Extraction of copper(II) from ammonium chloride solutions with capric acid

J. inorg, nucl Chem., 1978. Vol. 40, pp. 109-1t6. Pergamon Press. Printed in Great Britain

EXTRACTION OF COPPER(II) FROM AMMONIUM CHLORIDE SOLUTIONS WITH CAPRIC ACID ADAM BARTECKI and WIESLAW APOSTOLUK Institute of InorganicChemistryand Metallurgyof Rare Elements,TechnicalUniversity,Wroclaw,Poland (First received 15 January 1977;in revised form 31 January 1977;received [or publication 3 May 1977)

Abstract--Extraction of copper(II) with capric acid from ammoniumchloride solutions was examined. The composition of the complex extracted was determined and it was found that the extraction process can be adequately described by the equation: 2Cu(~+ 3(HR)2,(o~(CuR2'HR)2,(o)+ 4H(~)

INTRODUCTION Extraction of copper(lI) with carboxylic acids of general formula HR (where R denotes carboxylate anion) which have recently become a matter of great interest, has been dealt with in many papers of both fundamental and practical nature. Our main interest is in those papers where the composition and molecular structures of the complexes extracted and extraction mechanisms are discussed. Fletcher and Flett[1] have extracted copper with benzene solutions of naphthenic acids; Tanaka et al.[2, 3] used benzene solutions of aliphatic carboxylic acids and they have shown independently that the copper complexes extracted correspond to a formula (CuR2.HR)2 in the organic phase. Haffenden and Lawson[4] used pivalic (trimethylacetic) acid in toluene to extract copper from nitrate solutions and found that the composition of the complex extracted corresponds to a formula Cut.9R3.sHR2.t; in their opinion this is evidence that a certain amount of the monomeric species appears in the organic phase apart from the dimeric (CuR2"HR)2. Illuvieva and Kopylenko[5] have studied the magnetic properties of solutions containing some metal naphthenates in carbon tetrachloride and have found the formula Cu2(HR)4R4 for copper naphthenate, which is contradictory to the results obtained by Fletcher and Flett [1]. It should be mentioned that there are relatively few data available on the molecular structures of such complexes. Flett [6] as well as Tanaka et al.[7] assume that the dimeric copper carboxylates contain metal-metal bonds. This opinion stems from the fact that a number of authors, e.g. Kato et a/.[8], explain the low magnetic moments of copper acetate monohydrate and its homologues by assuming this kind of bonding. Tanaka[7] generalized this point of view by stating that not only copper caprate but also the dimeric nickel and cobalt caprates have such a structure. However, Gindin[9] excluded such a structure for nickel and cobalt carboxylates. It is interesting to note the results obtained by Flett et al. who extracted copper with naphthenic acids in the presence of certain alkylamines Primene JMT and Alamine 336[10], nonylphenol[6], and LIX 63111]. The extraction of copper in the presence of nonylphenol and LIX 63 takes place with the formation of mixed corn-

plexes whose compositions and molecular structures are unknown. The nonylphenol complex extracted from an ammoniacal medium by means of naphthenic acid was assigned the following formula by Flett and West[6]: Cu(NH3)4R2"n(C9H19~

OH)

Ashbrook[12] extracted copper from ammonium sulphate solutions with VERSATIC 911 acid and found that two types of complexes occur: Cu-VERSATIC 911 and Cu-NH~-VERSATIC 911, which were successfully separated. From UV and IR spectroscopic studies, be assigned to the ammonia complex a molecular structure similar to those described by Kokof and Martin[14]. Haffenden and Lawson[13] extracted copper, nickel and cobalt from sodium nitrate and ammonium nitrate solutions using VERSATIC 9 acid solutions in toluene. The results obtained by these authors indicate that extraction in the presence of ammonium nitrate is considerably inhibited because of the formation of metal amminocomplexes in the aqueous phase. Since there are few data available on the extraction of copper with carboxylic acids from ammonium salt and ammonia solutions, the main purpose of the present work was to investigate the extraction of copper from ammonium chloride solutions using capric acid solutions in carbon tetrachloride. EXPERIMENTAL Reagents. Capric acid (Reachim,USSR) was purifiedby three stage crystallization from carbon tetrachloride (m.p. 304.4K). The other acids: caprylic and palmitic (Reachim), stearic (J. T. Baker Chemicals)and 10-undecenoic(E. Merck AG) were used without additionalpurification.The solvents used: carbon tetrachloride, benzene, chloroform, trichloroethylene (POChl and n-hexane (Reachim) were of AnalaRgrade. All other reagents obtained from POCh Gliwicewere analyticallypure. Equilibrium studies. Studies on the extraction equilibriawere carried out at 298-+ 1K. The aqueous copper chloride solutions were made up by weight and the ionic strength adjusted to the predetermined value with ammoniumchloride. Since the copper ion concentrationswere small compared with overall concentration, the ionic strength was considered invariantin the experiments carried out. Capric acid solutions in carbon tetrachloride were standardizedby titrationwith standardNaOH solution to the phenolphtbaleinend-point. The separatory funnelswere filledwith 25 cm3 of the aqueous

