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Solvent extraction of Cd(II) with N-cyclohexyl-N-nitrosohydroxylamine and 4-methylpyridine into methyl isobutyl ketone
Tahra, Vol. 35, No. 5, pp. 413-418, 1988 Printed in Great Britain. All rights nscrved
Copyright 8
~3~91~/88 $3.00 + 0.00 1988 Fbgamon Rws pk
SOLVENT EXTRACTION OF Cd(I1) WITH lV-CYCLOHEXYL-N-NITROSOHYDROXYLAMINE AND 4-~~~FYRI~INE INTO METHYL ISOBUTYL KETONE G.
RMJRET,
L.
PINEDA,
R. C!~sns and R.
COMPAI~O
Department of Analytical Chemistry, University of Barcelona, Barcelona, Spain A.
SAMXEZ--
Department of Fundame&al Physics, University of Barcelona, Barcelona, Spain (Received I4 August 1987. Revised 6 Umber 1987. Accepted 8 &cenr&cr 1987) Summary--The distribution equilibria of the complexes cadmium-cnha and cadmimn-cnha-4methylpyridine in the water-methyl isobutyl ketone system have been studied at 25”, by using ‘“Cd as a radiotracer to measure the metal distribution ratio. A very sensitive method for detection of ‘““Cd, based on the use. of a liquid scintillator, has been developed. From the 8raphical treatment of the ~~b~~ data, it has been deduced that CdL, is the compiex extracted in the absence of ~~~ylpy~~ne, and that the adduct Cd&B is extracted when the second i&and is present. This model has been checked by treating the data with the program LETAGRGP-DISTR and the following equilibrium constants have been obtained: stability constants of CdL,, log& = 2.82 It 0.14, log & 3: 5.981 f 0.004, distribution constant of CdLz, log K, = -0.49 f 0.01; adduct formation constant of CdL,B, log & = 2.70 & 0.07.
The industrial applkatims of cadmium have increased considerably in recent years.’ Because this element is hi&ly toxic even at very low concentrations it must be determined in enviromental samples. Although some analytical methods for cadmium determination are very sensitive, low ~n~ntra~ons are generally determined after use of a preconcentration technique such as solvent extraction. Several authors have proposed systems for cadmium extraction.z-‘3 Reagents such as dithiione,2p3 thiouxineT3 and some di~~~at~,4,6,~ co-ordinating through sulphur atoms, have often been studied. When the co-ordination takes place through nitrogen or oxygen atoms, a second ligand (pyridine, picolines, l,lO-phenanthroline, TOPO, TBP, . . .) has often been added to the system to enhance the extraction as a result of formation of a ternary complex.b’O~lz In the present paper, as a part of a wider study to assess the utility of ~~yclohexyl*~-~trosohy~oxylamine @ha) as an agent for extracting metals into methyl isobutyl ketone (MIBK),‘4Js we report the study of the distribution equilibria of Cd-cnha and Cd-cnba-4-methylpyridine complexes. The cnhaMIBK system has proved useful for the determination of Cu(I1) in river water by atomicabsorption spectrometry*4 and its application to the determination of other heavy metals in natural and waste waters is now being studied. The use of radioactive tracers is one of the best methods for studying the distribution of metals be-CAL. 3Sif-F
tween immiscible phases. In the present work, rosCd was used and a very sensitive method for its detection, based on the use of a liquid scintillator, has been developed.
&?ag#zts The sodium salt of Ncycloh~l-N-~trosohydraxyfamine was obtained from the potassium salt (BASP) as described earlier.“’ A stock solution of cadmium (1 g/l.) was made by dissolving the metal (analytical-reagent grade) in a small vohtme of concentrated nitric acid, then adding perchloric acid and boiling until white fumes were evolved. The solution was cooled, transferred to a 1-Iitre standard thtsk and diluted to the mark. Carrier-free tWCd (t,,r 453 d) was supplied by Amersham International as CdCl,. The radiotracer was added to the cadmium solution before the extraction. Analytical~rea8ent grade pyridine (Fluka), tri-n-butyl phosphate (Carlo Erbaf, ~-n-~~lph~ oxide iMerck) and ~1~~~yI~~d~yl~~~~ chloride t&k& Trioctylamine, 95% (Merck) and cl-methvlnvridine. 96% (Fluk& The content bf 4-metbylpyridineVii ihe comtnercially available product was checked by gas chromatography, Liquid scintillation cocktail Insta-Gel (packard). All other chemicals wem of analytical grade and were used without further purl&&ion. A coaxial Ge(Li) semiconductor detector (Grtec), with an effective volme of 58 cm3, cormected to a 4092”chamrel multichannel analyser (Silena) was used for gamma-ray measurements~ A dual liquid-scintillation counter (Bertold LB 5048) was used for beta part& measurements.
413
414
ANALYTICAL
DATA
A pa-meter (Radiometer PHM 64), equipped with a glass+alomel electrode pair, and standardized with buffer solutions at pH 4.008 and 6.865 (25”) prepared from Merck salts according to DIN 19266, was used.
