Teacher asked for this yesterday to test for the presents of Chlorine.
It is available commercially but I needed to make some in a hurry .
Found this You-tube link https://www.youtube.com/watch?v=ztXFWo0DyWk but its bucket Chemistry
Ended up with 1 Gram starch powder , 0.5 Grams Potassium Iodide in 150mls water .
Bring to boil while stirring .
Dip in strips of filter paper and hang up to dry .
To use , moisten strips with distilled water and hold near gas . paper will turn a blue colour depending on concentration of Chlorine .
It is very sensitive . It will turn blue with chlorinated tap water .