Chemistry: Where did all the chlorine come from?

Factual Questions
1

At work I mixed up about 2 liters of aqua regina a mix of hydrochloric and nitric acid. I used it to etch stainless steel. When I was done I neutralized it with baking soda and washing soda (sodium bicarbonate and sodium carbonate). I checked the solution with litmus paper and when it indicated pH 7, I dumped the solution down the drain. The next day the factory reeked of chlorine, not enough to bring tears to your eyes but enough to know it was there. How could this happen? shouldn’t the acids have combined with the bases to form stable salts?. Where did all the chlorine come from? :confused:

2

How many kilos of carbonate did you use? About 5 kg to neutralise at a guess?

Maybe:

  1. did not use enough carbonate to neutralise the acid.
  2. there was something in the drains that formed chlorine or chloramine gas
  3. do you have iron pipes? (or should I say ‘did’ you)
  4. did you use enough water to flush the residue away
3

aqua regia always contains a small amount of chlorine probably caused by the oxidation of chloride. Upon neutralisation the chlorine is essentially untouched, so you will smell it for a while afterwards

4

Antechinus There is a storage pit below our biulding that only gets pumped out when full, so my solution had all night to de-gass. scm1001 , so the chlorine is bound to an oxygen atom and after a while that molecule falls apart releasing the chlorine? Or is the chlorine just dissolved in the water like seltzer?

5

Aqua regia decomposes to form Cl2.

HNO3(aq) + 3HCl(aq) --> NOCl(aq) + Cl2(g) + 2H2O

Additionally the nitrosyl chloride will breakdown.

2NOCl(aq) --> 2NO(g) + Cl2(g)

Additionally, nitrogen oxides may have a similar smell to chlorine. I would expect the decomposition to have slowed down after neutralization, but once you dump it down the drain there is no telling what it might come in contact with that could catalyze this reaction. I could easily see dissolved iron from the pipes having an effect.

You might want to look up better ways of disposing spent aqua regia. The neutralizing the acidity is one thing, but the oxidative power is still there. The wrong combination of things in your drain and the results could be quite explosive.

6

Thanks for those who took the time to answer my question. :slight_smile: I will use more caution when disposing of chemicals in the future. I Guess there is more to chemistry than what’s in the textbook!