Determination of ammonia nitrogen in the urine of small ruminants

The objective of the present work was to compare colorimetric and distillation-titration methods to determine the ammonia-nitrogen (NH3-N) concentration in sheep and goat urine samples. Colorimetric methods used were based on the indophenol reaction, whereas titrimetric methods were based on the alkali distillation of ammonia from the urine and its titrimetic determination with a standard acid. Colorimetric methods were only reliable when urine samples were diluted at least 1 : 20. Both colorimetric and alleali-distillation methods gave quantitative recoveries with standard NH3-N solutions, but when NH3-N was determined in urine samples there was a significant discrepancy between analytical methods on the measured concentrations. These were between 1.3 and 10-fold (on average 2.6-fold) higher with alkali distillation than with the colorimetric method. The difference between concentrations measured by both analytical methods was significantly related (R2 = 0.990; p < 0.001) to the concentration of urea in the urine samples. To study the effect of urea concentration, standard solutions containing variable concentrations of urea and NH3-N were prepared, and NH3-N concentration determined by both methods. Concentrations measured by colorimetry were similar to the actual concentrations irrespective of the urea concentration of the solutions. In urea-free solutions, alkali-distillation methods gave values similar to the actual concentrations, but when urea was present observed values were consistently higher than the expected concentrations. The overestimation increased with the urea concentration of the solutions, with a significant (P < 0.001) relationship between both variables. Colorimetric methods were more reliable to measure NH3-N concentrations in urine samples containing high urea concentrations.