Ask A Scientist Chemistry Archive

name         Douglas
status       other
location     TN

Question -   In the water treatment industry we use Chlorine Gas for
disinfection.  To check for Chlorine leaks we use Ammonium Hydroxide
which gives off a white puff of vapor when it reacts to Chlorine
Gas.  What is the make up of this white vapor?
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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.
It can be impossible to pour ammonium hydroxide and hydrochloric acid 
solutions into a bottle to mix
without filling the bottle with too much fog to see the liquid level.
     Cautions:  this (acid+base) mixing generates substantial heat,
so they should be pre-diluted or poured very slowly.
Both HCl and Ammonia have strong fumes which can hurt your nose, throat, 
lungs;
one must not get a close breath of either.
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.

Note that the effectiveness of this test depends on free availability of 
water vapor.
On a dry day (humidity <15%), of if the leak is very strong,
the Cl2 gas will not immediately find enough water vapor to convert to HCl 
gas.
Also the NH4Cl that forms will not collect as much water to make it more 
visible.
You could get 10 times less mist from the same leak.

Similarly, in a dry atmosphere chlorine gas is more dangerous humans will 
not sense it.
That tart, irritating, dangerous-feeling smell you sometimes notice is 
from HCl gas.
Cl2 gas itself is odorless even though it is strongly oxidizing (damaging 
to tissue).
On a dry day, or near a strong leak, you may never smell the Cl2 gas.

Jim Swenson
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Both ammonia and the leaking chlorine readily absorb water forming
aerosols. The chlorine reacts with water: Cl2 + H2O ----> HOCl + HCl. The
HCl formed reacts with ammonia: HCl + NH3 ----> NH4Cl.
to form ammonium chloride, NH4Cl, mist. It is this mist that you observe.

Vince Calder
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