Platinum metals-solution chemistry and separation methods (ion-exchange and solvent extraction)

0039s9140184 $3.00 + 0.00

Talanta, Vol. 31, No. IOA, pp. 815-836, 1984 Printed in Great Britain. All rights reserved

Copyright ~CI1984Pergamon Press Ltd

PLATINUM METALS-SOLUTION CHEMISTRY SEPARATION METHODS (ION-EXCHANGE AND SOLVENT EXTRACTION)

AND

S. J. AL-BAZI and A. CHOW* Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada (Received 14 October 1983. Revised 6 February

1984. Accepted 20 April 1974)

effects of knowledge of the solution chemistry of the platinum metals on their separation by solvent extraction and ion-exchange methods are reviewed, for the period 1950-1983. The review concentrates on the chloro-complexes of these metals and indicates those areas which need more investigation or interpretation to provide adequate separational methods.

Summary-The

The complex nature of the solution chemistry of the platinum metals has contributed to the difficulty of developing methods for their separation. With more knowledge of the complexes possible in solution, and of their stability and kinetic properties, the solvent extraction and ion-exchange methods should be better understood. The relationship between the solution chemistry of the platinum metals and the ion-exchange and solvent extraction methods used for their separation is therefore reviewed here, for the period 1950-1983. Because of the common use of the chloro-complexes of these metals in such investigations, the review is mainly restricted to these. In some publications, the solution chemistry has been extensively discussed, whereas in others it has been almost completely ignored. We have divided the methods into appropriate sections with illustrative examples and have tabulated similar procedures. An indication is given in each section of the areas which require additional investigation or interpretation. It is important that the chemistry be elucidated if progress is to be made in this difficult area of analytical chemistry. ION-EXCHANGE Ion-exchange methods have been developed for most of the platinum metals, based either on differences in the affinity of similar complexes for a resin or on larger differences caused by varying the solution constituents. The capacity of the resins used is usually large (up to 1 meq/g), which allows concentration of the metals, and the method is rapid atd offers separation from a variety of other ions. Electrostatic

efects

Several studies’-’ of the chloro-complexes of the platinum metals clearly indicate that the strength of interaction with anion-exchangers is highly dependent on the charge of the complex. Doubly*Author for correspondence. 815

charged complexes (PdCli-, PtCl:-, PtCli-, IrCli-, RuCli-, OsCli-) were found to be strongly sorbed by the resins, whereas the triply-charged (IrClz-, RhCli-, RuCli-) were only weakly bound in static systems used for preconcentrating these metals. The sorption of the chloro-complexes of rhodium and ruthenium also depends on the age of the solution, mainly because of the labile character of these metals towards aquation;4,8-‘0 complexes of the type [MC1,,Y(Hz0),]-Y-3 (x = l-6) are formed.“-26 This property might be useful in some separations but has mainly been considered a problem in dealing with solutions of rhodium and ruthenium. Several platinum metals have been separated with anionexchange resins, on the basis of the relative strengths of electrostatic interaction (Table 1). Palladium andplatinum. Palladium and platinum in dilute acid are both sorbed strongly by anionexchangers but it is possible to use the slight difference in interaction at high acid concentrations for sequential elution of palladium with concentrated hydrochloric acid and then platinum with perchloric acid.2 Alternatively2’ palladium can be recovered with sodium hydroxide solution and platinum with dilute nitric acid. Rhodium and iridium. By use of only the differences in the strength of electrostatic interaction with the anion-exchange resin, rhodium is typically eluted from the column first with a dilute acid or salt solution, and can be separated from palladium2 and platinum.’ The separation of rhodium and iridium by anion-exchange often requires a suitable oxidizing agent to maintain iridium in the quadrivalent state, which is strongly sorbed by the resin.8,9 Selective reducing agents can reduce Ir(IV) to the nonextractable Ir(II1) (E” = 1.02 V),‘* which permits a separation from palladium’ and platinum.5~27*29 Furthermore, by controlling the oxidation-reduction conditions it is possible to separate a multicomponent system. Table 1 outlines separations of rhodium and iridium from each other and from palladium and platinum.2,30

_ ._ - -

Ir(IV)-Pt(IV)

Ir(III)-Pd(I1) Ir(III)-Pt(IV) Ir(IV)-Pd(II)

_. - - _ _. -

_

ES(OH-)

IRA-400(Cll)

._ - - - - -

Pennutite

Amberlite

2M HCI

IRA-4OO(Ci -)

IRA-4OO(Cll)

Amberlite

Amberlite

DEAE

Cellulose

Dowex-l(Cll)

NaCl

IRA-4OO(Cll)

Amberlite

ES(OH-)

Permutite

separation

IRA-4OO(Cl-)

Exchanger

I. Ion-exchange

Amberlite

Table

IOM HCI

0.8M HCl,

pH 2.1, O.lM NaCl O.lM HCI, 2% NaCl

Rh(III)-Pt(IV)

Rh(IIIFIr(IV)

