Determination of potassium

Discussion. Potassium may be precipitated with excess of sodium tetraphenyl-borate solution as potassium tetraphenylborate. The excess of reagent is determined by titration with mercury(II) nitrate solution. The indicator consists of a mixture of iron(III) nitrate and dilute sodium thiocyanate solution. The end-point is revealed by the decolorisation of the iron(III)-thiocyanate complex due to the formation of the colourless mercury(II) thiocyanate. The reaction between mercury( II) nitrate and sodium tetraphenylborate under the experimental conditions used is not quite stoichiometric hence it is necessary to determine the volume in mL of Hg(N03)2 solution equivalent to 1 mL of a NaB(C6H5)4 solution. Halides must be absent. 

Procedure. Prepare the sodium tetraphenylborate solution by dissolving 6.0 g of the solid in about 200 mL of distilled water in a glass-stoppered bottle. Add about 1 g of moist aluminium hydroxide gel, and shake well at five-minute intervals for about 20 minutes. Filter through a Whatman No. 40 filter paper, pouring the first runnings back through the filter if necessary, to ensure a clear filtrate. Add 15 mL of 0.1M sodium hydroxide to the solution to give a pH of about 9, then make up to 1 L and store the solution in a polythene bottle. 

Standardisation. Pipette 10.0 mL of the sodium tetraphenylborate solution into a 250 mL beaker and add 90 mL water, 2.5 mL 0.1 M nitric acid, 1.0 mL iron(III) nitrate solution, and 10.0 mL sodium thiocyanate solution. Without delay stir the solution mechanically, then slowly add from a burette 10 drops of mercury(II) nitrate solution. Continue the titration by adding the mercury(II) nitrate solution at a rate of 1-2 drops per second until the colour of the indicator is temporarily discharged. Continue the titration more slowly, but maintain the rapid state of stirring. The end point is arbitrarily defined as the point when the indicator colour is discharged and fails to reappear for 1 minute. Perform at least three titrations, and calculate the mean volume of mercury(II) nitrate solution equivalent to 10.0 mL of the sodium tetraphenylborate solution. 

Pipette 25.0 mL of the potassium ion solution (about 10 mg K + ) into a 50 mL graduated flask, add 0.5 mL 1M nitric acid and mix. Introduce 20.0 mL of the sodium tetraphenylborate solution, dilute to the mark, mix, then pour the mixture into a 150mL flask provided with a ground stopper. Shake the stoppered flask for 5 minutes on a mechanical shaker to coagulate the precipitate, then filter most of the solution through a dry Whatman No. 40 filter paper into a dry beaker. Transfer 25.0 mL of the filtrate into a 250 mL conical flask and add 75 mL of water, 1.0 mL of iron(III) nitrate solution, and 1.0 mL of sodium thiocyanate solution. Titrate with the mercury(II) nitrate solution as described above. 

This determination is only suitable for students with analytical experience and should not be attempted by beginners. 

1 The sample is vaporised in an air-propane flame and the potassium compounds are atomised. The potassium atoms thus formed emit radiation of which the intensity is measured at a waveiength of 766.5 nm. 

1 This procedure yields a standard curve that is linear up to approximateiy 50 mg/L K. 

2 The detection limit is approximately 2 mg/L in the digest. The determination limit is approximately 6 mg/L (7.7-19 mmol/kg in the dried plant material). 

1 To prevent ionisation interferences, cesium is added to act as an ionisation buffer. 

1 The reproducibility of determinations by this procedure should give, at reasonable control and thorough sample preparation, a coefficient of variation within 5 % -r 5 mmol/kg. 

---

To evaluate the precision for the determination of potassium in blood serum, duplicate analyses were performed on six samples, yielding the following results. 

Now suppose that the determination of potassium chloride and potassium bromide in a mixture is desired. The total halide is determined by Mohr s method or with an adsorption indicator. Let the weight of the mixture be w3 g and w4 g, be the weight of silver nitrate required for complete precipitation,... 

