SKALAR METHODS

ANALYSIS: UREA
RANGE: 5 - 500 µg N/liter
SAMPLE: DRINKING, SURFACE, FRESH,
WASTE & SEA WATER
SAN++
Catnr. 612-001 issue 011912/MH/99270483

PRINCIPLE

The automated procedure for the determination of Urea is based on the following reaction; the sample is
diluted in a sodium chloride solution. Hereafter an acid catalyst reagent is added. The analytical stream is
heated to 90°C, where the chromogen is formed between the urea and the diacetyl monoxime. The
presence of semicarbazide intensifies the colour. The absorbance is measured at 520 nm.

LABORATORY FACILITIES

1. Maximum power consumption depending on the analyser configuration, 2000 VA. Check voltage at the
back of the instrument before installation.
2. Facilities for chemical wastes. Check environmental regulations for proper disposal of waste.

PROCEDURE SAMPLE PREPARATION

Water no. 1.1.3

Field of application

Sample preparation for the determination of total alkalinity, aluminium, amino acids, ammonia, anionic
surfactants (MBAS), bromide, calcium, C.O.D., chromium, conductivity, D.O.C., ethanol, hardness,
magnesium, manganese, methanol, nitrate + nitrite, nitrite, nonionics, ortho-phosphate, potassium
permanganate, potassium, total-N (UV digestion), protein, silicate, sodium, volatile acids and urea in water.

Principle

The sample is not preserved and analysed as soon as possible, at least within 24 hours after collection.
Store the sample at 4°C till analysis.

Note : The sample preparation in this method is only meant to be an example procedure. It requires
validation on its correct procedure for all types of samples to be analyzed, which is the responsibility of the
user.

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REAGENTS

A. Sodium chloride solution

Required chemicals: Sodium chloride ........... 150 g. Preparation: Dissolve the sodium chloride in ± 800 ml
NaCl distilled water. Fill up to 1 liter with
Distilled water * ....... 1000 ml. distilled water, add the Brij 35 and mix.
H2O
Brij 35 (30%)................... 3 ml. Note: Solution is stable for 1 week. Store at
4°C when the solution is not used.

B. Acid reagent

Required chemicals: Sodium dihydrogen Preparation: Dissolve the sodium dihydrogen

phosphate ...................... 50 g. Phosphate in ± 300 ml distilled water. Add
NaH2PO4.H2O very carefully, while cooling and constant
Sulfuric acid ............... 588 ml. swirling, the sulfuric acid. Fill up to 1000
H2SO4 (95-97%) ml with distilled water and mix.
Distilled water * ......... 412 ml.
H2O Note: Solution is stable for 1 month.

C. Colour reagent

Required chemicals: Diacetyl monoxime ..... 8.25 g. Preparation: Dissolve the diacetyl monoxime,
C4H7NO2 semicarbazide hydrochloride, manganese
Semicarbazide (II) chloride and the potassium nitrate in ±
hydrochloride ........... 100 mg. 400 ml distilled water. Fill up to 500 ml
CH6ClN3O with distilled water and mix thoroughly.
Manganese(II)
chloride ..................... 26.15 g. Note: Solution is stable for 1 week.
MaCl2.4H2O
Potassium nitrate...... 500 mg.
KNO3
Distilled water * ......... 500 ml.
H2O

D. Rinsing liquid sampler

Required chemicals: Distilled water * Note: Refresh weekly.

H2O

STANDARDS

Stock solution 100 mg N/liter

Required chemicals: Urea........................ 214.4 mg. Preparation: Dissolve the urea in ± 800 ml distilled
CH4N2O water. Fill up to 1 liter with distilled water
Distilled water * ....... 1000 ml. and mix.
H2O
Note: Solution is stable for 2 weeks.

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Working standards

500 µg N/liter: Dilute 0.5 ml stock solution 100 mg N/liter to 100 ml with rinsing liquid sampler.
400 µg N/liter: Dilute 0.4 ml stock solution 100 mg N/liter to 100 ml with rinsing liquid sampler.
300 µg N/liter: Dilute 0.3 ml stock solution 100 mg N/liter to 100 ml with rinsing liquid sampler.
200 µg N/liter: Dilute 0.2 ml stock solution 100 mg N/liter to 100 ml with rinsing liquid sampler.
100 µg N/liter: Dilute 0.1 ml stock solution 100 mg N/liter to 100 ml with rinsing liquid sampler.

Note:
1. Prepare the working standards fresh daily.
2. The water used for the reagents and standards must be degassed properly before making up the
reagents and standards. Especially water produced by reverse osmosis or ion exchange equipment
contains a lot of gasses, which must be removed by degassing (degassing procedure see operational
remarks and trouble shooting).

CATALOGUE NUMBERS REQUIRED CHEMICALS

Product Supplier and catnr. Danger classification

Sodium chloride Merck 106404

Brij 35 (30%) Skalar SC 13900
Sodium dihydrogen phosphate monohydrate Merck 106346
Sulfuric acid (95-97%) Merck 100731 corrosive
Diacetyl monoxime Merck 102918
Semicarbazide hydrochloride Merck 107722 toxic, irritant
Manganese(II) chloride tetrahydrate Merck 105927 harmful
Potassium nitrate Merck 105063 oxidizing
Urea Merck 108487
Sodium hypochlorite solution (13% active chlorine) Acros 21925 corrosive

RECOMMENDED OPERATIONAL SETTINGS

1. System sample time: 100 sec., wash time: 100 sec., air: 0 sec.
2. Module sample time: 60 sec., wash time: 60 sec., air: 0 sec.
3. Calibration type: 1st order.