2M HCI

NaCl

Medium

Rh(III~Pd(I1)

Pd(II‘t-Pt(IV)

Pd(II)-Pt(IV)

Metals separated*

Ir: Pd: Ir: Pt:

2M HCI 12M HCI 1M NaOH 2SM HNO,

Pd: 9-12M HCI Pt: 2.4M HCIO, Pd: IM NaOH Pt: 2.5M HNO, Rh: 2M HCI Pd: 12M HCI Rh: 0. I M NaCl Pt: IM HCI Rh: 2% NaCl in O.lM HCI and 5% bromine water Ir: 5M NH,OH followed by 6M HCI Rh: Cerium(IV) solution Ir: Soxhlet extraction with 6M HCI Ir: IOM HCI

_

on electrostatic

Eluent

based information

as oxidizing

..-

-

Sodium oxalate to reduce Ir(IV) selectively

99% Ir eluted: no attempt to recover Pd or Pt NH,OH to reduce Ir(IV)

Cerium(IV) agent

Quantitative separation; microgram amounts Error 1% for 4.7 mg Pt. 3% for 2.5 mg Rh Bromine water maintains iridium as Ir(IV)

Pd and Pt quantitatively sorbed Milligram amounts

Other

effects

-.

I_

27

2

1

9

Reference

.-

-

3h4 HCl

2M HCl, NaCl

2M HCl, NaCl

2M HCl, NaCl

HBr

Ir(IV)-Pt(IV)

Rh(III)-Ir(IV)Pd(I1)

Rh(III)-Ir(IVk Pt(IV)

Rh(III)-Ir(IV)Pd(II)-Pt(IV)

Ir(IV)-Pt(IV) Rh(III)-Pt(IV) Pd(II)-Pt(IV) Rh(III)-Ir(IV) Ir(IVEPd(IIkPt(IV) Rh(III)-Ru(IV)

*(-) means “separated from”.

O.lM HCl

Ir(IVkPt(IV)

Rh: 2M HCI Ru: Not possible even with 1% NH,OH.HCI

Amberlite IRA-4OO(Cl-)

Dowex-l(Brr)

Rh: 2M HCI Ir: 2M HCl Pd: 9M HCl Pt: 2.4M HClO, Pt: 8% thiourea

Ir: O.lkO.4M NaCl in O.Ol0. IM HCl, 0.035M ascorbic acid Pt: IMHCI Ir: 9M HCI Pt: 50% HClO, Pd: 9M HCl Rh: 2M HCl Ir: Reduced with NH,OH.HCl and eluted with 2M HCI Pt: 2.4M HCIO,

Amberlite IRA4OO(Cl-)

Amberlite IRA-4OO(Cl-)

Amberlite IRA-4OO(Cl-)

Dowex A-l(Cl-)

Cellulose DEAE(OH-)

30

2

Cerium(IV) to prevent Ru reduction by resin

29

Bromo-complexes separated; Br, as oxidant, N,H,.HCl as reductant

NH,OH.HCl to reduce Ir(IV) selectively Ir(IV) in mixture reduced with NH,OH.HCl Ir(II1) in effluent oxidized with cerium(IV) before recycling through column Ir(IV) in mixture reduced with NH,OH.HCI Rh and Ir separation as above Combination of the above two procedures used for separation

Ascorbic acid to reduce Ir(IV) selectively

IR-lOO(NH:)

Amberlite

NH,OH

Varion

Strongly basic anionexchanger (Cl-)

0.25M NH,Ac

PH 2

pH 3.5

pH 3.5, NaCl

Pd(II)Pt(IV)

Pt(IV)-Rh(II1)

Rh(IIIkPt(IV)

Pd(IIbRh(III) Pt(IV)-Rh(II1)

Dowex

50(H +)

KS(H+)

AGl-XI(CI-)

NH,OH 0.25 M NH,Ac

Pd(IIkPt(IV) Pt(IV)-Pd(I1)

Silica gel AGSOW-X4(NH:)

Dowex-2(Cl-)

IR-lOO(NH,+)

Amberlite

NH,OH

Pt(IV)-Pd(I1) Ir(IV), Pt(IVkPd(I1) Pd(II)-Rh(III)_ Pt(IV)-Ir(IV) [Rh(IIItPt(IV)_

~co1

IR-lOO(NH:)

Amberlite

Exchanger

NH,OH

Medium

Table 2. Ion-exchange

Ir(IV)-Pd(II)

Metals separated*

.

based

on kinetic

Pd or Pt: water Rh: 6M HCI with heating at 60^

Rh,Pt: removed one after another with 0.025M NH,OH-0.025M NH&I Pd: 0.5M HCl Pt: 0.25M NH,Ac Pd:O.IM HCI in 90% acetone Pd: 0.25M NH,Ac Pt: O.OlM HCI-O. I M thiourea mixture at 80’ Pt: water Rh: 1M HCl Rh: 0.2M HCI Pt: dilute NH,OH

