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Solvent extraction of divalent metal perchlorates by tributyl phosphate
J. inorg,nucl.Chem., 1969,Vol.31, pp. 2919 to 2926. PergamonPress. Printedin Great Britain
S O L V E N T E X T R A C T I O N OF D I V A L E N T METAL P E R C H L O R A T E S BY T R I B U T Y L P H O S P H A T E J. AGGETT, T. E. CLARK and R. A. RI(HARDSON Chemistry Department, University of Auckland, Auckland, N e w Zealand (Received 21 October 1968) A b s t r a c t - Cobalt(ll), copper(lI), nickel(ll), and zinc(lI) perchlorates extract into tributyl phosphate as solvent-separated ion triplets. Maxima in the distribution ratios at high sodium perchlorate concentration result from co-extraction of sodium perchlorate. T h e s e cations are extracted less efficiently from perchloric acid solutions as perchloric acid itself is preferentially extracted.
INTRODUCTION ALTHOUGH the solvent extraction of ion-pair systems involving nitrate ions and halide ions has been studied extensively there have been few reports on studies of perchlorate systems. The uranyl ion[l, 2] is reported to extract into tributylphosphate (TBP) as the species UO2(C104)2 2TBP which is more ionised and more hydrated than the corresponding nitrate. Cobalt(lI) and nickel(lI) extract into 2-octanol as solvent-separated ion triplets [3, 4]. Zirconium(IV), thorium(IV). promethium(Ill), and yttrium(Ill) also extract into TBP from aqueous solutions with high perchloric acid concentration [5]. In earlier studies on the extraction of divalent cations into TBP by salicylic acid[6] it was observed that the cations were extracted in the absence of salicylic acid when sodium perchlorate was present in the system. This paper reports subsequent studies on the extraction of cobalt(II), copper(IlL nickel(II), and zinc(lI) into TBP from aqueous sodium perchlorate and perchloric acid solutions. EXPERIMENTAL Distribution measurements in the cobalt(li), copper(ll), and zinc(ll) systems were made with the radiochemical tracers 6°Co. 64Cu, and ~sZn respectively. Both phases were counted; the distribution ratio is given by the expression Distribution ratio q = total metal conc. in organic phase total metal conc. in aqueous phase In the nickel(ll) system the concentration of nickel(ll) in the aqueous phase '~,as determined by titration with E D T A . The concentration of nickel(! 1) in T B P was obtained by subtracting the aqueous concentration from the total concentration. In all the distribution experiments equal phase volumes were equilibrated at 25°C. Organic solvents were pre-equilibrated with water to prevent volume changes during the extraction experiments. The distribution of soclium perchlorate between organic solvents and water was also measured radiochemically with 24Na. I. 2. 3. 4. 5. 6.
E.'Hesford and H. A. C. M c K a y , J . inorg, nuel. Chem. 13, 165 (1960). J. L. Woodhead and H. A. C. M c K a y , J . inorg, nucl. Chem. 27. 2247 (1965). P.C. Yates, R. J. Laran, R. E. Williams and T. E. Moore, J. 3 m. chem. Soc. 75, 2212 ( 1953). T. E. Moore, R. J. Laran and P. C. Yates, J. phys. Chem. 59, 90 (1955). S. Sieckierski,J. inorg, nucl. Chem. 12, 129 (1959). J. Aggett and P. Crossley, J. inorg, nucl. Chem. 29, 1113 (1967). 2919
2920
J. AGGETT, T. E. C L A R K and R. A. R I C H A R D S O N
Perchlorate analyses were done gravimetrically using nitron as the precipitating agent. Metal perchlorates were prepared by dissolving freshly precipitated metal hydroxides in aqueous perchloric acid; they were recrystallized from water. The metal hydroxides were obtained from AR metal salts by precipitation from homogeneous solution using the hydrolysis of urea to alter the pH. The precipitates were washed until there was no evidence of the anion in the filtrate. Hydroxides precipitated in this way were dense and much more readily purified than those obtained by normal precipitation methods. Conductivity measurements were made with a Philips PR9501 conductivity bridge. In order to make these measurements concentrated aqueous solutions of divalent metal perchlorates were equilibrated with TBP. Concentrated solutions of the extracts were then successively diluted with TBP which had previously been equilibrated with water. Viscosities of these same solutions were measured with an Ostwald viscometer. All measurements were made at 25°C. The amount of water extracted into TBP with the metal perchlorates was determined by Karl Fischer titration. Aqueous solutions containing 0,5-1 M metal perchlorates were extracted with an equal volume of TBP which had previously been equilibrated with water. The total amount of water in TBP is given by the expression H2OTotal =
H20 in extracted species + HzO associated with TBP.