109

ll0

ADAM BARTECKI and WIESLAW APOSTOLUK

phase and 25 cm3 of the organic phase. The pH of the aqueous phase was adjusted with a small amount of diluted ammonia• The funnels were shaken for about 15 rain. This time was sufficient since the equilibrium was reached rapidly (after 5 rain as a rule). After separation both phases were filtered through dense quantitative filters Filtrak 390. By varying the amount of ammonia added, the extraction was studied as a function of pH. The effect on the metal partition of changes in the ionic strength, initial copper content and capric acid concentration were also examined• Analysis. The copper content in the aqueous phase was analysed spectrophotometrically by means of 2,2'-diquinolyl[15]. The copper content in the organic phase was determined by difference or by measuring the absorption of the organic phase at a = 680 nm[l, 16]. The pH measurements on the aqueous phase were made with a N-517 digital pH-meter (Mera-Elmat) and SAgP-201/7W combined electrode. Spectroscopic studies in the visible and UV regions were made using a HITACHI 356 model spectrophotometer.

Equation (7) shows that the function log D = f(pH) for j > 1 is not a straight line and depends on the extracted metal concentration• For fixed values of [(HR)2]o and c ~ , eqn (7) become log D~/J(1 + D) ° - ' / j = B + npH

In the discussion of eqn (8), Gindin[9] provides two limiting cases• For D,~ 1 expression (8) gives log D = B ~+ nj pH

(9)

d log D d pH = nj

(9a)

and

For D >> 1 expression (8) gives logD = B2+ n pH

RESULTS AND DISCUSSION

The extraction process of the metal cations with carboxylic acids may be described by the following general equation:

(8)

(10)

and dlogD - - = dpH

n

(10a)

/Me(,,) + ~(n + x) (HR)2.~o) = (MeRn'xHR)j.co)+ njH~o>

(1)

where the subscripts (a) and (o) correspond to the aqueous and organic phases, respectively and j, degree of polymerization of the complex; x, number of monomeric acid molecules contained in the complex; n, cationic charge. The equilibrium constant of reaction (1) when j is constant may be expressed as follows: K~

[(MeR,,.xHR)j] [H+] "i = [Me.+]j[(HR)2](.+~)v2.

(2)

By defining the distribution coefficient D = cMo(o)

(3)

Equations (9a) and (10a) show that the function log D =/(pH) for small values of D is a straight line of slope nj, while for large values of D the slope is n. It should be noted that the derivation of eqns (7-10) known as a model of single equilibrium assumes that in the aqueous phase the metal appears exclusively as an aquo-ion. However, in fact it frequently happens that the metal ion is hydrolysed, complexed by the salt anions and carboxylic acid anions, complexed by other ligands present in solution (e.g. NH3, pyridine) and polynuclear species are formed. All these processes result in different shapes of the extraction curves than those foreseen by the simple theory and generally the slope of log D = f(pH) is a function of the average metal ion charge in the aqueous phase[17]. This may be taken into account quantitatively by introducing the quantity known as side reaction coefficient aMe defined as follows:

CMe(a)

CM,(a) = aMe X [Me n+]

(11)

and allowing for the metal balance in both phases CMe(o) +

CMe(a)=

CMe (total)

(4)

the following relationships are obtained: D x cu,t.) = cM,(o)

(5)

(D + I)cM.(`.) = CMe(total)

