Nitrogen determination in urea

Application NoteNo. 251/2016

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KjelDigester K-449 with Scrubber K-415, KjelMaster System K-375/K-376:Nitrogen determination in urea according to the Kjeldahl Method

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1. Introduction

Urea is a common source of nitrogen in all solid nitrogenous fertilizers and it is widely used as a nitrogen release fertilizer. The standard crop-nutrient rating (NPK rating) of urea is 46-0-0. Hence, it contains 46 % elemental nitrogen (N) 0 % elemental phosphorous (P), and 0 % elemental potassium (K). Therefore, it has very low transportation costs per unit nitrogen. [1] Urea decomposes as soon as it is applied to the soil due to the activity of the enzyme urease. In presence of soil, moisture and enzyme, urea normally hydrolyses and converts to ammonium and carbon dioxide (reaction 1). [2] If the soil is totally dry, the decomposition reaction does not occur. The chemical reaction takes place after the urea is dissolved in water. To complete the reaction it takes about 48 hours under alkaline conditions. CO(NH2)2 + H2O + [urease] → 2NH3 +CO2 (reaction 1) Ammonia gas will volatilize to the air if not protected. After the application of urea fertilizer placed

on the soil surface or plant it may loose from 50- 90 % of its nitrogen as ammonia, if not protected. Therefore, the urea is put into the soil and not on the soil to conserve fertilizer nitrogen. [3]

Here, the determination of nitrogen in a urea sample used as a fertilizer is performed using the

KjelDigester K-449 and KjelMaster System K-375 / K-376.

2. Equipment

⋅ KjelDigester K-449 (the parameters used are also valid for the K-446)

⋅ Scrubber K-415 TripleScrub

⋅ KjelMaster System K-375 / K-376

⋅ Analytical balance (accuracy ± 0.1 mg)

3. Chemicals and Materials

Chemicals:

⋅ Sulfuric acid conc. 98 %, Merck (1007482500)

⋅ Titanium BUCHI Kjeldahl Tablet

⋅ Sodium hydroxide 32 %, Merck (106469)

⋅ Boric acid 4 %, 400 g boric acid, Merck diluted to 10 L with deionized water,

pH adjusted to 4.65

⋅ Sulfuric acid 0.25 mol/L, Fluka (35357)

⋅ Neutralization solution for the Scrubber: 600 g sodium carbonate, calcined, technical,

Synopharm (0179420) about 2 mL ethanol and a spatula tip of bromthymol blue, Fluka

(18460) diluted to 3 L with distilled water

For a safe handling please pay attention to all corresponding MSDS.

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Samples:

Urea samples were supplied by a customer (Figure 1). The sample was used as received.

Figure 1: Urea sample used as fertilizer.

4. Experimental

The determination of nitrogen in urea includes the following steps:

⋅ Digestion of the sample, using the KjelDigester K-449.

⋅ Distillation and titration of the sample, using the KjelMaster System K-375 / K-376.

4.1. Digestion method

1. Weigh the sample directly in a 300 mL sample tube as described in Table1

Table 1: Weight of urea sample.

Sample Weight [g]

Urea_1 0.2028

Urea_2 0.2125

Urea_3 0.2079

Urea_4 0.2075

2. Preheat the KjelDigester K-449 according to Table 2. 3. Add 2 Titanium Tablets. 4. Add 20 mL of sulfuric acid (conc. 98 %). 5. Prepare additional blanks, chemicals without sample. 6. Connect the Scrubber K-415 to the K-449 for absorbing the acid fumes created during

digestion. 7. Insert the rack containing the samples into the preheated unit. 8. Digest the samples according to the parameters listed in Table 2.

Table 2: Temperature profile for digestion with the K-449.

Step Temperature [°C] Time [min]

Preheat 350 0

1 420 120

Cooling - 30

9. Let the samples cool down to ambient temperature and start the distillation according to Table 3.

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Figure 2: Digested urea samples.

4.2. Distillation and titration

Distill the samples according to the parameters listed in Table 3.

Table 3: Parameters for distillation and titration with the KjelMaster K-375.

H2O volume 50 mL Titration solution H2SO4 0.25 mol/L

NaOH volume 90 mL Sensor type Potentiometric

Reaction time 5 s Titration mode Standard

Distillation mode Fixed time Titration start time 0 s

Distillation time 240 s Measuring mode Endpoint pH

Stirrer speed distillation 5 End point pH 4.65

Steam output 100 % Stirrer speed titration 7

Titration type Boric acid Titration start volume 0 mL

Receiving solution vol. 60 mL Titration algorithm Optimal

4.3. Calculation

The results are calculated as a percentage of nitrogen. The following equations (1) and (2) are

used to calculate the results.

1000 m

Mfc z V-(Vw

Sample

NBlankSampleN

⋅

⋅⋅⋅⋅

=

) (1)

%N = wN 100 % (2)

wN : weight fraction of nitrogen

VSample : amount of titrant for the sample [mL]

VBlank : mean amount of titrant for the blank [mL]

z : molar valence factor (1 for HCl, 2 for H2SO4)

c : titrant concentration [mol/L]

f : titrant factor (for commercial solutions normally 1.000)

MN : molecular weight of nitrogen (14.007 g/mol)

mSample : sample weight [g]

1000 : conversion factor [mL/L]

%N : percentage of weight of nitrogen

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5. Results

The results of the nitrogen determination are presented in Table 4. The nitrogen content was

analyzed in fourfold. The theoretical nitrogen content was 46.63 %.

Table 4: Results of the nitrogen content in urea.

Urea mSample [g] VSample [mL] %N

Urea_1 0.2028 13.490 46.30

Urea_2 0.2125 14.322 46.93

Urea_3 0.2079 13.851 46.38

Urea_4 0.2075 13.806 46.32

Average [%] – – 46.49

SD – – 0.30

Rsd [%] 0.6

N Recovery [%] 99.70

The mean blank volume (VBlank) was 0.081 mL (n = 4).

6. Conclusion

The determination of nitrogen in urea, or fertilizer, using the KjelDigester K-449 and the KjelMaster

System K-375 / K-376 provides reliable and reproducible results with low relative standard

deviations (rsd).

These results correspond well to the labelled values of the nitrogen in urea which is 46.63%.

7. References

[1] https://en.wikipedia.org/wiki/Urea

[2] http://www.extension.umn.edu/agriculture/nutrient-management/nitrogen/fertilizer-urea

[3] http://extension.usu.edu/files/publications/publication/AG_283.pdf