Pd: IM HCI

Ir: 0.025M NH,OH0.025M NH&l Pd: 1M HCI

Eluent

separation

First made basic with NaOH and after 10 min made acidic with HCI

44 Made alkaline (pH 13) first with NaOH, then acidified with HNO, Made alkaline first with NaOH then acidified with HCI; 3.5”/, Rh left on column

46

45

43

42 43

40

Anion-exchanger sorbed PtCli- but not Pd(NH,):+

Effluent from cation-exchanger made acidic before passing through anion-exchanger; Ir not recovered Static system Heated for 10 min to ensure complete formation of Pd(NH$

as above

40

procedure

Similar

Reference 41

information

Pd retained as Pd(NH,):+; Ir as anionic chloride not sorbed

Other

effects

Pd(I1)

Sulphonylguanidine

Styrene-8-aminoquinoline-based copolymer

0.5M HCl, 0.1 M thiourea

0.3M HCl, thiourea

0.02M NH,SCN, 2M HCl

PH 2

3M HCl

Rh(III)/Ir(IV)Pd(II)/Pt(IV)

Ir(IV)-Rh(II1)

Pt(IV)-Pd(I1)

Pt(II)-Pd(I1)

Rh(III)/Ir(III)Pd(II), Pt(IV)

Rh or Ir: 3M HCI

Pt: 0.02M NH&N-2M HCI Pd: 0.05M thiourea Pt: water Pd: 334M HCl

Rh or Ir: 0.5M HClO.lM thiourea Pd or Pt: 4.5M HBr Ir: 3M HCl Rh: 6M HCl at 74”

Ir: 10% chlorine water Rh: 6M HCI

Sulphonylguanidine, which forms a neutral complex with Pd, was incorporated into a styrene-divinylbenzene copolymer Sorbent exhibits both ionexchange and complex-forming properties forming properties

Heating for 1 hr on steam-bath permits complete formation of the cationic Rl-thiourea complex Separation done at below S”, Pd forms Pd(SCN)i- and Pt remains as PtCli-

Rh reacts 6.5 times faster than Ir to form (Mpy,Cl,)+ With excess Cl-, Ir(IV) forms (Irpy,Cl,) which is reduced with ascorbic acid to (Irpy,Cl,)Rh and Ir do not react with thiourea under these conditons

Ir(IV) reduced with 1% hydroquinone before Rb precipitated and redissolved as cationic complex, then Ir(II1) oxidized with chlorine gas Combination of the two procedures above used for separation of Rh from the mixtures

52

51

50

49

48

53

47

46

46

*(-) means “separated from”; (,) means “and”; (/) means “or”; [ ] indicates “a subsequent separation of these metals after the preliminary step shown above it”.

Cellulose DEAE (SCN-)

Dowex 50W-X8(H+)

AG50W-X4(H+)

KU-2(H+)

Pyridine

Ir(IV)-Rh(II1)

KU-2(H+)

Dowex 50(H+)

Pyridine

pH 2.8

Rh(III)-Pt(IV), Pd(II), Ir(IV) Ir(IV)-Rb(II1)

Rh(III)-Pd(II), Ir(IV) Rh(III)-Pt(IV), Ir(IV)

Rh(III jPt(IV),

Ir(IV)-Rh(III)

820

S. J. AL-BAZI and A. CHOW

Ruthenium and osmium. The only description found of an anion-exchange separation of Ru(IV) is for its separation from Rh(III), both being present as chloro-complexes.* The ease of reduction of Ru(IV) to Ru(II1) by the resin made it essential to add cerium(IV) to maintain the higher oxidation state. However, because of the strong sorption of ruthenium on the resin, its complete recovery was impossible even with reducing agents such as hydroxylamine hydrochloride. If the ruthenium is reduced to Ru(II1) in solution, no separation is possible from Rh(II1) because of the similar low affinity of both tervalent metals for the exchanger. No work has been reported on the sorption of the chloro-complexes of osmium by anion-exchange resins, although quantitative liquid ion-exchange extraction of the quadrivalent metal has been achieved.“m37 No successful ion-exchange separation of osmium and ruthenium has been reported. The ease of reduction of Os(IV) and the reduction or partial hydrolysis of Ru(IV) prevent their quantitative separation3* However, liquid anionexchangers have been used for the separation of the chloro-complexes of Ru(II1) and OS(IV),~~,~~as discussed later. Because the ion-exchange resins are usually less reducing than the liquid ion-exchangers, it would be expected that the resin separation of ruthenium and osmium should indeed be possible. Kinetic effects The differences in the labile character of the platinum metals in the formation of cationic, anionic, and neutral species has been used for their separation (Table 2). For example, the rate of formation of the cationic ammine complex of Pd(I1) from its chlorocomplex is sufficiently different from that for other metals*43 to permit isolation of the complex. The rapid formation of Rh(H*O)i+ through precipitation of the hydroxide and redissolution in dilute acid has been used in several ion-exchange procedures.4M6 Incomplete conversion and instability in more acid conditions [resulting in formation of other complexes, RhCl,,(H20)~~3] cause major problems with these methods. Other studies on the separation of platinum metals have been reported where the differences in their labile character toward pyridine:’ thiourea,48.49 thiosulphonylguanidine incorporated into a cyanate, copolymer,” and styrene-divinylbenzene styrene-8-aminoquinoline-based copolymer’* have played an important role. Though several studies have indicated reasonable success in separation of several of the platinum metals, the advantage of the differences in their labile character could be further utilized. SOLVENT EXTRACTION Solvent extraction has been widely used for separation of the platinum metals.5”s5 The difficulty in

developing extraction procedures for the individual metals is due to the slow reaction of the chlorocomplexes. The solvent extraction methods for the separation of binary or multicomponent systems of these metals use the differences in their kinetic behaviour for the formation of extractable species, as well as the strength of electrostatic interaction of their chloro-complexes with liquid ion-exchangers or with oxygen-containing solvents. Common binary mixtures Electrostatic effects. The contribution of the charge of the complex and its labile character towards hydration has been useful in the separation of platinum metal mixtures. The inertness of the chlorocomplexes of palladium and platinum toward aquation plays an important role in their extraction from acidic solution by an anion-exchange mechanism with organic bases such as amines, quaternary ammonium salts, antipyrines and other nitrogencontaining exchangers.34.36.5”63 However, because both metals are highly extractable as chlorocomplexes their separation is difficult and has not been reported in the literature reviewed. The chloride complex of Ir(IV) is highly extractable into organic solvents,34.36.57,59,62-69 whereas the Ir(II1) form is much less extractab1e59.62,656Rowing to the increase in the charge on the complex. On the other hand, the Rh(II1) chloro-complex is also poorly extracted 34,36,57,59,63~66,69 which is due to the charge of the complex as well as its labile character toward aquation, i.e., formation of [RhC1,,(HZO),]rm3 (x = l-6). Ruthenium and osmium can also be separated as chloro-complexes because of their different affinities towards the extractant through either the anionexchange or hydration-solvation mechanism. Ru(IV) is highly extractable but Ru(II1) is not.34.36.59.7”7y The poor extraction of Ru(III), like that of Rh(III), is attributed to both the charge on the complex and to the ease of aquation to form [RuC~,~,(H,O),]‘~ (x = l-6). Os(IV) behaves similarly to Ir(IV) and is inert toward aquation and is quantitatively extracted by anion-exchangers3’m37 or by oxygen-containing extractants.8s83 Several procedures (Table 3) have utilized these differences in behaviour of the chloro-complexes of these common binary mixtures of platinum metals for their separation by different types of organic solvents. Although the separation of rhodium and iridium as their chloro-complexes has been widely investigated with the iridium kept in the quadrivalent state, the problem of recovery of the last traces of rhodium has not yet been solved. In addition, the nature of the extracted rhodium complex needs to be elucidated to provide the necessary information for developing efficient methods for its recovery. Kinetic efjcts. The differences in the formation rate of the extractable anionic, neutral, or cationic species of the binary mixtures of Pd-Pt, Rh-Ir, and

Separation of platinum metals

821

Table 3. Solvent extraction based on electrostatic effects Metals separated*

Aqueous phase

Organic phase

Other information

Reference

Rh(III~Ir(IV)

67M HCl

Tri-n-butyl phosphate

70

Rh(IIItIr(IV)

Tri-n-butyl phosphate

Rh(IIItIr(IV)

6M HCl saturated with NaCl 6MHCl

Rh(III)-Ir(IV)

6M HCl

Rh(III)-Ir(IV)

HCI

Ru(IVtOs(IV)

3-6M HCI

Diantipyrylpropylmethane in dichloroethane

Ru(III)-Os(IV)

Chloroform

Ru(III)-Os(IV)

0.1-0.3M HCI Ph, A&l 0.058M HCl

Aqueous phase treated with H,O, to oxidize Ir, heated at 90 to destroy excess peroxide For quantitative separation, the oxidation of Ir by H,O, is necessary in each of the nine extraction stages required Continuous treatment with chlorine gas to prevent Ir reduction by amine In presence of free chlorine, 98% Ir extracted while 99% Rh remained Acidic solutions containing NaCl and H,O, evaporated to a moist salt before dissolving for extraction Solutions containing N,H,.HCl or N,H,. H,SO, heated to reduce Ru selectively to Ru(II1) OS extracted as 2Ph,As+ ~OsCl~~

39

Ru(IIItOs(IV)

6M HBr

Methyl isobutyl ketone

Ru(IIItOs(IV)

_ 5M HCI

Triphenylphosphine in 1,2-dichloroethane

OS extracted then Ru oxidized to RuO; and extracted with Ph,AsCl in chloroform As bromo-complexes, OS extracted quantitatively whereas only 2% Ru was extracted A quantitative separation is possible

Tri-n-octylamine in benzene Tri-n-octylamine-loaded silicone rubber foam Diantipyrylpropylmethane in dichloroethane

Amberlite LA-I in chloroform

71

72 73 34 34

74

7s

64

(-) means “separated from”. Ru-0s with several complexing or chelating agents have been utilized for their separation from aqueous solution. Palladium and platinum. These will be discussed in terms of charge type. (i) Anionic complexes. The kinetically labile character of the chloro-complexes of palladium towards a number of hydrophobic anions allows the immediate formation of highly extractable anionic complexes at room temperature.8”86 Platinum, which reacts slowly under the same conditions,87 may thus be separated from palladium. For example, milligram amounts of these metals in ammonium thiocyanate-hydrochloric acid solutions can be separated by the weakly basic diethylaminoethyl cellulose ion-exchanger.50 The efficiency of separation increases with decreasing temperature, and below S’, about 98% of the platinum (as PtCli-) can be separated from palladium, which is quantitatively sorbed as Pd(SCN):-. Because the palladium complex is highly extractable by oxygen-containing solvents whereas the PtCli- is not, it should also be possible to separate them by using extractants such as phosphates, alcohols, ketones, esters, ethers, etc. (ii) Neutral complexes. Neutral complexes of the type MX,L, (n = 2 for Pd and 2 or 4 for Pt) are formed by the reaction of palladium*697 and platinum56,88,93,98,99 with suitable singly-charged anions (X) and organic bases (L) such as those containing N, S, P, As, Sb, etc. These complexes with ions such as halide or thiocyanate are soluble in carbon chloroform, 1,2-dichloroethane, tetrachloride, cvclohexane. benzene. etc. The extractable~~~snecies ~r..~__ of _:

pd(II)64.92.9”‘02 are produced rapidly even at room temperature, whereas Pt(II) complexes form more slowly and Pt(IV) complexes often require heating93,l00.lO2.l03 or the use of a catalyst such as stannous chloride.64*93*9”0’ Stannous chloride also acts by reducing Pt(IV) to the more reactive Pt(I1) species. The rate of formation of the neutral complexes is in the order Cll < Br- < I- in the presence of excess of the halo-acid; this corresponds to the order of decreasing bond strength and increasing reduction potential. For example, there is no reaction with 2-mercaptobenzothiazole”2 or diphenylthiourea’13 added to a hydrochloric acid solution of platinum, but a yellow precipitate is formed if potassium iodide is added to the solution either before or after the ligand. This can be attributed to reduction to Pt(I1) and the subsequent formation of the more labile PtI:- from the original PtClg-. Also, palladium can be extracted as PdX,(DOS), [DOS = di-n-octyl sulphide] from acidic chloride, bromide or iodide solution with cyclohexane,‘06 but platinum can only be extracted from iodide solution [probably as Pt12(DOS), rather than the reported PtI,(DOS),]. The separation of platinum and palladium is best accomplished by extracting the palladium complex before formation of the extractable platinum complex becomes significant. This can be done by starting with the chloro-complexes of the metal in neutral solution at room temperature or below. A low salt concentration reduces the formation of PtCl,L, and a weakly acidic solution restricts the extraction of PtC@ through the hydration-solvation or anionextraction mechanism. To minimize the extraction of

822

S. J.

AL-BAZI

platinum, it is best to start with Pt(IV) in the absence of any reducing agent, since this form is less labile, and the separation should be done quickly since the formation of PtX,L, is kinetically slow. On the basis of these differences in the behaviour of palladium and platinum toward different neutral ligands, several methods for their separation have been considered, some of which are indicated in Table 4. Several sulphur-containing ligands,“““6 including dialkylsulphides and dialkylsulphoxides,‘07~“7~“9 have been used to extract palladium and may be efficient for its separation from platinum. Though several derivatives of dithiocarbamic acid have been used in chloroform to extract palladium and platinum individually, it would appear12’ that diphenyldithiocarbamic acid can be used for quantitative separation of palladium from platinum. This should be possible in solutions containing sodium sulphide as a reducing agent, even in up to 2M hydrochloric acid, though at such high acid concentrations it is customary to use a nitrogen-containing ligand as counter-anion in an ion-association extraction. Some of the most promising ligands for the separation of palladium and platinum are dialkylsulphoxides, which can be used in oxidizing media, and dialkylsulphides, which can be used in basic or highly acidic solutions (> 6M hydrochloric acid), whereas ligands containing sulphur and nitrogen and/or oxygen as donor atoms can be used only in dilute acid solutions. More detail is needed on the effect of branched substituents on the behaviour of the ligand and on whether the different extractable species formed with the sulphoxides in highly acidic solutions are neutral PdX,(R,SO), or hydrated-solvated 2[H(R,SO)(H,O),+].[PdCl:~] complexes. Also there may be sufficient differences in the kinetic behaviour of platinum and palladium with different amines to allow separation of these metals, since the tendency of the amines to form the neutral complexes decreases in the order RNH, > R,NH > R,N. (iii) Cutionic complexes. The significant difference in the rate of formation in ammoniacal medium of the ammine complexes [M(NH,),“+] of palladium and platinum has been used for the separation of these metals.“’ High molecular-weight amines such as tri-n-octylamine and primene, tri-n-hexylamine, dissolved in chloroform or benzene, have been used as liquid anion-exchangers for the extraction separation of platinum from palladium, but careful timing was necessary to avoid significant formation of the platinum ammine complex. Other methods could be developed, based on the slower formation of the perhaps by enhancing the platinum-ammine, difference in the formation rates by using ammonium acetate rather than ammoniacal medium, or by use of different liquid anion- and cation-exchangers. Rhodium and iridium. The chloride complexes of both ~~~~~~~6'~"~65~70~87~93~l2l~l25 and iridium".bS,'3.'OO. '"'~'03~"5~'24~'26~'27

react very slowly at room temperature

and A.

CHOW

to form extractable anionic (M,Cl,X,)k[x = Br-, II, &Cl;, SnBr;, SCN-, etc.] or neutral MCl,L, [L = a neutral organic compound containing N, S, P, As, Sb] complexes. Heating the aqueous phase or introduction of a catalyst before extraction accelerates the formation of the extractable species. The relatively labile character of rhodium compared to iridium [Rh(III) > Ir(II1) > Ir(IV)] in the formation of the extractable anionic’28-‘30 or neutra164,98~99~‘3”32 complexes plays an important role in their separation; this commonly involves converting rhodium into the extractable form while maintaining iridium in the unextractable chloride form. In addition, the presence of excess of salt or stannous chloride affects the formation of the anionic and neutral complexes, respectively. Table 5 lists the separation of several rhodium and iridium complexes. (i) Anionic complexes. Rhodium has been extracted in the form Rh,Cl,Xzfrom solutions containing iridium, by oxygen-containing solvents. The addition of stannous bromide produces the rhodium complex much more rapidly than that of iridium in hydrochloric acid solution.‘33 Isopentyl alcohol can be used to remove the rhodium-tin(I1) bromide even in the presence of large amounts of sodium chloride, and from perchloric acid solutionThe difference in the reaction rates means that the rhodium complex takes 2 hr to form at room temperature (although obviously the time could be shortened by warming the solution) whereas the iridium complex is formed only by heating the solution at 90” for at least 3 min (and even then gives inconsistent results). Rhodium can also be extracted from iodide solution with tri-n-butyl phosphate in toluene.‘34 The formation of the iodo-complex requires heating at 70” for 1 hr at pH 2. The complex is extracted from through the 1M sulphuric acid, probably hydration-solvation mechanism, and can be stripped with ammonia solution. The iridium complex is unlikely to be appreciably formed under these conditions and a separation of rhodium and iridium should be possible. One of the difficulties in the extraction of rhodium and its separation from iridium is the labile character of the starting hexachloro-complex of rhodium towards aquation. With aging, species of the type [RhCl,(H20),]“3 (x = l-6) are formed, which react at different rates with a ligand and thus produce different complexes with varying extractability. To avoid this problem, rhodium can be converted into the hexachloro form by evaporating the solution to dryness in the presence of a chloride salt shortly before use.‘33 An example of the effect of aging13’ is the extraction of rhodium as a thiocyanate complex by polyurethane foam; in this case a one-day old solution, containing mainly [RhCl,(H20)]2~, was 92% extracted, whereas a seven-month old solution containing mainly RhCl,(H,O), was only 78% extracted. The use of a chloride salt for stabilizing rhodium

~-

,

,

Dimethylglyoximetreated silicone fubber foam Di-n-octyl sulphide in cyclohexane R,S in benzene; R = Bu-C,H,,,

PH 4

, - ._ .. .. ._ “_

NH,OH, cont. HCl, SnCl,

Cationic complexes

_. .

pH 6, 2-thenoyltrifluoroacetone

2-Mercaptobenzothiazole chloroform n-Butanol

PH 3

.~

-

Tri-n-hexylamine, tri-n-octylamine, or primene in benzene or chloroform

in

Di-n-octyl sulphoxide in benzene Di-n-heptyl sulphoxide in 1,1,2-trichloroethane

Up to 2M HCI 6M HCI

HCI or HNO,

HCI or HBr

Chloroform

Organic phase

Ph

_. .

-- --

._ - ..

- - ._ .- “_

Delay of 0.5-I min between addition of ammonia and HCl permits complete formation of Pd(NH,):+ while maintaining Pt in its chloro-form

Five min at room temperature for Pd-pnitrosodimethylaniline complex formation; no Pt reaction for several hr Quantitative separation; Pd extracted was completely recovered with 8M HNO, Selectivity of the ligand for Pd permits its separation from Pt High efficiency of Pd extraction as PdX,.2R,S (X = NO; or Cl-) from HCI or HNO, may isolate it from Pt A separation factor of lo3 permits selective removal of Pd Simultaneous extraction of Pd and Pt and subsequent back-extraction of Pt with water and Pd with 10% dimethylamine can separate Pd and Pt in solution containing Rh and Ir Pd was quantitatively extracted and since 1.8% Pt was extracted, a separation was suggested After Pd was extracted as PdCI,L,, solution was made 6M in HCI and Pt extracted with butanol-acetophenone mainly through the hydration-solvation mechanism

Other information

Table 4. Solvent extraction of palladium and platinum, based on kinetic effects

pH 2-5, alcoholic solution of p-nitrosodimethylaniline

Neutral complexes

Aqueous phase

_ I_ -

III

110

93

108 109

107

106

105

104

.

Reference

~- -

._

-

‘L9C;M KSCN,

KSCN,

HCI, SnCl,

2M HCl,

2M HCI

NaCl

Cationic complexes < 0.05M HCI

1-2M HCl, 4,5-dimethyl2-mercaptothiazole 3-9M HCl, SnCl,, 4,5dimethyl-2-mercaptothiazole IdM HCl, SnCI,, diphenylthiourea

0.25-2M

Neutral complexes 4-6M HCI

;;

2 x lO-jM

NaI, IM H,SO, HClO,

or

phase

Anionic complexes SnBr,, HBr-HCIO,,

Aqueous

foam

Dinonylnaphthalenesulphonic acid in n-heptane

3:2 v/v Chloroform-acetone mixture

Chloroform

extraction phase

2-Mercaptobenzothiazole in chloroform 2-Mercaptobenzothiazole in chloroform Chloroform

Polyurethane

foam

phosphate

alcohol

Polyurethane

Tri-n-butyl in toluene

Isopentyl

Organic

Table 5. Solvent

and iridium. Other

effects

information

based on kinetic

not required;

quantitative

for Rh, 610% for Rh; negligible

boiled for 7 min; quantitative

Ir extraction

Ir extracted

Rh(OH), precipitated by 6M NaOH and redissolved with O.lM HCI; 90% Rh extracted and recovered with 6M HCI; 99% Ir in aqueous phase

Vigorous boiling for I hr required for reduction and maximum formation of complex; both Rh and Ir extractable Reduction and formation of complex at room temperature; no interference by Ir Standing for 20 min at room temperature for complete Rhdiphenylthiourea complex formation; quantitative separation of Rh and Ir.

Heating

Solution

Extractable Rh-stannous bromide complex formed at room temperature; heating required for Ir Under optimum conditions for RI-iodide complex formation (pH 2, heating at 70” for I hr), a distribution coefficient ratio of Rh/Ir of 650 suggests an efficient separation possible Acid added after heating the solution at 90 for 4 hr; an average of 88% Rh extracted while 91% of Ir remained in aqueous phase Acid added after heating the solution at 90” for 30 min; an average of 93% Rh extracted; 95% Ir remained in aqueous phase

of rhodium

138

113

137

136,137

93

93

125

135

134

133

Reference

R P

Separation of platinum metals as the hexachloro-complex in solutions is of considerable importance. Because rhodium solutions usually require heating to form the extractable species, the aquation of the rhodium complex is facilitated. The different aquation complexes react at various rates with a ligand, because the water molecule is less labile than the chloride ion, although eventually the same complex may be produced. When a chloride salt is present in a freshly-prepared rhodium solution, it ensures the rhodium is present as the hexachloro-complex, which is the most labile form. The use’25 of 2M lithium chloride medium reduces the heating time for formation of Rh(SCN)iat 90” to 30 min from the 4 hr required in absence of the chloride. However, with iridium solutions even 4M lithium chloride does not decrease the required heating time of 4 hr and in fact decreases the extraction of the thiocyanate complex by polyurethane foam;‘25,‘35 this is due to the inertness of the hexachloroiridate towards aquation, resulting in a competition between the chloride and thiocyanate ions for iridium. The tin(I1) bromide complexes of rhodium and iridium, which are extracted by several nitrogencontaining solvents in chloroform,‘29 are also affected by chloride ion. The presence of 4M chloride almost completely inhibits the formation of the extractable iridium-tin(I1) bromide complex while not affecting the formation of the rhodium complex, for which these are the ideal formation conditions. In this case the competition between SnBr; and Cll for iridium is the most likely reason for the decrease in formation of the bromostannate(I1) complex. The role of chloride in the reaction of iridium and in labilizing the formation of anionic complexes of rhodium with different hydrophobic anions, e.g. Br, II, SnCl;, SnBr;, etc. needs to be further studied and should be extended to neutral ligands containing N, S, P, As, Sb, etc. (ii) Neutral complexes. Neutral sulphur-containing complexes of rhodium can be formed, and extracted by an organic solvent, while iridium is maintained as the unextractable chloro-complex. Heating is required to accelerate the formation of the rhodium complex and reduction to the more labile Rh(II) form, but this also results in the partial formation of an extractable iridium complex. Though quantitative extraction of rhodium has been obtained93 by boiling a solution iridium with of rhodium and 2-mercaptobenzothiazole, cooling, and extracting with chloroform, 610% of the iridium was coextracted. To avoid co-extraction of the iridium with rhodium, acidic solutions of stannous chloride have been used to labilize formation of the rhodium complex. The catalytic behaviour of stannous chloride arises mainly because of its ability to reduce rhodium to Rh(I1) and form relatively labile complexes of the type MCl,(SnCl,), x4”+m).The formation of the extractabie neutral sulphur-containing complex (MCI, L,)

