Analytical Method: Urea in Water and Wastewater, Enzymatic Determination

Reagent 1:
Dissolve 1.5 g of tri-sodium phosphate dodecahydrate, 0.5 g sodium dihydrogenphosphate monohydrate, and
1.5 g Titriplex^® III in 100 ml water for analysis.

Reagent 2:
Urease

Reaction:
Urease causes ammonium to be released from the urea; the ammonium can be photometrically determined according to the indophenol blue method. When ammonium is present in the sample, the sum determination and the determination of the ammonium concentration can be used to calculate the urea concentration.

Measuring range:
0.5 - 25.0 mg/l urea at 690 nm in the 16-mm round cell

Analysis:
To 5.0 ml of the sample solution add 2 drops of reagent 1 as well as 1 drop of reagent 2 and mix; leave to stand for 5 min. Take 0.50 ml of this solution for the determination of ammonium by means of the Spectroquant^® Ammonium Cell Test.

Parameter list for programming the photometer Spectroquant^® NOVA 60, Pharo 100, and Pharo 300

Parameter list for programming the photometer Spectroquant^® Prove 100, Prove 300 and Prove 600

Calculation when measuring the absorbance value using the photometers without programming the method in the memory of the instrument:

Differentiation between urea and ammonium:

A. Measurement of the concentration of ammonium without the addition of reagent 1 and 2: Measurement value A in mg/l NH₄⁺ (using method for Spectroquant^® Ammonium Cell Test)
= Measuring value A in mg/l NH₄

B. Measurement of the sum of ammonium + urea after the addition of reagent 1 and 2: Measurement value B in mg/l NH₄⁺ (using parameter list for programming photometer NOVA 60, Prove and Pharo instruments
= Measuring value B in mg/l urea

C. Calculation of the urea concentration:
urea concentration in mg/l urea = B - (A x 1.66)