3500-K POTASSIUM* 3500-K A. Introduction 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. 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. 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. * Approved by Standard Methods Committee, 1997. Editorial revisions, 2011. Joint Task Group: 20th Edition—See Section 3500-Al. 3500-K B. Flame Photometric Method 1. General Discussion 3. Reagents 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. 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 2. Apparatus Make determination as described in Section 3500-Na.B.4, but measure emission intensity at 766.5 nm. See Section 3500-Na.B.2. 1 POTASSIUM (3500-K)/Potassium-Selective Electrode Method 5. Calculation in 33 laboratories by the flame photometric method, with a relative standard deviation of 15.5% and a relative error of 2.3%. See Section 3500-Na.B.5. 6. Precision and Bias 7. Bibliography 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 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 trode 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. 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 3500K: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 elecTABLE 3500-K:I. CONCENTRATION OF CATIONS INTERFERING VARIOUS CONCENTRATIONS OF POTASSIUM 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 3500K.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 AT 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 3500K.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 Concentration Causing 10% Error mg/L Cation K conc ⫽ 1 mg/L K conc ⫽ 10 mg/L K conc ⫽ 100 mg/L CS⫹ NH4⫹ T1⫹ Ag⫹ Tris⫹ Li⫹ Na⫹ H⫹ 1.0 2.7 31.4 2 765 3 105 356 1 179 3.6* 10 27 314 27 650 31 050 3 560 11 790 2.6* 100 270 3 140 276 500 310 500 35 600 117 900 1.6* * pH. 2 POTASSIUM (3500-K)/Potassium-Selective Electrode Method (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 K⫹ concentration from calibration curve. 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). 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. 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. 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 3