FIGURE 5-17 Flow injection potentiometric determination of potassium in serum. (Reproduced with permission from reference 47.)... 

Ward GM and Heeney HB (1960) A collaborative study of methods for the determination of potassium, calcium and magnesium in plant materials. Canad J Plant Sci 40 589-595. 

Especially sensitive and selective potassium and some other ion-selective electrodes employ special complexing agents in their membranes, termed ionophores (discussed in detail on page 445). These substances, which often have cyclic structures, bind alkali metal ions and some other cations in complexes with widely varying stability constants. The membrane of an ion-selective electrode contains the salt of the determined cation with a hydrophobic anion (usually tetraphenylborate) and excess ionophore, so that the cation is mostly bound in the complex in the membrane. It can readily be demonstrated that the membrane potential obeys Eq. (6.3.3). In the presence of interferents, the selectivity coefficient is given approximately by the ratio of the stability constants of the complexes of the two ions with the ionophore. For the determination of potassium ions in the presence of interfering sodium ions, where the ionophore is the cyclic depsipeptide, valinomycin, the selectivity coefficient is Na+ 10"4, so that this electrode can be used to determine potassium ions in the presence of a 104-fold excess of sodium ions. 

Experience shows that samples weighing 0.2-20g are usually sufficient for dating by this method. The determination of potassium-40 is, at present,... 

Charlton S.C., Fleming R.L., Zipp A., Solid-phase colorimetric determination of potassium, Clin. Chem. 1982 28,1857. 

Ng R.H., Sparks K.M., Statland B.E., Colorimetric determination of potassium in plasma and serum by reflectance photometry with a dry-chemistry reagent, Clin. Chem. 1992 38 1371. 

R.J. Mortimer, P.J.S. Barbeira, A.F.B. Sene, and N.R. Stradiotto, Potentiometric determination of potassium cations using a nickel(II) hexacyanoferrate-modified electrode. Talanta 49, 271-275 (1999). 

Potentiometric titration has been applied to the determination of potassium in seawater [532-534], Torbjoern and Jaguer [533-544] used a potassium selective valinomycin electrode and a computerised semiautomatic titrator. Samples were titrated with standard additions of aqueous potassium so that the potassium to sodium ion ratio increased on addition of the titrant, and the contribution from sodium ions to the membrane potential could be neglected. The initial concentration of potassium ions was then derived by the extrapolation procedure of Gran. 

Polarography has also been applied to the determination of potassium in seawater [535]. The sample (1 ml) is heated to 70 °C and treated with 0.1 M sodium tetraphenylborate (1 ml). The precipitated potassium tetraphenylborate is filtered off, washed with 1% acetic acid, and dissolved in 5 ml acetone. This solution is treated with 3 ml 0.1 M thallium nitrate and 1.25 ml 2M sodium hydroxide, and the precipitate of thallium tetraphenylborate is filtered off. The filtrate is made up to 25 ml, and after de-aeration with nitrogen, unconsumed thallium is determined polarographically. There is no interference from 60 mg sodium, 0.2 mg calcium or magnesium, 20 pg barium, or 2.5 pg strontium. Standard eviations at concentrations of 375, 750, and 1125 pg potassium per ml were 26.4, 26.9, and 30.5, respectively. Results agreed with those obtained by flame photometry. 

Molecular probe dyes for the determination of potassium, lithium, and sodium have been identified. Additionally, an NIR probe selective for potassium has been fabricated. The detection limits of this probe are in the ppm range. Lower detection limits may be achieved by varying the matrix which allows the entrapment of ions. Preliminary data for the detection of lead and cadmium demonstrate the potential capability of these probes for environmental applications. The development of OFMP for the detection of other ions of environmental interest such as Be2+, Hg2+, As3+, and Ni2+ is currently underway. 