Since the amount of water extracted by TBP is a function of the water activity of the aqueous phase the following method was used for its determination. A plot of [H~O]TBP VS. aw was prepared, using NaCI which does not extract into TBP, to vary aw. The water activity of the aqueous solution containing the divalent cation was obtained from data in Robinson and Stokes[7] and Christie[8] and the value of [H20]TBp corresponding with this aw was read off the original curve. It was also necessary to make a correction for TBP coordinated to the cation in the organic phase. Since data presented later in this paper suggested that two TBP molecules are associated with each cation extracted the final value for H20 associated with TBP was obtained by the expression [~
nl
[ [TBP] - 2 [M(CIO4)s] oi
where [TBP] is the concentration of pure TB P. This procedure should be clarified by the following example. [Zn~+]o = 0.198 M;
[Zn2+]a= 0.344 M.
Total H20 cone. in TBP = 5.08 M
awat 0.344 M Zn(CIO4)2
0.98.
From aw vs. [H20]xap curve, [H20]TBp = 3.02 M (3.64--2 × 0.198'~ .-. H20 assoc, with TBP = 3.02 × \. 3--64 ] = 2.69 M .'. [H20] associated with Zinc(ll) = 2.39 M and the hydration number = 12.1, RESULTS
In preliminary experiments the distribution of cobalt(II) between aqueous sodium perchlorate solutions and a number of organic solvents containing oxygen was determined (Fig. 1). These results show that the TBP system is more 7. R.A. Robinson and R. H. Stokes, Electrolyte Solutions. Butterworths, London (1959). 8. L. D. Christie, Thesis, University of New Zealand (1958).
Solvent extraction of divalent metal perchlorates
2921
.O
~a
-I i
OI
LOG O NaC[O4 Fig. I. Extraction of cobalt(If). O - T r i b u t y l phosphate; G - C y c l o h e x a n o l ; isobutyl ketone; • - n -octanol; (ll - n -decanol; [ ] - n -hexanol.
~-methyl
effective than the other systems in promoting extraction and that it differs from the other systems in that there is a maximum in the value of q at high sodium perchlorate concentration. Extraction profiles of log q vs. log aqueous sodium perchlorate concentration were determined for cobalt(II), copper(II), nickel(If), and zinc(II) in the T B P system. Experiments were done with different metal ion concentrations (10 -3l0 -2 M) and at different pH (2-6). Neither of these factors affected the profiles. Figure 2 shows typical results for these systems. T h e s e profiles are very similar for the four cations; such a similarity was also observed in the cobalt(II) and nickel(II)-2-octanol systems[3]. T h e variation of q with sodium perchlorate concentration at a constant ionic strength viz. 1. is shown in Fig. 3. Sodium chloride was used to adjust the ionic strength in these experiments.
O
c
--I
.
-
I
LOG CILNeCI04
Fig. 2. Extraction of copper(II), nickel(I1), zinc(II) into tributyl phosphate. ~-copper(II); O-- nickel(I1); ~ - zinc(II).
2922
J. A G G E T T , T. E. C L A R K and R. A. R I C H A R D S O N
.O
ed J
:1
I
-2
-I LOG [NaCIO4]
0
Fig. 3. Extraction of cobalt(ll), nickel(ll) and zinc(II) from NaCI-NaCIO4 mixtures. - cobalt(I I); O - nickel(II); ~ - zinc(I I).
