Extraction of copper(II) with aliphatic carboxyclic acids

J. inorg, nucl. Chem., 1970, Vol. 32, pp. 1333 to 1340. Pergamon Press.

EXTRACTION

Printed in Great Britain

OF COPPER(II) WITH CARBOXYLIC ACIDS

ALIPHATIC

ISAO KOJIMA, M A S A O U C H I D A and M O T O H A R U T A N A K A Laboratory of Analytical Chemistry, Faculty of Science, Nagoya University, Chikusa-ku, Nagoya, Japan

(Received 28 April 1969) Abstract-Extraction of copper(II) with benzene solution of aliphatic carboxylic acids from 0.1 M (Na, H)C104 solution have been studied at 25°C. Aliphatic carboxylic acids used are n-butyric, nvaleric, n-caproic, n-heptanoic, n-caprylic, n-pelargonic, and n-capric acids. The extracted species being dimeric complex, (CuRs" HR)2, the extraction equilibrium is written as: K

2CuZ++3(HR)2.o ~- (CuR2.HR)~o÷4H *. Despite the increase in the distribution coefficient of carboxylic acid with increasing number of carbon atoms in carboxylic acid, the extraction constant, K~x, remains constant irrespective of the number of carbon atoms in carboxytic acid. This is quantitatively explained by means of the regular solution theory: the distribution coefficient of metal carboxylates increases with increasing number of carbon atoms involved in the carboxylates to the same extent as in the case of carboxylic acids, INTRODUCTION

THE EXTRACTED species involved in the extraction of copper, nickel, cobalt and

iron(II 1) with carboxylic acids has been recently established [ 1-4]. The extracted species of copper with capric acid dissolved in benzene[l] and with naphthenic acid [2] has been found to be the dimerized copper carboxylate mono (carboxylic acid), (CuR~. H R)2. After the distribution study of carboxylic acids between 0.1 M (Na, H)CIO4 solution and different solvents, we estimated distribution coefficients and dimerization constants of carboxylic acid and showed that the increment of distribution coefficient of carboxylic acid for an added methylene group is almost constant, A log KJa CH2 -- 0.56--0.60, irrespective of organic solvents. This was quantitatively explained by means of the regular solution theory [5], In the present paper, extraction of copper with aliphatic carboxylic acids with different number of carbon atoms is studied and extraction equilibrium is interpreted on the basis of the regular solution theory. EXPERIMENTAL

Reagents All of carboxylic acids and benzene was obtained from Wako Pure Chemical Co., Osaka, Japan. These acids were purified by distilling twice and by collecting a fraction distilled at constant tempera1. M. Tanaka and T. Niinomi, J. inorg, nucl. Chem. 27, 431 (1965). 2. A. W. Fletcher and D. S, Flett, Solvent Extraction Chemistry of Metals, p. 359. MacMillan. New York (1966). 3. M. Tanaka, N, Nakasuka and S. Sasane, J. inorg, nucl. Chem. 31,2591 (1969). 4. M. Tanaka, N. Nakasuka and S. Goto, Solvent Extraction Chemistry, p. 154, North-Holland, Amsterdam (1967). 5. I. Kojima, M. Yoshida and M. Tanaka, J. inorg, nucl. Chem. 32, 987 (1970). 1333

1334

I. KOJIMA, M. U C H I D A and M. T A N A K A

ture. Benzene of G.R. grade was purified by shaking first with concentrated sulfuric acid, by washing with dilute sodium hydroxide solution, then with dilute hydrochloric acid solution and finally five times with distilled water and was used without dehydration. Copper perchlorate used was prepared as described previously [6]. Procedure All experiments were carried out in a room thermostatted at 25 _+ I°C. 4-(2-thiazolylazo)resorcinol (TAR) was used as an indicator in the compleximetric determination of copper in the aqueous phase. Stock solution of carboxylic acids were prepared by dilution of the acids with benzene. The ionic strength of the aqueous phase was kept constant at 0.1 M (Na, H)CIO4. The initial volume of the aqueous and organic phases was always 15 ml. The concentration of copper in both phases was below 10 -2 M. The aqueous and organic phases were contacted in a separatory funnel on a reciprocating shaker for overnight (200-250 strokes/min). After the equilibrium, both phases were allowed to stand for more than 30 min for a complete phase separation. The concentration of copper in the organic phase was directly determined photometrically at 680 nm and that in the aqueous phase compleximetrically at pH 4-8. Hydrogen ion concentration was measured with Radiometer type priM-22 pH meter, 10-2 M HCIO4 solution being used as a standard.

