/ TECHNICAL AUXILIARY MATERIALS / Disinfectants

T-751.01.032 [2024-08] Disinfectants containing active chlorine

The microbicidal effect of the chlorine available in disinfectants is mainly based on the irreversible oxidative effect on the cellular components of microorganisms. Structural and enzyme proteins and nucleic acids are so fundamentally altered that microorganisms die after a short exposure time and regeneration of partially damaged cells can be ruled out.

Due to the high reactivity with protein-like cell components, a similar reaction with organic residues in the systems can lead to partial inactivation of the active ingredient, whereby protein residues are more effective than carbohydrates. In general, protein residues have a stronger effect than carbohydrate residues ("the protein error").

In aqueous solutions, chlorine can occur in the form of elemental chlorine (Cl₂), hypochlorous acid (HClO), hypochlorite ions (ClO^-) and oxidizing chlorine-substitution compounds.

Dissolved elemental chlorine, hypochlorous acid and hypochlorite ions are referred to as "free effective chlorine", while the oxidizing chlorine-substitution compounds are called "combined effective chlorine".

The sum of "free effective chlorine" and "combined effective chlorine" is referred to as "effective chlorine".

Equilibrium states dependent on the pH value

Cl₂ + H₂O	⇌	HClO + H⁺ + Cl^-
		Hypochlorous acid
		HClO ⇌ H+ + ClO^-
		Hypochlorite

Cl₂ + 2 OH^-	⇌	Cl^- + H⁺OCl^- + H₂O

2 ClO₂ + H₂O	⇌	HClO₂ + HClO₃
Chlordioxid		Chlorous acid Chloric acid

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Application/Purpose

Determination of available chlorine in disinfectants.

Scope of Application

Suitable for all solutions containing active chlorine with the exception of solutions containing chlorine dioxide.

Principle

Disinfectants containing active chlorine, e.g. hypochlorites, oxidize iodide to iodine, which is titrated with sodium thiosulfate (Na₂S₂O₃) and converted to active chlorine.

Sodium thiosulphate reacts to form sodium tetrathionate (Na₂S₄O₆)

NaClO + 2 NaI+ H₂SO₄	→	I₂ + NaCl + Na₂SO₄ + H₂O
I₂ + 2 Na₂S₂O₃	→	2 NaI+ Na₂S₄O₆