An attempt was made to determine the role of TBP in the extraction process by measuring the variation in log q in TBP-diluent mixtures. Two diluents were chosen viz. cyclohexane which is generally considered to be an inert solvent, and n-decanol which has a dielectric constant similar to that of TBP but which is also capable of extracting perchlorate. The results of these experiments for the zinc(I I) system are shown in Fig. 4. The TBP-n decanol data has been corrected for extraction of zinc perchlorate by n decanoi itself. Although there is some scatter at very low q values in the TBP-n decanol system both these sets of data show a second order dependence on TBP concentration. Curves for the cobalt(II) system had similar slopes.
.o
¢
jO
I
LOG IT B P'J Fig. 4. Extraction of zin¢(II) into solvent mixtures. O - T r i b u t y ] phosphate-n-decanol; -Tribuytl phosphate-cyciohexane, [NaCIO4I = 3.2 M.
Solvent extraction of divalent metal perchlorates
2923
Table 1 contains the hydration numbers for the extracted species. These show some variation with metal ion concentration, the number generally decreasing with increasing metal ion concentration in TBP. Table 1. Hydration numbers of extracted cations Metal
[Metal],rap
Co
Hydration number
0-331 M 0-249 0.183 0-320 0-141 0.129 0-722 0.406 0.343 0.366 0.269 0.198
Cu
Ni
Zn
12.6 14.4 16.9 11-0 16-7 17.8 8-9 11.6 12.1 10.8 13.2 14-1
The conductivities and viscosities of the extracted species are shown in Fig. 5. Figure 6 shows data for the extraction of cobalt(I I), copper(I I), and zinc(l I) into TBP from aqueous perchloric acid solutions. Distribution ratios for nickel(11) were less than 10 -2 and have not been included in this figure. The distribution ratios observed in these systems are much less than those of the corresponding sodium perchlorate systems. Sodium perchlorate distribution data are shown in Fig. 7. Measurements were made on all the solvent systems for which the distribution of divalent cations had previously been investigated. (see Fig. 1).
J
'
a
I
i
I
.I ....
_~ !
I
I
I
a
-2
-3
-I
-2
LOG
CONC.
I
-3
Fig. 5. Conductivities and viscosities of extracts in tributyl phosphate. • - E q u i v a l e n t conductance (ohms -1 cm2). Q - Viscosity (poises). DISCUSSION
The accumulation of evidence presented in the previous section and particularly the conductivity and hydration number data suggests that these divalent metal perchlorates are extracted as ion-triplets. Moreover, both the large hydra-
2924
J. AGGETT, T. E. CLARK and R. A. RICHARDSON
:1
o-~e
o o._o_
LOG [HCIO I Fig. 6. Extraction of cations by HCIO4. @ - zinc(l I); ~ - cobalt(1 I); O - copper(l I).
-I LOG [NaClo~.O~
Fig. 7. Extraction of sodium perchlorate. C)-Tributyl phosphate; ~-Cyclohexanol; @ - methyl isobutyl ketone; • - n-octanol; [ ] - n-decanol; ~ - n-hexanol.
