If your sample contained chlorine residual there will not be any nitrite. Chlorine oxidise nitrise quite rapidly. Anyway thiosulfate doesn't react with nitrite.

Thiosulfate also doesn't react with nitrate and ammonium.

Beware that thiosulfate does disturb several common analytical methods used to determine both nitrate and ammonia. Check with your laboratory's chemist.

Thank you Mr. Anderson. However, there is also ammonia in the water. So, before chlorination, the water sample has both ammonia and nitrite and thus, spiked chlorine splits between nitrite and ammonia. Therefore, after the injection of chlorine there will be both free ammonia, nitrite ion and nitrate ion in the water for a long time. (Cl2:NH-3 molar ratio is 0.5 and NO2-N:NH3-N molar ratio is 1.0). Some papers say that thiosulfate creates additional free ammonia by neutralizing monochloramines and that may interfere the ammonia readings.

I don't follow " spiked chlorine splits between nitrite and ammonia". What do you mean?

Once water is chlorinated nitrite is oxidized to nitrate. Ammonia is completely converted to chloramines in excess of chlorine.

Thiosulfate does bring ammonia back from chloramine. Perhaps this process is slow but you could add a bit of iodide to speed the reaction up.

What I meant by saying ‘spiked chlorine splits between nitrite and ammonia’ is that, when chlorine is injected into the artificial dionized water that has ammonia and nitrite ion in it, some portion of free chlorine instantaneously oxidized some portion of nitrite to nitrate. However, the rest of the injected free chlorine reacts with ammonia and forms monochloramine and dichloramine. But there will be still nitrite in the water because not all the free chlorine reacts with nitrite, but some portion of the free chlorine will form chloramines and chloramines oxidizes nitrite ion to nitrate ion much slower than free chlorine.

I do not know whether I was clear but thank you very much for your answers again..

That is difficult to understand. If it is chlorination there should be excess free chlorine compared to the demand and thus no nitrite left.

I am trying to stay on the left side of the break point curve, where the Cl2:NH3-N molar ratio is between 0.1-1.0. That is also why the injected free chlorine is not enough to oxidize all the nitrite to nitrate instantaneosly. Chlorine will be the ‘bottleneck’of this reaction and it will be shared between nitrite and ammonia and after the reactions that does occur in miliseconds right after the addition of free chlorine, we will have free ammonia, nitrite ion, nitrate ion, and chloramines. My question is, that when we dechlorinate this sample would sodiumthiosulfate creates additional ammonia, while neutralizing the monochloramine in the sample? Thank very much.

I think I got that the essence of your question is that happens to the chloramine with thiosulfate. I tried to answer that as it is reduced it will form ammonium again, but the reaction is not so fast and its progress depends on other solutes in the sample (among other is iodide) so you cannot just assume the conversion is quantitative. If you like to be sure add some iodide to make the reaction quantitative. Iodide catalyzes reduction of chloramine.

Thank you again Professor for your time. I am using DPD-FAS Titrimetric method to determine the chloramines (by adding potassium iodide) and then adding thiosulfate to determine the ammonia concentration. The spectrophotometer that I use (HACK DR3900) has an interference saying that the device shows monochloramine as ammonia during the analysis. That is why I asked that maybe thiosulfate generates ammonia while neutralizing chloramines. I will keep searching or use manual phenate method to determine the ammmonia. Once again thank you for your time I appreciated.