ANALYST. APRIL 1989, VOL. 113 533 Ion-pair Extraction and Spectrophotometric Determination of Potassium Using Dibenzo-18-crown-6 and Bromothymol Blue Rosario Escobar, Concepcion Lamoneda, Fernando de Pablos and Alfonso Guiraum Department of Analytical Chemistry, Faculty of Chemistry, University of Seville, 4 lOI2-Seville, Spain A sensitive extractive - spectrophotometric method for the determination of potassium using dibenzo-I 8- crown-6 and bromothymol blue as the counter ion is described. The absorbance was measured at 410 nm and the value of the molar absorptivity was 18 000 I mol-1 cm-1. A linear working range from 0.1 to 3.0 pg ml-1 of potassium was obtained and the relative standard deviation was 2.3%. Rubidium and ammonium ions gave the highest interference. The method was applied to the determination of potassium in fruits and beverages. Keywords Potassium determination; crown ether complex; potassium extraction; spectrophotometry; fruits and beverages Crown ethers show a remarkable ability to extract alkali and alkaline earth metal ions selectively' and also other ions such a s Ag'.Cu7+ and Cu+' and organic cations and anions.3 These macrocyclic ligands have been widely used and scveral revieus+g have been published. The determination of potass- ium by means of ion-selective electrodes".lO involving the use of crown ethers has been reported. as has their use as potentiometric titrants. 1 1 Crown ethers have also been used for chromatographic ~eparations.3~12 Photometric or fluori- metric determinations, particularly of alkali and alkaline earth metal ions, can be carried out by solvent extraction of the ion pair formed between the crown ether complex and a col- ouredl3 or fluorescent14 counter ion.Also, direct photometric determinations have been performed by using crown ethers having a chromogenic crown.2.15-17 This paper describes the extractive - spectrophotometric determination of potassium using dibenzo- 18-crown-6 and bromothymol blue as the coloured counter ion. In addition, a study of the extraction of the crown ether - K+ complex using various counter ions and organic solvents was carried out. The method was applied successfully to the determination of potassium in fruits and beverages. Experimental Apparatus A Perkin-Elmer 554 spectrophotometer with 1 .O-cm glass cells and a Philips PW-9408 pH meter with a combined glass - calomel electrode were used.Reagents All chemicals used were of analytical-reagent grade or better, and water purified with a Millipore Nanopure system was used throughout. Dibenzo-18-crown-6 was obtained from Merck. A 0.1 'YO im V solution in chloroform was prepared. A 0.04% rrzlV bromothymol blue (BTB) solution was prepared by dissolving 0.04 g of BTB in 10 ml of ethanol and diluting to 100 ml with water. A 1 .0 g 1 - 1 stock solution of potassium was prepared from potassium chloride. Working standard solutions were pre- pared fresh by dilution of the stock solution. A buffer solution (pH 5.1) was prepared by mixing EDTA (2.0 g), lithium hydroxide (4.8 g) and glacial acetic acid (13 ml) and diluting to 1 1 with water.Recommended Procedure Determinution of potassium To a solution containing between 1 and 30 pg of potassium in a 50-ml separating funnel add 1 ml of buffer solution and dilute to 7 ml with water. Add 10 ml of dibenzo-18-crown-6 solution and shake for 2 min. Then add 3 ml of BTB solution and shake for a further 5 min. Discard the aqueous layer and measure the absorbance of the chloroform extract at 410 nm against a blank prepared in the same way but without the potassium solution. Use this procedure to construct a calibration graph. Fruit. Mineralise about 1 g (weighed accurately) of fruit in a muffle furnace for 30 min at 500 "C. Allow the residue to cool, then dissolve it in water, filter the solution and dilute to 100 ml with water. Take suitable aliquots of this solution and proceed as described under Determination of potassium.Orange urid lemon juice. Evaporate about 2 g of the sample to dryness, then follow the procedure described above for fruit . Beer. Take a 5-ml aliquot of the sample and dilute it to 100 ml; allow the C02 evolved to escape at room temperature overnight. Use suitable aliquots of this solution and proceed as decribed under Determination of potassium. Results and Discussion Selection of Counter Ion and Extraction Solvent Several acidic, coloured dyes, including methyl orange, congo red, bromocresol green, bromophenol blue, thymol blue and BTB, were used as counter ions; the highest absorbance was obtained with BTB. In addition, a study of the influence of the solvent on the efficiency of the extraction was carried out; chloroform was found to be the best solvent.The dibenzo-18- crown-6 - K+ complex, extracted into chloroform. with BTB as the counter ion, had an absorption spectrum with a maximum at 410 nm (Fig. 1). 300 400 500 600 Wavelengthin m Fig. 1. (A) Absorption spectrum of the dibenzo-18-crown4 - K+ - BTB system in chloroform. (B) Blank against water. cK = 1.5 pg ml-I534 ANALYST, APRIL 1989, VOL. 114 Table 1. Effect of foreign ions on the determination of 10 ug of potassium 1 0.8 I I L 0 4 PH 2 8 Fig. 2. BTB system. cK = 2.0 pg ml-1 Absorbance - pH graph of the dibenzo-18-crown-6 - K+ - Influence of pH Dibenzo-18-crown-6 and other crown ethers have been reported to be proton complexing agents18319 and this, together with the fact that BTB behaves as a weak acid, probably explains the influence of pH on the absorbance of the chloroform extract. Fig.2 shows the absorbance - pH graph; as can be seen the absorbance is independent of the pH in the range 4.2-5.8. A pH 5.1 buffer solution was used to adjust the pH of the aqueous phase within this range. Several buffer solutions were tested (glycine - HC1, imidazole - HC1) but none of them had a good buffer capacity in the optimum pH range. As the Li+ ion does not interfere with the formation of the dibenzo-18-crown4 - K+ complex14 an acetic acid - lithium acetate buffer was used; good results were obtained. Further, as EDTA also produces no interference, a buffer solution consisting of acetic acid, lithium hydroxide and EDTA (pH 5.1) was employed.This solution had a good buffer capacity and a better selectivity. Extraction of Potassium and Extraction Variables The influence of the crown ether and BTB concentrations was studied for a constant amount (2.6 X 1 0 - 5 ~ ) of potassium. The maximum absorbance was obtained when equal concen- trations of potassium and crown ether were used (with an excess of counter ion). The absorbance remained constant at higher crown ether to potassium ratios. Hence a concentration of 2.7 x 10-3 M of crown ether was used. For a reproducible extraction, an excess of the counter ion is essential; however, a very large excess would lead to the blank being highly coloured and would introduce errors into the absorbance measurements. Hence 3 ml of a 0.04% BTB solution, diluted to 10 ml to obtain a BTB concentration of 1.9 X 10-4 M, were used.In order to avoid the formation of emulsions between the organic and aqueous phases, the BTB must be added after complexation of the K+ ion with the crown ether; the formation of emulsions is undesirable because the dye is then adsorbed and irreproducible values for the absorbance are obtained. For this reason a procedure consisting of two extraction steps was employed. The first step is performed in the absence of the counter ion and the second after adding BTB. Constant absorbances are obtained after shaking for 2 min in the first extraction step, and for 5 min in the second. Linearity, Sensitivity and Precision A linear relationship between absorbance and potassium concentration is obtained in the range 0.1-3.0 pg ml-l Foreign ion Tole r a ncei Pg Mg*+, Fe*+, Fe3+, Mn*t, Ni*t, Ag+, Hg*+, Pb*+, Zn*+, Cd2+ .. . . . . . . . . . . . . . . 1000 Ca2+ Co2+, Cu2+, Be2+ . . . . . . . . . . . . 500 Bi3+ . . . . . . . . . . . . . . . . . . 250 N a i , C s + . . . . . . . . . . . . . . . . . . 100 Sr*+, Ba*', T1+ . . . . . . . . . . . . . . 20 Rb', NH,+ 10 . . . . . . . . . . . . . . . . Table 2. Determination of potassium in fruits and beverages Potassium contenthequiv. per 100 g Sample Banana fruit . . Orangeflesh . . Lemonflesh . . Orange juice Lemon juice . . Applejuice . . Beer . . . . Low alcohol beer Proposed method . . . . 10.33 . . . . 3.69 . . . . 3.22 . . . . 3.91 . . . . 3.34 . . . . 2.12 . . . . 0.80 . . . . 0.52 Flame photometry 10.36 3.71 3.21 3.93 3.27 1.99 0.82 0.53 of potassium and the molar absorptivity is 1.8 X 104 1 mol-1 cm-1.The precision of the method was checked by analysing 11 samples as described under Recommended Procedure; a relative standard deviation of 2.3% (p = 0.05) was obtained. Effect of Foreign Ions The influence of alkali and alkaline earth metal ions and also other metal ions was investigated. An ion was considered to interfere if it caused a variation in the absorbance of more than twice the relative standard deviation obtained for the determi- nation of potassium alone. Table 1 shows the results obtained. The degree of interference for alkali metal ions is in the order Rb+ > Cs+ =: Na+. This is consistent with the order of the stability constants4 of the corresponding metal - crown ether complexes.The NH4+ ion produces a large interference but this can easily be removed by making the sample solution alkaline with lithium hydroxide and boiling. For some alkaline earth metal ions, e.g., Mg2+, and heavy metal ions, e.g. Agt , Hg2+ and Pb2+, the tolerance level is higher than might be expected from consideration of the stability constants of the corresponding metal - crown ether or ion-association com- plexes. This is due to the masking effect of EDTA in the buffer solution. Applications In order to ascertain the utility of the proposed method, the potassium content of some fruit juices and beverages (orange, apple and lemon juices and beer) was determined. The results are summarised in Table 2 and are compared with those obtained by flame photometry; there was good agreement between the two methods.Conclusion The sensitivity of the proposed method compares favourably with that of flame photometry, the latter being one of the most commonly used methods for the determination of potassium in food quality control. The tolerance level of the method towards transition metal ions and several heavy metals ions isANALYST. APRIL 1989, VOL. 114 535 very high; in addition, some of the alkali metal ions, e.g., Na+, which is normally present in many food samples, can be tolerated at high levels. The authors thank the Comision Asesora de Investigacion Cientifica y Tecnica del Ministerio de Educacion y Ciencia de Espaiia for supporting this study (PR 84-1024). 1. 2. 3. 4. 5. 6. 7. 8. 9. References Pedersen, C. J ., J. Am. Chem. SOC., 1967, 89,7017. Muroi, M.. Harnaguchi, A., and Sekido, E., Anal. Sci., 1986, 2, 351. Yoshio, M., and Noguchi, H., Anal. 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Shchori, E., and Grodzinski, J. J.. J. Am. Chem. Soc., 1972, 94, 7957. Nae, N. N., and Grodzinski, .I. J.,J. Am. Chem. Soc., 1977.99, 489. Paper 8102777C Received July I l t h , 1988 Accepted November I1 th, 1988