO2 + H2O→6 ClO2– + ClO3– + 2H+

    NaOCl + H2O → Na+ + HOCl + OH–

     

    Answered on by

  4. Hi Anvar.

    Sodium hypochlorite causes an increase in the pH value. This increase can also be very substantial after the dosage since the commercial formulation of sodium hypochlorite is stabilized at about pH 12.

  5. If you are using RO water as a water source or a natural water source with low carbonate levels and mineral dissolved salts, then all of them can have an impact on the pH.  If you are treating a water based on borehole or river water with higher levels of mineral salts, then there is little impact on pH.

    1 Comment

    1. thank you. My query is in regards to seawater desalination.  The permeate from the membrane will treated in Calcite filters to reach a overall TDS of 130.  After this we need to maintain some residual chlorine in the tank farm.  After the tank farm, the water will metered and sold.  The catch is we need maintain many parameters, pH is one of those.  So I needed some guidance on selection of post treatment chlorination. What chemical to use, so that pH remains in the range of 7.2 to 8.5.

  7. I think it does especially if the water has more of carbonate and bicarbonate constituents which precipitated at a certain level oxidation by chlorine. 

    1 Comment

  9. The comment by  Dwight Hanson is correct.   You also asked about chlorine dioxide - there will be no pH reactivity with ClO2.  However, it is difficult to work with due to its lack of stability and gas phase toxicity and residual chlorite that can be formed.  

    1 Comment

    1. Thank you. With Chlorine dioxide what are the chances of chlorine by products being formed?

      1 Comment reply

      1. One of the advantages of using chlorine dioxide is that it does not form halogenated disinfection byproducts such as trihalomethanes.    However, you are applying chlorine to reverse osmosis permeate of seawater which will be very low in dissolved organic carbon.   Therefore trihalomethane products will not be formed regardless of which type of chlorine that you choose.   

  10. Sodium and Calcium hypochlorite will raise the pH.  Gaseous Chlorine will lower the pH.  If you are treating at normal levels for potable water the change will be minimal.

    If you are shock chlorinating the change will be greater.

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