Phosphorus test ascorbic acid reduction method – LaMotte DCL-20 Colorimeter User Manual
Page 23
PHOSPHORUS TEST
ASCORBIC ACID REDUCTION METHOD
QUANTITY
CONTENTS
CODE
60 mL
*Phosphate Acid Reagent
*V-6282-H
5 g
*Phosphate Reducing Reagent
*V-6283-C
1
Pipet, 1 mL, plastic
0354
1
Spoon, 0.1 g
0699
*WARNING: Reagents marked with a * are considered hazardous substances. Material Safety Data Sheets (MSDS) are
supplied for these reagents. For your safety read label and accompanying MSDS before using.
Phosphorus is necessary for the hardy growth of the plant and activity of the cells. It encourages root development, and by
hastening the maturity of the plant, it increases the ratio of grain to straw, as well as the total yield. It plays an important
part in increasing the palatability of plants and simulates the formation of fats, convertible starches and healthy seed. By
stimulating rapid cell development in the plant, phosphorus naturally increases the resistance to disease. An excess of
phosphorus does not cause the harmful effect of excessive nitrogen and has an important balancing effect upon the plant.
RANGE:
0-99 lbs/A
METHOD:
Am mo nium molybdate and an ti mony po tas sium tartrate re act in a fil tered acid me dium with di lute
so lu tion of PO4-2 to form an an ti mony-phosphomolybdate com plex. This com plex is re duced to an
in tense blue col ored com plex by ascor bic acid. The color is pro por tion ate to the amount of phos -
phate pres ent. (Only orthophosphate forms a blue color in this test.) Polyphosphates (and some
or ganic phos pho rus com pounds) may be con verted to the orthophosphate form by sul fu ric acid di -
ges tion. Or ganic phos pho rus com pounds may be con verted to the orthophosphate form by
per sul fate di ges tion.
IN TER FER ENCES:
High iron con cen tra tions can cause pre cip i ta tion of and sub se quent loss of phos pho rus.
PROCEDURE
1. Use the 1 mL pipet (0354) to add 1 mL of the soil filtrate to a clean colorimeter tube (0967) and dilute to the 10 mL
line with deionized water.
2. Select setting 4 on the “Select Wavelength” knob.
3. Insert the tube containing the diluted soil filtrate into chamber and press the “30 Second Read” button. Adjust
“100"%T with the ”Set Blank" knob.
4. Remove the tube containing the diluted soil filtrate and add one measure of *Phosphate Acid Reagent (6282) with the
1 mL pipet (0354), cap tube and mix.
5. With the 0.1 g spoon (0699), add one level measure of *Phosphate Reducing Reagent (6283) and shake until
dissolved. Allow five minutes for full color development. Phosphates exhibit a clear blue color.
6. At the end of the waiting period, insert test sample into colorimeter chamber, press the “30 Second Read” button and
measure %T as soon as reading stabilizes.
7. Consult the chart to find the concentration of Phosphorus (P), in the soil, in pounds per acre.
PHOSPHORUS CALIBRATION CHART (LBS/A)
%T
9
8
7
6
5
4
3
2
1
0
90
0.2
1.0
1.7
2.4
3.1
3.9
4.6
5.4
6.2
6.9
80
7.7
8.5
9.3
10.1
10.9
11.8
12.6
13.5
14.3
15.2
70
16.1
17.0
17.9
18.8
19.7
20.7
21.6
22.6
23.6
24.6
60
25.6
26.7
27.7
28.8
29.9
31.0
32.1
33.2
34.4
35.6
50
36.8
38.0
39.3
40.5
41.8
43.2
44.5
45.9
47.3
48.8
40
50.2
51.8
53.3
54.9
56.5
58.2
59.9
61.7
63.5
65.4
30
67.3
69.3
71.3
73.4
75.6
77.9
80.2
82.6
85.1
87.8
20
90.5
93.3
96.3
99.4
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1985-01 • 10/01