AOAC Official Method 941.04 Urea and Ammoniacal Nitrogen in Animal Feed

Urease Method
First Action 1941
Final Action
A. Reagents
(a) Defoaming solution.—Dow Corning Corp. Antifoam B Emulsion.
(b) Urease solution.—Prepare fresh solution by dissolving standardized urease in H2O so that each 10 mL neutralized solution will
convert N of ³0.1 g pure urea.

Standardization.—To determine alkalinity of commercial urease preparation dissolve 0.1 g in 50 mL H2O and titrate with 0.1M HCl, using methyl red, 984.13B(c) (see 4.2.09). Add same volume 0.1N HCl to each 0.1 g urease in preparing urease solution. To determine enzyme activity, prepare ca 50mLneutralized1%solution. Add different amounts of solution to 0.1 g samples pure urea and follow with enzyme digestion and distillation as in determination. Calculate activity of urease preparation from amount of this urease solution that completely converted urea, as determined by complete recovery of N by distillation.
(c) Calcium chloride solution.—Dissolve 25 g anhydrousCaCl2 in 100 mL H2O.