cid with Benzoic A the Extraction of Copper(II) Department of Applied Chemistry, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466, Japan

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1 ANALYTICAL SCIENCES OCTOBER 1995, VOL Solvent Effects with Benzoic A on cid the Extraction of Copper(II) Hiromichi YAMADA, Kyoko YAJIMA and Hiroko WADA Department of Applied Chemistry, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466, Japan The extraction of copper(ii) with benzoic acid using chloroform, benzene, l-octanol, and 2-octanone as a solvent was carried out at 25 C and at an aqueous ionic strength of 0.1 mol dm-3 (NaCIO4). Copper(II) has been found to be extracted in any of these solvents as monomeric and dimeric copper(ii) benzoates though the extent of dimerization of copper(ii) benzoate differs from one another. The ease of the extraction was according to the order: chloroform> 1- octanol>2-octanone>benzene; thus, chloroform is superior to the other solvents. In extraction using benzene the emulsion appeared even in the lower percent of the extraction region. The superiority of chloroform over the other solvents can be attributed to the low polarity and moderate solvating ability of chloroform, as well as the low solubility of water in it. Keywords Solvent extraction, solvent effect, copper(ii) benzoates, partition constant, dimerization constant Some reagents had been already prohibited from use, since they are harmful to human beings. Recently the preservation of the natural environment has been strongly advocated. Because of the possibility of damaging the natural environment on an Earth-wide scale, other agents are becoming newly controlled regarding their use. Halogenated solvents, such as chloroform, which have been widely used in the field of solvent extraction, are also on the course of being voluntarily controlled regarding use. Especially chloroform has been widely used as the most useful solvent in solvent extraction. It is therefore urgently necessary for us to develop a useful substitute solvent for chloroform. In the present work the extraction equilibrium on the extraction of copper(ii) with benzoic acid using chloroform, benzene, 1-octanol, and 2-octanone was studied based on a slope analysis, and useful information concerning solvent effects on the extraction of copper(ii) with benzoic acid has been obtained. A pronounced difference in the extraction equilibrium between benzene and chloroform has been found. Experimentals Reagents Reagent grade chloroform was washed successively with concentrated sulfuric acid and distilled deionized water just before use. Benzene, 1-octanol, and 2- octanone were purified by the same method as that previously described.) HYDRANAL-Coulomat A and C (Hayashi Pure Chemical Ind., Ltd.) were employed for determining the saturated water concentration in each organic solvent. Aqueous solutions were prepared with distilled deionized water. All other reagents were of reagent grade and were used without further purification. Procedure The extraction of copper(ii) with benzoic acid in each solvent was carried out in a thermostatic bath kept at 25±0.2 C under the following conditions: an initial copper(ii) concentration of 5X103 mol dm 3 and a total concentration of benzoic acid in the region of mol dm 3 for chloroform, mol dm 3 for benzene and 1-octanol, and mol dm 3 for 2-octanone, respectively. A volume of 15 ml each of the aqueous and organic phases in a 50-ml centrifuge tube was shaken for about one hour, which was sufficient for complete equilibration. After shaking, each centrifuge tube was centrifuged for 5 min at 3000 rpm. Subsequently, the concentrations of hydrogen and copper(ii) ions in the aqueous phase were determined by the same method as that previously described.2 The partition constant of benzoic acid between chloroform and 0.1 mol dm 3 perchloric acid and the dimerization constant of benzoic acid in a chloroform were determined in a similar manner to that described in the preceding paper.) The complex formation constants of copper(ii) ion with benzoate anion were determined by emf measurements using a cupric ion-selective electrode in the same method as that described previously.3 Apparatus A Cool Bath Shaker ML-10 (TAITEC Koshigaya) was used for extraction at 25±0.2 C. centrifugation, a Table Top Centrifuge Model Co., For 5100

