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Determination of NH4+ ions using spectrophotometry

Dosage principle

In the presence of potassium or sodium hydroxide (alkaline environment), there is a reaction between the Nessler reagent (alkaline potassium iodo-mercurate) and NH3 ions, creating a compound with a color ranging from orange-yellow to brown. This chemical can be analyzed with a spectrophotometric measurement at a wavelength close to 420 nm.

The complex formation reaction is as follows:

H4+ + OH- in excess
NH3 + H2O

2 (HgI4)2- + 2 NH3
Mercury iodide
2 NH3HgI2 + 4 I-
2 NH3HgI2
NH2Hg2I3 + I- + NH4+
Mercury ammonium iodide (yellow-orange)

Illustration of a standard range for the determination of NH4+ ions. A series of 7 beakers is arranged from left to right, each containing a liquid with an increasingly pronounced coloration, and bearing handwritten inscriptions ranging from 0 to 4.

To prevent any interference from any calcium or magnesium ions that might be present, double sodium and potassium tartrate must first be added to the sample.

Warning pictogram
Precautions for use: Filtering the water before dosing is preferable since the presence of suspended matter is capable of falsifying the colorimetric measurement.
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