tion numbers and the close similarity between the extraction curves for the different cations indicate that the species are solvent-separated. The extraction process can therefore be represented by an expression of the type M 2++ 2CIO4- + mH20 + 2TBP ~ [M2+IH20)m(TBP)z ] [CIO4-]2. Admittedly it is possible that the perchlorate ion may also be hydrated but it is felt that the hydration number is not likely to be significant because of the low charge-density of the perchlorate ion. If it is assumed that there is no ion-association in the aqueous phase then the slopes of the curves in Fig. 2 might be expected to approach a value of 2 at least at low aqueous perchlorate concentration. However, all these slopes are about 1.4-1.5. This negative deviation is considered to be due to the fact that the water
Solvent extraction of divalent metal perchlorates
2925
activity falls with increasing aqueous sodium perchlorate concentration and although the fall itself is relatively small the magnitude of the effect becomes significant through the high power dependence of the equilibrium on the water activity. This hypothesis was examined by investigating extraction from solutions of constant ionic strength and water activity (Fig. 3). The sodium perchloratesodium chloride system is suitable for this experiment because the variation of water activity with concentration for these two electrolytes is almost identical [8]. In this system plots of log q vs. log[NaCIO4] had slopes of 1.9 and 2.0 for cobalt (I1) and nickel(ll) which appears to confirm the hypothesis. The extraction curve for zinc(II) shows that considerable extraction occurs at low perchlorate concentration. This is undoubtedly due to extraction of chioro-complexes which has been reported by Morris and Short [19]. The conductivity curves have shapes common for electrolytes in media of low dielectric constant. The region of minimum conductance corresponds with existence of the undissociated ion-triplet; at lower concentration dissociation of the species produces charged species and a consequent increase in conductance. In the region above about 10 -1 M there is at first a slight increase in conductance but this is followed by further decrease. In this same region there is a large continuous increase in viscosity. This suggests the formation of ion-multiplets. It would appear that the smallest of these species e.g. [M2+(H20)n][(C104)3]- do contribute to the overall conductance but that larger species formed at higher concentration make much less contribution to the conductance because of their greatly increased bulk. Since TBP itself extracts a considerable amount of water it was necessary to use salt concentrations/> 0-1 M in order to obtain reliable data for the hydration numbers. At these concer~trations some association of the extracted species undoubtedly occurs. And this degree of association is dependent on the concentration. The observed variation in hydration numbers with concentration therefore appears to be associated with the formation of ion-multiplets, and furthermore the hydration numbers quoted are presumably not those of the ion-triplets themselves but of more highly-associated species. But for each of the cations the hydration number increases with decreasing concentration and it therefore seems reasonable that the ion-triplets will have hydration numbers at least as great as those of the associated species formed at higher concentration. The differences in the shapes of extraction curve for different solvent systems appear to be caused by co-extraction of sodium perchlorate. Data in Fig. 7 show that TBP is the only solvent into which sodium perchlorate is extracted in significant quantities e.g. at 5 M aqueous sodium perchlorate the concentration of sodium perchlorate in TBP is about 1 M. Hence as the sodium perchlorate concentration in these TBP systems is increased the amount of free TBP is diminished and competition between sodium perchlorate and the divalent metal perchlorate is intensified. This results in extraction of less divalent metal perchlorate than might be anticipated from mass action effects. The existence of this competition is also demonstrated by the effect of increasing divalent metal ion concentration on the extraction of sodium perchlorate by TBP. (Table 2). In the other solvents 9. D. F. C. Morris and E. L. Short,J. c h e m . Soc ~. 2662 (1962).
2926
J. A G G E T T , T. E. C L A R K and R. A. R I C H A R D S O N Table 2*. Distribution of sodium perchlorate in the presence of divalent metal perchlorates Metal Co
Cu
Zn
Molarity
Distribution ratio
0.29 0.58 0.72 0.33 0.59 1.00 0.23 0.50 1-00
0.149 0.112 0.095 0.144 0.116 0-090 0.171 0.125 0-082
*[NaCIO4] = 3 M.
where extraction of sodium perchlorate is not significant the extraction curves for divalent cations show no significant decrease in slope at high aqueous sodium perchlorate concentration. Kertes and Kertes [10] have shown that perchloric acid is extracted into TBP as HCIO4.4TBP, HCIO4.2TBP, and HC104.TBP and their results were confirmed during the present investigation. Since perchloric acid is extracted to an even greater extent than sodium perchlorate it is not unexpected that extraction of the divalent cations from perchloric acid solutions does not occur as readily as it does from the corresponding sodium perchlorate solution. It does appear that in the presence of perchloric acid TBP shows some selectivity towards divalent cations. This may be associated with the ability of the cations to extract as species with coordination numbers less than six. This hypothesis is supported by the fact that zinc(II) which commonly assumes a tetrahedral configuration extracts considerably more readily than the other cations under these conditions. 10. A. Kertes and V. Kertes,J. appl. Chem. 10, 287 (1960).
