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Add 1 mL 10 000 μg/mL Sc stock standard [(
j
) above], 3 g cesium chloride [(
k
) above], 20 mL
nitric acid [(
d
) above], and 1 mL 0.5% Triton-X [(
n
) above] to a 1 L volumetric flask containing
approx. 500 ml deionized (or equivalent) water. Bring flask to volume with deionized (or
equivalent) water and mix. If Be is used as an internal standard, add 4 mL
of 10 000 μg/mL Be
[(
i
) above] stock standard to obtain a concentration of 40 μg/mL Be.
(p)
2500
μ
g/mL Phosphorus as orthophosphate (PO
4
).–
Commercial custom standard
prepared in a water matrix preserved with a biocide (Inorganic Ventures,
Christiansburg, VA,
USA). Note a commercial stock standard preserved in acid is not acceptable as the acid will
change the matrix of the pH neutral ammonium citrate EDTA and produce erroneous results.
(q)
7500
μ
g/mL Potassium from potassium chloride.–
Commercial custom standard prepared
in a water matrix preserved with a biocide (Inorganic Ventures
). Note a commercial stock
standard preserved in acid is not acceptable as the acid will change the matrix of the pH
neutral ammonium citrate EDTA and produce erroneous results.
C.
Calibration
(a)
Standard solution
.—Prepare calibration standards from potassium dihydrogen phosphate
[Alternative A, B(
e
)], potassium chloride [Alternative A, B(
f
)], and potassium nitrate
[Alternative A, B(
g
)] as recommended in Table
2015.18A
. Several calibration standards are
required since multiple ICP-OES wavelengths are utilized, since some wavelengths are split
into multiple calibration segments, and since a minimum of five points per curve is
recommended. Table
2015.18A
provides the P and K concentrations, expressed as µg/mL and
the percent oxide forms.
(b)
Stock standards
.—A 2500 mg/L custom blend commercial phosphorus standard [Alternative
A, B(
p
)] and a 7500 custom blend commercial potassium standard [Alternative A, B(
q
)] can also
be used, but commercial stock standards preserved in acid should not be used as the acid
changes the pH and matrix of the calibration standards and can produce erroneous results.
Table
2015.18B
provides the details for preparing standards from custom purchased standards.
(c)
ICP-OES calibration
.—Emission intensity for each of the calibration standards is plotted
against concentration. A minimum of five calibration standards is recommended for each
wavelength. Use an internal standard [Alternative A, B(
o
)] to adjust the concentration of the
calibration standards and test solutions. The recommended wavelengths, standards,
concentration ranges, curve fit, and neighboring wavelengths that may produce spectral
interference are listed in Table
2015.18C
. Linear regression is preferred, whenever possible.
Quadratic curve fit may be necessary because of the dynamic range in fertilizer K concentration,
Candidates for 2016 Method of the Year
247