2 716 ANALYTICAL SCIENCES OCTOBER 1995, VOL. 11 (Kubota Seisakusho Ltd., Tokyo) was employed. A determination of the hydrogen ion concentration in the aqueous phase was made with a DKK ION Meter, Model 1OL-50 (DKK Co., Tokyo). For an emf measurement of cupric ion activity, the same DKK ION Meter as mentioned above was used with a DKK cupric ionselective electrode (Model 7140) and a DKK reference electrode (Model 4083). The saturated water concentration in each solvent was coulometrically measured using a Karl-Fischer Moisture Titrator MKC-210 (Kyoto Electronics Manufacturing Co., Kyoto). A spectrophotometric determination of benzoic acid in the aqueous phase in the partition of benzoic acid between chloroform and 0.1 mol dm 3 perchloric acid was performed on a UVIDEC-430A Double Beam Spectrophotometer (JASCO, Tokyo). Results and Discussion Partition of benzoic acid between the organic and aqueous phases Taking into account the dimerization of benzoic acid in the organic phase and the dissociation of the acid in the aqueous phase, the partition ratio of benzoic acid is expressed as Fig. 1 Determination of the partition constant of benzoic acid between chloroform and the aqueous phases and the dimerization constant of benzoic acid in the chloroform phase. The solid line is a straight line drawn by the leastsquares method. Aqueous phase: 0.1 mol dm 3 perchloric acid solution. CHA,0 D= _ CHA,W KD,HA(1 +2KD,HAK2,HA[HA]) 1 + Ka/ [H+] where KD,HA, K2,HA and Ka represent the partition constant between the organic and aqueous phases, the dimerization constant in the organic phase, and the dissociation constant in the aqueous phase of benzoic acid, respectively; [HA], CHA,o and CHA,W denote the concentration of the monomeric benzoic acid in the aqueous phase, and the total concentrations of the acid in the organic and aqueous phases, respectively. Since, under the condition that the dissociation of benzoic acid in the aqueous phase can be neglected (i.e.,[ha]=cha,w), organic phase. the denominator on the right-hand side of Eq. (1) can be approximated as being equal to one. Thus, the following equation is obtained: (1) D = KD,HA + 2KD,HA2K2,HA[HA]. (2) The plots of D against [HA] fit well with a straight line drawn by the least-squares method (Fig. 1). From the intercept (KD,HA) and slope (2KD,HA2K2,HA) the partition constant of benzoic acid between chloroform and 0.1 mol dm-3 perchloric acid and the dimerization constant of the acid in chloroform phase were determined. The values are summarized in Table 1, together with those in benzene, 1-octanol, and 2-octanone, which were estimated in a previous study.' Although the partition constant of benzoic acid for a chloroform system is comparatively larger than that for a benzene system, the dimerization constant of the acid in the former solvent is smaller than that in the lattter Table 1 acid This suggests that the chloroform molecule (though the extent of solvation is not as large as 1-octanol and 2-octanone) may solvate to monomeric benzoic acid resulting in an increase in the partition constant of monomeric benzoic acid and a decrease in the dimerization constant of the acid in the chloroform phase. Complex formation of copper(ii) ion with benzoate ion in the aqueous phase The formation constants of copper(ii) benzoates were determined in a similar manner to that previously described.3 The following equation can be obtained: acu(a) -1 Partition and dimerization constants of benzoic K1 + K1K2[A-], (3) where K1 and K2 denote the following stepwise formation constants: Kl=[CuA+]/([Cu2+][A-]) and K2=[CuA2]/

