Neutralization of Phosphoric Acid Waste

[emccoy]

Our brewery tried using phosphoric acid to un-flocculate our cells for counting, and I've acquired some waste of it. I talked with the DEP branch of our area and they told me to neutralize it and then it's safe to put down the drain. I planned to neutralize it with a strong base, like sodium hydroxide or potassium hydroxide. The only problem is phosphoric acid is triprotic, and the third pKa of this acid is 12.3,¹ meaning the pH of the final equivalence point is going to be above 12.3,² but I'm having trouble determining what exactly the value would be. I can't think of a pH indicator that I could add during titration to tell me, since Alizarin seems to only work between 11<pH<12.5, and is the highest-pH-working indicator that I can find.³

An entry in ChemLibre Texts also shows that the third equivalence point is difficult to pin-point graphically:⁴


Does anyone know how to determine what the pH of the solution should be at the third equivalence point? Or, if there's a pH indicator that works between 10<pH<14?

Thanks!

1- https://organicchemistrydata.org/han..._pKa_table.pdf
2- https://www.youtube.com/watch?v=cOuqLXezSS0
3- https://openstax.org/books/chemistry...ase-titrations (Figure 14.19)
4- https://chem.libretexts.org/Bookshel...dium_Hydroxide (Figure 4)

[dick murton]

Just how much have you got? You only need to treat a few ml of sample, not the whole batch of yeast for a yeast count. I cannot imagine you need to get the pH anything other than roughly neutral, certainly not up in the pH 10 - 14 as this is way outside (UK) effluent specs. So litmus will get you in the right ballpark. Just keep adding and stirring batches of caustic at a time, then checking. Don't forget if you are adding concentrated caustic to ad it slowly because of potentially explosive boiling, and wear all the safety gear, especially face protection. One of my guys lost an eye when he (first and only time I had ever seen him without it) didn't wear eye protection.

[emccoy]

I only have about 600mL of each at the moment. Yeah, that makes sense. I think I was going in too many brain circles. Thanks!! And will do-- caustic definitely scares me.

Edit: Actually, I'm still a bit confused. Wouldn't the phosphoric acid still be potentially reactive at a pH of 7? It's a weak acid, so neutralizing it doesn't necessarily mean the pH will be 7. This only happens with neutralization reactions between strong acids and bases. Isn't it more important to stop the reactivity of the acid then to keep the pH "neutral," since it won't truly be neutral at that pH?

[dick murton]

For that amount, I wouldn't even bother neutralising it - just put it down the drain. Even on a septic tank / cess pit system, if you put it down the drain with virtually anything else it will not be noticed by the bugs.

[emccoy]

Yeah, but now that I've asked the DEP I have to meet what they say.

I also asked this Q in r/chemistry, and someone explained to me that the last two protons are very weakly acidic and not hazardous, so they agree with you that I can just get the pH neutral and it should be fine!! Thanks for the help! I was thinking those last two protons were way stronger than they are.