Iodine vs Chlorine

Iodine and chlorine are both disinfectants, both used to kill pathogens in water and on surfaces. Why do we think Iodine is far superior in practical applications? There are some serious scientific facts that support our vision....

• Active Killing Agent
• Effect of pH
• Reactions to Organic Matter
• Measuring iodine

Active Killing Agent:

Chlorine in solution breaks down to two compounds: Hypolchlorous Acid (HOCl) and Hypochlorite (OCl^-). It is only HOCl that is biocidal. In contrast, iodine breaks down into principally four compounds: Iodine (I₂), Hypoiodious Acid (HIO), Tri-iodide (I₃^-) and Iodate (IO₃^-). Both I₂ and HIO are strong biocidal agents. I₃^- and IO₃^- are only present in very low concentrations and only significant at very high pH >8.5

Effect of pH:

One of the main disadvantages of chlorine use is the need to keep the wash water in a narrow pH range (6.5 to 7.5) to maximize the HOCl compound and consequently effective biocidal action. The effect of even a minor increase or decrease in pH is damaging to effective disinfection. In addition the natural chemical reaction of the wash water with the chlorine solution will be to raise the pH level thereby compounding the problem. In addition, chlorine will tend to gas off as it moves out of the pH range of 6.5 to 7.5 (see Table 1 below)

The active iodine compounds, I₂ and HIO, in combination are much less sensitive to variations in pH being able to remain effective over such a broad range as to make pH monitoring and adjustment in most cases unnecessary. In addition, the Isan System constantly removes the iodide compounds formed which further enhances the concentration of the active iodine compounds remaining in solution.

Table -1: Effect of pH on Active Iodine and Chlorine Compounds

Table-1: Comparison of active iodine and chlorine compounds at different pH at a constant 25^oC. (* the range of percentage of the active iodine compounds is due to the effect of the presence of iodide in the solution – see above)(** At low pH HOCl will disassociate and start to gas off)

Reactions to Organic Matter / Effectiveness in Turbid Water:

Both chlorine and iodine react to organic matter. Turbidity is often made up of Natural Organic Matter (NOM) in water. Organic matter is present in significant levels in fruit and vegetable washing, as well as any surface water source such as dams, lakes and rivers.

These side reactions to NOM consume active biocide thus reducing the available active biocide present to kill the bacteria. These reactions can have some sever impacts on water quality, including causing the formation of Disinfection By Products (DBPs).

However, it has been shown that chlorine reacts three times faster than iodine to proteins and bromine reacts four times faster than iodine. The effect of this reactivity is to substantially and quickly reduce the available chlorine in solution for biocidal activity and therefore increasing the necessary amount of chlorine needed to effectively disinfect the water. This will in turn cause an increase in DBP formation as well as other issues.

Due to iodine’s lack of reactions to NOM it has been proven to be more effective in higher turbidity levels at killing micro-organisms.

Measuring iodine:

The unique behavior of iodine at the face of an redox/reduction electrode means that the biocidal species of iodine; namely diatomic iodine and hypoiodous acid, cause an electric potential to be generated in an exact relationship dependent on their concentration. The relationship which links the electrode potential to the iodine concentration is known as the Nernst equation and means that the iodine concentration can be directly calculated from the voltage potential generated on the electrode face.

The electrodes themselves are flat-surface and low maintenance in comparison to the glass bulb style electrodes traditionally used for oxidation/reduction potential (ORP) measurements. The Ioteq electrodes allow for the accurate measurement of iodine to within one or two parts per million (ppm) in direct contrast to ORP sensors which merely gives a reading of the oxidative milivolt (mV) potential and therefore apparent estimated biocidal activity of the water. For chlorine solutions, the accuracy of these ORP measurements is only maintained up to a concentration of 5 to 8ppm. Any chlorine concentration above this level cannot be accurately measured by ORP. Only a manual measurement from titration can achieve this result.

Statutory regulatory directives for sanitation efficacies are typically written in terms of ppm and therefore the fact that the Isan system actually has a real-time display of ppm is highly significant. The Ioteq electrodes consequently allow for very accurate data logging of the iodine levels during the course of the day a fact highly regarded in the scientific community.

Nernst Equation:

The Nernst equation is a straight line logarithmic curve generated by mV in solution. That is, the more iodine added to a solution the more mVs are generated. These mVs behave as such to follow the Nernst Equation which is a straight line curve. No other biocide behaves in this manner. This means that the Isan System is the only system in the world able to generate very accurate ppm readings in solution.

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