Procedure Ten ml of aqueous phase saturated with the organic solvent and containing an appropriate concentration of cadmium (labelled with ‘@‘Cd),the sodium salt of cnha and Cmethylpyridine (the last only when the distribution of the ternary complex was studied) were shaken with 10 ml of MIBR, saturated with water, in a thermostatic bath at 25.0 + - 0.1” for 15 min. The DH of the aaueous chase was adjusted with either perchloric acid or an acetic aid-sodium acetate buffer, and sodium perchlorate was added to give a constant ionic strength of 0.1. After phase separation, the metal distribution ratio (0) was measured by one of the following procedures. (a) For D > 0.1 equal volumes (4 ml) of both phases were measured by pipette into suitable counting tubes and their activities measured at constant geometry. Then
D = AJA,
(1)
where A, and A, are the activities of the organic and aqueous phases respectively. Q) For D < 0.1, 4 ml of each aqueous phase were taken and their y-activities determined; the same volume of each organic phase was mixed with 15 ml of the liquid scintillator and the activity measured. The distribution ratio was calculated by means of the expression D = A, (liquid scintillator) x F
A, (gamma) where F is defined as
F=
Activity of organic phase (gamma) Activity of organic phase (liquid scintillator)
(3)
and is found for each series of experiments by use of organic phases with activities measurable accurately by means of both techniques. The quench of the samples was determined by the channel ratio-external standard method. All the samples of each series showed the same quench. RESULTS AND DISCUSSION
Measurement of ‘09Cd activity The nuclear transformation
of ‘@‘Cd takes place by
electron capture to give “‘@“Ag,which decays to the ground state by an 88-keV gamma transition (N 3.8%) or emission of internal conversion electrons ( w96.2%).16 When the metal solution is labelled with ‘09Cd, the distribution ratio may be determined, after phase equilibration, as the ratio of the gamma activity of the organic phase to that of the aqueous phase. However, the low probability of gamma-ray emission, together with the low efficiency (-4%) of the Ge(Li) detector for the geometry used, made it necessary, when D was lower than 0.1, to increase the quantity of radiotracer added to the system or to use long counting periods in order to measure the activity of the organic phase with adequate accuracy. The advantageous features of liquid scintillation measurements for detecting low-energy beta emitters seemed favourable for the detection of internal conversion electrons of lBCd and also the subsequent Auger electrons. Since these electrons are monoenergetic, their pulse-height distribution curve will be
Channel
number
Fig. 1. Spectrum of the Auger and internal conversion electrons of ‘09Cd.
peak-shaped and the area under the curve will give their total number. Figure 1 shows the spectrum obtained by mixing a solution of known activity of ‘09Cd with the liquid scintillator. The lowest energy peak, at 18 keV, corresponds to the Auger electrons and the 65 and 84 keV peaks correspond to the internal conversion electrons. The spectrum may contain a small contribution from the gamma-rays, which can interact with the scintillator and produce an electric pulse. From the total number of counts, obtained from integration of the spectrum and the activity of the ‘@Cd solution, an overall apparent efficiency of 120% was obtained, because of the simultaneous detection of the Auger and internal conversion electrons. Hence, this method is 800 times as sensitive as the Ge(Li) detector method, for the same counting time, and is suitable for measuring the activity of the organic phases when the distribution ratio is very low. Distribution of the cacimium-cnha binary complex in the water-MIBK system Influence of shaking time. The distribution ratio values obtained at pH 7.20, with shaking times between 1 and 60 min, show that the distribution equilibrium is attained after 5 min of shaking. In subsequent experiments an extraction time of 15 min was adopted. Influence of ionic strength. The ionic strength of the aqueous phase was varied over the range O.Ol-l.OM by the addition of sodium perchlorate. The distribution ratio, at constant pH and cnha concentration, was independent of ionic strength up to 0.3M; at higher values, the distribution ratio slightly decreased. In the rest of the work the ionic strength was adjusted to 0.1&f. Injluence of metal concentration. Varying the cadmium concentration between 8.93 x 10w6M and
415
ANALYTICAL DATA
suggests that acetate forms non-extractable cadmium complexes in the aqueous phase, and that this decreases the distribution ratio. A later series of experiments done in the absence of cnha confirmed that cadmium is not extracted with acetate into MIBK. Composition of the extracted species and calculation of the equilibrium constants. Since variation of the
cadmium concentration demonstrated that only mononuclear species are extracted, the metal distribution ratio is given by ;;
[ML,(HL)J,
D=
(4) [Ml, + C [ML,], + c
P
PH
Fig.
2. Influence of pH on the distribution c* = 1.0 x IO-*M. c, = 9.0 x lo-*A4.
7.14 x tration bution species
ratio.
10m4M, at pH 7.1 and with a reagent concenof 1.0 x lo-‘M, had no effect on the distriratio, suggesting that only mononuci~r were extracted.
Influence
of pH
and ligand concentration.
The
influence of pH on the distribution ratio was studied for 1.0 x 10-zM cnha. The various pH values were obtained by addition of dilute perchloric acid to the aqueous phase. The results are plotted in Fig. 2. Because of the low buffer capacity of the aqueous phases, the pH measurements were very slow; therefore, in the subsequent experiments an acetic mixture, at total concentration acid-acetate 6.0 x lo-‘M, was added to the aqueous phases to facilitate the pH me~urement. Figure 3 shows values of log I), obtained for four different ligand concentrations, plotted us. pH. If the curve, in Fig. 2 is compared with that for the same reagent concentration in Fig. 3, it is evident that the curve for the acetate medium is displaced by about 0.4 pH units to higher values, although both curves coincide in the region of maximum extraction. This
I
I
I
I
4
5
6
7
1 6
PH
Fig. 3. Influence of pH on the distribution ratio in the presence of an acetic acid-acetate buffer. c, = 9.0 x to-JM. CC, = 05.0 x lo-‘M; cl 1.0 x lo-*M; 0 2.5 x 10-2M; A 5.0 x lo-‘M.