Jackson SE, Gunther D (2003) The nature and sources of laser induced isotopic fractionation in laser ablation-multicollector-inductively coupled plasma-mass spectrometry. J Anal At Spectrom 18 205-212 Jiang S-J, Houk RS, Stevens MA (1988) Alleviation of overlap interferences for determination of potassium isotope ratios by Inductively-Coupled Plasma Mass Spectrometry. Anal Chem 60 1217-1220 Lam JWH, Horlick G (1990) A comparison of argon and mixed gas plasmas for inductively coupled plasma-mass spectrometry. Spectrochim Acta Part B 45 1313-1325 Langmuir I, Kingdon KH(1925) Thermionic effects caused by vapours of alkali metals. Phil Trans R Soc A107 61-79... 

Altria, K. D., Wood, T, Kitscha, R., and Roberts-Mcintosh, A. (1995). Validation of a capillary electrophoresis method for the determination of potassium counter-ion levels, in an acidic drug salt. /. Pharm. Biomed. Anal. 13(1), 33 — 38. 

Noteworthy are the articles from Altria et al. and Assi et al. describing the robustness and validation of the determination of potassium as a counterion. An intercompany cross-validation of the determination of sodium in an acidic drug salt was also published. 

Examples of the use of FIA with ISE detection involve the determination of nitrate and total nitrogen in environmental samples [48, 49, 125, 166], potassium, sodium [125], calcium [51] and urea [124] in serum or major nutrients in fertilizers [73]. An interesting combination of an ISFET sensor with the FIA principle [52] is shown in fig. 5.17. This is a simultaneous determination of potassium, calcium and pH in serum during dialysis on an artificial kidney. 

Fig. 5.17. (a) A combination of FIA with ISFET detection [52]. For a description, see the text, (b) A recording of simultaneous determination of potassium, calcium and pH in serum by FIA with ISFET detection [52]. A - potassium standard B - calcium plus pH standard 0 - baseline values in the carrier solution (physiological... 

The most important application of the valinomycin macroelectrode is for the determination of potassium in serum [9, 126,141,174] and in whole blood [45, 71, 224]. This electrode with a polymeric membrane is a component of most automatic instruments for analysis of electrolytes in the serum. It has also been used for monitoring the K level during heart surgery [168]. The valinomycin ISE is also useful for determination of Rb [33]. 

Method 6.3c. Determination of potassium in the acid digest from Methods 6.1a. or 6.1b. 

Method 7.11b. Determination of potassium in piant materiai by fiame photometry (dry ashing extract)... 

Schwer, E.W. and Conan, H.R. (1950) Fertiliser analysis. The determination of potassium. In Joint Symposium on Fertiliser Analysis. Proceedings No. 62. 

Figure 5.22 — Reversible flow-through fluorimetric sensor for the determination of potassium in human blood plasma based on the mechanism shown in Fig. 5.21.3. (A) Flow-cell containing the lipophilic membrane. (B) Flow injection conflguration. P pump IV injection valve W waste. For details, see text. (Reproduced from [86] with permission of Elsevier Science Publishers).

Oxidation of Potassium Peroxide. Determination of Potassium Superoxide. Potassium peroxide was prepared by the addition of a tert-butyl alcohol solution of 90% hydrogen peroxide to potassium tert-butoxide in DMSO or tert-butyl alcohol. Oxygen absorption was followed in the standard manner (20). Analysis of solid precipitates for potassium superoxide followed exactly the method of Seyb and Kleinberg (23). Potassium superoxide formed in the oxidation of benzhydrol was determined in a 15-ml. aliquot of the oxidation solution. To this aliquot 10 ml. of diethyl phthlate was added to prevent freezing of the solution. The mixture was cooled to 0°C., and 10 ml. of acetic acid-diethyl phthlate (4 to 1) added over a period of 30 minutes with stirring. The volume of the evolved oxygen was measured. 

Note, — Regarding the quantitative determination of potassium ferricyanide, see Mohr s Lehrb. Chem.-anal. Titriermeth., 7 ed., p. 249 Sutton, Volumet. Anal., 9 ed., p. 210. 

---