(6)

where cM,(o), analytical metal concentration in the organic phase; cMao), analytical metal concentration in the aqueous phase; c~(tot~), total metal concentration in the aqueous phase before extraction. By assuming a constant ionic strength in the aqueous phase, constant activity coefficients in the organic phase and the volume ratio of both phases equal to unity, on substituting the relations (5) and (6) into eqn (2) and taking logarithms the following expression was derived log Dl/J(I + D) ° - l m = A + (j - 1)/j log CM, (toua) + (n + x)12 log [(HR)2]o + npH

where A is a constant•

where crae(`.), analytical metal concentration in aqueous phase; [Men+], metal aquo-ion concentration in aqueous phase; aMo, side reaction coefficient. The values of a~e may be calculated from stability constants of the respective complexes and from concentrations of all ligands. For the organic phase processes, the above considerations are valid for the case j = const. This condition is satisfied when one metal complex species of a definite degree of polymerization is extracted, or, when its predominance over other species is very clear[7]. The organic phase equilibria are complicated and their effect is quantitatively discussed by Tanaka et al.[18]. In order to determine the values of j and x, eqn (7) is solved for the case with D = 1, pHo.d9]. Thus: pHo.5 = B3 - ~ j-I

(7)

log [(HR)2]o

n./ log cM~(tot~).

(12)

On differentiating eqn (11), the following equations are

Extraction of copper(II) from ammonium chloride solutions with capric acid obtained

1

IgD 2,0

[0

0pHo.s ] _ log CM~ttot.l)J t~nR)21o

./-1

nj

(13)

0pHo.5 ] n+x 0 log [(HR)2]oJ,.,~,,o,,~ = - 2 ~

1,6

;/

(14)

from which the values of j and x may be determined. The values of A, B, B~, Bz, B3 in eqns (7-11) are constant. The distribution coefficient for reaction (1) may be expressed as follows[7]:

°/

1,2

,,3,8

j

O = ~ ~ i[(MeR,cxHR)~lolcM~,,, 1

:

0

2 2 I

JK.x[CM~'~']i-'[(HR)2]o'"+')"2(°tMJ[H+]

"

o

(15) where: cMo(., analytical metal concentration in the aqueous phase at the equilibrium state; aM~, side reaction coefficient allowing for metal complexation in the aqueous phase; K~, equilibrium constant of reaction (1). Assuming that (MeR..xHR)j is the only metal complex species in the organic phase, the summing up procedure in eqn (15) may be ignored. Then

@,0

-@,L

-0,8

!2

[log D + log cM~(.)] = log jK~ + ~ (n + x) log [(HR)2]o

i a OC

+ ][log CM~(.) -- log t~t. + n pH].

(16)

i .L??

i L 8@

, r v,-

~ _ : r~,: ~,!

Fig. 1. Effect of ammonium chloride concentration on the extraction of copper by capric acid in carbon tetrachloride. ¢cu(totab 1 × 10 3 M, [(HR)2]o = 0.377 M; O, 0.1 M NHaCI: ,~, 1.0NH4CI; O, 3.0M NH4C1; II, 4.0M NH4C1;0.5.0 M NH4('I. =

With a predetermined carboxylic acid concentration in the organic phase, the relationship [log D + log CMe(,)] = ]'[log CMeta)-- log aM~ + n pH]

(17)

should give a straight line of slope j. On the other hand, at a predetermined metal concentration in the aqueous phase, the relationship [log D - (] - 1)log cMet,)+ j log aM~- nj pH] = ]'(log [(HR)2lo)

(18)

should yield a straight line of slope (n +x) which intersects the axis of coordinates at log jKex.