g
[MX,lw
where M represents the metal, HL the extracting agent, X another ligand present in the aqueous phase and the subscripts o and w denote the organic and the aqueous phases respectively. Charges have been omitted for simplicity. If only the species ML,,(HL), is extracted and the metal is present in the aqueous phase predominantly as the cation Mui, the following equation can be obtained: log D = log & + npH + (m + n)log[HL],
(5)
where X;,, the extraction constant of ML,(HL),, is K,,,KrJ3,K~IK$a (j,, = formation constant of ML,; Km =~s~bu~on constant of ML,; Km= adduct formation constant in the organic phase, Ka and KDR= dissociation and distribution constants of the reagent, respectively). When species other than ML, can be neglected in the aqueous phase, the following equation is valid: log I) = Iog(K, K,)
+ m log [HLf,
(6)
(a) Graphicul treatment The curve shown in Fig. 2 is linear up to pH 5.8, with a slope of 1.7. In accordance with equation (S), this suggests that n = 2, although the presence of a mixture of Cd2+ and CdL’ in the aqueous phase must be considered. The small variations of [HL], over the pH range 4.9-5.8 have been neglected in the slope analysis of the curve log D vs. pH. At higher pH values the slope of this curve decreases and finally, at pH 7.0, the distribution ratio becomes inde~ndent of the pH, minting to the validity of equation (7). The curves obtained in the presence of acetate in the aqueous phase (Fig. 3) are very similar to the curve in Fig. 2, with a linear segment having a slope of 1.5-1.6. To examine the iniiuence of cnha ~on~ntmtion on the distribution ratio, at constant pH, the values of log D corresponding to each reagent concentration were calculated from the curves in Fig. 3 at pH 5.10, 5.40, 5.50, 5.70, 5.90 and 6.00. These values, plotted against log [HLL, give straight lines with slopes of 1.8-1.9, and this, according to equation (S), suggests
ANALYTICAL
416
DATA
Table 1. Survey of the equilibrium constants for the extraction of Cd(I1) with cnha into methyl isobutyl ketone Method Graphical LETAGROPDISTR
-0.49
3.0
6.0
-0.49 f 0.01
2.60
5.981+ 0.004
0.0146 0.0618 0.0680 0.0673
2.82 rf:0.14 3.35 f 0.14* 2.70 & 0.07
*Value obtained in the presence of acetate in the aqueous phase. The limits eiven for the constants
(Wag 4;
correspond
- log D,,,,)2/W’2
that m + n = 2. Since all the curves in Fig. 3 tend to the same maximum value of log D, it is unlikely that any adduct species is extracted, so m = 0 and n = 2 and log D = log K,
(7)
The slopes of the distribution curves suggest that the species extracted is the simple 1:2 chelate CdL,. Its distribution constant, evaluated as the average of the distribution ratio values obtained at pH values above 7.0, is log K, = -0.489 + 0.008. To check the proposed model, as well as to calculate Koc and the stability constants /I1 and /I2 of CdL2 in the aqueous phase, Sillen’s curve-fitting method” was applied to the data obtained in the absence of acetate. The normalized curves are given by D* = u*/(l +pu + u*)
to log[B f 3a(jI)];
c(logD) =
tions where the complex CdL, is the predominant species in both phases. The final value of /I1 is given in Table 1. The distribution of cadmium among the various species was calculated by HALTAFALL.*’ Figures 4 and 5 show distribution diagrams calculated for an aqueous phase with and without acetate.
(8)
where the normalized variables and the parameter p are defined by D* = D/K,, II =fl:‘*[L-] and P = B1/SY2.
The values of the constants obtained by this method are given in Table 1. The good agreement between the experimental points and one of the normalized curves confirms the proposed model. (b) Numerical treatment
The values of the constants obtained graphically were refined by means of the program LETAGROPDISTR.‘* The data obtained in the absence of acetate were subdivided into two groups. From the group of points obtained at pH above 7.0, K, and /I2 were refined. The points at lower pH were used to obtain /Il. Treatment of all the data at once leads to less precise values of /.I2and Km. The results are given in Table 1. In the computer treatment of the data obtained in the presence of acetate in the aqueous phase, the formation of cadmium-acetate complexes must be taken into account. Literature values for the stability constants of these complexes” were adjusted for an ionic strength of 0.1 by the method of Linder and Murray.2Q The data obtained in the presence of acetate were used only to find the constant /I,; the other constants were not refined because, as can be seen in Fig. 3, there were few experimental points for the condi-
6
5
I
6
PH
Fig. 4. Distribution diagram of species.ccnhs= 1.0 x 10-2M.
a 0.4 -
0.3 CCdL& 0.2 -
0.1 -
5
6
7
6
PH
Fig. 5. Distribution diagram of species in the presence of acetate. cEllr = 1.0 x lo-*A4; caatpte= 0.06M.
417
ANALYTICAL DATA
Table 2. Influence of the presence of a second ligand on the degree of extraction (R) of cadmium; cFnh.= 1.Ox lo-‘A4 Ligand
Concentration
pH
R, %
5.0 x IO-2M
7.73 7.82 7.96 7.92 8.06 7.40 7.98 7.92 8.02 7.73
44 25 71 25 88
Trioctylamine Pyridine
1.0 x 10-3M
and y -picoline concentrations were kept constant at 9.0 x lo-rM and 5.0 x 10m2M, respectively. The results are shown in Fig. 6. Effect of pH and y -picoline concentration
Similarly, three series of extractions at varied pH and different y -picoline concentrations were carried out. The metal concentration was 9.0 x 10-SM and Zephiramine that of cnha 1.O x IO-*A4 in all cases. These results Tributyl phosphate ; are shown in Fig. 7. lo-2iu 28 Trioctylphosphine lo-)A!f 25 The influence of y-picoline concentration, in the oxide lo-2lu 93 region where the distribution ratio is independent of pH, was also studied. [B], was calculated from the values of the distribution constant of y -picoline (6.59) DISTRIBUTION OF CADMIUM-a&a-4-METHYLPYRIDINEand the dissociation constant of the picolinium ion TERNARY COMPLEX IN THE WATER-MIRKSYSTEM (7.00 x lo-‘) previously determined.22 Figure 8 shows a plot of log D us. log[B],. To enhance the extraction of cadmium by formation of a ternary complex, a second ligand was Composition of species extracted and calculation of the added to the system. The influence of various comadduct formation constant pounds on the degree of metal extraction was studied Taking into account the formation of ternary and the results are shown in Table 2. Of the complexes with y -picoline, the metal distribution substances found to enhance extraction, Cmethylratio is given by pyridine (y -picoline) was chosen as synergic agent because it is more readily available. WLI, + 1 W-J,Io To find out whether any cadmium-y -picoline complexes are extracted into MIBK in the absence of r ,? cnha, some extractions for pH = 5.4-7.5, [metal] = 9.0 x 10V5M and [y-picoline] = 5.0 x lo-*A4, were 1+ CKPI", ~,B,K:[HLI: tried. Since 99.0 + 0.5% of cadmium remained in the s i = aqueous phase at all pH values, the extraction of KnDrJHK1+ ~&[L]pw+ ~B,[xlt + CBJBI:, cadmium in the absence of cnha can be neglected. 5.0 x 1.0 x 5.0 x 5.0 x 1.0 x 5.0 x 1.0 x 5.0 x
4-Methylpyrldine
lo-2A4 lo-‘A4 lo-2M lo-2M 10-3M
)
i
E$ect of pH and cnha concentration
Three series of extractions at varied pH were performed, each series corresponding to one ligand concentration. In all the experiments the cadmium
P
4
I
(9) where, besides the symbols previously defined, B represents y -picoline and KS is the adduct formation constant in the organic phase. When the cation M”+ is the predominant species in the aqueous phase and [B], is kept constant,
1
logD=logK+nlog[HL],+npH
(10)
where K=(l+~KJB]:)~
a
If the metal is present in the aqueous phase predominantly as the complex ML,, equation (12) is deduced:
a B
(11)
-1
D = K,+ -2
5
6
7
6
Fig. 6. Influence of pH on the distribution ratio for Ccd= 9.0 x lo-SM. CrPloo,,M = the ternary complex. 5.0 x lo-*Iv. c,, = 0 5.0 x 10-3M; A 1.0 x 10-2M; 0 5.0 x lo-2M.
