The Percentage of Chlorine in Sodium Hypochlorite?

Currently, many municipal water treatment facilities use Sodium Hypochlorite (NaOCl) to disinfect water in one of the stages of the water treatment. I have observed over the years that in practice, most users cannot actually measure the percentage of Chlorine in Sodium Hypochlorite before use for appropriate dosing or injection.

What is the most accurate method or instrument that can measure the percentage of Chlorine in Sodium Hypochlorite with a negligible error margin?

Regards,

Justin