Chloramination and detecting chloramine with a total chlorine test.

Rexmundi

Chloramines are produced by the reaction between ammonia and chlorine like this,

NH3 + HOCl → NH2Cl + H2O

I have read papers on water treatment that say that in application you need about 10mg of chlorine to react with each 1mg of ammonia.

Using a jug of household ammonia and a jug of bleach with a spectrophotometer to do the measurements I have prepared two extremely dilute solutions. The first has an ammonia concentration of 2 mg/L and the second has a total chlorine concentration of 10 mg/L and free chlorine concentration of 8 mg/L.


In theory I believe that mixing these two solutions at a 1:1 ratio would initially result in concentrations 50% of the original values because the amount of the substances isn't changing but the volume would double. Then as the reaction between the ammonia and free chlorine took place the concentrations of these two things would decrease until one of them became zero. This reaction changes the free chlorine to chloramine, which is also supposed to be picked up by total chlorine tests, so I thought the final value of total chlorine should be quite close to 5 mg/L.

When really trying it the final results I obtained after taking measurements every few minutes for an hour were 0.2 mg/L for ammonia and 0.3 mg/L for free chlorine. I do not know why these did not go to zero but maybe the reaction just becomes very slow at lower concentrations.
The big problem is that the total chlorine dropped to 0.6 mg/L instead of staying relatively constant.

Where did all the chlorine go?
Is this just an example of experimental yield being much lower than theoretical yield?
Could there be chloramine in the final sample which is just not being seen by the test?

If anyone has any insight I'd love to hear your thoughts.

chemisttree

Could it be that chloramine isn't that stable?

Rexmundi

Chemisttree, thank you, this was very interesting and helpful. As I said I had been mixing the ammonia and chlorine with an excess of Cl but now I have tried it with an excess of ammonia as Audrieth and Rowe suggest in that paper.
As might be expected the final mixture still has a lot of ammonia in it but the free Cl goes to zero and the total Cl stays steady - exactly what I was looking for.