Effect of the ionic strength on copper extraction In order to explain the effect of the ionic strength of the aqueous phase on the extraction process, a series of copper extractions was run with 0.377 M capric acid solution in carbon tetrachloride from solutions of varying ammonium chloride concentrations. The copper concentration in the aqueous phase was 1 x 10-3 M while NH4Cl concentrations were 0.1, 1.0, 3.0, 4.0 and 5.0 M, respectively. The results obtained are shown in Fig. 1. An increase in the ionic strength has a significant effect on the relationship log D = [(pH) which is a straight line of slope 2 only for 0.1 M NH4CI over the entire pH range. For 1.0, 3.0 and 4.0 M NH4CI solutions a gradually increasing deviation from this slope value is observed• For 5.0 M NH4CI the change in the shape of the function log D =/(pH) is so significant that this relation yields a straight line of slope I. This may suggest that an increase

in ammonium chloride concentration decreases the extraction and in order to maintain the constant ionic strength it is not necessary to use NH4CI solutions of concentrations exceeding 1.0 M. A decrease in copper extraction with the increasing ionic strength could be related to an increase in the copper complexation in the aqueous phase and to the effect on the capric acid solubility in the aqueous phase. A similar effect was observed by Ashbrook[12] who has found that an increase in ammonium sulphate concentration has an adverse effect on the extraction of copper with the VERSATIC 911 acid. From the data[19-22] the values of side reaction coefficients were calculated for the pH range of 4-6 according to the following formula: ao, = 1 + flx[NH3] + f12[NH312+/~3[NH3] 3 +/34[N H3]4 +/3'[Ct ]+/3'[C1--12+/3"[C1 ]3+/3'[C1 ]4

t191

where /3, stability constants of copper amminocomplexes; /3', stability constants of copper chloride complexes. The ammonia concentration was calculated from the formula: [NH4 +] [NH3] = KNm [H +] •

(20)

Formula (20) indicates that the terms related to the formation of amminocomplexes depend on pH while calculations showed that over the pH range from 4 to 5.5

112

ADAM BARTECKI and WIESLAW APOSTOLUK

the values of acu are only slightly dependent on pH. Thus, it should be assumed that under these conditions the complexation of copper by the chloride ions is of importance whereas the formation of amminocomplexes proceeds only to a small extent. Assuming that the prevailing cationic species of the metal in 5 M NH4CI solution is CuC1÷ the extraction of copper from that solution may be described by the scheme:

P%,~ ~,20

5,0o

/.,80

+ 2+x CuClta) + ~ (HR)2.(o) 1

i

+

= _(CuR2.xHR)j,(o) + 2H(a) + CI~-,,) 1

(21)

which explains fairly well the slope of 1 that is found. Enhanced complexation of copper in solutions containing high ammonium chloride concentrations was also supported by the spectroscopic studies (vide infra).

Dependence of extraction on copper concentration In order to elucidate the effect of the copper concentration on the extraction process, a number of tests were carried out using 0.107 M capric acid solution in carbon tetrachloride at various copper concentrations ranging from 1 x 10-4 to 5 x 10-3 M. The NH4CI concentration in all cases was 1.0 M while pH of the aqueous phase varies from 4.1 to 5.5. The functions log D = f(pH) presented in Fig. 2 are in all cases straight lines of slope 2. The values of pHo.5 found were used to plot the function pHo.5 =f[log Cc,,ot,,)] shown in Fig. 3. This function is linear and its slope calculated by the method of least squares is about -0.26. From eqn (13) the degree of polymerization for the complex extracted was calIgD 1,5

-~,o

-3,o

-2,o

[gCcul totGI

Fig. 3. Function pH0.5= ./[log Ccuttot~t)]. culated; ./= 2. For comparison the relationship [log D + log Ccu to)] =/[log Ccu(a) + 2 pH] corresponds to a straight line of slope 2.06. From the results it was deduced that the extracted copper complex with capric acid is a dimer. For comparison it is worthwhile quoting the data reported by Fletcher and Flett[1] who found that the extraction of copper over the concentration range from 5x 10-3 to 5x 10-2M/dm 3 from nitrate solutions by means of naphthenic acid solutions in benzene does not depend on the initial metal concentration and the overlapped log D = f(pH) curves have a slope of 2.37. Since the attempted interpretation on the basis of eqns (12-14) led to erroneous conclusions, these authors assumed that the model of a single equilibrium has only limited applicability. The only indication of a dimeric structure of the extracted complex was that the slope of the extraction curves exceeded 2 and some physicochemical properties of copper carboxylates.

Dependence of copper extraction on capric acid concentration Figure 4 shows the results obtained for the extraction of copper with solutions of various capric acid concen-

1,2

log D

0,8

1.2

0,8 0,0

0./-. -0/,

0.0

-0,/.

