It should be selective to NH4+. However, if you have an NH4+ electrode available and with a good reference electrode, there is a simple solution: test it. Make a calibration curve with NH4+ and afterwards add NH2Cl to each standard in the higher level you expect to find in your samples. If your readings more or less stay the same in all concentration levels and your curve is more or less the same after it, there is no significant interference. The best way to be sure, though, is to make at least three readings for each standard in both situations (with and without interferent), calculate averages and standard deviations as well as apply statistical tests to compare results and curves in order to know whether they are statistically different.

1. NH2Cl is a weaker base (pKb = 15) than NH3, so you may ignore the ionic form +NH3Cl as an interferent.

2. But I expect a significant interference due to the production of NH4+, depending on pH and on the presence of reducing reagents. Think to these reactions:

3NH2Cl = N2 + NH4Cl + 2 HCl (decomposition in neutral and alkaline solution) or

2NH2Cl + H+ ⇌ NHCl2 + NH4+

3NHCl2 + H+ ⇌ 2 NCl3 + NH4+ (disproportionation in acidic solution) or

NH2Cl + 2 H+ + 2e → NH4+ + Cl− (reduction in acidic solution)

3. Test directly the electrode answer to a series of diluted solutions of NH2Cl .

4. Be aware of the compound toxicity.

A fellow chemist was finally able to test. After calibrating with NH4+ standards, he used hypochlorite to convert ammonium into monochloroamine at high pH. A 100ppm standard read less than 0.1ppm.

So the answer is no, an ammonium ISE will not respond to monochloroamine. Therefore chlorinating agents are negative interferents.

Good to know! If there is no previous literature on it, publish it as a technical note.

@Stephen

The test must be made at low pH (10) NH4+ is converted to NH3 ; ISE is not responding to neutral forms NH3 and NH2Cl.

Oops, thanks for catching that, Cornel. Yes, the ammonium standard was acidified for stability prior to chlorination.