Calculation of Available Chlorine（Active Chlorine）

Calculation of Available Chlorine

I. Definition

The available chlorine content in chlorine-based disinfectants does not refer to the amount of chlorine but rather to the oxidizing capacity of the disinfectant, expressed as the equivalent oxidizing capacity of chlorine. Specifically, it represents the amount of chlorine gas whose oxidizing capacity matches that of a given quantity of the disinfectant when fully reacted with acid. For example, if 100g of calcium hypochlorite reacts completely with hydrochloric acid to release 99.3g of chlorine, the available chlorine content of the calcium hypochlorite is said to be 99.3%. Thus, the available chlorine content reflects the oxidizing capacity of chlorine-based disinfectants. The higher the available chlorine content, the stronger the disinfecting power, and vice versa.

Available chlorine content is typically expressed in milligrams per liter (mg/L) or parts per million (ppm). For instance, 100 ppm indicates that the oxidizing capacity of one liter of disinfectant solution (100g) is equivalent to 0.1g (100 mg) of chlorine. Therefore, it can also be denoted as 100 mg/L or simply mg/L.

When the available chlorine content of a chlorine-based disinfectant is very high, it may be expressed as a percentage. For example, an available chlorine content of 1% corresponds to 10,000 ppm (10,000 mg/L, meaning 10g of available chlorine per liter).

II. Calculation of Available Chlorine

Calculation Based on Electron Transfer

Available Chlorine = Efficiency Coefficient × Chlorine Content

Where:

Efficiency Coefficient = Number of Electrons Gained / Number of Cl Atoms (the number of chlorine atoms in the disinfectant molecule; the number of electrons gained is the net electron change per chlorine atom).

Chlorine Content = Atomic Weight of Chlorine / Molecular Weight of the Disinfectant Compound.

The result represents the number of electrons gained per chlorine atom multiplied by the percentage of chlorine in the disinfectant compound.

Examples:

Cl₂: (2/71) × 35.5 = 1

NaClO: (2/74.5) × 35.5 = 0.953

Ca(ClO)₂: (4/143) × 35.5 = 0.993

ClO₂: (5/67.5) × 35.5 = 2.63

Available chlorine refers to chlorine present in a positive oxidation state in chlorides. Its calculation can be derived from the decrease in oxidation state in redox reactions. By convention, the available chlorine of Cl₂ is defined as 100%. Cl₂ can be viewed as consisting of one Cl⁺ and one Cl⁻. In ClO₂, chlorine is in the +4 oxidation state and must gain 5 electrons to reach the -1 oxidation state. Thus, its available chlorine content is 5 times its actual chlorine content. The actual chlorine content of ClO₂ is 35.5/67.5 ≈ 52.6%, so its available chlorine content is 52.6% × 5 = 2.63. This explains why the available chlorine of ClO₂ is 2.63 times that of Cl₂.

However, if Cl₂ is taken as 100%, chlorine in Cl₂ has an oxidation state of 0 and gains 1 electron to reach -1, while ClO₂ gains 5 electrons. Thus, for the same amount of substance, ClO₂ has 5 times the oxidizing capacity of Cl₂.

The available chlorine in sodium hypochlorite (NaClO) solution is due to hypochlorous acid (HClO). Sodium hypochlorite is a weak electrolyte that undergoes slight hydrolysis in solution, maintaining an ionization equilibrium:

NaClO + H₂O ⇌ HClO + NaOH

or: ClO⁻ + H₂O ⇌ HClO + OH⁻

Therefore, the active component is HClO, which constitutes the available chlorine.

Can the available chlorine of sodium hypochlorite be calculated using the electron transfer method?

In sodium hypochlorite, chlorine changes from +1 to -1, gaining 2 electrons.

Chlorine content = 35.5 / (23 + 35.5 + 16) = 47.65%

Thus, its available chlorine content is 47.65% × 2 = 95.3%.

Calculation Based on Reaction Equations (Experimental Determination)

① The "available chlorine" content is defined as the ratio of the mass of Cl₂ required to oxidize the same amount of I₂ from KI to the mass of the specified compound, often expressed as a percentage. For example, the "available chlorine" of pure LiClO is 121%.

Reactions:

Ca(ClO)₂ + 4HCl → 2Cl₂↑ + CaCl₂ + 2H₂O

Cl₂ + 2KI → 2KCl + I₂

2Na₂S₂O₃ + I₂ → Na₂S₄O₆ + 2NaI

② Available chlorine is calculated based on the amount of chlorine gas produced when the disinfectant reacts with hydrochloric acid.

For sodium hypochlorite reacting with hydrochloric acid:

NaClO + 2HCl → Cl₂↑ + NaCl + H₂O

74.5 g → 71 g

1 g → 0.953 g

Thus, 1g of sodium hypochlorite produces 0.953g of chlorine gas, which aligns with the result from the electron transfer method.