3500-K POTASSIUM*
3500-K A. Introduction
1.
Occurrence and Significance
Potassium (K) is the fourth element in Group IA of the
periodic table; it has an atomic number of 19, an atomic
weight of 39.10, and a valence of 1. The average abundance
of K in the earth’s crust is 1.84%; in soils it has a range of 0.1
to 2.6%; in streams it is 2.3 mg/L, and in groundwaters it has
a range of 0.5 to 10 mg/L. Potassium is commonly associated
with aluminosilicate minerals such as feldspars.
40
Kisa
naturally occurring radioactive isotope with a half-life of 1.3
⫻10
9
years. Potassium compounds are used in glass, fertil-
izers, baking powder, soft drinks, explosives, electroplating,
and pigments. Potassium is an essential element in both plant
and human nutrition, and occurs in groundwaters as a result of
mineral dissolution, from decomposing plant material, and
from agricultural runoff.
The common aqueous species is K
⫹
. Unlike sodium, it does
not remain in solution, but is assimilated by plants and is
incorporated into a number of clay-mineral structures.
2.
Selection of Method
Methods for the determination of potassium include flame atomic
absorption (3111B), inductively coupled plasma (3120), flame pho-
tometry (B), and selective ion electrode (C). The inductively cou-
pled plasma/mass spectrometric method (3125) usually may be
applied successfully (with lower detection levels), even though
potassium is not specifically listed as an analyte in the method. The
preferred methods are rapid, sensitive, and accurate; selection de-
pends on instrument availability and analyst choice.
3.
Storage of Samples
Do not store samples in soft-glass bottles because of the
possibility of contamination from leaching of the glass. Use
acid-washed polyethylene or borosilicate glass bottles. Adjust
sample to pH ⬍2 with nitric acid. This will dissolve potassium
salts and reduce adsorption on vessel walls.
3500-K B. Flame Photometric Method
1.
General Discussion
a. Principle: Trace amounts of potassium can be determined
in either a direct-reading or internal-standard type of flame
photometer at a wavelength of 766.5 nm. Because much of the
information pertaining to sodium applies equally to the potas-
sium determination, carefully study the entire discussion dealing
with the flame photometric determination of sodium (Section
3500-Na.B) before making a potassium determination.
b. Interference: Interference in the internal-standard method
may occur at sodium-to-potassium ratios of 5:1 or greater. Cal-
cium may interfere if the calcium-to-potassium ratio is 10:1 or
more. Magnesium begins to interfere when the magnesium-to-
potassium ratio exceeds 100:1.
c. Minimum detectable concentration: Potassium levels of
approximately 0.1 mg/L can be determined.
d. Quality control (QC): The QC practices considered to be
an integral part of each method can be found in Section 3020.
2.
Apparatus
See Section 3500-Na.B.2.
3.
Reagents
To minimize potassium pickup, store all solutions in plastic
bottles. Shake each container thoroughly to dissolve accumu-
lated salts from walls before pouring.
a. Reagent water: See Section 1080. Use this water for pre-
paring all reagents and calibration standards, and as dilution
water.
b. Stock potassium solution: Dissolve 1.907 g KCl dried at
110°C and dilute to 1000 mL with water; 1 mL ⫽1.00 mg K.
c. Intermediate potassium solution: Dilute 10.0 mL stock potas-
sium solution with water to 100 mL; 1.00 mL ⫽0.100 mg K.
Use this solution to prepare calibration curve in potassium
range of 1 to 10 mg/L.
d. Standard potassium solution: Dilute 10.0 mL intermediate
potassium solution with water to 100 mL; 1.00 mL ⫽0.010 mg
K. Use this solution to prepare calibration curve in potassium
range of 0.1 to 1.0 mg/L.
4.
Procedure
Make determination as described in Section 3500-Na.B.4, but
measure emission intensity at 766.5 nm.
* Approved by Standard Methods Committee, 1997. Editorial revisions, 2011.
Joint Task Group: 20th Edition—See Section 3500-Al.
1
5.
Calculation
See Section 3500-Na.B.5.
6.
Precision and Bias
A synthetic sample containing 3.1 mg K
⫹
/L, 108 mg
Ca
2⫹
/L, 82 mg Mg
2⫹
/L, 19.9 mg Na
⫹
/L, 241 mg Cl
⫺
/L, 0.25
mg NO
2
⫺
-N/L, 1.1 mg NO
3
⫺
-N/L, 259 mg SO
4
2⫺
/L, and 42.5
mg total alkalinity/L (contributed by NaHCO
3
) was analyzed
in 33 laboratories by the flame photometric method, with a
relative standard deviation of 15.5% and a relative error of
2.3%.
7. Bibliography
MEHLICH, A. & R.J. MONROE. 1952. Report on potassium analyses by means
of flame photometer methods. J. Assoc. Offic. Agr. Chem. 35:588.
Also see Section 3500-Na.B.7.
3500-K C. Potassium-Selective Electrode Method
1.
General Discussion
a. Principle: Potassium ion is measured potentiometrically by
using a potassium ion-selective electrode and a double-junction,
sleeve-type reference electrode. The analysis is performed with
either a pH meter having an expanded millivolt scale capable of
being read to the nearest 0.1 mV or a specific ion meter having
a direct concentration scale for potassium.
