Nitrite by Electrode: Path and Control

The electrode method for nitrite in water is built around a nitrite-selective membrane whose potential responds to the activity of NO₂⁻ according to the Nernst equation. The analytical path itself is not elaborate; reliability depends more on controlling the chemical environment at the membrane surface than on instrumentation sophistication.

The measurement system pairs a nitrite ion-selective indicator electrode with a reference electrode, read on an ion meter or a pH/mV meter capable of 0.1 mV resolution. Calibration follows the same sequence as sample measurement: a series of nitrite standards spanning the expected range is prepared, treated identically, and measured in order of increasing concentration. The resulting potential versus log concentration plot should yield a slope in the range of approximately −55 to −60 mV per decade; a slope outside this window signals electrode degradation or a medium mismatch rather than a calibration error per se.

The single most consequential operational step is the addition of an equal volume of nitrite interference suppressor solution (NISS) to both standards and samples. NISS performs three inseparable functions: it adjusts the solution to a pH range in which hydroxide interference is suppressed and nitrite remains predominantly as NO₂⁻; it equalizes ionic strength so that activity coefficients are comparable across standards and samples; and it masks or reduces the effect of coexisting anions such as chloride and nitrate. 

The practical pH window is narrow: below about pH 4.5 a significant fraction of nitrite converts to HNO₂, to which the membrane does not respond, while at higher pH hydroxide begins to compete for membrane sites. Omitting NISS from standards while adding it to samples, or vice versa, places calibration and measurement in different chemical media and renders the calibration inapplicable.

Sample handling before NISS addition also matters. Turbid samples should be filtered to protect the membrane from particulate fouling, and strongly oxidizing or reducing constituents that react with the membrane surface must be absent or removed. Temperature should be held constant within approximately ±1 °C between standards and samples, because electrode potential carries a measurable temperature coefficient; at low nitrite levels a 1 °C difference can translate into a few percent error. Stirring must be steady but not so vigorous as to introduce bubbles onto the sensing surface, which would interrupt the membrane–solution contact. Between readings the electrode is rinsed and blotted, not wiped, to preserve the sensitive membrane.