Solvent extraction of salicylates-III

J. inorg,nucl.Chem.,1969,Vol.31, pp. 3241to 3246. PergamonPress. Printedin Great Britain

SOLVENT EXTRACTION

OF S A L I C Y L A T E S - I I I

THE DISTRIBUTION OF T R I V A L E N T CATIONS BETWEEN A Q U E O U S S O L U T I O N A N D T R I B U T Y L P H O S P H A T E IN T H E PRESENCE OF SALICYLIC ACID J. A G G E T T , P. CROSSLEY and R. H A N C O C K Chemistry Department, University of Auckland, New Zealand (Received 24 January 1969) Abstract-The distribution of iron(Ill), aluminium(Ill), and promethium(Ill) between TBP and aqueous solution in the presence of salicylic acid has been examined. Both iron(I 1I) and aluminium(lll) which exist in aqueous solution as chelates with the doubly-charged salicylate ion extract as species of the type M(CTH4Oa)(CrHsOa). Promethium(llI) which shows little tendency to chelate extracts as Pm(CTH503).~. INTRODUCTION

ALTnOUGrI the salicylate ion generally functions as a doubly-charged bidentate ligand in aqueous solution[l] it can function either as a singly-charged, or a doubly-charged anion in solvent extraction systems[2-4]. In the previous two papers in this series [3, 4] the role of salicylic acid in the extraction of a number of divalent cations has been studied. It has been established that the nature of the extracted species depends on the magnitude of the formation constant for the uncharged monochelate species M(C7H4Oa)* in the aqueous phase; and this in turn depends significantly on the size of the cation. Thus beryllium(II) which has a small ionic radius and a large formation constant for the reaction M 2+ + Sal 2- ~

MSal

(1)

extracts into tributyl phosphate (TBP) as BeSal. 2H20 while manganese(II) and other larger cations with much smaller formation constants for reaction I extract as species of stoichiometry M(HSal)2.2TBP. This study has now been extended to the trivalent cations iron(III), aluminium(III), and promethium(III) to determine both the nature of the extracted species and the factors which affect the mode of extraction of these ions. EXPERIMENTAL Extraction data were obtained by equilibrating equal volumes of aqueous and organic phases at 25°C. In the iron(Ill) and promethium(III) systems distribution data were obtained radiochemically using SaFe and 147Pm as tracers. In the aluminium(lII) system data were obtained spectrophotometrically by the following methods: *In this paper the species CrH6Oa, CrHsOa-, and CTH4Oaz- will be donated by H2Sal, HSai-, and Sal2- respectively. 1. D. D. Perrin, Nature, Lond. 182, 741 (1958). 2. H.J. De Bruin, D. Kairaitis and L. Szego, Aust.J. Chem. 15, 218 (1962). 3. J. Aggett and P. Crossley,J. inorg, nucl. Chem. 29, l 113 (1967). 4. J. Aggett, D. Evans and R. Hancock,J. inorg, nucl. Chem. 30, 2529 (1968). 3241

3242

J. AGGETT, P. CROSSLEY and R. H A N C O C K

Aqueous phases. The pH was adjusted to 7 and the aluminium(lll) extracted into chloroform by 10-1 M 8-hydroxyquinoline. The absorbance of the chloroform extracts was measured at 385 m#. Salicylate ions did not interfere at concentration -< 10-t M. Organic phases. These were diluted with an equal volume of 2 x 10-1 M 8-hydroxyquinoline in TBP. The absorbance was measured at 385 my,. The presence of salicylic acid at concentrations -< 10-1 M had no significant effect on the absorbance of these solutions. Higher concentrations of salicylic acid lowered the absorbance, the absorbance in the presence of 1 M salicylic acid being 8 per cent lower than that in the presence of 10 -1 M salicylic acid. For solutions containing salicyclic acid between 10-t M and 1 M the aluminium(IlI) concentrations were obtained from calibration curves. In this work the distribution ratio is defined as: q=

