Potassium test tetraphenylboron method – LaMotte DCL-20 Colorimeter User Manual
Page 25
POTASSIUM TEST
TETRAPHENYLBORON METHOD
QUANTITY
CONTENTS
CODE
30 mL
*Sodium Hydroxide, 0.1N
*4004WT-G
5g
*Tetraphenylboron Powder
*6364-C
1
Pipet, 1 mL
0354
1
Spoon, 0.05g
0696
*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.
Potassium is not a component of the structural makeup of plants, yet it plays a vital role in the physiological and
biochemical functions of plants. The exact function of potassium in plants is not clearly understood, but many beneficial
factors, implicating the involvement and necessity of potassium in plant nutrition have been demonstrated. Some of these
factors are: it enhances disease resistance by strengthening stalks and stems; activates various enzyme systems within plants;
contributes to a thicker cuticle (waxy layer) which guards against disease and water loss; controls the turgor pressure within
plants to prevent wilting; enhances fruit size, flavor, texture and development and is involved in the production of amino
acids (the building blocks for protein), chlorophyll formation ( green-color), starch formation and sugar transport from
leaves to roots.
When present in large amounts, ammonia salts will produce a precipitate similar to that produced by potassium. If fertilizer
containing ammonia salts has recently been applied, or if the soil pH is below pH 5.0, perform the ammonia test before
performing the potassium test. A high ammonia nitrogen test result will alert the operator to a probable false high reading
in the potassium test; actual potassium tests will be somewhat lower.
RANGE:
0-500 lbs/A
METHOD:
Po tas sium re acts with so dium tetraphenylboron to form a col loi dal white pre cip i tate in quan ti ties
pro por tional to the po tas sium con cen tra tion mea sured as tur bid ity.
IN TER FER ENCES:
Cal cium and Mag ne sium at very high con cen tra tions.
PROCEDURE
1. Use the 1 mL pipet (0354) to add 2 ml of the soil filtrate to a clean colorimeter tube (0967) and dilute to the 10 mL
line with deionized water.
2. Select setting 1 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 4 drops of *Sodium Hydroxide, 1.0 N (4004) and mix.
5. With the 0.05 g spoon (0696), add 1 level measure of *Tetraphenylboron Powder (6364). Cap the tube and shake
vigorously until all of the powder has dissolved.
6. After standing 5 minutes, shake the tube to suspend any settled precipitate and immediately place it in the colorimeter
chamber, press the “30 Second Read” button and measure the %T as soon as the reading stabilizes.
7. Consult the chart to find the concentration in pounds per acre of Potassium in the soil.
POTASSIUM CALIBRATION CHART (LBS/A)
%T
9
8
7
6
5
4
3
2
1
0
90
22.9
26.5
30.1
33.6
37.1
40.5
43.9
47.3
50.5
53.8
80
57.0
60.1
63.2
66.2
69.2
72.1
75.0
77.8
80.6
83.3
70
86.0
88.6
91.2
93.7
96.2
98.6
100.9
103.2
105.5
107.7
60
109.8
111.9
114.0
116.0
118.0
119.9
121.7
123.6
125.4
127.1
50
128.8
130.5
132.1
133.7
135.3
136.8
138.4
139.9
141.4
142.9
40
144.3
145.8
147.3
148.9
150.4
152.0
153.7
155.3
157.1
159.0
30
160.9
163.0
165.2
167.5
170.1
172.8
175.8
179.1
182.7
186.6
20
191.0
195.8
201.1
207.0
213.5
220.9
229.1
238.2
248.6
260.2
10
273.2
288.0
304.7
323.7
345.4
370.1
398.3
430.8
468.3
511.6
25
1985-01 • 10/01