copper(II) Complexing capacity of natural water and lability of soluble copper(II) complex was .... chlorate was taken in a separatory funnel, and an equal.
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Simultaneous Determination of of Soluble Copper(II) Complex Dithizone
263
Complexing in Natural
Capacity Water by
and
Lability
Extraction
Hideyuki ITABASHI, Hiroshi KAWAMOTO, Norihiro NIIBE, Kin-ichi Department of Applied Chemistry, Faculty of Engineering, Gunma
TsUNODA University,
and Kiryu,
Hideo Gunma
AKAIWA 376, Japan
Based on a kinetic aspect of the extraction of copper(II) with dithizone, a method for the simultaneous determination of copper(II) Complexing capacity of natural water and lability of soluble copper(II) complex was developed. The feasibility of the proposed method was confirmed using some chelating agents such as nitrilotriacetic acid, ethylenediaminetetraacetic acid and citrate instead of naturally occurring ligands. The proposed method was applied to river water samples. The copper(II) complexing capacity of the water sample from urban area was larger than that from up-stream, due to a ligand which originated from human activity. Keywords
Copper(II) complexing capacity, lability, soluble copper(II) complex, river water sample, dithizone extraction
Recent advances in instrumental analysis enables us to determine total concentration of trace metals in natural waters without any preconcentration. From an environmental point of view, however, the speciation of trace metals is more important because bioavailability of these elements depends on their chemical forms, which are predominantly affected by naturally occurring ligands.l-4 To estimate a complexation ability of the naturally occurring ligands, the concept "Complexing Capacity" has been introduced.5_g In the case of copper(II) as a trace metal, copper(II) complexing capacity (CuCC) of a water sample is the ability of the sample to remove added copper(II) from the free ion pool.9 The authors have developed a method for estimating copper(II) complexing capacity and have applied the method to river water samples.lo-12 This method is simple and useful for the estimation of copper(II) complexing capacity of natural water samples. On the other hand, the estimation of lability of the copper(II) complex is also important because the labile complex may behave as a free copper(II) and become bioavailable. Thus, a method for the simultaneous determination of copper(II) complexing capacity of natural water samples and lability of soluble copper(II) complex by dithizone extraction was developed. The feasibility of the proposed method was confirmed using nitrilotriacetic acid (nta), ethylenediaminetetraacetic acid (edta) and citrate as chelating agents. The established method was successfully applied to river water samples, and important information on the behavior of a ligand which originated from human activities was obtained.
Principle In the case in which a ligand (L) in a water sample forms a complex CuL by adding an excess amount of copper(II) to the water sample, the extraction reaction of CuL with dithizone (H2dz) can be expressed by CuL + 2H2dzorg
Cu(Hdz)2,org+ 2H++ L
(1)
where the subscript org denotes the organic phase. Assuming that the extraction rate of copper(II) is the first order with respect to CuL, the reaction rate is defined as rate = v = -
d[CuL] d t = kobsd[CuL]
(2)
where kobsdrepresents the rate constant for the extraction. Integration results in -ln[CuL] t = -ln[CuL]t=o + kobsdt
(3)
where the subscript t represents shaking time t. According to Eq. (3), a plot of -ln[CuL]t against t should give a straight line having a slope of kobsdand an intercept of -ln[CuL]t-o. Here, kobsdand [Cul]t=o were defined as lability of CuL and copper(II) complexing capacity (CuCC), respectively. Experimental Reagents A stock solution of copper(II) (1X102 -mol
dm 3) was
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prepared by dissolving 2.5 g of copper(II) sulfate pentahydrate in 1 dm3 of 5X102 mol dm 3 sulfuric acid. Stock solutions of nta, edta and citrate (1X102 -mol dm 3) were prepared by dissolving appropriate amounts of sodium salts in water. A stock solution of dithizone (5X10-3mol dm 3) was prepared by dissolving 0.64 g of dithizone in 500 cm3 of chloroform. All of the reagents were of analytical reagent grade and were used without further purification. The water used to prepare the reagent and buffer solutions was obtained from a Milli-Q water purification system (Millipore Co.). Apparatus A Hitachi (Model 170-30) atomic absorption spectrophotometer equipped with copper hollow cathode lamp (Hamamatsu Photonics) was used for the determination of copper(II). A Denki Kagaku Keiki (Model HGC-10) pH meter and an Iwaki (KM-type) shaker were used for pH measurements and shaking of the separatory funnels, respectively. Procedure A river water sample was filtered through a 0.45 sm membrane filter. When chelating agents such as nta, edta and citrate were used, the pH of the solutions was adjusted to 7.0 by adding 0.01 mol dm-3 3-morpholinopropanesulfonic acid (MOPS) buffer solution. Ten cubic centimeters of a sample solution containing 2X 10-5mol dm 3 copper(II) and 0.1 mol dm 3 sodium perchlorate was taken in a separatory funnel, and an equal volume of chloroform containing 5X10-4 mol dm 3 dithizone was added. The mixture was then shaken vigorously for a definite time. After the phases were allowed to separate, copper(II) in the aqueous phase was determined with an atomic absorption spectrometer. According to Eq. (3), the reciprocal of the natural logarithm of the copper(II) concentration was plotted as a function of shaking time, and the values of kobsdand CuCC were obtained from the slope and the intercept of the resulting straight line. All experiments were carried out at room temperature (ca. 293 K).