Total metal concentration in organic phase, Total metal concentration in aqueous phase

All chemicals were purified by standard methods[5]. Sulphuric acid was used for pH adjustment as preliminary results showed that iron(IIl) was extracted into TBP by perchloric acid in the absence of salicylic acid. RESULTS AND DISCUSSION

Figures 1-3 show the distribution ratios of iron(III), aluminium(III) and promethium(III) as a function of total salicylate concentration and pH. 1.0

O

-I.O

-2"C 3

pH

I

I

I

4

5

6

7

Fig. 1. Distribution profile or iron(III). (3 : H=SaIT -- 1 M; ~ : H=SaIT = 3 x 10-1 M; • : H2Salr = 10-1 M; () : H~SaIT = 3 X 10-~ M; ~ : H=Salr = 10-2 M.

In the iron(Ill) system the dependence of the distribution ratio on total metal ion concentration was investigated over the range 10-s-10 -2 M. At concentrations ~< 10-a M the distribution ratio was found to be independent of metal ion con5. D. D. Pen-in, W. L. F. Armarego and D. R. Pert'in, Pergamon Press, Oxford (1966).

Purification of Laboratory Chemicals.

Solvent extraction of s a l i c y l a t e s - 111

3243

q

o t# t.9 o.J

7

-I<2

I

-2"C 3

4

I

pH

I

5

6

I

7

Fig. 2. Distribution profile of aluminium(liD. ©: H2SalT ----- 1 M; ~ : HsSalT = 3 × 10-l M; 0 : H2SalT = 10-1 M; O: H2SalT = 3 X 10-~ M; @: H2SaJT = 10-2 M.

I

-I -2 -3

3

I

m

6 pH 5 Fig. 3. Distribution of promethium(III). © : H2SaJT = 4

2 X 10-~ M; (~:H2Sal T -~- 10-~ M; O : H ~ S a l w = S X 10-2M; ~ : H2SaIT = 2.5 × 10-2 M.

centration; at higher concentrations an increase in distribution ratio was observed. That this increase is due to the formation of a different species was shown by the fact that the extracts at high iron(lII) concentration were orange-red while the extracts at low iron(llI) concentration were purple. The purple species showed maximum light absorption at 530 rn~. In the aluminium(lII) and promethium(Ill) systems the distribution was independent of metal ion concentration for solutions ~
3244

J. AGGE'I'T, P. C R O S S L E Y and R. H A N C O C K

The fact that extraction of trivalent cations does occur suggests that species of the type M(HSal)a are probably formed. However the detailed evidence supports this hypothesis only in the case of promethium(IIl). In the iron(Ill) system plots of logq vs. log [HSal]~ at constant pH have slopes of about one in the region between pH2.5 and 4 (Fig. 4).* And beyond pH6 log q is essentially independent of the salicylate concentration. Since d log q d log [HSal-]~ = ~ o - fia where ~o and ~a are the numbers of ligands associated with each cation in the organic and aqueous phases respectively, this behaviour implies that the number of salicylate ions per iron(Ill) moiety in the organic phase is greater by about I

0

--I

-I

-2

-6

I

-5

I

I

-4 -3 • oG C.s=~]=~

I

-2

Fig. 4. Iron(ill) system: Iogq vs. log [HSal-]aq. • pH 2.5; @ pH 3.0; O pH 4.0.

one than the number bound to iron(lll) in the aqueous phase in the low pH region, and is equal to the aqueous ligand number in the vicinity of pH 6 and 7. Now calculations using known equilibrium constants [3, 6] show that between pH 2 and 4 in the extraction systems under study the predominant species in the aqueous phase are Fe 3+ and FeSal + i.e. in this pH region ~a ~< 1. In the region between pH 5 and 7 the species FeSal2- is formed in higher concentrations and ~ increases to about 2 in all systems by about pH 7, although the exact variation of ~ with pH for each system depends on the salicylate concentration in the system concerned. It therefore seems reasonable to conclude that the species extracted contains 2 salicylate ions per iron(Ill) and since only uncharged species are normally extracted into organic solvents this species is believed to be FeSal. HSal. • In Figs. 4 and 5 the concentrations of HSal~- have been calculated from data presented in Part I of this series [3]. 6. A. MarteU and L. G. Sill6n Stability Constants, Spec. publ. No. 17, The Chemical Society London (1964).

