Salicylate Method for Ammonia Quantification in Nitrogen Electroreduction Experiments: The Correction of Iron III Interference

Juan José Giner-Sanz,Graham M Leverick,Yang Shao-Horn,Valentín Pérez-Herranz

doi:10.1149/1945-7111/abbdd6

Abstract

The salicylate method is one of the ammonia quantification methods that has been extensively used in literature for quantifying ammonia in the emerging field of nitrogen (electro)fixation. The presence of iron in the sample causes a strong negative interference on the salicylate method. Today, the recommended method to deal with such interferences is the experimental correction method: the iron concentration in the sample is measured using an iron quantification method, and then the corresponding amount of iron is added to the calibration samples. The limitation of this method is that when a batch of samples presents a great iron concentration variability, a different calibration curve has to be obtained for each sample. In this work, the interference of iron III on the salicylate method was experimentally quantified, and a model was proposed to capture the effect of iron III interference on the ammonia quantification result. This model can be used to correct the iron III interferences on ammonia quantification. The great advantage of this correction method is that it only requires three experimental curves in order to correct the iron III interference in any sample provided the iron III concentration is below the total peak suppression concentration.

Highlights

The salicylate method is a modification of the indophenol method, in which sodium salicylate is used as the phenolic compound, sodium hypochlorite is used as the hypochlorite source, and sodium nitroprusside is used as a catalyst
The new feature can be attributed to the formation of Fe(OH)[3] and Fe-Salicylate complex(es) when the reagents of the salicylate method were added to the sample
For a 0.14 mM TAN concentration sample, the 652 nm peak absorbance is around 2.4 when no Fe III is present in the sample, and is around 1.8 when the sample contains 20 mM of Fe III. This hypochromic shift of the 652 nm peak was found to considerably reduce the effective molar attenuation coefficient when Fe III was present in the sample, from 17.4 mM-1 · cm-1 in pure water to 12.0 mM-1 · cm-1 in the 20 mM Fe III solution, which would translate to a 45% error in the ammonia quantification if the calibration curve in water was used in the latter case

Summary

Introduction

Berthelot, observed in 1859 the formation of a blue-colored dye when ammonia, phenol and hypochlorite were mixed,[13,14] a great number of studies have investigated this reaction, which is known nowadays as the Berthelot or indophenol reaction.[15] This term has since been generalized to refer to any reaction in which a phenolic compound reacts with ammonia and a hypochlorite source to form an indophenol-like dye, where the family of dyes is characterized by the indophenol group This highly conjugated group strongly absorbs visible light in the range between 630 nm and 720 nm. The amount of ammonia in a sample can be determined by converting the ammonia contained in the sample into an intense blue indophenol-like dye, which can be quantified by UV–Visible spectrometry.[34,35]

Methods

Results

Conclusion