-0,B

-0.8

-1,2

/-,,00 ,

':j50

r

5,00

i

%50 pH

Fig. 2. Dependence of the copper extraction on the total metal concentration, csn,cI = 1.0 M, [(HR)2]o = 0.107M; O, 5 x 10-3 M CuCI2;r-q,2.5 x 10-3 M CuCI2;A, I x 10-3 M CuCI2;~7, 2 x 10-4 M CuCI2; (>, I x 10-4 M CuCI2.

/..50

5,00

5.50

pH

Fig. 4. Dependence of the copper extraction on the capric acid concentration. CNa,O=1.0M, Ccuao~)= l X 10-3M; @, 1.21M capric acid; ©, 0.85M capric acid; l , 0.682M capric acid', D, 0.405 M capric acid; A, 0.377M capric acid; A, 0.282M capric acid; @, 0.241M capric acid; ¢, 0.148 M capric acid; T, 0.107M capric acid.

113

Extraction of copper(II) from ammonium chloride solutions with capric acid trations. The concentrations of copper and ammonium chloride in the aqueous phase were 1 x 10-3 and 1.0 M, respectively. Capric acid concentrations in carbon tetrachloride varied from 0.1 to 1.21 M. For capric acid concentrations which do not exceed the value of 0.4 M, the curves log D =/(pH) are straight lines of slope 2. For concentrations exceeding the value of 0.4 M these functions are straight lines of slope about 3. According to data reported in paper[l] an increase in naphthenic acid concentration results in a decrease in the slope of the log D = f(pH) curves describing the extraction of the copper from 2.46 to 2.25. It seems that a change in the slope of log D = flpH) found in both cases is related to the physicochemical properties of carboxylic acid solutions. Figure 5 shows the results of dielectric constant measurements for capric acid solutions in CCI 4 as a function of acid concentration. They represent two straight lines intersecting at about 0.4 M/dm3. Thus, the results obtained are evidence that the above mentioned view is correct. Figure 6 presents a plot of pHo.5 =/(log [(HR)2]o). The function for acid concentrations ranging from 0.1 to 0.4 M is linear and its slope calculated by the method of least squares is -0.78. From eqn (14) the value of x = 1.12 was calculated. It may thus be assumed that the extracted copper complex corresponds to a formula (CuR2.HR)2. For comparison, the relationship [log D log C c , ~ - 4 p H ] =/(log[(HR)2]o) for capric acid concentrations below 0.4 M is linear with a slope of 3.06 so the value of (n + x) may be assumed to be 3. These data show that for capric acid concentrations up 0.4M/dm 3 the composition of the extracted complex corresponds to a formula (CuR2.HR)2 while the extraction of copper with capric acid from ammonium chloride solution may be described by an equation: 2Cu~2~+ 3(HR)2,~o~(CuR2'HR)2,~o)+ 4H~+~. (22)

2.25

2.2/.

2.23

pH~ 4,90 4,70 /.,.50 4,30 t,.10 -

-

1.0

0,5

0,0 log IIHR)2lo

Fig. 6. Function pH0.5=/(log [(HR)2]o). A decrease in the slope of pHo.5 =/(log [(HR)2]) for acid concentrations exceeding 0.4 M/dm3 indicates that it is possible to extract dimeric copper caprate (CUR2)2. The possibility of simultaneous extraction of (CuR2.HR)2 and (CUR2)2 was demonstrated by Jaycock et a1.[23]. Using the values of acu found, the values of log K,x were determined from eqn (16). The average value of log Kex is 10.97 and is in a good agreement with that provided for capric acid solutions in CC14 by Tanaka and Yamada [24]. The results obtained are in good agreement with those reported in papers[I-4, 23, 24] and their discussion both on the basis of eqn (8) and eqn (15) yields consistent conclusions. -

Effect of the type o[ ammonium salt on copper extraction Table 1 shows the results obtained for the extraction of copper from various ammonium salt solutions with 0.148M capric acid solution in carbon tetrachloride. Initial copper and ammonium ion concentrations in aqueous phase were 1 × 10-3 M and 1.0 M respectively. The relationships log D = f ( p H ) are straight lines of slope 2. The extraction of copper was found to proceed most readily from ammonium perchlorate solution and with most significant difficulties from ammonium sulphate solution. This fact may be explained qualitatively as due to salting out effect. In this specific case, that effect is mainly due to the anion influence. It is generally assumed that the non-hydrated or weakly hydrated anions are not good salting out agents [25]. Therefore, the arrangement of the ammonium salts according to increasing pHo5 agrees with the arrangement according to increasing anion free hydration energy[26]. According to the data reported by Jaycock et a/.[23] the type of salt anion Table 1. Extraction of copper with 0.148M capric acid solution from various ammonium salts

012

0',/.'