S O L V E N T E X T R A C T I O N OF D I V A L E N T METAL P E R C H L O R A T E S BY T R I B U T Y L P H O S P H A T E J. AGGETT, T. E. CLARK and R. A. RI(HARDSON Chemistry Department, University of Auckland, Auckland, N e w Zealand (Received 21 October 1968) A b s t r a c t - Cobalt(ll), copper(lI), nickel(ll), and zinc(lI) perchlorates extract into tributyl phosphate as solvent-separated ion triplets. Maxima in the distribution ratios at high sodium perchlorate concentration result from co-extraction of sodium perchlorate. T h e s e cations are extracted less efficiently from perchloric acid solutions as perchloric acid itself is preferentially extracted.
INTRODUCTION ALTHOUGH the solvent extraction of ion-pair systems involving nitrate ions and halide ions has been studied extensively there have been few reports on studies of perchlorate systems. The uranyl ion[l, 2] is reported to extract into tributylphosphate (TBP) as the species UO2(C104)2 2TBP which is more ionised and more hydrated than the corresponding nitrate. Cobalt(lI) and nickel(lI) extract into 2-octanol as solvent-separated ion triplets [3, 4]. Zirconium(IV), thorium(IV). promethium(Ill), and yttrium(Ill) also extract into TBP from aqueous solutions with high perchloric acid concentration [5]. In earlier studies on the extraction of divalent cations into TBP by salicylic acid[6] it was observed that the cations were extracted in the absence of salicylic acid when sodium perchlorate was present in the system. This paper reports subsequent studies on the extraction of cobalt(II), copper(IlL nickel(II), and zinc(lI) into TBP from aqueous sodium perchlorate and perchloric acid solutions. EXPERIMENTAL Distribution measurements in the cobalt(li), copper(ll), and zinc(ll) systems were made with the radiochemical tracers 6°Co. 64Cu, and ~sZn respectively. Both phases were counted; the distribution ratio is given by the expression Distribution ratio q = total metal conc. in organic phase total metal conc. in aqueous phase In the nickel(ll) system the concentration of nickel(ll) in the aqueous phase '~,as determined by titration with E D T A . The concentration of nickel(! 1) in T B P was obtained by subtracting the aqueous concentration from the total concentration. In all the distribution experiments equal phase volumes were equilibrated at 25°C. Organic solvents were pre-equilibrated with water to prevent volume changes during the extraction experiments. The distribution of soclium perchlorate between organic solvents and water was also measured radiochemically with 24Na. I. 2. 3. 4. 5. 6.
E.'Hesford and H. A. C. M c K a y , J . inorg, nuel. Chem. 13, 165 (1960). J. L. Woodhead and H. A. C. M c K a y , J . inorg, nucl. Chem. 27. 2247 (1965). P.C. Yates, R. J. Laran, R. E. Williams and T. E. Moore, J. 3 m. chem. Soc. 75, 2212 ( 1953). T. E. Moore, R. J. Laran and P. C. Yates, J. phys. Chem. 59, 90 (1955). S. Sieckierski,J. inorg, nucl. Chem. 12, 129 (1959). J. Aggett and P. Crossley, J. inorg, nucl. Chem. 29, 1113 (1967). 2919
2920
J. AGGETT, T. E. C L A R K and R. A. R I C H A R D S O N
Perchlorate analyses were done gravimetrically using nitron as the precipitating agent. Metal perchlorates were prepared by dissolving freshly precipitated metal hydroxides in aqueous perchloric acid; they were recrystallized from water. The metal hydroxides were obtained from AR metal salts by precipitation from homogeneous solution using the hydrolysis of urea to alter the pH. The precipitates were washed until there was no evidence of the anion in the filtrate. Hydroxides precipitated in this way were dense and much more readily purified than those obtained by normal precipitation methods. Conductivity measurements were made with a Philips PR9501 conductivity bridge. In order to make these measurements concentrated aqueous solutions of divalent metal perchlorates were equilibrated with TBP. Concentrated solutions of the extracts were then successively diluted with TBP which had previously been equilibrated with water. Viscosities of these same solutions were measured with an Ostwald viscometer. All measurements were made at 25°C. The amount of water extracted into TBP with the metal perchlorates was determined by Karl Fischer titration. Aqueous solutions containing 0,5-1 M metal perchlorates were extracted with an equal volume of TBP which had previously been equilibrated with water. The total amount of water in TBP is given by the expression H2OTotal =
H20 in extracted species + HzO associated with TBP.
