Concentrações de hidróxido de sódio nas soluções de destilação do método de Kjeldahl sobre a quantificação do nitrogênio total

Abstract

The objective of this study was to evaluate the effects of sodium hydroxide concentrations in the distillation stage on the total nitrogen (N) contents of the Kjeldahl method in different materials. Eight samples of different materials were used: four samples with low-N content (sugar cane in natura, cattle feces, corn grain, and corn silage) and four samples with high-N content (sheep carcass, casein, sunflower meal, and soybean meal). Moreover, analytical two standard samples with known N content (HCl-Lysine and ammonium sulfate) were evaluated. The experimental period lasted nine days, where each group of samples (standards, high and low-N content samples) were analyzed for three consecutive days (round of analysis). Four concentrations of sodium hydroxide were used in the distillation step: 300, 400, 500, and 600 g/L. The analytical standards were evaluated separately according to recovery of N by using a model including the fixed effect of sodium hydroxide concentration and the random effect of round of analysis. The low- and high-N samples were also evaluated separately using a model that included the fixed effect of sodium hydroxide concentration and the random effects of round of analysis, samples and sodium hydroxide × samples interaction. A complete recovery of N from standards was observed (P>0.01). There was no effect of samples as well as interaction between sodium hydroxide concentrations and samples on the N contents in the different materials (P>0.01). No effects of the concentrations of sodium hydroxide were observed on the N contents (P>0.01). The variation in sodium hydroxide concentrations did not affect the repeatability of the results. It is concluded that sodium hydroxide concentrations from 300 to 600 g/L in the solution used in thedistillation step of the Kjeldahl method provide complete recovery of N with a similar pattern of precision. Thus, considering non-technical aspects, it is recommended the utilization of a solution with a concentration of 300 g/L due to the lower cost of reagents.