The Fertilisers (Sampling and Analysis) Regulations 1996

Ureic nitrogen

7.6—7.6.1 Urease method

Transfer by pipette into a 500 ml graduated flask, an aliquot portion of the filtrate (7.1) containing not more than 250 mg of ureic nitrogen. To remove phosphates, add a suitable quantity of saturated barium hydroxide solution (4.18) until further addition does not cause the production of more precipitate. Eliminate excess barium ions (and any dissolved calcium ions) with 10% sodium carbonate solution (4.19). Allow to settle and check whether precipitation is complete. Make up to the mark, mix and filter through a fluted filter paper. Transfer by pipette 50 ml of filtrate into the 300 ml Erlenmeyer flask of the apparatus (5.3). Acidify with 2 M hydrochloric acid (4.20) to pH 3.0, measured by means of the pH meter (5.5). Raise the pH to 5.4 by the addition of 0.1 M sodium hydroxide (4.17). To avoid ammonia losses when hydrolysis by urease occurs, close the Erlenmeyer flask by means of a stopper provided with a dropping funnel and a small bubble trap containing exactly 2 ml standard 0.1 M hydrochloric acid solution (4.21). Introduce through the separating funnel, 20 ml urease solution (4.22). Allow to stand for one hour at 20 – 25 C. Place 25.0 ml of the standard 0.1 M hydrochloric acid solution (4.20) in the dropping funnel, allow to run into the solution, then rinse with a little water. Transfer quantitatively the contents of the bubble trap to the solution contained in the Erlenmeyer flask. Titrate the excess acid using standard 0.1 M sodium hydroxide solution (4.17), until a pH of 5.4 is obtained, measured on the pH meter.

• Remarks

  1.	After precipitation by barium hydroxide and sodium carbonate solutions, make up to the mark, filter and neutralise as quickly as possible.
  2.	The titration may also be carried out using an indicator (4.26), although the change of colour is more difficult to observe.

7.6.2 Blank test

See 7.2.3.

7.6.3 Expression of result

No math image to display

where:

• a = ml of standard solution of sodium or potassium hydroxide (0.1 M) (4.17) used for the blank, carried out in exactly the same conditions as the analysis.

• A = ml of standard solution of sodium or potassium hydroxide (0.1 M) (4.17) used for the analysis.

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

7.6.4 Gravimetric method using xanthydrol

Transfer by pipette into a 100 ml beaker an aliquot 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, using a slight reduction in pressure, through a sintered glass crucible (5.6) which has been previously dried and weighed. 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.

7.6.5 Expression of result

No math image to display

where:

• m = mass of the precipitate in grams.

• M = mass 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.

7.6.6 Difference method

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

Case	Nitric N	Ammoniacal N	Ureic N
1	Absent	Present	(7.2.4) – (7.5.3)
2	Present	Present	(7.3.3) – (7.5.3)