Abstract

 
 
 
 Ammonium ion of digested samples of food, soil, fertilizer and water was determined based on the reaction of the ammonium ion with formaldehyde, and then titrated with sodium hydroxide solution. The optimization of variables such as the amount of formaldehyde and EDTA concentration carried out through univariate analysis. Under optimized conditions, the method allowed ammonium determination with the achieved detection limit of 1.83 mg L-1, a quantification limit of 6.11 mg L-1 and precision of 6.0 - 0.5% for ammonium solutions of 0.400 - 3.773 mg L-1 concentration, respectively. The procedure was validated using the Kjeldahl method. The method was successfully applied to determinate ammonium in samples of water, soil, fertilizer and oat. The proposed procedure resulted in a simple, fast and cost effective method to the determination of ammonium in routine analysis.
 
 
 
Highlights
Nitrogen is an element essential for life as part of the vital processes of plants and animals, this significantly participating in various biological and non-biological processes in the environment (Vargas et al 1993)
The ammonium ion in aqueous solution is in equilibrium with ammonia, according to the reaction represented by equation II (Figure 1)
At the end of the reaction, the H3O+ used is fully recovered, as it is shown in the mechanism of Figure 2 (Solomons, 1992), which illustrates the decrease in pH after the addition of formaldehyde
Summary
Nitrogen is an element essential for life as part of the vital processes of plants and animals, this significantly participating in various biological and non-biological processes in the environment (Vargas et al 1993). The distillation and titration stages are methods traditionally performed, as in the British Standards Institution (1986), for the determination of total nitrogen, in the form of ammonia, from Kjeldahl digests of soils, plants, cereals, food, fertilizer products and many other matrices. The method consists essentially of the digestion of organic matter by hot concentrated sulfuric acid in the presence of potassium sulfate and mercury catalyst, neutralization of the excess of sulfuric acid with the use of methyl red in alkali medium, and the addition of formaldehyde and titrated with alkali, in order to reach the pink color of phenolphthalein In this paper, it is proposed the modification and the optimization of Marcali and Rieman (1946) method as to assess the total nitrogen in order to eliminate the distillation stage in the Kjeldahl method and perform the titration using a pH meter for the measurement of the final point. Several real samples of food, soil, fertilizer and water were processed and the results obtained were compared with those provided by the Kjelddahl reference method
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