EQUILIBRIUM

TREATMENT

Suppose that the extracted species of c o p p e r with carboxylic acid used in the present study is the same as in the extraction with capric acid[l], i.e. the dimerized copper caprate mono (capric acid), then the extraction equilibrium can be written as: K,x

2Cu 2+ + 3 (HR) 2,o ~ (CuRs.HR)2.o + 4 H +

( 1)

where overall extraction constant, Kex, is given by

[(CuR2"HR)s]o[H+]4 [CuS+]2[(HR)2]oZ

K~x =

(2)

This constant can be rewritten as: Kex

--

~ C2u R 2

.

--4. K s --2 "Kd --4 K o 2 . Kf 2 Kdi m. Krla

(3)

where the constants involved are given as follows: Cu 2+ + 2R- ,--- CuR2

(4)

CuR2 ~ CuR2 o

(5)

CuR2,o + (HR)2,o ~ CuR2"2HR(o~

(6)

2 C u R s ' 2 H R ( o ) K"~~--S ( C u R s ' H R ) 2 , o + (HR)2,o

(7) (8)

Km~

H++R - ~ HR Kd

H R ~ HR(o)

2HR(o) ~ (HR)s,o

,

6. I. Kojima, J. Fukuta and M. Tanaka, J. inorg, nucl. Chem. 31, 1815 (1969).

(9) (10)

Extraction of copper( I I )

13 3 5

In these equilibria, the subscript O refers to the organic phase. Then the concentration of copper in the aqueous and organic phases are given by: Ccu,w = [Cu 2+] + [Cu R+] + [CUR2] (ll)

2R -- 2 = [Cu2+](l+flca.a[R-] +flC.R2[R ] )

= [Cu~+],~R(c.)

(12)

Ccu.o = 2[(CuR2"HR)2]o

where aR(cu) is the side reaction coefficient taking into account only the complex formation of copper with carboxylate ion, no hydrolysis of copper occurring under the present experimental condition. When carboxylic acids are distributed between both phases, the following equations hold: C.r~.o =

[HR]o

+

(13)

2[(HR)2]o

(14)

CHR.w = CHR--CHR.o = [HR] ÷ [ R - ] . Combining Equations (13) and (14), we have CUR-- CHR w

CHR w

1

-'

Then the concentration of carboxylic acid in the aqueous phase is calculated by means of Equation (16). C.R.w = -- (Ho~-dR)+ 1)/4Kd 2"K2"a~-r~R) -2 1/2 . --2 "+-[(KdO~-IR)÷I)2 ÷ 8 K d2. K2"OqHR)'CHR] /4Kd2. K20t(HR)

where ~(.m = 1 + 1/K.R[H+].

(16)

Combining Equations (2), (11 ) and (12), we have 2 --2 Ccu.o = 2Kex[(Hr)2]o 3 [H + ] - - 4 Ccu,wt~a(cu).

(17)

Then the plot of log Cc~,o against (log Ccu.w+ 2 p H - log aR(Cu)) at constant concentration of carboxylic acid dimer in benzene should yield a straight line with a slope of 2 and the plot of log Ccu.o- 2(log Ccu.w+ 2pH--log aR(Cu)) against log [(HR)2] should yield a straight line with a slope of 3. RESULTS AND DISCUSSION

Extraction of copper as a function of pH is demonstrated in Fig. 1 - the greater the distribution coefficient of an acid used as an extractant, the better the extraction of copper. In the case of capric acid, the plot of log Ccu.o against (log Ccu.w+ 2pH - log aa(cu)) at constant [(HR)2]o gives rise to a straight line with a slope of 2 at the concentration of copper down to 10 -5 M in the organic phase. For the same system,

1336

I. K O J I M A , M. U C H I D A and M. T A N A K A

IOO E%

C3HrCOOH

Z~ C4H9COOH o C5H,,COOH

80~

.

8

:=

J.~,,/"Jf~ / / / /

c,,,coo.

; c,..,coo.

60

x

4C

d~ 20

i

3'.1

3-3

i

3-7

3.5

i

I

i

3-9 pH

4"1

4"3

I

4"5

i

4.7

Fig. 1. Extraction of copper with carboxylic acids. Initial concentration of carboxylic

acids in benzene: 1 M.

the plot of log Ccu.o- 2(log Ccu,w + 2 p H - log o~n(cu))against log [(HR)2] yields a straight line with a slope of 311]. Thus the extracted species is the dimerized copper caprate mono (capric acid): (CuR~.HR)~. Exactly the same results were obtained in the case of carboxylic acid having seven to nine carbon atoms. In the extraction with a carboxylic acid having the number of carbon atoms less than or equal to 6, the plot of log Ccu,o against (log Ccu,w + 2pH - log an(cu)) yielded a straight line with a slope of less than 2 (see Fig. 2). This phenomenon is due to the

-2.0

CBHIrCOOH ~ C6HI3COOH

-2.0

/~//

i!!!;:= /./

qI~,/ -2.5

-2.5 o

o

d (..) _Q

d

¢J

-3-5

-55

-4.G

~ 50

4-5

IogCcu..

+

'

5~.5

2pH- log ~RIc,)

-4-0

•

I

J

4.5

IogCc=.,

+

I

!

5O

5-5

2oH- Iog~,(c. }

Fig. 2. Determination of the degree of polymerization of the extracted species. Initial

concentration of carboxylic acids in benzene: 1M.

Extraction of copper(ll)

1337

increase of the partition of carboxylic acid into the aqueous phase and the decrease in the concentration of the dimerized species of carboxylic acid according to the monomer-dimer equilibrium in benzene. After the correction for the decrease in the concentration of the dimerized acid, the plot of log Ccu.o - 2(log Ccu. + 2 p H - log cq~tcu)) against log [(HR)2]o yielded a straight line with a slope of 3. From these results demonstrated in Fig. 3, it is confirmed that the extracted species is the dimerized copper carboxylate mono-(carboxylic acid) and that the extraction equilibrium is expressed as Equation (1). The extraction constant, Kex, for various copper carboxylates are summarized in Table 1 together with the distribution coefficient, the dimerization constant, the formation constant of carboxylic acids and the formation constants of copper carboxylates. It is evident from Table 1 that K~x is constant irrespective of the number of carbon atoms in carboxylic acids, despite the steady increase in the partition constant of carboxylic acid with increasing number of carbon atoms involved. Table 1. Formation c o n s t a n t s of carboxylic acids and copper carboxylates, distribution coefficient and dimerization constant of carboxylic acids and overall extraction c o n s t a n t s Acids Acetic Propionic Butyric Valeric Caproic Heptanoic Caprylic Pelargonic Capric

log K~R Iog/3cuR? 1og/3cuR2t log Kd 4.78 4.87 4.82 4.86 4.88 4.89 4.89 4.95

1.79 1.86 1.82 1.92 1-92

2.94 3.00 2.98 < 3 2.98

log K2 log Kex

-2,07 - 1'36 --0.79 --0.16 0.31

2-16 2.21 2.28 2.36 2.45

2-40

2-85

-- 11 "50 --11.5~ -- 11.4~ - 11.5s -- 11 "65 -- 11-5~ - 11 "5~

*G. Kortiim, W. Vogel and K. A n d r e s s o w . Dissociation constants of organic acids in aqueous solution. Butterworths, l.ondon (1961). t L. G. Sillrn and A. E. Martell, Stability constants of metal-ion complexes. T h e Chemical Society, L o n d o n (1964).