K,xK,[B]:,
s
(12)
(a) Graphical treatment The curves of log D vs. pH, plotted in Fig. 6, have linear segments with slope 1.8, so according to equation (lo), it is probable that n = 2. At higher pH values the slope decreases progressively to zero and the three curves coincide, a fact which confhms that self-adduct complexes with cnha are not extracted. The slope of the linear segments of the curves of log D vs. pH in Fig. 7 is 1.8. The values of log D corresponding to each y -picoline concentration were
ANALYTICAL DATA
418
concentration, the complex CdL, is the predominant species in both phases and equation (12) becomes IogD = K,; at higher concentrations the adduct CdLrB is the main species in the organic phase and equation (12) becomes log D = log&,&)
+ log[BJ,
(14)
which explains the existence of a linear segment with a slope close to 1. To calculate the adduct formation constant, a curve-fitting methodI was applied to the data of Fig. 8, The normalized curve D * = 1 f u was obtained by substituting the normalized variables D* = D/K, and u = &[B], in equation (12) for s = 1. The value log i& = 2.60 was obtained. (b) Numerical treatment The value of J& was refined by means of the program LETAGROP-DISTR,‘* the final result being log K, = 2.70 + 0.07 with o(log d) = 0.0673 (Table 1). I
I
6
6
PH
Fig. 7. Influence of pH on the distribution ratio for the ternary complex. c, = 9.0 x lO_jM. c,, = 1.0 x 10e2M. I+&,~ = 4 1.0 x lo-2M; A 5.0 x lo-2M; @ 1.0 x lo-‘M.
calculated from the curves in Fig. 7 at pH 5.30, 5.60, 5.90 and 6.20. These values, plotted against log [B],, give straight lines with a slope of 1.1. The former results suggest that, in the presence of y-picoline, the distribution equilibrium can be represented by the equation Cd;;W + 2HL,, + B.j=CdL&,j
+ 2H;:,
(13)
The data plotted in Fig. 8 were obtained in conditions where s = 1 in equation (12). At low y-picoline
I
I
I
I
-3
-2
-1
log
REFERENCES 1. J. W. Moore and S. Ra~moo~y,
CBI,
Fig. 8. Distribution of Cd(H) as a function of the equilibrium concentration of y-picoline in the organic phase. c, = 9.0 x IO-‘M. c&.&,= 1.0 x lo-*M. pH = 7.65-7.95.
Hearty Met&s in Natural Waters: Applkd Monitoring qnd Impact Assessment, pp. 29-31. Springer Verlag, New York, 1984. 2. T. Sekine and Y. Hasegawa, Solvent Extraction Chemistry; Fundamentals and Applications, pp. 615-617. Dekker, New York, 1977. 3. H. Onishi, Photometric Determination of Traces of Metals* Part IIA, 4th Ed., pp. 305-315. Wiley, New York, 1986. 4. A. Domeman and H. Kleist, Analyst, 1979, 104, 1030. 5. J. Kommarek, J. Have1 and L. Sommer, Collection Czech. Chem. Common., 1979, 44, 3241. 6. R. E. Sturgeon, S. S. Berman, A. Desaulniers and D. S. Russell, Taianta, 1980, 27, 85. 7. B. Nikolova and N. Jordanov, &id., 1982, 29, 861. 8. E. Yamada, E. Nakayama, T. Kuwamoto and T. Fujinaga. Anal. Chim. Acta, 1982, 138,409. 9. Idem, Bull. Chem. Sot. Japan, 1982, SS, 3155. 10. G. N. Rao and R. Lahiri, Proc. Indian Acad. Sci. Ser. Chem. Sci., 1983, 92A, 167. 11 M. Silva and M. Valcarcel, Mikrochim. Acta, 1983 I, 3i5. Ku. K. Onishchenko, I. V. Pyt2. T. A. ~ishchenko, atnitskii and V. V. Sukhan, Zh. A&it. I&m., 1986,41, 1040. Y. K. Agrawal and T. A. Desai, Analyst, 1986,111,305. t:: 0. Rauret. L. Pineda. M. Ventura and R. Comnafio, Talanta, 1986, 33, 14i. 15. G. Rauret, R. Compaiio, L. Pineda and J. M. Falgueras, An. Quim., 1987, s3, 82. 16. C. M. Lederer and V. S. Shirley (eds), Ta&Ieo~Isotopes, 7th Ed., p. 503. Wiley, New York, 1978. 17. L. G. Sill&n, Actu Chem. Stand., 1956, 10, 186. 18. D. Hay Liem, ibid., 1971, 25, 1521. 19. A. E. Martell and R. M. Smith, Critical Stability Constants, Vol. 3, Plenum Press, New York, 1977. 20. P. W. Linder and K. Murray, Talanta, 1982, 29, 377. 21. N. Ingri, W. Kakolowicz, L. G. Sillen and B. Wamquist, Ta~~ta, 1967, 14, 1261; errata, 1968, 15, No. 3, ix. 22. G. Rauret, L. Pineda and R. Cornpatio, An. Quim., in the press.