Results
and
Discussion
To confirm the feasibility of the proposed method, chelating agents such as nta, edta and citrate were used instead of naturally occurring ligands. The extractability (E) of copper(II) in the absence and the presence of the above ligands is plotted as a function of shaking time. The results are given in Fig. 1, where it is seen that the extraction rate of copper(II) in the presence of chelating agents is smaller than that of free copper(II) which is quantitatively extracted to chloroform within 5 s. Therefore, the concentration of copper(II) in the aqueous phase at shaking time t can be assumed to be equal to [CuL]t. A plot of -ln[CuL]t against t in which nta as a chelating agent is shown in Fig. 2, where four straight lines show the validity of Eq. (3). When edta and citrate
Fig. 1 Plot of extractability (E) of copper(II) against shaking time (t). (0), free copper(II); (S), in the presence of citrate (2X10-5 mol dm-3); (E), in the presence of nta (2X105 mol dm-3); (/), in the presence of edta (2X10-5mol dm 3).
Fig. 2 Plot of -ln[CuL]t against shaking time (t) in the presence of nta. Concentration of nta: (0), 4X106 mol dm3; (•), 8X10-6 mol dm-3; (fl), 1.2X10-5mol dm3; (/), 1.6X10-5 mol dm-3.
were used, the plots also fall on straight lines. The kobsd and the CuCC values for the chelating agents obtained by the proposed method are summarized in Table 1, where the kobsdvalues are conditional rate constants under these experimental conditions (pH=7.0, [H2dz]org=5X10-4 mol dm 3). The CuCC values approximately agreed with the concentration of ligand added, indicating the usefulness of the method for the estimation of complexing capacity. The kobsdvalues are independent of the ligand concentration, but depend only on the kind of
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Copper(II) complexing capacity (CuCC) and k obsd values
for chelating
agents
[L]added,CuCC/ 10-6mol dm3, kobsd/s-'.
Table
2
Results
of the Kirvu
River
water
samples
CuCC/ 10-' mol dm 3, kobsd/s-r.
Fig. 3 Plot of -ln[CuL]r against shaking time (t) in the Kiryu River water samples. Sampling location: (0), point A (pH=6.98); (S), point B (pH=7.01).
ligand. This means that one can use the kobsdvalue for the speciation of naturally occurring ligands. Finally, the present method was applied to natural water samples. The water samples were taken from the Kiryu River; the location of point A is up the stream and that of point B is in an urban area. In this instance, the buffer solution was not added to the samples because the pH of the samples was around 7. A plot of -ln[CuL]t against t consists of two different slopes at point A, indicating that two kinds of ligands exist (Fig. 3). On the other hand, the plot consists of three different slopes at point B, showing the existence of an additional ligand. The kobsdand the CuCC values obtained are summarized in Table 2. The total CuCC value obtained from point B is larger than that at point A; such a tendency is consistent with the previous studies.11,12 From the comparison of kobsdvalues at points A and B, it is estimated that two corresponding ligands are present in water samples from both sampling places. A ligand having intermediate kobsdat point B is absent in the water sample at point A. From this result, we can estimate that a ligand which originated from human activities is
present in the point B water. Consequently, larger CuCC values of point B water may be due to the ligand of human activity origin.
References
1. A. G. Lewis, R. H. Withfield and A. Ramnarine, Mar. Biol., 17, 215 (1972). 2. D. M. Anderson and F. M. M. Morel, Limnol. Oceanogr., 23, 283 (1978). 3. M. Gnassia-Barelli, M. Romeo, F. Laumond and D. Pesando, Mar. Biol., 47, 15 (1978). 4. N. S. Fisher and D. Frood., Mar. Biol., 59, 85 (1980). 5. E. W. Davey, M. J. Morgan and S. J. Erickson, Limnol. Oceanogr., 18, 993 (1973). 6. M. S. Shuman and G. P. Woodward, Anal. Chem., 45, 2032 (1973). 7. R. Chau and K. Lum-Shue-Chen, Water Res., 8, 383 (1974). 8. Y. K. Chau, J. Chromatogr. Sci., 11, 579 (1973). 9. P. G. C. Campbell, M. Blisson, R. Gagne and A. Tessior., Anal. Chem., 49, 2358 (1977). 10. H. Akaiwa, H. Kawamoto and H. Ogura, Chem. Lett., 1986, 605. 11. M. Uchiyama, H. Kawamoto, H. Itabashi and H. Akaiwa, Nippon Kagaku Kaishi, 1989, 309. 12. H. Kawamoto, W. Ishida, K. Tsunoda and H. Akaiwa, Bull. Chem. Soc. Jpn., 65, 3485 (1992). (Received November 22, 1994) (Accepted January 6, 1995)