0',6'

018 ' 1;0 [( H R)~.~rnole.dme']

Fig. 5. Dielectric constant of capric acid solutions in CCI4 as function of acid concentration. JINCVol.40.No.I--H

a

Type of salt

PH0.5

AG° of salt anion hydration [kcal/mole]

IM NH4CIO4 1MNH4NO3 1MNH4CI 0.5M (NH4)2SO4

4.05 4.74 4.77 5.31

50 69 79 249

114

ADAM BARTECKIand WIESLAWAPOSTOLUK

affects the extraction conditions but has no influence on the composition of the complex extracted.

Effect of solvent on the extraction of copper Table 2 shows the data obtained for the extraction of copper with capric acid solutions in carbon tetrachloride, benzene, n-hexane, chloroform and trichloroethylene. Copper concentration in the aqueous phase was 1 x 10-3 M, NH4C1 concentration 1.0M, while the capric acid concentration 0.214 M was the same in all solvents. Table 2. Extraction of copper with capric acid solutions in various solvents Solvent n-Hexane Carbon tetrachloride Benzene Trichloroethylene Chloroform

pHo.5 Slope 4.78 4.64 4.53 4.84 4.46

2.0 2.0 2.0 2.0 1.87

% Extn. at pH = 5 75.0 80.5 90.0 67.7 90.0

It should be pointed out that there is no simple relationship between the extraction effects and the dielectric constant of the solvent. This is supported also by the results obtained by Cattrai and Walsh[27] for the extraction of iron(Ill) from nitrate solutions with capric acid. Copper was most readily extracted into chloroform and benzene solutions whilst the poorest results were obtained for capric acid solutions in n-hexane. The effect of the solvent may be explained on the basis of the regular solution theory[24]. The extracts obtained were used for spectroscopic studies whose results will be discussed later.

solutions of several carboxylic acids in carbon tetrachloride. Copper and ammonium chloride concentrations in the aqueous phase were I x 10-3 and 1.0 M respectively, while acid concentrations in the organic phase were the same and equal to 0.282 M. The data presented in Fig. 7 show that an increase in the length of the acid hydrocarbon chain results in an increase of copper extraction which is in agreement with the tendency found by Tanaka et al.[3]. Nevertheless, stearic acid appeared to be a less effective extraction agent than paimitic acid. The properties of 10-undecenoic acid, which appeared to be the most effective extraction agent of the group of acids under investigation, should be explained as due not only to the length of the hydrocarbon chain but also to the fact that this is an unsaturated acid. The ditlerences in the extraction properties of carboxylic acids result from the length and structure of the hydrocarbon chain, types and number of substituents and the existence of multiple bonds. These factors affect the acid strength, its hydrophobic properties or the hydrophobic properties of its metal complex and the stability of the complex itself.

