See 7.2.3.

Transfer by pipette into a 100 ml beaker a portion of the filtrate (7.1) containing not more than 20 mg urea. Add 40 ml acetic acid (4.11). Stir with a glass rod for one minute. Allow any precipitate to settle for five minutes. Filter, wash with a few ml acetic acid (4.11). Add 10 ml xanthydrol solution (4.23) to the filtrate drop by drop, stirring continuously with a glass rod. Allow to stand until the precipitate appears, then stir again for one or two minutes. Allow to stand for one and a half hours. Filter through a sintered glass crucible (5.6) which has been previously dried and weighed, using a slight reduction in pressure. Wash three times with 5 ml ethanol (4.28), without trying to remove all the acetic acid. Place in an oven at a temperature of 130°C for one hour (do not exceed 145°C). Allow to cool in a desiccator and weigh.

$\mathrm{\%}\mathrm{N}\left(\mathrm{urea}\right)=\genfrac{}{}{0.1ex}{}{6.67\times \mathrm{m}}{\mathrm{M}}$

where:

• m = weight of the precipitate in grams.

• M = weight of the sample, in grams, present in the aliquot part taken for analysis.

Correct for the blank.

Note: Although biuret will also be precipitated by xanthydrol, this should not give rise to a significant error in the determination since its level is generally low in absolute value in compound fertilisers.

Ureic N can also be calculated as indicated in the following table: