Anal. Chem. 1900, 52, 1668-1671
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Complexation and Extraction of Alkali Metal Ions by 4’-Picrylaminobenzo- 18-crown-6 Derivatives Hiroshi Nakamura, Makoto Takagi, and Keihei Ueno” Department of Organic Synthesis, Faculty of Engineering, Kyushu University, Higashi-ku, Fukuoka, 8 12 Japan
An extraction study of alkali metal ions was made with the crown ether reagents, 4’-picrylaminobenro-18-crown-6 derivatives (HL; l , 2). The reagents extract alkali metals into chloroform as colored complexes of composition ML. The ease of extraction followed the order, K+ > Rbf > Cs’ > Na’ >> Li’. The colorimetric determination of 4 to 40 ppm Kf was possible in the presence of other alkali and alkaline earth metal ions.
T h e application of crown ethers and related compounds in analytical chemistry is expanding rapidly ( I ) . Our previous paper reported the synthesis and the extraction study of 4’picrylaminobenzo-15-crown-~ derivatives (HL; 3-5) for use in the photometric determination of alkali metal ions (2, 3 ) . These crown ether reagents extracted alkali metals into chloroform forming the 1:2 complexes in metal to crown ether ratio. T h e extraction was thus highly dependent on the reagent concentration, and a relatively high concentration of 3-5 had to be employed t o achieve the extraction photometric determination of potassium in aqueous solution. In the present report, 4’-picrylaminobenzo-18-crown-6 derivatives, 1 and 2, which are expected t o form 1:l complexes, were studied in the hope to avoid such concentration dependence and t o improve the extraction behavior a t lower reagent concentration.
1
n~ 2
2
2
3
1 1 1
4 5
H
NO2 H Br NO2
EXPERIMENTAL Materials. The synthesis of 1 and 2 has been described ( 4 ) (the reagent 2 is now available either from Dojindo Labs. Inc., 2861 Kengun, Kumamoto-shi, 862 Japan, or from Polysciences, Inc., Paul Valley Industrial Park, Warrington, Pa. 18976). The tetralithium salt solution of EDTA was prepared by dissolving EDTA.3Li (Dojindo Labs. Inc.) in a calculated volume of lithium hydroxide solution. Other reagents, either analytical or guaranteed grade, were used as received. Acid Dissociation Constants. The acid dissociation constants of the crown ether reagents in 10% dioxane were determined photometrically. Several milligrams of the reagent were dissolved in 10 mL of dioxane in a 100-mL volumetric flask. Lithium chloride or other alkali metal salt was added and the solution was diluted to 100 mL with water. The lithium hydroxide solution (0.05 M, 1 M = 1 mol/dm3) was then added, and the optical absorbance and the pH of the solution were measured ( 4 ) . The ionic strength was in the range 0.074.15. When the effect of alkali metal ions on the proton dissociation process is to be excluded, 0003-2700/80/0352-1668$01 .OO/O
lithium salt was used in a minimum concentration to maintain the pH. A Orion Digital Ion Meter 801A combined with a TOA GST-155C glass electrode was used for pH measurement. Alkali errors in the high pH region were not corrected. A Hitachi model 200 spectrophotometer with 1-cm standard cell was used for spectral measurement. Solvent Extraction. The high pH conditions were necessary t o ensure the extraction of alkali metals. The following two procedures were adopted ( 3 ) . (A) An aqueous solution of alkali metal salt (5 mL) was shaken with a chloroform solution (5 mL) of 1 (4X lo-’ M) and triethylamine (1M). The optical absorbance of the organic and the aqueous layer was measured at 550 nm and 440 nm, respectively, using a 1-cm standard cell at 25 O C . In this medium, the pH of the aqueous layer was constant at 11.46. (B) A mixture of alkali metal salt solution (2.5 mL) and 0.1-0.2 M EDTA.4Li salt solution (2.5 mL) was shaken with a 1-4 X M chloroform solution of 1 or 2 (5 mL). The absorbance of the organic and the aqueous layer was measured a t 550 nm and 440 nm for 1 and 490 nm and 470 nm for 2, respectively. In this medium, the pH of the aqueous phase was constant a t 12.35. Determination of Potassium in Portland Cement. A two-tenths-gram sample of Portland cement was dissolved in 1 mL of concentrated hydrochloric acid, and the solution was heated to dryness on a water bath. The residue was digested in 10 mL of water, and the mixture was neutralized to phenolphthalein by 0.1 M LiOH. After dilution to 100 mL, the solution was filtered. The potassium concentration in the filtrate was in the order 1-3 x M depending on the cement samples. Sodium was present a t a similar level, while the concentration of calcium and magnesium was about 0.02 M and 7 X M, respectively. Two milliliters of this solution and 2 mL of 0.2 M EDTA.4Li were taken in a 20-mL centrifuge tube, and 5 mL of chloroform solution of 2 (1 x M) was added. The mixture was shaken for 5 min. After phase separation by centrifugation, the absorbance of the organic layer was measured at 490 nm against the reagent blank using a 1-cm standard cell. TREATMENT OF EQUILIBRIUM DATA Complexation in Aqueous Solution. The pKa values of the acid H L can be calculated in a standard manner if t h e interaction of alkali metal ions with HL or L- is negligible. However, for the compounds of the type treated in this study, the values are strongly dependent on the nature of alkali metals and their concentration. T h e equilibria in aqueous solution are illustrated in Figure 1, and the corresponding equilibrium constants are defined by Equations 1-4. T h e subscript “a” for the chemical species indicates t h a t those species in aqueous solution are concerned.