Copyright 8
~3~91~/88 $3.00 + 0.00 1988 Fbgamon Rws pk
SOLVENT EXTRACTION OF Cd(I1) WITH lV-CYCLOHEXYL-N-NITROSOHYDROXYLAMINE AND 4-~~~FYRI~INE INTO METHYL ISOBUTYL KETONE G.
RMJRET,
L.
PINEDA,
R. C!~sns and R.
COMPAI~O
Department of Analytical Chemistry, University of Barcelona, Barcelona, Spain A.
SAMXEZ--
Department of Fundame&al Physics, University of Barcelona, Barcelona, Spain (Received I4 August 1987. Revised 6 Umber 1987. Accepted 8 &cenr&cr 1987) Summary--The distribution equilibria of the complexes cadmium-cnha and cadmimn-cnha-4methylpyridine in the water-methyl isobutyl ketone system have been studied at 25”, by using ‘“Cd as a radiotracer to measure the metal distribution ratio. A very sensitive method for detection of ‘““Cd, based on the use. of a liquid scintillator, has been developed. From the 8raphical treatment of the ~~b~~ data, it has been deduced that CdL, is the compiex extracted in the absence of ~~~ylpy~~ne, and that the adduct Cd&B is extracted when the second i&and is present. This model has been checked by treating the data with the program LETAGRGP-DISTR and the following equilibrium constants have been obtained: stability constants of CdL,, log& = 2.82 It 0.14, log & 3: 5.981 f 0.004, distribution constant of CdLz, log K, = -0.49 f 0.01; adduct formation constant of CdL,B, log & = 2.70 & 0.07.
The industrial applkatims of cadmium have increased considerably in recent years.’ Because this element is hi&ly toxic even at very low concentrations it must be determined in enviromental samples. Although some analytical methods for cadmium determination are very sensitive, low ~n~ntra~ons are generally determined after use of a preconcentration technique such as solvent extraction. Several authors have proposed systems for cadmium extraction.z-‘3 Reagents such as dithiione,2p3 thiouxineT3 and some di~~~at~,4,6,~ co-ordinating through sulphur atoms, have often been studied. When the co-ordination takes place through nitrogen or oxygen atoms, a second ligand (pyridine, picolines, l,lO-phenanthroline, TOPO, TBP, . . .) has often been added to the system to enhance the extraction as a result of formation of a ternary complex.b’O~lz In the present paper, as a part of a wider study to assess the utility of ~~yclohexyl*~-~trosohy~oxylamine @ha) as an agent for extracting metals into methyl isobutyl ketone (MIBK),‘4Js we report the study of the distribution equilibria of Cd-cnha and Cd-cnba-4-methylpyridine complexes. The cnhaMIBK system has proved useful for the determination of Cu(I1) in river water by atomicabsorption spectrometry*4 and its application to the determination of other heavy metals in natural and waste waters is now being studied. The use of radioactive tracers is one of the best methods for studying the distribution of metals be-CAL. 3Sif-F
tween immiscible phases. In the present work, rosCd was used and a very sensitive method for its detection, based on the use of a liquid scintillator, has been developed.
&?ag#zts The sodium salt of Ncycloh~l-N-~trosohydraxyfamine was obtained from the potassium salt (BASP) as described earlier.“’ A stock solution of cadmium (1 g/l.) was made by dissolving the metal (analytical-reagent grade) in a small vohtme of concentrated nitric acid, then adding perchloric acid and boiling until white fumes were evolved. The solution was cooled, transferred to a 1-Iitre standard thtsk and diluted to the mark. Carrier-free tWCd (t,,r 453 d) was supplied by Amersham International as CdCl,. The radiotracer was added to the cadmium solution before the extraction. Analytical~rea8ent grade pyridine (Fluka), tri-n-butyl phosphate (Carlo Erbaf, ~-n-~~lph~ oxide iMerck) and ~1~~~yI~~d~yl~~~~ chloride t&k& Trioctylamine, 95% (Merck) and cl-methvlnvridine. 96% (Fluk& The content bf 4-metbylpyridineVii ihe comtnercially available product was checked by gas chromatography, Liquid scintillation cocktail Insta-Gel (packard). All other chemicals wem of analytical grade and were used without further purl&&ion. A coaxial Ge(Li) semiconductor detector (Grtec), with an effective volme of 58 cm3, cormected to a 4092”chamrel multichannel analyser (Silena) was used for gamma-ray measurements~ A dual liquid-scintillation counter (Bertold LB 5048) was used for beta part& measurements.
413
414
ANALYTICAL
DATA
A pa-meter (Radiometer PHM 64), equipped with a glass+alomel electrode pair, and standardized with buffer solutions at pH 4.008 and 6.865 (25”) prepared from Merck salts according to DIN 19266, was used.
