How can I measure the chlorine content?

To determine the residual Halamid® concentration in water one can do Halamid ion testing in which you measure the chlorine content: Chlorine can be easily measured using strips, like you can find here.This gives you a rough indication if chlorine is still present at ppm level. You could also use a titration test like:

This gives you an exact figure. There are also electronic devices available in the market: