Oxidising Biocides

Chlorine vs Bromine vs Chlorine Dioxide

The Importance of pH

The pH level of a system significantly impacts the effectiveness and stability of an oxidising biocide. To maximise the efficiency of your biocide, it is essential to monitor pH levels and select the most suitable biocide for those conditions.

Chlorine (Sodium Hypochlorite)

Sodium hypochlorite (NaOCl), a common product used in water treatment, reacts with water to from hypochlorous acid (HOCl), an effective biocide. At a pH greater than 7, HOCl dissociates (splits) into a hypochlorite ion (OCl⁻), which is a less effective biocide, and a hydrogen ion (H+). As the pH continues to increase, so does the dissociation of HOCl, meaning less effectiveness of your biocide treatment. From pH 8 upwards, HOCl is not working efficiently. At this point you may want to consider alternative treatment or lower the system pH.

Bromine (Sodium Bromide)

In systems where the pH has risen above 8 and the chlorine is no longer effective, sodium bromide (NaBr) and other forms of bromine are used. By adding sodium bromide, hypobromous acid (HOBr) is formed via a quick reaction with the HOCl. HOBr is a strong biocide that is less likely to dissociate into ions compared to HOCl at a higher pH. HOBr remains as an effective biocide beyond pH 9, and so, is the preferred choice in systems with relatively high pH.

Chlorine Dioxide

A reliable option as an oxidising biocide is chlorine dioxide (ClO2) due to its effectiveness at high pH ranges that are likely to be encountered in water treatment. Chlorine dioxide is typically generated using sodium chlorite (NaClO2) and hydrochloric acid (HCl). The reaction between these substances produces chlorine dioxide gas which is then dissolved in water to form a chlorine dioxide solution.

*USE BIOCIDES SAFELY. ALWAYS READ THE LABEL*

Please see graph below.

Figure 1: Graph showing pH against percentage active biocide - Hypochlorous Acid (HOCl), Hypobromous Acid (HOBr) and Chlorine Dioxide (ClO₂). Adapted from ‘The Nalco Water Handbook’, 4th ed., McGraw Hill, and ‘Using Chlorine Dioxide for Effective Water Treatment’, A. Gupta and S. Muench, in ‘Process Cooling’, published 4/3/2016.