Procedure Ten ml of aqueous phase saturated with the organic solvent and containing an appropriate concentration of cadmium (labelled with ‘@‘Cd),the sodium salt of cnha and Cmethylpyridine (the last only when the distribution of the ternary complex was studied) were shaken with 10 ml of MIBR, saturated with water, in a thermostatic bath at 25.0 + - 0.1” for 15 min. The DH of the aaueous chase was adjusted with either perchloric acid or an acetic aid-sodium acetate buffer, and sodium perchlorate was added to give a constant ionic strength of 0.1. After phase separation, the metal distribution ratio (0) was measured by one of the following procedures. (a) For D > 0.1 equal volumes (4 ml) of both phases were measured by pipette into suitable counting tubes and their activities measured at constant geometry. Then
D = AJA,
(1)
where A, and A, are the activities of the organic and aqueous phases respectively. Q) For D < 0.1, 4 ml of each aqueous phase were taken and their y-activities determined; the same volume of each organic phase was mixed with 15 ml of the liquid scintillator and the activity measured. The distribution ratio was calculated by means of the expression D = A, (liquid scintillator) x F
A, (gamma) where F is defined as
F=
Activity of organic phase (gamma) Activity of organic phase (liquid scintillator)
(3)
and is found for each series of experiments by use of organic phases with activities measurable accurately by means of both techniques. The quench of the samples was determined by the channel ratio-external standard method. All the samples of each series showed the same quench. RESULTS AND DISCUSSION
Measurement of ‘09Cd activity The nuclear transformation
of ‘@‘Cd takes place by
electron capture to give “‘@“Ag,which decays to the ground state by an 88-keV gamma transition (N 3.8%) or emission of internal conversion electrons ( w96.2%).16 When the metal solution is labelled with ‘09Cd, the distribution ratio may be determined, after phase equilibration, as the ratio of the gamma activity of the organic phase to that of the aqueous phase. However, the low probability of gamma-ray emission, together with the low efficiency (-4%) of the Ge(Li) detector for the geometry used, made it necessary, when D was lower than 0.1, to increase the quantity of radiotracer added to the system or to use long counting periods in order to measure the activity of the organic phase with adequate accuracy. The advantageous features of liquid scintillation measurements for detecting low-energy beta emitters seemed favourable for the detection of internal conversion electrons of lBCd and also the subsequent Auger electrons. Since these electrons are monoenergetic, their pulse-height distribution curve will be
Channel
number
Fig. 1. Spectrum of the Auger and internal conversion electrons of ‘09Cd.
peak-shaped and the area under the curve will give their total number. Figure 1 shows the spectrum obtained by mixing a solution of known activity of ‘09Cd with the liquid scintillator. The lowest energy peak, at 18 keV, corresponds to the Auger electrons and the 65 and 84 keV peaks correspond to the internal conversion electrons. The spectrum may contain a small contribution from the gamma-rays, which can interact with the scintillator and produce an electric pulse. From the total number of counts, obtained from integration of the spectrum and the activity of the ‘@Cd solution, an overall apparent efficiency of 120% was obtained, because of the simultaneous detection of the Auger and internal conversion electrons. Hence, this method is 800 times as sensitive as the Ge(Li) detector method, for the same counting time, and is suitable for measuring the activity of the organic phases when the distribution ratio is very low. Distribution of the cacimium-cnha binary complex in the water-MIBK system Influence of shaking time. The distribution ratio values obtained at pH 7.20, with shaking times between 1 and 60 min, show that the distribution equilibrium is attained after 5 min of shaking. In subsequent experiments an extraction time of 15 min was adopted. Influence of ionic strength. The ionic strength of the aqueous phase was varied over the range O.Ol-l.OM by the addition of sodium perchlorate. The distribution ratio, at constant pH and cnha concentration, was independent of ionic strength up to 0.3M; at higher values, the distribution ratio slightly decreased. In the rest of the work the ionic strength was adjusted to 0.1&f. Injluence of metal concentration. Varying the cadmium concentration between 8.93 x 10w6M and
415
ANALYTICAL DATA
suggests that acetate forms non-extractable cadmium complexes in the aqueous phase, and that this decreases the distribution ratio. A later series of experiments done in the absence of cnha confirmed that cadmium is not extracted with acetate into MIBK. Composition of the extracted species and calculation of the equilibrium constants. Since variation of the
cadmium concentration demonstrated that only mononuclear species are extracted, the metal distribution ratio is given by ;;
[ML,(HL)J,
D=
(4) [Ml, + C [ML,], + c
P
PH
Fig.
2. Influence of pH on the distribution c* = 1.0 x IO-*M. c, = 9.0 x lo-*A4.
7.14 x tration bution species
ratio.
10m4M, at pH 7.1 and with a reagent concenof 1.0 x lo-‘M, had no effect on the distriratio, suggesting that only mononuci~r were extracted.
Influence
of pH
and ligand concentration.
The
influence of pH on the distribution ratio was studied for 1.0 x 10-zM cnha. The various pH values were obtained by addition of dilute perchloric acid to the aqueous phase. The results are plotted in Fig. 2. Because of the low buffer capacity of the aqueous phases, the pH measurements were very slow; therefore, in the subsequent experiments an acetic mixture, at total concentration acid-acetate 6.0 x lo-‘M, was added to the aqueous phases to facilitate the pH me~urement. Figure 3 shows values of log I), obtained for four different ligand concentrations, plotted us. pH. If the curve, in Fig. 2 is compared with that for the same reagent concentration in Fig. 3, it is evident that the curve for the acetate medium is displaced by about 0.4 pH units to higher values, although both curves coincide in the region of maximum extraction. This
I
I
I
I
4
5
6
7
1 6
PH
Fig. 3. Influence of pH on the distribution ratio in the presence of an acetic acid-acetate buffer. c, = 9.0 x to-JM. CC, = 05.0 x lo-‘M; cl 1.0 x lo-*M; 0 2.5 x 10-2M; A 5.0 x lo-‘M.
