As a water disinfectant, the strength of chlorine is measured in terms of:

Concept

The strength of a chlorine-releasing disinfectant is expressed as available chlorine: the amount of active chlorine the product can release for disinfection.

Related measurements describe different things. Total chlorine is a residual measured in treated water, whereas terms such as reactive chlorine and pure chlorine describe chemical species or composition rather than the standard product-strength measure.

Application

The stem asks for the measure used to express the disinfecting strength of chlorine. That measure is available chlorine.

Contrast

• Available chlorine: oxidizing capacity expressed as equivalent elemental chlorine.

• Total chlorine: free chlorine plus combined chlorine in a water sample.

• Reactive chlorine: a broad description of chemically reactive chlorine species.

• Pure chlorine: elemental chlorine considered without dilution or admixture.

Cross-check

Standard disinfectant guidance specifies bleach or hypochlorite concentrations in units such as percent, grams per litre, or parts per million of available chlorine.

Therefore, the strength is measured in terms of available chlorine.