The extraction constant Kex is composed of various constants as seen from Equation (3). Among these constants, formation constants of carboxylic acid and copper carboxylates are nearly constant regardless of the number of carbon atoms in carboxylates. In addition, for aliphatic monodentate ligands L F E R (Linear Free Energy Relationship) of unit slope is reasonably expected between the stepwise formation constant of a complex and the formation constant of the conjugate acid of the ligand. Therefore flCua2KHa 2 -4 is expected to be constant for a series of carboxylic acids used in the present study. Quite analogously Kf2-K2-z is also anticipated to be constant. It is likely that a carboxylic acid dimer acts as a bidentate in the monomeric extracted species as in the extraction of copper with di-(2-ethylhexyl)-phosphoric acid[6]. The Equation (7) can be rewritten as:

log Cc..,, - 2 { I o g C c . . .

I

+

2 pH - l o g # ( m c . ; )

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IogCcu.o-2(IogCcu.,,,

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Extraction of copper(II) R

R

R

o/C~o

o/C~o

2 0

"~c<.

O~c/O R

1339 /R

Oo/ O" O j / O

0

,., n..H..~. ,.,

.. ~ RCOOH..../,Cu . . . /ICu .. . . . . HOOCR + R-C,,o_I.r.o,C-R 0 O,.O ~O ,,. O...-~, C..-R" " R

O'-.c/O R

where carboxylic acid are denoted as R C O O H . In the production of the dimerized copper carboxylate mono-(carboxylic acid) according to Equation (18), the net change is: (1) the formation of a copper-copper bond; (2) the formation of two copper-carboxylic acid bonds; (3) the break-down of two hydrogen bonds - H . . . . O - . The contribution of the first item can be regarded constant for all the carboxylates in the present study. And between the latter two L F E R of unit slope is reasonably expected. Then Kdim is constant regardless of the number of carbon atoms in the carboxylic acid used. From these considerations, it follows 2

2

--4.

flCuR2"Kf "Kdim'KHR K2

--2

= constant.

(19)

In the preceding paper[6], the distribution coefficient of acids increases with increasing number of carbon atoms in acids and its increment was almost constant irrespective of the solute and solvent, i.e. A log K J a CHz = 0.56-0.60. This was quantitatively explained by means of the regular solution theory [7]: A log Kd =

AVHR

2.30 RT [ (6~q - 6HR)2-- (6o~g-- 6HR)2]

(20)

where 6aq, 6org and ~HR are solubility parameters of 0.1 M (Na,H)CIO4 solution, of organic solvents and of the carboxylic acids, respectively, Vrm molar volume of carboxylic acids and AVHR increment of molar volume per CH2 group. Similarly, the increment of KD with increasing number of carbon atoms in carboxylic acids is quantitatively explained by means of the regular solution theory, i.e. A VcuR2

A log KD = 2"30 RT [ ( 6 a " - 6cua2)2- (6arg-- 6cuR~)2]

(21)

where AVcua2 refers to the increment of molar volume of the copper carboxylate for an added CH2 group in the carboxylic acid used in the extraction and 6CuR~ solubility parameter of the copper carboxylate. In this case, we can put AVcuR~ ---2AVHR and 6cua~ ----6HR[8]. Then it is evident from Equations (20) and (21) that IOgKD--2A 1ogKd= 0: Kd-4"KD2 = constant for all the system in the present study despite the steady increase in Kd with increasing number of carbon atoms in carboxylic acid. 7. J. H. Hildebrand and R. L. Scott, The Solubility ofNonelectrolytes, 3rd Edn. Dover. New York (1964). 8. T. Omori, T. Wakahayashi, S. Oki and N. Suzuki, J. inorg, nucl. Chem. 26, 2265 (1964).

1340

I. KOJIMA, M. UCHIDA and M. TANAKA

The above consideration based on the regular solution theory together with Equation (19) can interpret quantitatively the experimental results: in the extraction of copper with fatty acids with C 4 - C10 the extraction constant is constant irrespective of the carboxylic acid as an extractant. In the extraction of metals with a series of similar extractants having the same functional group into a given solvent, it is expected that the overall extraction constant is constant irrespective of extractant, provided L F E R of unit slope holds between the stepwise formation constant of the extractable species and the formation constant of the conjugate acid of the extractant. Acknowledgement-The financial support given by the Ministry of Education (Japan) is gratefully acknowledged.