g
[MX,lw
where M represents the metal, HL the extracting agent, X another ligand present in the aqueous phase and the subscripts o and w denote the organic and the aqueous phases respectively. Charges have been omitted for simplicity. If only the species ML,,(HL), is extracted and the metal is present in the aqueous phase predominantly as the cation Mui, the following equation can be obtained: log D = log & + npH + (m + n)log[HL],
(5)
where X;,, the extraction constant of ML,(HL),, is K,,,KrJ3,K~IK$a (j,, = formation constant of ML,; Km =~s~bu~on constant of ML,; Km= adduct formation constant in the organic phase, Ka and KDR= dissociation and distribution constants of the reagent, respectively). When species other than ML, can be neglected in the aqueous phase, the following equation is valid: log I) = Iog(K, K,)
+ m log [HLf,
(6)
(a) Graphicul treatment The curve shown in Fig. 2 is linear up to pH 5.8, with a slope of 1.7. In accordance with equation (S), this suggests that n = 2, although the presence of a mixture of Cd2+ and CdL’ in the aqueous phase must be considered. The small variations of [HL], over the pH range 4.9-5.8 have been neglected in the slope analysis of the curve log D vs. pH. At higher pH values the slope of this curve decreases and finally, at pH 7.0, the distribution ratio becomes inde~ndent of the pH, minting to the validity of equation (7). The curves obtained in the presence of acetate in the aqueous phase (Fig. 3) are very similar to the curve in Fig. 2, with a linear segment having a slope of 1.5-1.6. To examine the iniiuence of cnha ~on~ntmtion on the distribution ratio, at constant pH, the values of log D corresponding to each reagent concentration were calculated from the curves in Fig. 3 at pH 5.10, 5.40, 5.50, 5.70, 5.90 and 6.00. These values, plotted against log [HLL, give straight lines with slopes of 1.8-1.9, and this, according to equation (S), suggests
ANALYTICAL
416
DATA
Table 1. Survey of the equilibrium constants for the extraction of Cd(I1) with cnha into methyl isobutyl ketone Method Graphical LETAGROPDISTR
-0.49
3.0
6.0
-0.49 f 0.01
2.60
5.981+ 0.004
0.0146 0.0618 0.0680 0.0673
2.82 rf:0.14 3.35 f 0.14* 2.70 & 0.07
*Value obtained in the presence of acetate in the aqueous phase. The limits eiven for the constants
(Wag 4;
correspond
- log D,,,,)2/W’2
that m + n = 2. Since all the curves in Fig. 3 tend to the same maximum value of log D, it is unlikely that any adduct species is extracted, so m = 0 and n = 2 and log D = log K,
(7)
The slopes of the distribution curves suggest that the species extracted is the simple 1:2 chelate CdL,. Its distribution constant, evaluated as the average of the distribution ratio values obtained at pH values above 7.0, is log K, = -0.489 + 0.008. To check the proposed model, as well as to calculate Koc and the stability constants /I1 and /I2 of CdL2 in the aqueous phase, Sillen’s curve-fitting method” was applied to the data obtained in the absence of acetate. The normalized curves are given by D* = u*/(l +pu + u*)
to log[B f 3a(jI)];
c(logD) =
tions where the complex CdL, is the predominant species in both phases. The final value of /I1 is given in Table 1. The distribution of cadmium among the various species was calculated by HALTAFALL.*’ Figures 4 and 5 show distribution diagrams calculated for an aqueous phase with and without acetate.
(8)
where the normalized variables and the parameter p are defined by D* = D/K,, II =fl:‘*[L-] and P = B1/SY2.
The values of the constants obtained by this method are given in Table 1. The good agreement between the experimental points and one of the normalized curves confirms the proposed model. (b) Numerical treatment
The values of the constants obtained graphically were refined by means of the program LETAGROPDISTR.‘* The data obtained in the absence of acetate were subdivided into two groups. From the group of points obtained at pH above 7.0, K, and /I2 were refined. The points at lower pH were used to obtain /Il. Treatment of all the data at once leads to less precise values of /.I2and Km. The results are given in Table 1. In the computer treatment of the data obtained in the presence of acetate in the aqueous phase, the formation of cadmium-acetate complexes must be taken into account. Literature values for the stability constants of these complexes” were adjusted for an ionic strength of 0.1 by the method of Linder and Murray.2Q The data obtained in the presence of acetate were used only to find the constant /I,; the other constants were not refined because, as can be seen in Fig. 3, there were few experimental points for the condi-
6
5
I
6
PH
Fig. 4. Distribution diagram of species.ccnhs= 1.0 x 10-2M.
a 0.4 -
0.3 CCdL& 0.2 -
0.1 -
5
6
7
6
PH
Fig. 5. Distribution diagram of species in the presence of acetate. cEllr = 1.0 x lo-*A4; caatpte= 0.06M.
417
ANALYTICAL DATA
Table 2. Influence of the presence of a second ligand on the degree of extraction (R) of cadmium; cFnh.= 1.Ox lo-‘A4 Ligand
Concentration
pH
R, %
5.0 x IO-2M
7.73 7.82 7.96 7.92 8.06 7.40 7.98 7.92 8.02 7.73
44 25 71 25 88
Trioctylamine Pyridine
1.0 x 10-3M
and y -picoline concentrations were kept constant at 9.0 x lo-rM and 5.0 x 10m2M, respectively. The results are shown in Fig. 6. Effect of pH and y -picoline concentration
Similarly, three series of extractions at varied pH and different y -picoline concentrations were carried out. The metal concentration was 9.0 x 10-SM and Zephiramine that of cnha 1.O x IO-*A4 in all cases. These results Tributyl phosphate ; are shown in Fig. 7. lo-2iu 28 Trioctylphosphine lo-)A!f 25 The influence of y-picoline concentration, in the oxide lo-2lu 93 region where the distribution ratio is independent of pH, was also studied. [B], was calculated from the values of the distribution constant of y -picoline (6.59) DISTRIBUTION OF CADMIUM-a&a-4-METHYLPYRIDINEand the dissociation constant of the picolinium ion TERNARY COMPLEX IN THE WATER-MIRKSYSTEM (7.00 x lo-‘) previously determined.22 Figure 8 shows a plot of log D us. log[B],. To enhance the extraction of cadmium by formation of a ternary complex, a second ligand was Composition of species extracted and calculation of the added to the system. The influence of various comadduct formation constant pounds on the degree of metal extraction was studied Taking into account the formation of ternary and the results are shown in Table 2. Of the complexes with y -picoline, the metal distribution substances found to enhance extraction, Cmethylratio is given by pyridine (y -picoline) was chosen as synergic agent because it is more readily available. WLI, + 1 W-J,Io To find out whether any cadmium-y -picoline complexes are extracted into MIBK in the absence of r ,? cnha, some extractions for pH = 5.4-7.5, [metal] = 9.0 x 10V5M and [y-picoline] = 5.0 x lo-*A4, were 1+ CKPI", ~,B,K:[HLI: tried. Since 99.0 + 0.5% of cadmium remained in the s i = aqueous phase at all pH values, the extraction of KnDrJHK1+ ~&[L]pw+ ~B,[xlt + CBJBI:, cadmium in the absence of cnha can be neglected. 5.0 x 1.0 x 5.0 x 5.0 x 1.0 x 5.0 x 1.0 x 5.0 x
4-Methylpyrldine
lo-2A4 lo-‘A4 lo-2M lo-2M 10-3M
)
i
E$ect of pH and cnha concentration
Three series of extractions at varied pH were performed, each series corresponding to one ligand concentration. In all the experiments the cadmium
P
4
I
(9) where, besides the symbols previously defined, B represents y -picoline and KS is the adduct formation constant in the organic phase. When the cation M”+ is the predominant species in the aqueous phase and [B], is kept constant,
1
logD=logK+nlog[HL],+npH
(10)
where K=(l+~KJB]:)~
a
If the metal is present in the aqueous phase predominantly as the complex ML,, equation (12) is deduced:
a B
(11)
-1
D = K,+ -2
5
6
7
6
Fig. 6. Influence of pH on the distribution ratio for Ccd= 9.0 x lo-SM. CrPloo,,M = the ternary complex. 5.0 x lo-*Iv. c,, = 0 5.0 x 10-3M; A 1.0 x 10-2M; 0 5.0 x lo-2M.