Solvent extraction of saficylates- III

3245

This is supported by the absorption spectra data. The peak at 530 m~ in the extract corresponds with that of FeSal + in aqueous solution (530 r ~ ) [ 7 ] ; species FeSal~- and FeSala 8- absorb at 490 m/z and 430 n ~ respectively. Furthermore since these peaks are associated with charge transfer transitions involving phenolic oxygen atoms[8] it seems that in the extracted species the phenolic oxygen of the HSal ion is not coordinated to the iron(Ill). In the iron(Ill)-benzoic acid-TBP and iron(Ill)-o-nitro benzoic acid-TBP systems where chelation cannot occur the extracted species have no visible absorption spectra and the pH profiles are similar to those of the promethium (III)-Salicylic acid-TBP system in which Pm(HSal)3 is believed to be extracted. The pH profiles for the aluminium(lll) system are similar to those of the iron(Ill) system. Plots of log q vs. log [HSal-]~ at constant pH (Fig. 5) have slopes

o

/

-I

-I

-2

-6

I

-S

G4. L H ' ~o-3 LO

-2

Fig. 5. Aluminium(IIl) system: iogq vs. log [HSal-]a q. • Ph 3.5; I~ pH 4.0; O pH 4-5.

of about one. Calculations show that ~a ~< 1 for all salicylic acid concentrations at pH ~< 4.2, while it has a value of about 2 in the vicinity of pH 5.5-7. Hence it appears that the extracted species is of the type (AISal)(HSal). The pH profiles of promethium(Ill) are characteristically different. The slopes of the curves are about 3 and at constant pH d log q/d log [ n s a l ] a ~ 3. In this system the extraction equilibrium appears to be kd

Pm hI+ 3HSal- .

" Pm(HSal)3.

It has previously been established[9] that other trivalent cations viz. scandium (III), yttrium(Ill), and Lanthanum(Ill) are extracted into amyl alcohol as species M(HSal)3. The difference in behaviour of the trivalent ions studied is related to their 7. L. Vareille,Bull. Soc. chim. Fr. 1493 (1955). 8. D. D. Perrin Organic Complexing Reagents. Wiley-lnterscience (1964). 9. B. N. Sudarikov, V. A. Zaytsev and Yu. G. Puchtov, Nauch. Dokl. vyssh. Shk., khim i khim TekhnoL 1, 80 (1959).

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J. AGGETF, P. CROSSLEY and R. HANCOCK

ability to form chelates with the doubly-charged salicylate ion. And in this respect their behaviour is similar to that of divalent cations. The magnitude of the formation constants for iron(III)[6] and aluminium(III) [10] shows quite clearly that they form such chelates readily. Unfortunately there is no data available for the promethium(Ill)- salicylic acid system. However there is no reason to suggest that the formation constants for this system are significantly different from those of other lanthanide-salicylate system. And the magnitude of these shows that lanthanides have little tendency to chelate with salicylate ions[11]. It seems probable that this type of behaviour will apply to other trivalent cations. Those which readily form chelates with the salicylate ion being extracted as species MSal. HSal, and those with little tendency to chelate being extracted as species MHSala. From a practical point of view cations are extracted much more effectively by HSal- than by Sal~-. However any degree of selectivity originating from differences in the magnitude of formation constants is likely to be lost unless chelates are formed. 10. J. Pecci andW. O. Foye, J. Pharm. Sci. 49, 411 (1960). 11. M. Cefola, A. S. Tompa, A. V. Celiano and P. S. Gentile, Inorg. Chem. 1,290 (1962).