3 ANALYTICAL SCIENCES OCTOBER 1995, VOL Fig. 2 Determination of the complex formation constants of copper(ii) ion with benzoate ion in the aqueous phase (0.1 mol dm 3 perchlorate medium). The solid line is a straight line drawn by the least-squares method. Fig. 3 Confirmation of the extraction of the monomeric and dimeric copper(ii) benzoates. The open and closed symbols refer to 1-octanol and 2-octanone, respectively. The dotted curves are the normalized curves, log(1+x) vs. log X, and the solid lines are the asymptotes of the respective normalized curves. CHA: (1) 0.1, (2) 0.2, (3) 0.3, (4) 0.4 and (5) 0.5 mol dm-3, respectively. ([CuA+][A-]), respectively. In addition, acu(a) and [A-] (the concentration of benzoate ion) were calculated under the respective conditions according to the following expressions: and acu(a) = Ccu/ [Cu2+] (4) [A]= _ CHA 1 + H+ Ka ' (5) o where [Cu2+] was determined from emf measurements using a cupric ion-selective electrode, and Ccu and CHA denote the total concentrations of copper(ii) and benzoic acid, respectively. Benzoic acid was in large excess compared to copper under the present experimental conditions. Plots of (acu(a)-1)/[a-] against [A-] according to Eq. (3) fit a straight line, as shown in Fig. 2. The formation constants were determined to be K1=54.3 and K2=1.1 from the intercept (Kl) and slope (K1K2) of a straight line drawn by the least-squares method. Extraction of copper(ii) with benzoic acid Each extraction equilibrium on the extraction of copper(ii) with benzoic acid for the monomeric and dimeric benzoic acids can be expressed as follows: and icu2+ + (2i + a)(ha)o ;=± (CuiA2i (HA)a)o + 2iH+ [CuiA2i(HA)a]o[H+]2i Kex(ia) - [C u2+]i[ha](2i+a) (6) ( 2j+ a' jcu2++ 2 ( HA)2,0 (Cu11A2J(HA)a-)o +2 'H+ Fig. 4 Confirmation of the extraction of the monomeric and dimeric copper(ii) benzoates. The open and closed symbols refer to benzene and chloroform, respectively. The dotted curves and solid lines are the same as in Fig. 3. CHA: (1) 0.1, (2) 0.15, (3) 0.2, (4) 0.25, (5) 0.3, (6) 0.4, (7) 0.5 and (8) 0.7 mol dm3, respectively. [Cu1A2j(HA)a']o[H+]2' J[(HA)2](oJ+a')I2 ' (7) where Kex(ia) and Kex(ja') denote the extraction constants designating benzoic acid as the monomeric (HA) and dimeric ((HA)2) forms, respectively. Then, the relationship between these extraction constants can be written as a')k2 (2j+a')l2 Kex(ia)-Kex(j,HA '

4 718 ANALYTICAL SCIENCES OCTOBER 1995, VOL. 11 Since similar plots to those described previously3 on the basis of the following Eqs. (8) and (9) (corresponding to Eqs. (6) and (7), respectively) fit well in the normalized curve, log(1+x) vs. log X(as shown in Figs. 3 and 4), the monomeric and dimeric copper(ii) benzoates have been found to be responsible for the extraction of copper(ii) with benzoic acid in any solvent system: log Cc,0 - (log[cu2+] - 21og[H+]) - log Kex(la)-(2+a)log[HA]o = l0 g 1 -I- 2Kex(Zb) K HA (z+b-a) Cu2+ H+ -2 8~ ex la [ ]o [ ][ ] ( for the extraction systems using 1-octanol and 2- octanone as a solvent, and log Cc,0 - (log[cu2+] - 2log[H+]) - l og Kex(la') (2+a') - 2 log[(ha)2]a 2Kex(2b') = l0 g 1 + HA (2+b'-a')/2 Cu2+ H+ -2 Kex la' [~ )2]0 [ ][ ] (9) for those using benzene and chloroform. Here, a (or a') and b (or b') refer to the number of benzoic acid molecules involved in each monomeric and dimeric extracted species, respectively. In a similar manner (as noted previously3) the following expressions are obtained in the region where the monomeric copper(ii) benzoates prevail: log Cc,0 - (log[cu2+] - 21og[H+]) =log EKex(la)[HA]o2+a) (l0) Fig. 5 Estimation of the number of benzoic acid molecules involved in the monomeric and dimeric copper(ii) benzoates. Symbols: 0 and D refer to the monomeric and dimeric copper(ii) species in 1-octanol, and and refer to the monomeric and dimeric species in 2-octanone, respectively. The solid and dotted lines are straight lines with slopes of 2.0 and 4.0, respectively. for Eq. (6), and log Cc,0 - (log[cu2+] - 21og[H+]) =log ~Kex(la'>[(HA}2]( o2+a')/2 (11) for Eq. (7), respectively. On the other hand, for determining the number of benzoic acid molecules involved in the dimeric copper(ii) species, the following expressions are available: log Cc,0-2(log[Cu2+] - 21og[H+]) = log 2 b Kex(2b) [HA]oa+b) (12) for the extraction systems using 1-octanol and 2- octanone, and log Ccu,a - 2(log[Cu2+] - 2log[H+]) =log 2EKex(Zb')[(HA)2]oa+b')IZ (13) b for benzene and chloroform systems. According to Eqs. (l0) - (13), plots for estimating the number of Fig. 6 Estimation of the number of benzoic acid molecules involved in the monomeric and dimeric copper(ii) benzoates. Symbols: 0 and D refer to the monomeric and dimeric copper(ii) species in benzene, and and refer to the monomeric and dimeric species in chloroform, respectively. The solid, dotted and dashed lines are straight lines with slopes of 1.5, 1.0 and 3.0, respectively.

