A formaldehyde-based assay for ammonium N in fertilizers. Interactive exercise.

Abstract

Ammonium salts react with formaldehyde with quantiative release of protons that can be titrated with NaOH solution to phenolphthalein endpoint. In this interactive exercise it is made use of this property to describe an assay for ammonium N in fertilizers. After weighing the sample (selecting one of three weighing ranges), the analyst must dissolve the fertilizer in a volumetric flask of his choice and take an aliquot of this solution for analysis. The addition of excess neutral formaldehyde brings about the release of protons that are titrated with NaOH solution. For calculations, a puzzle-like correct stoichiometric expression must be set up which allows entering the found value of N percentage in the fertilizer sample. This result is finally checked. All the data are generated randomly.