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Analytical Method: Urea in milk (Photometry)

Reagents and accessories:
Cat.No. 100807  Trichloroacetic acid for analysis
Cat.No. 116754  Water for analysis
Cat.No. 100573  ortho-Phosphoric acid 85 % for analysis
Cat.No. 100731  Sulfuric acid 95 - 97 % for analysis
Cat.No. 103943  Iron(III) chloride hexahydrate for analysis
Cat.No. 102918  Diacetyl monoxime for analysis
Cat.No. 108156  Thiosemicarbazide for analysis
Cat.No. 114724  Empty cells 16 mm with screw caps
Folded filter
Thermoreactor
 
Reagent 1:
Mix 20 g of trichloroacetic acid and 80 ml of water for analysis and dissolve.
 
Reagent 2:
Mix 100 ml of phosphoric acid 85 %, 100 ml of sulfuric acid 95 - 97 %, and 0.45 g of
iron(III) chloride hexahydrate and dissolve.
 
Reagent 3:
Mix 27 g of diacetyl monoxime and 0.77 g of thiosemicarbazide and homogenize.
 
Sample preparation:
Pipette 50 ml of milk into a 100-ml volumetric flask. With swirling add 20 ml of reagent 1, make up to the mark with water for analysis, and mix. Centrifuge this mixture for 15 minutes (4000 rpm). Also filter the supernatant through a folded filter. The result is a slightly turbid solution; dilute this solution 1 + 49 with water for analysis. The total dilution is 1+99.
 
Analysis:
Mix 5.0 ml of the clarified and diluted solution with 2.0 ml of reagent 2 and 50 mg of reagent 3 and heat in the closed blank cell in the thermoreactor to 100 °C for 6 minutes. Subsequently immediately cool the blank cell to room temperature in an ice-water bath and leave to stand for 10 minutes. In the presence of urea the solution turns red. Swirl the sample and measure the absorbance at 525 nm against a blank solution prepared using the respective reagents.
 
Calculation:
Urea content in mg/l = analysis value in mg/l x 100
 
Parameter list for programming the photometer Spectroquant® NOVA 60, NOVA 400, Pharo 100, and
Pharo 300

 

Method number ----
Method-designation urea
Wavelength 525 nm
Citation urea
Units mg/l
Resolution 0.01
Zero point (E0) 0.000
Slope (E/C) 0.465
Meas. range begin 0.50
Meas. range end 5.00
Reference cell 16 mm round

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

 

Measured absorbance
of samples in A
Result in mg/l
[A sample * 1 / 0.465]
1.000 2.15

 

Parameter list for programming the photometer SQ 118

 

01 Method urea
02 Cat.No. -------
03 Measuring range 0.50 - 5.00
04 Units mg/l
05 Reaction time 6 10 min
06 Cell 16 mm round
07 Wavelength 525 nm
08 Calibration Factor
09 Evaluation Linear
10 Factor 2.15

 
 
 
 
Calculation when measuring the absorbance value using the photometer SQ118 without programming the method in the memory of the instrument:

 

Measured absorbance
of samples in A
Result in mg/l
[A sample * 2.15]
1.000 2.15

 

Materials

     
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