OPERATIONAL REMARKS AND TROUBLE SHOOTING

1. The stabilising time of the system is approximately 30 minutes.

2. The sensitivity of the highest standard 500 µg N/liter is ± 0.360 AU.
3. Persons using this Skalar method should be familiar with normal laboratory practice. This Skalar
method does not purport to address all of the safety problems, if any, associated with its use. It is the
responsibility of the user to establish appropriate safety and health practices and to ensure compliance
with any national regulatory conditions.
4. The connection between the sampler and the sample pump tube is made of SA 5141 tube.
5. Avoid any turbidity in the reagents, filter if necessary.
6. After 6 months the amount of Brij 35 can be reduced to 1 ml/liter.
7. If a digital matrix photometer 28505911, is in use, a correction interference filter of 630 nm ± 10 nm is
advised.

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8. The reagent bottles must be rinsed thoroughly with distilled water before refilling with fresh reagents.
This is done to remove precipitation of micro organism and other interferences.
9. If solvents or high concentrated acids (> 2M) must be controlled by a rinsing valve, use the solvent/acid
resistant type.
10. Degassing procedures;
a) Good: degas the water with a helium gas flow of approximately 20 liter/hour with aid of a sparger.
Degas 5 liter water for 15 minutes. This procedure removes all dissolved air or, degas the water by
boiling the water for 10 minutes. Cool down to room temperature before usage.
b) Medium: degas the water with a nitrogen gas flow of approximately 20 liter/hour with aid of sparger.
Degas 5 liter water for 15 minutes. This procedure will remove most of the dissolved gasses.
c) Bad: degas the water by ultrasonic waves. This procedure will remove a small part of the dissolved
gasses.

GENERAL REMARKS

1. In most analyses, the first peak coming from the baseline is lower than it should be. The first peak
coming from the baseline is rejected and will not be included in the calculation of the CV-value.
2. If the sample take up volume is less than 1.00 ml/min, a bypass is required to increase the sample
stream to approximately 1.00 ml/min. For SA 5210 debubbler, when the sample is 1.00 ml/min and
higher use 0.23 ml/min debubble tube.

MAINTENANCE

1. Daily; When daily analysis is finished rinse all reagent lines with distilled water for 30 minutes.
2. Weekly; To decontaminate the system, rinse with 1% diluted hypochlorite solution for 30 minutes and
with distilled water for another 30 minutes.
3. Monthly; Replace all pump tubing (or shift pump tube one colour bridge on pump).

MODULE CONSUMABLES

silicone tube catnr. SA 3150 acid flex sleeves catnr. SA 5414

polyethylene tube catnr. SA 3142 acid flex sleeves catnr. SA 5415
polyethylene tube catnr. SA 5141 pump tube 0.80 ml/min. catnr. SA 3030
polyethylene tube catnr. SA 5142 pump tube 1.00 ml/min. catnr. SA 3031
sleeves catnr. SA 5400 pump tube 0.60 ml/min. catnr. SA 3029
sleeves catnr. SA 5401 pump tube 0.42 ml/min. catnr. SA 3028
sleeves catnr. SA 5406 pump tube 1.60 ml/min. catnr. SA 3034

MODULE COMPONENTS

manifold holder catnr. 25200105 reactor catnr. SA 5523

module catnr. SA 5107 debubbler catnr. SA 5250
end block catnr. SA 5109 glass coil catnr. SA 5321
inlet connector catnr. SA 5216 clamps small catnr. SA 5111 7x
inlet connector catnr. SA 5247 2x sinkers catnr. SA 5380
inlet connector catnr. SA 9242 flow cell 50 mm catnr. SA 6275
glass coil catnr. SA 5324 2x filter 520 nm catnr. SA 6537
connector catnr. SA 5221 2x filter 630 nm catnr. SA 6559
glass coil catnr. SA 5325

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REFERENCES

Chemical method (no. 612)

1. Sullivan, D.M., Havlin, J.L., “Flow injection analysis of urea nitrogen in soil extracts:, Soil sci, Soc. Am. J.,
Vol. 55, January-February 1991, page 109-113.
2. Wybenga, D.R., Di Glorgio, J.& Pileggi, V.J. (1971). Clinical Chem., 17, 891-895.
3. Seaton, B & Ali. A. (1984) Med. Lab. Sciences 41, 327-336.
4. ISO 3696:1987, Water for analytical laboratory use. Specification and test methods.
5. ASTM, D1193, Standard Specification for Reagent Water.

Sample preparation (water no. 1.1.3)

1. Environmental Protection Agency, methods for chemical analysis of water and wastes, 1983.
2. Standard Methods for the determination of water and waste water, 17th edition, 1989.
3. ASTM, 1990.
4. International Organisation for Standardisation, ISO-5667-3.

FLOW DIAGRAM
waste
ml/min

Flow cell 50 mm
5321 Filter520 nm
Cor. filter 630 nm

Air
Resample 1.60 9242
5250
waste 5523 5325

90°C
Colour reagent 0.60 5247
5221
Acid reagent 0.42 5247

Air
1.00 5216 5324 5324
Sodium chloride solution
0.60 5221
Sample
sleeved in acid flex

0.80
waste
Sampler

Revision: Rev 1 Rev 2 Rev 3 Method:

Date: Analysis: Urea
Chemists Range: 5-500 µg N/liter
Authorization Sample: (Sea)water
First issued: 06-12-2011

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