Application Notes Reflectometry – Nitrate in Soil Samples

Reflectometric determination after reduction to nitrite and reaction with Griess reagent

Reagents:
Catalog No. 116971 Reflectoquant® Nitrate Test
Catalog No. 116995 Reflectoquant® Nitrate Test
Catalog No. 117246 Reflectometer RQflex® 20

This application note pertains to RQflex®20 and all discontinued instruments (RQflex® 10, RQflex® plus)

Sample preparation:
Homogenize ca. 100 g (exactly weighed) of soil sample with 100 ml of 0.01 M CaCl2 solution by shaking for 30 minutes. Filter through nitrate-free folded filter.

RQflex® 20 Reflectoquant®
RQflex® 20 Reflectoquant®

Analysis:
Press the reflectometer START key and simultaneously dip the test strip into the sample (15 - 30 °C) for ca. 2 seconds, ensuring that both reaction zones are immersed. Allow excess liquid to run off via the long edge of the strip on to an absorbent paper towel.

Wait 60 seconds and then measure the strip in the reflectometer. The value [mg/l] will be stored automatically.

(Please refer to the RQflex® operating instructions and the package insert of the Reflectoquant® Nitrate Test.)


Calculation:

Nitrate content [mg/kg] = Measured value [mg/l] x Vol. CaCl2 sol.[ml]
Weight of sample [g]


Conversion to kg Nitrate-N/hectare (layer of soil):

kg Nitrate-N/ha = A x BF x 3 x D x 0.226

where:

A = Measured value
BF = Factor for wetness of soil and extraction BF = 1.41 at an extraction ratio of 1+1 and 83 %dry mass
3 = For a 30-cm thick layer of soil
D = Soil density (1.5 kg/dm3)
0.226 = Conversion factor NO3 into NO3-N


Results - comparison with standard method (photometry):

Sample Reflectoquant® [kg Nitrate-N/ha] Standard method [kg Nitrate-N/ha]
1 92 97
2 89 93
3 86 94
4 100 99
5 46 44
6 34 33
7 48 45
8 46 43
9 31 28
10 32 28
11 36 32
12 30 33
13 39 39
14 76 77
15 44 42
16 61 63
17 39 37
18 66 68
19 44 42
20 68 69
21 48 45
22 87 84
23 46 45
24 53 51
25 116 110
26 111 110

 

Influence of the extraction medium on the nitrate recovery

Extraction medium Required value
[mg/l]
Measured value
[mg/l]
Mean
[mg/l]
Recovery
[%]
Distilled water 25
80
25 27 27 24
85 84 82 83
25.8
83.5
103
104
0.015 M KCl 25
80
25 26 24 25
80 81 80 83
25.0
81.0
100
101
0.025 M KCl 25
80
25 22 26 25
81 80 81 79
24.5
80.3
98
100
0.1 M KCl 25
80
17 18 17 17
53 55 56 58
17.0
55.5
68
69
0.01 M CaCl2 25
80
24 24 24 24
80 77 81 80
24.0
79.5
96
99
Lactate extract 25
80
24 23 24 23
74 77 74 78
23.5
75.8
94
95


Preparation of lactate extract:

Dissolve 15.4 g Calcium lactate in 250 ml distilled water.
Dissolve 7.9 g Calcium acetate in 250 ml distilled water.
Dissolve 18.75 g glacial acetic acid in 150 ml distilled water.
Mix the solutions and make up to 1 l.


Reagents:

Catalog No. 104936 Potassium chloride
Catalog No. 102103 Calcium lactate
Catalog No. 109325 Calcium acetate hydrate
Catalog No. 100063 Acetic acid 100 %

Materials

     
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