Blog

What is Isothiazolone

What is Isothiazolone

  1. Isothiazolone is a powerful broad-spectrum biocide used in open cooling tower systems to control microbial growth, including:
    1. Bacteria (like Pseudomonas and Legionella)
    2. Algae
    3. Fungi
  1. What Is Isothiazolone?
    1. “Isothiazolone” usually refers to a group of related compounds, primarily:
      1. CMIT: 5-chloro-2-methyl-4-isothiazolin-3-one
      2. MIT: 2-methyl-4-isothiazolin-3-one
  • These are often blended (e.g., CMIT/MIT 3:1) and sold under trade names like Kathon, Acticide, or Proxel.
  1. Function in Cooling Towers
    1. Isothiazolones are used to prevent and control biological fouling, which can cause:
    1. Reduced heat transfer efficiency (biofilms act as insulation)
    2. Microbiologically Influenced Corrosion (MIC)
    3. Clogging of nozzles, strainers, and fill material
    4. Legionella risk
  1. Mechanism of Action
    1. Isothiazolones kill microorganisms by disrupting critical cellular functions, specifically:
      1. Enzyme Inhibition (Thiol Reactivity)
        1. Isothiazolones react with thiol (-SH) groups in proteins and enzymes inside microbial cells.
        2. These thiol groups are essential for enzyme function.
        3. When isothiazolones oxidize or bind to these groups, the enzymes are inactivated, and metabolism shuts down.
        4. Result: The cell can’t produce energy or maintain its structure → rapid cell death
      2. Membrane Disruption
        1. Isothiazolones also damage the cell membrane, making it leaky.
        2. This causes loss of vital ions, nutrients, and cellular contents.
        3. The cell can no longer regulate its internal environment, which is lethal.

 

  • Broad-Spectrum Kill
    1. This dual action (enzyme + membrane damage) works against:
    2. Gram-negative and gram-positive bacteria
    3. Fungi
    4. Algae
  1. It’s particularly effective against biofilm-forming bacteria, like Pseudomonas.
  1. Best Practices:
    1. Feed as slug dose or continuous, depending on system loading.
    2. Use test kits or dip slides to monitor biological activity.
  1. If operating at pH > 9
    1. consider switching to biocides like glutaraldehyde, DBNPA, or TTPC.

 

 

 

 

  1. Key Features of Isothiazolone:

Property

Details

Effective Range

1–15 ppm active / 66 to 1000 ppm as product

Spectrum

Broad (bacteria, fungi, algae)

Synergy

Often paired with oxidizing biocides (e.g., bleach, bromine)

Non-oxidizing

Yes

Degrades

Biodegradable under normal conditions

pH Tolerance

Stable up to ~pH 8.5; degrades faster at high pH

Residual Control

Short-lived—requires frequent or continuous feed

  1. Why It Needs Careful Dosing
    1. At high enough concentrations, isothiazolone kills quickly.
    2. But if overdosed or exposed to high pH (>8.5) or with strong strong oxidizers, it:
  1. Degrades quickly
  2. May lose efficacy
    1. Could cause material compatibility issues (especially soft elastomers or certain plastics)
  1. In Practice:
    1. Dosed intermittently (slug dose) or continuously
    2. Often alternated with other biocides (oxidizers or different non-oxidizers) to prevent resistance
    3. Monitored using microbiological testing (e.g., dipslides or ATP meters)