Nitrites chlorine demand.

WolfeNY3213

Member
Apr 10, 2017
8
Sarasota, FL
So after being taught by the industry that ammonia/nitrates were the culprits for high chlorine demand for so many years working in the industry, I have finally learned the truth.

My professor at the University of Florida informed me that it is actually the nitrites that exert chlorine demand in their transient state between ammonia and becoming nitrates. Nitrites are NO2, and in order for them to convert to Nitrates NO3, they need oxygen. Where do they get that oxygen? They strip it off NaOCl.

Just thought I'd share if anybody was interested to hear that.
 

JamesW

TFP Expert
Mar 2, 2011
20,372
The oxidation of ammonia by chlorine produces about 90 % nitrogen and about 10 % nitrate.

If the only product were nitrogen, then the molar ratio would be 1.5 chlorine to ammonia.

The formation of nitrate increases the needed ratio to about 1.75.

Since chlorine is about 5.07 times the mass per mole of nitrogen, the needed ratio of ppm chlorine to ppm ammonia nitrogen is about 8.9.

The chlorine demand comes from oxidizing the nitrogen atom from -3 in ammonia to 0 in nitrogen gas and to +5 in nitrate.

The oxygen can come from OCL or water, but it's not part of the oxidation reaction.

Every chlorine atom can take 2 electrons from the nitrogen atom.

So, it takes 1.5 chlorine moles of chlorine to oxidize one mole of ammonia nitrogen to nitrogen gas and 4 moles of chlorine to oxidize one mole of ammonia nitrogen to nitrate.

Nitrite is an intermediate species where the nitrogen is in the +3 state. So, it takes one mole of chlorine to oxidize one mole of nitrite to nitrate.

Overall, the chlorine demand comes from chlorine oxidizing (removing electrons from) the nitrogen atom to higher oxidation states.

Assuming that you have 100 moles of ammonia, you would need about 175 moles of chlorine to oxidize the ammonia.

10 moles would be needed to oxidize nitrite to nitrate, which is 5.7% of the total chlorine demand.

The oxidation of nitrite to nitrate is a relatively small amount of the total chlorine demand.
 
Last edited:
  • Like
Reactions: gregsfc

JamesW

TFP Expert
Mar 2, 2011
20,372
Nitrates and phosphates can promote algae growth in water with favorable conditions, such as inadequate chlorine.

In those cases, chlorine demand will be increased due to algae growth.

Maintaining adequate chlorine will prevent algae growth.

Nitrates and phosphates are fully oxidized. Since nitrates and phosphates cannot be further oxidized, they present no direct chlorine demand.