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. 40K is a

naturally occurring radioactive isotope with a half-life of 1.3

 109 years. Potassium compounds are used in glass, fertilizers, 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 photometry (B), and selective ion electrode (C). The inductively coupled 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 depends 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 potassium 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. Calcium may interfere if the calcium-to-potassium ratio is 10:1 or

more. Magnesium begins to interfere when the magnesium-topotassium 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 accumulated salts from walls before pouring.

a. Reagent water: See Section 1080. Use this water for preparing 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 potassium 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.

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5. Calculation

See Section 3500-Na.B.5.

6. Precision and Bias

A synthetic sample containing 3.1 mg K/L, 108 mg

Ca2/L, 82 mg Mg2/L, 19.9 mg Na/L, 241 mg Cl/L, 0.25

mg NO2

-N/L, 1.1 mg NO3-N/L, 259 mg SO42/L, and 42.5

mg total alkalinity/L (contributed by NaHCO3) 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 manually. The electrode response in sample solutions is measured following 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 0.12N sodium 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 electrode for 1 h in distilled 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.3c and 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 semilogarithmic graph paper by plotting observed potential in millivolts

TABLE 3500-K:I. CONCENTRATION OF CATIONS INTERFERING AT

VARIOUS CONCENTRATIONS OF POTASSIUM

Concentration Causing 10% Error

mg/L

Cation K conc  1 mg/L K conc  10 mg/L K conc  100 mg/L

CS1.0 10 100

NH4

2.7 27 270

T131.4 314 3 140

Ag2 765 27 650 276 500

Tris3 105 31 050 310 500

Li356 3 560 35 600

Na1 179 11 790 117 900

H3.6* 2.6* 1.6*

* pH.

POTASSIUM (3500-K)/Potassium-Selective Electrode Method

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(linear scale) against concentration (log scale). Alternatively, calculate 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 Kconcentration

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 concentration 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 potassium 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.

POTASSIUM (3500-K)/Potassium-Selective Electrode Method

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