Since the amount of water extracted by TBP is a function of the water activity of the aqueous phase the following method was used for its determination. A plot of [H~O]TBP VS. aw was prepared, using NaCI which does not extract into TBP, to vary aw. The water activity of the aqueous solution containing the divalent cation was obtained from data in Robinson and Stokes[7] and Christie[8] and the value of [H20]TBp corresponding with this aw was read off the original curve. It was also necessary to make a correction for TBP coordinated to the cation in the organic phase. Since data presented later in this paper suggested that two TBP molecules are associated with each cation extracted the final value for H20 associated with TBP was obtained by the expression [~
nl
[ [TBP] - 2 [M(CIO4)s] oi
where [TBP] is the concentration of pure TB P. This procedure should be clarified by the following example. [Zn~+]o = 0.198 M;
[Zn2+]a= 0.344 M.
Total H20 cone. in TBP = 5.08 M
awat 0.344 M Zn(CIO4)2
0.98.
From aw vs. [H20]xap curve, [H20]TBp = 3.02 M (3.64--2 × 0.198'~ .-. H20 assoc, with TBP = 3.02 × \. 3--64 ] = 2.69 M .'. [H20] associated with Zinc(ll) = 2.39 M and the hydration number = 12.1, RESULTS
In preliminary experiments the distribution of cobalt(II) between aqueous sodium perchlorate solutions and a number of organic solvents containing oxygen was determined (Fig. 1). These results show that the TBP system is more 7. R.A. Robinson and R. H. Stokes, Electrolyte Solutions. Butterworths, London (1959). 8. L. D. Christie, Thesis, University of New Zealand (1958).
Solvent extraction of divalent metal perchlorates
2921
.O
~a
-I i
OI
LOG O NaC[O4 Fig. I. Extraction of cobalt(If). O - T r i b u t y l phosphate; G - C y c l o h e x a n o l ; isobutyl ketone; • - n -octanol; (ll - n -decanol; [ ] - n -hexanol.
~-methyl
effective than the other systems in promoting extraction and that it differs from the other systems in that there is a maximum in the value of q at high sodium perchlorate concentration. Extraction profiles of log q vs. log aqueous sodium perchlorate concentration were determined for cobalt(II), copper(II), nickel(If), and zinc(II) in the T B P system. Experiments were done with different metal ion concentrations (10 -3l0 -2 M) and at different pH (2-6). Neither of these factors affected the profiles. Figure 2 shows typical results for these systems. T h e s e profiles are very similar for the four cations; such a similarity was also observed in the cobalt(II) and nickel(II)-2-octanol systems[3]. T h e variation of q with sodium perchlorate concentration at a constant ionic strength viz. 1. is shown in Fig. 3. Sodium chloride was used to adjust the ionic strength in these experiments.
O
c
--I
.
-
I
LOG CILNeCI04
Fig. 2. Extraction of copper(II), nickel(I1), zinc(II) into tributyl phosphate. ~-copper(II); O-- nickel(I1); ~ - zinc(II).
2922
J. A G G E T T , T. E. C L A R K and R. A. R I C H A R D S O N
.O
ed J
:1
I
-2
-I LOG [NaCIO4]
0
Fig. 3. Extraction of cobalt(ll), nickel(ll) and zinc(II) from NaCI-NaCIO4 mixtures. - cobalt(I I); O - nickel(II); ~ - zinc(I I).
