[jmc13]

Disclaimer: I am not a chemist so please excuse my ignorance.

So here is the situation. The company I work for does contract blending so we have a very, very diverse product base. This mean means that we have strange waste water issues from time to time. We collect waste water in a 1500 gal tank, adjust the pH to 6.5-8.5 and the send it off to be disposed (it must be in pH range to send off).

Now i have been here since march and we have never had any issues. Yesterday we had a problem with getting the pH in range. No matter how much acid we added it would not drop below ~13. I assume that there must be some buffering happening. Typically we use citric acid and they have been for the past 30 years with out issue. So I had my lab pull a sample and adjust it down to get an idea of what is going on.

She was working with a 2000ml sample and adding 20g of citric acid at a time to adjust it. She had added 100g and there was still little to no change in the pH. When she added the next 20g the sample reacted somewhat violently, foaming up and over flowing. The pH immediately dropped to ~5.5.

We do a lot of NaOCl products so I am worried that it is releasing Cl gas for obvious reasons. We have never experienced this before and I do not know why this is happening all of the sudden. I would greatly appreciate any guidance on what the potential reaction is as well as any suggestions for a different method of lowering the pH that would keep this from happening.

Again, I apologize for my ignorance on this. Thanks in advance.

[Borek]

Have you smelled chlorine?

Anyway, if the pH is around or above 13, there is nothing unusual in the fact pH initially changes very slowly with the addition of acid, and then rapidly. That's what is to be expected, as such solution contains a lot of OH^- and its buffering capacity is pretty high (even if there is no classic buffer made of acid and conjugate base).

[jmc13]

Yea but we do this every week and it has never been like this.

I ran a %Cl on both the sample and the product. the sample was at .17 and 0 for the product so it was certainly Cl gas.

Is there any way to lower the pH with out the liberating the Cl?

Thanks for your reply. I appreciate any help i can get wrapping my head around this.

[Borek]

Honestly - it is not a subject I feel comfortable about.

Some things I feel confident about are: you can get rid of chlorine adding sulfite. If you have to go with pH below 8.5 chlorine evolution is probably unavoidable. The lower the pH, the worse the situation (ie the faster the chlorine evolution). pH changes between pH 8.5-6.5 will be very fast (small amount of acid added will yield a huge change).

[jmc13]

I can appreciate that you don't feel comfortable about the subject and a appreciate your input.

I did a bit of poking around and i came across this equations for sodium sulfite/Cl: Na2SO3 + Cl2 + H2O -> 2NaCl + SO4^-2 + 2H⁺

Perhaps if i added sodium sulfite prior to the pH adjustment and slowly added the citric acid, the sodium sulfite would pull the Cl2 out as it was liberated. Is this plausible?

Also i came across a paper that discussed Sodium Hypocchlorite + sodium sulfite. It presented the equation NaOCl + Na2SO3 -> NaCl + Na2SO4. Which would essentially solve the problem. However, they did mention in the paper that it was an exothermic reaction so depending on how much heat is generated it could potentially melt the tank.

Does this make sense? Am I missing something?

[Borek]

Exact reaction doesn't matter much - whether it is Cl₂ or OCl^-, products are Cl^- and SO₄^2-, so it is basically the same chemistry (and the same stoichiometry per chlorine equivalent), just occurring at different pH.

Adding sulfite first is a must, that's the only way to be sure there will be no chlorine to evolve when the solution is acidified.

I doubt you will be able to melt the tank, as you won't get the temperature above the boiling point of the solution (won't be higher than just a few degrees over 100 °C), but you should definitely watch the temperature and not add sulfite too fast. You don't want the solution boiling to not risk splashes (although I don't know if the reaction is exothermic enough even to get the temperature that high).

[jmc13]

Great. I'm going to give this a shot on a small scale and see how it goes. You have been a huge help to this biologist pretending to be a chemist haha. Thanks a lot for your input.