Question

How can you detect the presence of chlorine in a pesticide?

Answer 1

For example, the reaction of aqueous NaOH with the chlorine yields sodium hypochlorite and sodium chloride NaCNO NaCl:
Cl₂ + 2 NaOH = NaClO + NaCl + H2O
Then silver nitrate AgNO₃ should be added:
NaCl + AgNO₃ = NaNO₃ + AgCl (insoluble white sediment) - it's characteristic reaction of chlorine presence.