Determination of NH4+ ions using spectrophotometry
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:
Mercury ammonium iodide (yellow-orange)
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.