(3) (4) 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
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Table I. Association Constants of 1 with Alkali Metal Ions ( 2 5 " C ) M' Na' K' Rb' CS' Figure 1.
Equilibria in solvent extraction
If A is the absorbance of the aqueous solution as measured against the solvent blank (pH and the salt concentration adjusted), and [ (HL);] represents the total concentration of the crown ether reagent, the mean molar absorptivity E defined by A/[(HL);] is a quantity which is experimentally readily accessible. This quantity can be expressed by Equation 5 in terms of the molar absorptivity of the chemical species and the equilibrium constants involved. The notation tHLa represents the molar absorptivity of the species HL in aqueous solution and so on.
[(L-la1(
t
log log K M L K M H L l o g K,' 1.30 1.92 1.52 1.36
1.00 1.62 1.20 1.08
-10.28 -10.28 -10.26 -10.30
1% KblCa'
Na+) are not much different from 3-5 (K+ > Rb+ > Na+ > Cs'). T h e difference in the ordering of Na+ and Cs' should not be taken seriously, since these metals so far cannot be compared on the same basis (extraction solvent and stoichiometry). Since the quantities KDiHL), K D i M L ) , KML,and KMLeX are interrelated by Equation 14, the self-consistency of the experimental values in Table I11 can be tested. If K , values in 10% dioxane are temporarily used. the KMLeX values as included in the last column of Table I11 were calculated. The agreement with the experimental values is good. Two factors, K M L and KD(MLI, are considered to contribute in determining the metal selectivity in extraction constant KMLeX.
Both factors are responsible for rendering sodium the most poorly extractable, while the low value of KMLis responsible for the poor extraction of cesium (reagent I ) . It is to be noted in passing that the KMLvalues determined by the solvent extraction method are somewhat lower than those in Table I. A difference in the medium (10% dioxane in Table I and water saturated with chloroform in Table 111) may explain a part of the reason. The association constants of crown ether complexes increase remarkably as the reaction medium is changed from aqueous to organic solvents ( 5 ) . T h e introduction of the nitro group in the benzo moiety of the crown ether 1 resulted in a remarkable increase in the value of KMLeX. This is due to the increase in the K , of 2. However, this effect is not fully reflected in the KhILeX, since KD(HL) contributes in a negative manner in KMLeX. Thus, the increased distribution of HL in the organic solution for 2 contributed -0.71 ( = 4.03-4.74; log K D ( H L ) , Table 111) in log of 2. The effect of nitro group on K D ( M L ) units t o the KMLeX is only slight. Table I11 includes the related equilibrium constants obtained by Motomizu and Tdei (6) for the extraction of alkali metals by dipicrylamine in nitrobenzene. The comparison of K M L clearly indicates t h a t the crown ether complexation is involved in the formation of ML for 1 and 2. That the KD(ML)
values for sodium are invariably low suggests that sodium ion is still strongly hydrated in the complex with the crown ether as well as in the ion-pair complex with dipicrylamine anion. D e t e r m i n a t i o n of P o t a s s i u m i n P o r t l a n d Cement. Three commercial portland cement samples were analyzed for potassium photometrically using reagent 2 by the procedure described in the Experimental section. The calibration line was prepared by extracting the standard aqueous solution of potassium chloride with chloroform ( A = 4.832 X lo2 X [(K'),] 0.3472; std. dev. = 0.0073). The analytical results are shown along with those values by standard flame photometry (sample no., K,O % by the present method, K 2 0 % by flame photometry): 1,0.61, 0.60; 2, 0.54, 0.54; 3, 0.26, 0.30. The agreement is good, proving the utility of the present extraction photometry for practical analysis. The potassium concentration of 4-40 ppm in aqueous solution is well covered in the calibration line. The interference from calcium which often poses a problem in the flame photometry of potassium, is effectively eliminated by the use of EDTA buffer in the extraction photometry. This reagent can be used for the absorption photometric determination of potassium in various materials, including soil, fertilizer, and biological samples.
+
ACKNOWLEDGMENT The authors are grateful to Onoda Cement Co. Ltd. for the supply of the analyzed cement samples. LITERATURE CITED (1) Kolthoff, I. M. Anal. Chem. 1979, 5 1 , 1R-22R. (2) Takagi. M.; Nakarnura. H.; Ueno, K. Anal. Lett. 1977. 10, 11 15-1122. (3) Nakamura, H.; Takagi, M.; Ueno, K . Talanfa 1979, 26, 921-927. (4) Yarnashita, T.; Nakamura, H.; Takagi, M.; Ueno, K. Bull. Chem. SOC. Jpn. 1980, 53,1550-1554. (5) Christensen, J. J.; Eatough. D. J.; Izatt, R . M. Chem. Rev. 1974, 7 4 , 35 1-304. (6) Motornizu, S.;Toei, K. Bull. Chem. SOC.Jpn. 1969, 42, 1006-1010.
RECEIVED for review March 10,1980. Accepted June 5,1980. Research partially supported by the Grant in Aid for Scientific Research (B) No. 347055 from the Ministry of Education, Science and Culture.
