Detecting Chlorine Leakage With Aqueous Ammonia.

In most factory that use a cooling water system or water treatment facility, we often use chlorine gas as disinfection. Due to that, we often detect the leakage of chlorine in a pipe line, but it’s hard to detect because of the gas is not visible, only the smell we can detect.

As a plant operator we often us an aqueos ammonia, to detect the leakage. It will form somekind of fog, when aqueous ammonia meet the chlorine gas. Base on this fog we know where the leakage is. It’s a normal detecting procedure for chlorine leakage. But i’m wondering where’s the gas came from. After searching a while in the internet then i found this :

Chlorine gas makes two kinds of acids in the air:

Cl2(gas) + H2O(humidity) <--> HCl(gas) + HOCl (gas?)

Concentrated Ammonium hydroxide solution has a large equilibrium vapor pressure of Ammonia gas:

NH4OH(aq.) <--> NH3(gas) + H2O(liq.)

Ammonia fumes react avidly with acid gasses:

HCl(gas) + NH3(gas) --> NH4Cl (solid)
[white mist particulates](not sure if HOCl does same thing, or something a little different including redox reaction.)

Ammonium Chloride salt is solid, but aggressively collects water vapor (is hygroscopic):

NH4Cl(solid) + H2O(humidity) <--> NH4(+) + Cl(-) + n.H2O(liq.)
[white mist droplets, more visible]

This fog-making tendency is very strong. "n" in that last formula can be greater than 10, so I like to think of it as chemical amplification, resulting in greater visibility.
We should be more carefull because this (acid+base) mixing generates substantial heat. The fog itself is probably a bit acidic and lung-irritating too, and will usually contain a surplus of one of the stronger constituent vapors. We must use safety equipment such as gas mask and  rubber gloves when detecting chlorine leakage.