An attempt was made to determine the role of TBP in the extraction process by measuring the variation in log q in TBP-diluent mixtures. Two diluents were chosen viz. cyclohexane which is generally considered to be an inert solvent, and n-decanol which has a dielectric constant similar to that of TBP but which is also capable of extracting perchlorate. The results of these experiments for the zinc(I I) system are shown in Fig. 4. The TBP-n decanol data has been corrected for extraction of zinc perchlorate by n decanoi itself. Although there is some scatter at very low q values in the TBP-n decanol system both these sets of data show a second order dependence on TBP concentration. Curves for the cobalt(II) system had similar slopes.
.o
¢
jO
I
LOG IT B P'J Fig. 4. Extraction of zin¢(II) into solvent mixtures. O - T r i b u t y ] phosphate-n-decanol; -Tribuytl phosphate-cyciohexane, [NaCIO4I = 3.2 M.
Solvent extraction of divalent metal perchlorates
2923
Table 1 contains the hydration numbers for the extracted species. These show some variation with metal ion concentration, the number generally decreasing with increasing metal ion concentration in TBP. Table 1. Hydration numbers of extracted cations Metal
[Metal],rap
Co
Hydration number
0-331 M 0-249 0.183 0-320 0-141 0.129 0-722 0.406 0.343 0.366 0.269 0.198
Cu
Ni
Zn
12.6 14.4 16.9 11-0 16-7 17.8 8-9 11.6 12.1 10.8 13.2 14-1
The conductivities and viscosities of the extracted species are shown in Fig. 5. Figure 6 shows data for the extraction of cobalt(I I), copper(I I), and zinc(l I) into TBP from aqueous perchloric acid solutions. Distribution ratios for nickel(11) were less than 10 -2 and have not been included in this figure. The distribution ratios observed in these systems are much less than those of the corresponding sodium perchlorate systems. Sodium perchlorate distribution data are shown in Fig. 7. Measurements were made on all the solvent systems for which the distribution of divalent cations had previously been investigated. (see Fig. 1).
J
'
a
I
i
I
.I ....
_~ !
I
I
I
a
-2
-3
-I
-2
LOG
CONC.
I
-3
Fig. 5. Conductivities and viscosities of extracts in tributyl phosphate. • - E q u i v a l e n t conductance (ohms -1 cm2). Q - Viscosity (poises). DISCUSSION
The accumulation of evidence presented in the previous section and particularly the conductivity and hydration number data suggests that these divalent metal perchlorates are extracted as ion-triplets. Moreover, both the large hydra-
2924
J. AGGETT, T. E. CLARK and R. A. RICHARDSON
:1
o-~e
o o._o_
LOG [HCIO I Fig. 6. Extraction of cations by HCIO4. @ - zinc(l I); ~ - cobalt(1 I); O - copper(l I).
-I LOG [NaClo~.O~
Fig. 7. Extraction of sodium perchlorate. C)-Tributyl phosphate; ~-Cyclohexanol; @ - methyl isobutyl ketone; • - n-octanol; [ ] - n-decanol; ~ - n-hexanol.