5 ANALYTICAL SCIENCES OCTOBER 1995, VOL benzoic acid molecules involved in the monomeric and dimeric copper(ii) species are depicted in Figs. 5 and 6. As can be seen from Fig. 5 for the 1-octanol and 2- octanone systems, in both solvents the plots for the monomeric copper(ii) species fall on a straight line with a slope of 2.0; those for the dimeric species fall on that with a slope of 4.0, respectively. Thus, the monomeric and dimeric species were found to be CuA2 and Cu2A4, respectively, in both systems using 1-octanol and 2- octanone as a solvent. The respective extraction constants were estimated from the individual intercepts of the straight lines: log Kex(lo) and log Kex(2o)= for 1-octanol, and log Kex(lo) and log Kex(2o)= for 2-octanone, respectively. These constants correspond to Eq. (6). The extraction constants for 1- octanol were newly determined in the present work. The present values are more reliable than our previous results, which were estimated by neglecting the dimeric copper(ii) species.2 The plots for benzene and chloroform systems are shown in Fig. 6. The plots for the monomeric copper(ii) extracted species fit well in the respective straight lines with slopes of 1.5 and 1.0 for the benzene and..chloroform systems, respectively; those for the dimeric species fit well in each straight line with a slope of 3.0 in both solvent systems. These results suggest that the monomeric copper(ii) species are CuA2HA for the benzene system, CuA2 for the chloroform system; the dimeric one is Cu2A4(HA)2 for both systems. The respective extraction constants were estimated from the intercepts of each straight line as follows: log Kex(Il)= and log Kex(22)= for benzene system, and log Kex(lo) and log X X(22)= for chloroform system, respectively. These constants correspond to Eq. (7). In the extraction system using benzene as a solvent, the extraction equilibrium was analyzed while keeping the percent extraction of copper below approx. 20%, since the emulsion appeared in the region where the percent extraction of copper exceeds about 20% under the present conditions. The extraction constants obtained in the present work Dimerization of copper(ii) benzoates Thus, the dimerization of copper(ii) benzoates in the organic phase can be formulated as follows: in 1-octanol and 2-octanone phases, 2(CuA2)o in benzene Kdim-1 phase, 2(CuA2HA)o (Cu2A4)a, Kd~ lm-2` (Cu2A4(HA)2)o, and in chloroform phase, 2(CuA2)o + 2(HA)0 Kdim-3 (Cu2A4(HA)2)a with the dimerization constants calculated from the corresponding extraction constants: log Kdim_1=2.28 for 1-octanol, log Kdim_1=3.78 for 2-octanone, log Kdim-2= 3.88 for benzene, and log Kdim-3=6.41 for chloroform, respectively. As noted above, the dimerization reaction of copper(ii) benzoates in each organic phase has been found to depend on the solvent, except for 1-octanol and 2-octanone, while not to do on the concentration of benzoic acid in each organic phase with the exception of chloroform phase. The extent of dimerization of copper(ii) benzoate in 1-octanol and 2-octanone, in which the dimerization of copper(ii) benzoate proceeds through the same mechanism, has been proved to be appreciably higher in 2-octanone than in 1-octanol, according to each dimerization constant. Such a tendency was also observed in previous results concerning the extraction of copper(ii) decanoates (discussed in detail by the regular solution theory4-6). The lower dimerization constant of the copper(ii) species in 1-octanol than in 2-octanone is attributable to the more favorable solvation of the monomeric copper(ii) species by 1- octanol molecules. On the other hand, in chloroform and benzene phases the ease of dimerization of copper(ii) benzoates cannot be directly judged based on the values of their dimerization constants, since the dimerization reaction of the are summarized in Table 2. For a comparison, the values for chloroform and benzene are converted by Kex(lo)=Kex(lo)K2,HA, Kex(11)=Kex(11)K2,HA312, and Kex(22)= Kex(22)K2,HA3, respectively. copper(ii) species differs from one another. Table 2 Extraction constants of copper(ii) benzoates Then, the distribution curves of the dimeric and monomeric copper(ii) species for the total concentration of copper(ii) benzoates in the organic phase are depicted at CHA=0.5 mol dm 3 in Fig. 7; as can be seen, the order of the facility in the dimerization of copper(ii) benzoates in each organic phase is as follows: chloroform>benzene>2- octanone>1-octanol. This order is the same as that of the dimerization constants of the different dimerization reaction described above. Although the dimerization constant of copper(ii) benzoate in the chloroform phase is much larger than that in the other solvents, there is not so much difference in the extent of dimerization in the three kinds of solvents, except for 1-octanol. The lower dimerization of copper(ii) benzoate in the 1-octanol