Spectroscopic studies The results of the electronic absorption spectra measurements, in the organic phase, are presented in Table 3. Some spectra of the organic and aqueous phases are presented in Fig. 8. A comparison of the spectra of both phases indicates certain fundamental differences in their compositions. In the aqueous phase the main absorption band lies at about 820-860 nm and a distinct shoulder appears at about 1000 nm. A high chloride ion concentration suggests that the aqueous phase contains a tetrahedral form, e.g. Effect of the type of acid on the copper extraction CuC142- or mixed forms in equilibrium with the ocFigure 7 shows the results of copper extraction with tahedral forms. An increase in ammonium chloride concentration enhances considerably the overall absorpIgD ! tion resulting simultaneously in the formation of a band at about 1000nm. Since in the spectrochemical series the 1,6 CI- anion corresponds to lower values of the Dq parameter, bathochromic shift is justifiable in this case. Simultaneously the band intensity increases which refl1,2 ects the equilibrium shift towards the tetrahedral form, whose molar absorption coefficients are considerably higher than those of the octahedral forms. 0,8 A different pattern is presented by the absorption spectra of the organic phase. In the range 250--1000nm four bands can be distinguished with the main band at A = 675-680 nm. This band proves beyond any doubt, 0,4 that the organic phase contains octahedral species and no tetrahedral forms appear in this phase. This is consistent with the dimeric structure of the complex determined O,O above. A more detailed discussion of the electronic spectra and assignment of particular bands together with magnetochemical studies will be provided in our next paper. -Od. With reference, however, to Graddon's studies[28] and his suggestions concerning the molecular structure of the complex with the above mentioned acids, it may be -0,8 noted that on assuming a metal-metal bond the coordination number of Cu2÷ is 6, or 5 if this bond is I I I I I 4,00 /,,40 4,80 5,20 5,50 pH neglected. In the former case, this would correspond to a Fig. 7. Influence of the type of acid on the copper extraction. distorted octahedral structure since then the coordination cna4ct= 1.0M, Ccuooua~= 1 x 10-3M; [(HR)2]o= 0,282M; (3, 10- sphere of the complex has a composition CuLL'Cu. In undecenoic acid; @,palmitic acid; r-l, stearic acid; A, capric acid; such a case, the electronic transitions should be regarded II, caprylic acid. as transitions in the two-holes system, i.e, allowed

Extraction of copper(lI) from ammonium chloride solutions with capric acid

115

Table 3. Data on the absorption spectra of the copper complex with capric acid Band I Solvent

Band II

Band III

Band IV

/t raax

fmax

~max

Emax

~max

l[raax

~'max

[nm]

[dm3mole-~cm-I]

[nm]

[dm3mole-Icm -I]

[nm]

[dm3mole-tcm -t]

[nm]

~max

[dm3 mole-I cm-l] Benzene Chloroform Carbon tetrachloride Trichloroethylene Cyclohexane

810 810 820

88 70 66

680 675 680

260 232 230

380 373 380

115 87 76

285 260 275

815 810

66 66

680 680

220 240

380 375

105 114

295 260

/ /

/

\

/

\ \

/

06

/

/

\

/

\ \

/ /

/

4.8 x 103

\

/

\

\

0.4

E

\

02

/

/

/ /

I I000

I

//

~

•

I 800

I

I 600

// ~-

I .~//

I

400

X [nm] Fig. 8. Absorption spectra of the aqueous and organic phases in the extraction system CuCl2-NH4Cl-capric acid in carbon tetrachloride. , I x 10-2 M CuCI2 in 1M NH4C1,pH = 4.5 d = 2 cm; . . . . . . ,1 × 10-2 M CuCI2 in 3M NH4CI, pH = 4.55 d = 2 cm; . . . . . , organic phase, [(HR)2]o = 0.682 M, Ccu~o)= 9.69 x 10-4 M, d = 1 cm; .... , organic phase, [(HR)2]o = 0.377 M, Cc,(o)= 8.06 x 10- M, d = I cm. triplet-triplet transitions. If, however, the Cu-Cu bond is neglected, then the complex under consideration is pentacoordinated for the d 9 electron configuration.

CONCLUSION 1. The extraction of copper with capric acid from ammonium chloride solutions has been shown to proceed according to eqn (22). A quantitative discussion of the results on the basis of eqns (7) and (15) yields consistent conclusions. 2. The dependence of the copper extraction on the capric acid concentration has been shown to be complicated. The simple relationships resulting from a detailed discussion of eqn (7) were satisfied only for relatively low acid concentrations.

3. The results show that the extraction of copper by the capric acid from ammonium chloride solutions, after introducing ammonia into the aqueous phase, it not accompanied by the formation of mixed complexes which contain ammonia in the pH region under investigation. In addition, it has been shown that the type of the ammonium salt in the aqueous phase affects significantly the extraction process and there is a simple correlation between the extraction data and the free hydration energy of the anions of these salts. An increase in the ammonium chloride concentration was also found to decrease the extraction of copper because of formation of the chloride complexes of copper. 4. Spectroscopic studies are in a very good agreement with the results of the studies on the extraction equilibria and support the conclusions drawn earlier on the composition of the complex extracted.

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