Percent active chlorine

Percent active chlorine is a unit of concentration used for hypochlorite-based bleaches. One gram of a 100% active chlorine bleach has the same bleaching power as one gram of chlorine. The term "active chlorine" is used because most commercial bleaches contain substantial amounts of chlorine in the form of chloride ions, which have no bleaching properties.

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Liquid bleaches sold for domestic use are typically 3–10% active chlorine, and should be diluted to 1–2% active chlorine before use. Commercial domestic bleaching powder is typically about 40% active chlorine. Values for some industrial solid bleaches are given in the table below.

Values can be higher than 100% because hypochlorite ion have higher oxidation state (+1) than chlorine(0) and so high oxidation power .
Percent active chlorine values have now virtually replaced the older system of chlorometric degrees: 1% active chlorine is equivalent to 3.16 °Chl. Taking the (reasonable) assumption that all active chlorine present in a liquid bleach is in the form of hypochlorite ions, 1% active chlorine is equivalent to 0.141 mol/Kg ClO⁻(0.141 mol/L if we assume density=1). For a solid bleach, 100% active chlorine is equivalent to 14.1 mol/kg  ClO⁻: lithium hypochlorite has a molar mass of 58.39 g/mol, equivalent to 17.1 mol/kg or 121% active chlorine.

Active chlorine values are usually determined by adding an excess of potassium iodide to a sample of bleach solution and titrating the iodine liberated with standard sodium thiosulfate solution.

Cl₂ + 2I⁻ → I₂ + 2Cl⁻

or

ClO⁻ + 2I⁻ + 2 H⁺ → I₂ + H₂O + Cl⁻

then

2S₂O₃²⁻ + I₂ → S₄O₆²⁻ + 2I⁻

From the above equations it can be seen that 2 mole of thiosulfate is equivalent to 70.9 grams of active chlorine.