А вы качественно или количественно хотите определить? Из того, что попалось под руку - статья в ЕР (Европейской Фармакопее), но там не сточная вода анализируется.
Думаю, если уделить больше времени поиску по моей библиотеке, то можно найти больше и точнее.
Вот держите методику на качественное определение:
1) добавить раствор 125 г/л азотной кислоты к анализируемому раствору. Должен выпасть белый кристаллический осадок в случае присутствия мочевины.
Количественное определение [EP 7., V.2, Urea 0743, p. 3176]
ASSAY
Dissolve 0.2000 g in water R and dilute to 50.0 mL with the same
solvent. Introduce 1.0 mL of this solution into a combustion
flask. Add 4 g of a powdered mixture of 100 g of dipotassium
sulfate R, 5 g of copper sulfate R and 2.5 g of selenium R,
and 3 glass beads. Wash any adhering particles from the neck
into the flask with 5 mL of sulfuric acid R, allowing it to run
down the sides of the flask, and mix the contents by rotation.
Close the mouth of the flask loosely, for example by means of a
glass bulb with a short stem, to avoid excessive loss of sulfuric
acid. Heat gradually at first, then increase the temperature
until there is vigorous boiling with condensation of sulfuric
acid in the neck of the flask; take precautions to prevent the
upper part of the flask from becoming overheated. Continue
the heating for 30 min. Cool, dissolve the solid material by
cautiously adding to the mixture 25 mL of water R, cool again
and place in a steam-distillation apparatus. Add 30 mL of strong
sodium hydroxide solution R and distil immediately by passing
steam through the mixture. Collect the distillate in 15 mL of a
40 g/L solution of boric acid R to which has been added 0.2 mL
of methyl red mixed solution R and enough water R to cover
the tip of the condenser. Towards the end of the distillation,
lower the receiver so that the tip of the condenser is above the
surface of the acid. Take precautions to prevent any water on
the outer surface of the condenser from reaching the contents
of the receiver. Titrate the distillate with 0.01 M sulfuric acid.
1 mL of 0.01 M sulfuric acid is equivalent to 0.6006 mg of
CH4N2O