tion numbers and the close similarity between the extraction curves for the different cations indicate that the species are solvent-separated. The extraction process can therefore be represented by an expression of the type M 2++ 2CIO4- + mH20 + 2TBP ~ [M2+IH20)m(TBP)z ] [CIO4-]2. Admittedly it is possible that the perchlorate ion may also be hydrated but it is felt that the hydration number is not likely to be significant because of the low charge-density of the perchlorate ion. If it is assumed that there is no ion-association in the aqueous phase then the slopes of the curves in Fig. 2 might be expected to approach a value of 2 at least at low aqueous perchlorate concentration. However, all these slopes are about 1.4-1.5. This negative deviation is considered to be due to the fact that the water
Solvent extraction of divalent metal perchlorates
2925
activity falls with increasing aqueous sodium perchlorate concentration and although the fall itself is relatively small the magnitude of the effect becomes significant through the high power dependence of the equilibrium on the water activity. This hypothesis was examined by investigating extraction from solutions of constant ionic strength and water activity (Fig. 3). The sodium perchloratesodium chloride system is suitable for this experiment because the variation of water activity with concentration for these two electrolytes is almost identical [8]. In this system plots of log q vs. log[NaCIO4] had slopes of 1.9 and 2.0 for cobalt (I1) and nickel(ll) which appears to confirm the hypothesis. The extraction curve for zinc(II) shows that considerable extraction occurs at low perchlorate concentration. This is undoubtedly due to extraction of chioro-complexes which has been reported by Morris and Short [19]. The conductivity curves have shapes common for electrolytes in media of low dielectric constant. The region of minimum conductance corresponds with existence of the undissociated ion-triplet; at lower concentration dissociation of the species produces charged species and a consequent increase in conductance. In the region above about 10 -1 M there is at first a slight increase in conductance but this is followed by further decrease. In this same region there is a large continuous increase in viscosity. This suggests the formation of ion-multiplets. It would appear that the smallest of these species e.g. [M2+(H20)n][(C104)3]- do contribute to the overall conductance but that larger species formed at higher concentration make much less contribution to the conductance because of their greatly increased bulk. Since TBP itself extracts a considerable amount of water it was necessary to use salt concentrations/> 0-1 M in order to obtain reliable data for the hydration numbers. At these concer~trations some association of the extracted species undoubtedly occurs. And this degree of association is dependent on the concentration. The observed variation in hydration numbers with concentration therefore appears to be associated with the formation of ion-multiplets, and furthermore the hydration numbers quoted are presumably not those of the ion-triplets themselves but of more highly-associated species. But for each of the cations the hydration number increases with decreasing concentration and it therefore seems reasonable that the ion-triplets will have hydration numbers at least as great as those of the associated species formed at higher concentration. The differences in the shapes of extraction curve for different solvent systems appear to be caused by co-extraction of sodium perchlorate. Data in Fig. 7 show that TBP is the only solvent into which sodium perchlorate is extracted in significant quantities e.g. at 5 M aqueous sodium perchlorate the concentration of sodium perchlorate in TBP is about 1 M. Hence as the sodium perchlorate concentration in these TBP systems is increased the amount of free TBP is diminished and competition between sodium perchlorate and the divalent metal perchlorate is intensified. This results in extraction of less divalent metal perchlorate than might be anticipated from mass action effects. The existence of this competition is also demonstrated by the effect of increasing divalent metal ion concentration on the extraction of sodium perchlorate by TBP. (Table 2). In the other solvents 9. D. F. C. Morris and E. L. Short,J. c h e m . Soc ~. 2662 (1962).
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J. A G G E T T , T. E. C L A R K and R. A. R I C H A R D S O N Table 2*. Distribution of sodium perchlorate in the presence of divalent metal perchlorates Metal Co
Cu
Zn
Molarity
Distribution ratio
0.29 0.58 0.72 0.33 0.59 1.00 0.23 0.50 1-00
0.149 0.112 0.095 0.144 0.116 0-090 0.171 0.125 0-082
*[NaCIO4] = 3 M.
where extraction of sodium perchlorate is not significant the extraction curves for divalent cations show no significant decrease in slope at high aqueous sodium perchlorate concentration. Kertes and Kertes [10] have shown that perchloric acid is extracted into TBP as HCIO4.4TBP, HCIO4.2TBP, and HC104.TBP and their results were confirmed during the present investigation. Since perchloric acid is extracted to an even greater extent than sodium perchlorate it is not unexpected that extraction of the divalent cations from perchloric acid solutions does not occur as readily as it does from the corresponding sodium perchlorate solution. It does appear that in the presence of perchloric acid TBP shows some selectivity towards divalent cations. This may be associated with the ability of the cations to extract as species with coordination numbers less than six. This hypothesis is supported by the fact that zinc(II) which commonly assumes a tetrahedral configuration extracts considerably more readily than the other cations under these conditions. 10. A. Kertes and V. Kertes,J. appl. Chem. 10, 287 (1960).