6 720 ANALYTICAL SCIENCES OCTOBER 1995, VOL. 11 Fig. 8 Extraction curves for the present extraction systems. The numbers are the same as in Fig. 7. The total concentrations of copper(ii) and benzoic acid are 5X103, and 0.5 mol dm-3, respectively. Fig. 7 Distribution curves of dimeric and monomeric copper(ii) extracted species in the organic phase. (1) benzene, (2) chloroform, (3) 1-octanol and (4) 2-octanone, respectively. The ordinate depicts the content of dimeric copper(ii) benzoates to total copper(ii) concentration in the organic phase. The total concentration of benzoic acid, (CHA) is 0.5 mol dm3. phase suggests that the monomeric copper(ii) species is strongly solvated by some 1-octanol molecules. Extraction curves The extraction curves for the present extraction systems at CHA=0.5 mol dm 3 and Ccu=5X 10-3 mol dm 3, which are depicted on the basis of the results obtained by the present work, are shown in Fig. 8. In the lower percent extraction region, where the emulsion does not appear, benzene is slightly superior to chloroform concerning the percent extraction of copper, and is thus the best solvent for the extraction of copper(ii) benzoates, as expected from the values of the extraction constant of the monomeric copper(ii) benzoate. However, benzene turns out to be the poorest solvent for the extraction of copper(ii) benzoates, because the emulsion appears under the conditions where the percent extraction of copper(ii) exceeds about 20%. Then, the order of decreasing extractability is as follows: chloroform>loctanol>2-octanone>benzene. In 1-octanol the extent of the dimerization of copper(ii) benzoate is markedly low compared to that in the other three kinds of solvents, as shown in Fig. 7. In spite of this quantitative relation, l-octanol is slightly superior to 2-octanone with respect to the extractability of copper(ii) benzoates. Although the dimerization of copper(ii) benzoate is one of the important factors regarding extractability, the larger extraction constant of monomeric copper(ii) Fig. 9 Distribution ratio of benzoic acid between the organic and aqueous phases. The numbers are the same as in Fig. 7. The total concentration of benzoic acid is 0.5 mol dm-3. The aqueous phase is 0.1 mol dm-3 perchlorate medium. benzoate in the 1-octanol system than in the 2-octanone system is anticipated to be more effective for the extractability of copper(ii) benzoates than the dimerization of copper(ii) benzoate. On the other hand, the partition ratio of benzoic acid against the hydrogen ion concentration in the aqueous phase is depicted for each solvent system in Fig. 9. Since the partition ratio of benzoic acid is regarded as being the ratio of the affinity of the acid for the organic and aqueous phases, it depends on not only the partition of monomeric benzoic acid between the organic and aqueous phases, but also the dimerization of the acid in the organic phase. As shown in Fig. 9, the partition ratio of benzoic acid decreases in the order: 2- octanone>1-octanol>chloroform>benzene. This is the same order of the partition constant of monomeric benzoic acid between each organic solvent and 0.1 mol dm 3 perchloric acid listed in Table 1. The emergence