K,xK,[B]:,
s
(12)
(a) Graphical treatment The curves of log D vs. pH, plotted in Fig. 6, have linear segments with slope 1.8, so according to equation (lo), it is probable that n = 2. At higher pH values the slope decreases progressively to zero and the three curves coincide, a fact which confhms that self-adduct complexes with cnha are not extracted. The slope of the linear segments of the curves of log D vs. pH in Fig. 7 is 1.8. The values of log D corresponding to each y -picoline concentration were
ANALYTICAL DATA
418
concentration, the complex CdL, is the predominant species in both phases and equation (12) becomes IogD = K,; at higher concentrations the adduct CdLrB is the main species in the organic phase and equation (12) becomes log D = log&,&)
+ log[BJ,
(14)
which explains the existence of a linear segment with a slope close to 1. To calculate the adduct formation constant, a curve-fitting methodI was applied to the data of Fig. 8, The normalized curve D * = 1 f u was obtained by substituting the normalized variables D* = D/K, and u = &[B], in equation (12) for s = 1. The value log i& = 2.60 was obtained. (b) Numerical treatment The value of J& was refined by means of the program LETAGROP-DISTR,‘* the final result being log K, = 2.70 + 0.07 with o(log d) = 0.0673 (Table 1). I
I
6
6
PH
Fig. 7. Influence of pH on the distribution ratio for the ternary complex. c, = 9.0 x lO_jM. c,, = 1.0 x 10e2M. I+&,~ = 4 1.0 x lo-2M; A 5.0 x lo-2M; @ 1.0 x lo-‘M.
calculated from the curves in Fig. 7 at pH 5.30, 5.60, 5.90 and 6.20. These values, plotted against log [B],, give straight lines with a slope of 1.1. The former results suggest that, in the presence of y-picoline, the distribution equilibrium can be represented by the equation Cd;;W + 2HL,, + B.j=CdL&,j
+ 2H;:,
(13)
The data plotted in Fig. 8 were obtained in conditions where s = 1 in equation (12). At low y-picoline
I
I
I
I
-3
-2
-1
log
REFERENCES 1. J. W. Moore and S. Ra~moo~y,
CBI,
Fig. 8. Distribution of Cd(H) as a function of the equilibrium concentration of y-picoline in the organic phase. c, = 9.0 x IO-‘M. c&.&,= 1.0 x lo-*M. pH = 7.65-7.95.
Hearty Met&s in Natural Waters: Applkd Monitoring qnd Impact Assessment, pp. 29-31. Springer Verlag, New York, 1984. 2. T. Sekine and Y. Hasegawa, Solvent Extraction Chemistry; Fundamentals and Applications, pp. 615-617. Dekker, New York, 1977. 3. H. Onishi, Photometric Determination of Traces of Metals* Part IIA, 4th Ed., pp. 305-315. Wiley, New York, 1986. 4. A. Domeman and H. Kleist, Analyst, 1979, 104, 1030. 5. J. Kommarek, J. Have1 and L. Sommer, Collection Czech. Chem. Common., 1979, 44, 3241. 6. R. E. Sturgeon, S. S. Berman, A. Desaulniers and D. S. Russell, Taianta, 1980, 27, 85. 7. B. Nikolova and N. Jordanov, &id., 1982, 29, 861. 8. E. Yamada, E. Nakayama, T. Kuwamoto and T. Fujinaga. Anal. Chim. Acta, 1982, 138,409. 9. Idem, Bull. Chem. Sot. Japan, 1982, SS, 3155. 10. G. N. Rao and R. Lahiri, Proc. Indian Acad. Sci. Ser. Chem. Sci., 1983, 92A, 167. 11 M. Silva and M. Valcarcel, Mikrochim. Acta, 1983 I, 3i5. Ku. K. Onishchenko, I. V. Pyt2. T. A. ~ishchenko, atnitskii and V. V. Sukhan, Zh. A&it. I&m., 1986,41, 1040. Y. K. Agrawal and T. A. Desai, Analyst, 1986,111,305. t:: 0. Rauret. L. Pineda. M. Ventura and R. Comnafio, Talanta, 1986, 33, 14i. 15. G. Rauret, R. Compaiio, L. Pineda and J. M. Falgueras, An. Quim., 1987, s3, 82. 16. C. M. Lederer and V. S. Shirley (eds), Ta&Ieo~Isotopes, 7th Ed., p. 503. Wiley, New York, 1978. 17. L. G. Sill&n, Actu Chem. Stand., 1956, 10, 186. 18. D. Hay Liem, ibid., 1971, 25, 1521. 19. A. E. Martell and R. M. Smith, Critical Stability Constants, Vol. 3, Plenum Press, New York, 1977. 20. P. W. Linder and K. Murray, Talanta, 1982, 29, 377. 21. N. Ingri, W. Kakolowicz, L. G. Sillen and B. Wamquist, Ta~~ta, 1967, 14, 1261; errata, 1968, 15, No. 3, ix. 22. G. Rauret, L. Pineda and R. Cornpatio, An. Quim., in the press.