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.