7 ANALYTICAL SCIENCES OCTOBER 1995, VOL of emulsion in the extraction of copper(ii) benzoates using benzene can be firstly attributed to this lower partition constant of monomeric benzoic acid for benzene. It was found that the extraction of benzoic acid, itself, is more strongly affected by the partition of monomeric benzoic acid than by its dimerization in the organic phase as well as the extraction of copper(ii) benzoates. Comparison of benzene with the other solvents In general, it has been known that the partition constant of an extracting agent is associated with the partition and extraction constants of an extracted species. Then, in the three kinds of solvent systems, except for the benzene system, in which the presence of CuA2 was not identified as an extracted species, the partition constant of CuA2(KD,cUA2) can be calculated from the expression as Kex(lo) = KD,CUA2 KD,HA-2/3CUA2 Ka2, (14) where /3cuA2 denotes the overall formation constant of CuA2 in the aqueous phase, and is calculated to be log f3cua2=1.78 under the present conditions by using J3CUA2=K1K2. The values of KD,CUA2 were estimated to be log KD,CUA2=2.17 for 1-octanol, 1.59 for 2-octanone, and 1.17 for chloroform, respectively. In all cases they have been found to be significantly smaller than those expected from the relation between the partition constants of CuA2 and the monomeric benzoic acid, which is represented by KD,CUA2=KD,HA2. This is attributable to an increase in the partition constant of monomeric benzoic acid (KD,HA) due to solvation with the respective solvent molecules. The solvation of monomeric benzoic acid was described in detail in a previous paper.' This is also supported by the fact that the partition constant of monomeric benzoic acid for these three kinds of solvent systems is considerably larger than that for the benzene system, as shown in Table 1. Not only the monomeric benzoic acid, but also the extracted copper(ii) species, is expected to be solvated in these three kinds of solvents. The extraction of the monomeric CuA2 species in these solvent systems, in contrast to the benzene system, suggests that copper(ii) benzoate is also solvated by the respective solvent molecules. On the other hand, in the benzene system the lower partition constant of monomeric benzoic acid can be expected to result in the lower partition constant of copper(ii) benzoate, and, in addition, the poor solubility of copper(ii) benzoate in benzene. It is convenient to imagine that this arises from the lack of a solvating ability in benzene. In the system using chloroform, which is regarded as being a so-called inert solvent, as well as benzene, the emulsion did not appear, in contrast to the benzene system. This discrepancy between the chloroform and benzene systems can be presumed to result from the difference in the solvating ability on these solvents. That is to say, chloroform possesses a solvating property, though not as strong as 1- octanol and 2-octanone. This is also demonstrated by the values of the partition constant of monomeric benzoic acid in Table 1. In spite of the emergence of the emulsion in the extraction system using benzene as a solvent, the extraction curve for benzene is located at the highest percent extraction in the region where the emulsion is not formed, as shown in Fig. 8. Judging from these results obtained from the present work, the mixed solvent prepared by the addition of 1- octanol or 2-octanone to benzene can be expected to have some solvating ability, leaving the advantage of benzene: the extraction constants of copper(ii) benzoates using benzene as a solvent are considerably larger than in the other solvent systems. Then, the extraction of copper(ii) benzoates using these mixed solvents is anticipated to result in the disappearance of the emulsion and the high extractability. Such a mixed solvent will be effective for solvent extraction, and will serve for available halogenated solvents, such as chloroform which have been pointed out to have the possibility to damage the natural environment. In the preliminary experiments it was confirmed that mixed solvents prepared by adding a certain content of 1-octanol or 2- octanone to benzene are superior to chloroform in the extraction of copper(ii) with benzoic acid. The present work was supported by a Grant-in-Aid for Scientific Research (No ) from the Ministry of Education, Science and Calture. References 1. H. Yamada, Kyoko Yajima, H. Wada and G. Nakagawa, Talanta, 42, 789 (1995). 2. H. Yamada, K. Adachi, Y. Fujii and M. Mizuta, Solvent Extr. Ion Exch., 4,1109 (1986). 3. H. Yamada, Y. Taguchi and H. Wada, Talanta, 41, 573 (1994). 4. H. Yamada, S. Suzuki and M. Tanaka, J. Inorg. Nucl. Chem., 43,1873 (1981). 5. H. Yamada, R. Kitazaki and I. Kakimi, Bull. Chem. Soc. Jpn., 56, 3302 (1983). 6. H. Yamada, K. Takahashi, Y. Fujii and M. Mizuta, Bull. Chem. Soc. Jpn., 57, 2847 (1984). (Received July 3, 1995) (Accepted August 11, 1995)