Before measurement, an ionic strength adjustor reagent is
added to both standards and samples to maintain a constant ionic
strength. The electrode response is measured in standard solutions
with potassium concentrations spanning the range of interest using
a calibration line derived either by the instrument meter or manu-
ally. The electrode response in sample solutions is measured fol-
lowing the same procedure and potassium concentration determined
from the calibration line or instrument direct readout.
b. Interferences: Although most sensitive to potassium, the
potassium electrode will respond to other cations at high
concentrations; this can result in a positive bias. Table 3500-
K:I lists the concentration of common cations causing a 10%
error at various concentrations of potassium chloride with a
background ionic strength of 0.12Nsodium chloride. Of the
cations listed, ammonium ion is most often present in samples
at concentrations high enough to result in a significant bias. It
can be converted to gaseous ammonia by adjusting to pH ⬎
10.
An electrode exposed to interfering cations tends to drift and
respond sluggishly. To restore normal performance soak elec-
trode for1hindistilled water and then for several hours in a
standard potassium solution.
c. Detection limits: Samples containing from 0.1 to 1000 mg
K
⫹
/L may be analyzed. To measure higher concentrations dilute
the sample.
2.
Apparatus
a. Expanded-scale or digital pH meter or ion-selective meter.
b. Potassium ion-selective electrode.
c. Sleeve-type double-junction reference electrode: Fill outer
sleeve with reference electrode filling solution (see 3500-
K.C.3b). Fill inner sleeve with inner filling solution provided
with the electrode.
d. pH electrode.
e. Mixer, magnetic, with a TFE-coated stirring bar.
3.
Reagents
a. Ionic strength adjustor (ISA): Dissolve 29.22 g NaCl in
reagent water and dilute to 100 mL.
b. Reference electrode outer sleeve filling solution: Dilute 2
mL ISA solution to 100 mL with reagent water.
c. Stock potassium solution: See 3500-K.B.3b.
d. Sodium hydroxide, NaOH, 6N.
e. Reagent water: See 1080.
4.
Procedure
a. Preparation of standards: Prepare a series of standards
containing 100.0, 10.0, 1.0, and 0.1 mg K
⫹
/L by making serial
dilutions of the stock potassium solution as in Section 3500-
K.B.3cand 3d.
b. Instrument calibration: Fill reference electrode according to
the manufacturer’s instructions using reference electrode filling
solution. Transfer 100 mL 0.1 mg K
⫹
/L standard into a 150-mL
beaker and add 2 mL ISA. Raise pH to about 11. Stir gently with
magnetic mixer. Immerse electrodes, wait approximately 2 min for
potential stabilization and record meter reading. Thoroughly rinse
electrodes and blot dry. Repeat for each standard solution in order
of increasing concentration. Prepare calibration curve on semiloga-
rithmic graph paper by plotting observed potential in millivolts
TABLE 3500-K:I. C
ONCENTRATION OF
C
ATIONS
I
NTERFERING AT
V
ARIOUS
C
ONCENTRATIONS OF
P
OTASSIUM
Concentration Causing 10% Error
mg/L
Cation K conc ⫽1 mg/L K conc ⫽10 mg/L K conc ⫽100 mg/L
CS
⫹
1.0 10 100
NH
4
⫹
2.7 27 270
T1
⫹
31.4 314 3 140
Ag
⫹
2 765 27 650 276 500
Tris
⫹
3 105 31 050 310 500
Li
⫹
356 3 560 35 600
Na
⫹
1 179 11 790 117 900
H
⫹
3.6* 2.6* 1.6*
* pH.
POTASSIUM (3500-K)/Potassium-Selective Electrode Method
2
POTASSIUM (3500-K)/Potassium-Selective Electrode Method
(linear scale) against concentration (log scale). Alternatively, calcu-
late calibration line by regression analysis.
c. Analysis of samples: Transfer 100 mL sample into a
150-mL beaker and follow procedure applied to standards in ¶ b
above. From the measured response, calculate K
⫹
concentration
from calibration curve.
5.
Precision
Reproducibility of potential measured, over the method’s
range, can be expected to be ⫾0.4 mV, corresponding to about
⫾2.5% in concentration.
6.
Quality Assurance
The slope of the calibration line should be ⫺56 mV/10-fold con-
centration change. If the slope is outside the range of ⫺56 ⫾3 mV, the
electrode may require maintenance (replace filling solutions). If the
proper electrode response cannot be obtained, replace electrode.
The quality control (QC) practices considered to be an integral
part of each method can be found in Section 3020.
Analyze an independent check standard with a mid-range potas-
sium concentration throughout analysis of a series, initially, every
ten samples, and after final sample. If the value has changed by
more than 5%, recalibrate electrode. Analyze a reagent blank at the
same frequency. Readings must represent a lower concentration
than the lowest concentration standard (0.1 mg/L).
7. Bibliography
PIODA, L., V. STANKOVA &W.SIMON. 1969. Highly selective potassium
ion responsive liquid membrane electrode. Anal. Lett. 2 (12): 665.
MIDGLEY,D.&K.TORRANCE. 1978. Potentiometric Water Analysis. John
Wiley & Sons, New York, N.Y.
BAILEY, P.L. 1980. Analysis with Ion-Selective Electrodes. Heyden &
Son Ltd., Philadelphia, Pa.
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