825

is facilitated by substitution of a sulphur-containing ligand (L) for the more labile SnCl; rather than for Cl-. In the presence of stannous chloride the rhodium complex forms rapidly and does not require heating, which would result in the simultaneous formation of a similar iridium complex. From acid solutions containing 2-mercaptobenzothiazole and stannous chloride, more than 99% of the rhodium can be extracted into chloroform, but no iridium.93 It is possible that stannous bromide may be even more effective in accelerating the formation of an extractable rhodium complex, since it is a stronger reducing agent and is more labile than stannous chloride. The use of sulphur-containing ligands has not been studied extensively and the effectiveness of several alkyl and aryl derivatives of thiourea which are now available should be considered. Dialkyl sulphides and other sulphur-containing reagents with a relatively high electron density should be examined, for example, since dibenzyldithiocarbamate has been noted as introducing an extreme interference from rhodium in the extraction of palladium and platinum’*’ because of an analogous rapid reaction. Other neutralacceptor ligands which are weakly protonated in acidic media, e.g., derivatives of phosphine, amine and stibine, may also be useful for separations. (iii) Cationic complexes. Liquid cation-exchangers have been used for the separation of the chlorocomplexes of rhodium and iridium, utilizing the ease of formation of the cationic Rh(H,O)i+ complex. This complex has usually been produced by precipitation of Rh(OH), and formation of Rh(H,O)i+ by redissolution in dilute acid. The latter complex can be extracted with dinonylnaphthalenesulphonic acid in heptane.“’ These methods are limited because of incomplete Rh(OH), precipitation and instability of in hydrochloric acid, where neutral or Rh(H,O);+ anionic chloro-complexes can also be formed. Ruthenium and osmium. The chloro-complexes of Ru(III or 1~)98.99.116.139-141 and OS(IV)~‘.~~.‘~’react with hydrophobic anions or neutral ligands to form extractable species. Several hours are required for the reaction of Ru(III or IV) chloride complex with hydrophobic’4’ and neutral complexing agents14’ at room temperature but only a few minutes with heating; the rate of reaction with Os(IV) chloride is much slower.‘*‘46 Excess of chloride also plays an important role in the separation, since the chloride complexes of Ru(III or IV), like those of Rh(III)14’ are labile toward aquation, whereas osmium is inert like iridium. Therefore the addition of chloride ions increases the rate of ruthenium reaction with hydrophobic or neutral complexing agents and decreases that of osmium. (i) Anionic complexes. The maximum formation of Ru(SCN& requires only 5 min heating at 90” while that of Os(SCN)requires 3 hr. In addition, the presence of 3M lithium chloride increases the formation of the ruthenium complex and decreases

826

S. J. AL-BAZI and A. CHOW

that of the osmium complex, so a 95% complete separation is possible.‘42 It would be useful to see the effect of other complexing agents in this type of separation.

nium and their spectrophotometric determination need to be developed on the basis of the effect of excess of halide and the reaction with organic compounds containing weak donor atoms. At present, the nature of the extractable species formed under (ii) Neutral complexes. The chloro-complexes of reducing conditions is unclear and should be eluciRu(II1 or IV) and Os(IV) react with neutral organic dated. compounds (L) such as those containing N, S, P, As, (iii) Cationic complexes. The reactions between Sb, etc. to form neutral complexes of the type64,98.99.‘40 thiourea and hexabromo-osmate”’ and hexachloroMCl,L, which are highly extractable into organic osmate’46 are extemely slow at room temperature. solvents (Table 6). The difference in formation of the However, though the osmium complex is not formed complexes of ruthenium and osmium is mainly due to in 6.7M hydrobromic acid for at least 4 hr, the ,the reducing nature of the solution and has played an maximum formation of cationic the important role in their separation. ruthenium-thiourea complex occurs in only 15 Osmium can be quantitatively extracted from 6M min.15’ Solvent extraction and ion-exchange sepahydrochloric acid with triphenylphosphine in ration of osmium and ruthenium should thus be 1,2-dichloroethane.@ Ruthenium is not extracted in possible, although neither has been reported. the absence of stannous chloride, because of its low The labile nature of the chloride complex of ruthelability at room temperature. If stannous chloride is nium toward aquation to give [RuCl,,(H,O),]“’ present, the extraction of osmium is reduced to less (x = l-6), which is like that of rhodium, results in than 20x, while ruthenium becomes up to 95% ruthenium interference in the extraction of rhodium extractable from 4M hydrochloric acid. Mojski sugby the liquid cation-exchanger dinonylnapthalenegested that in the presence of stannous chloride sulphonic acid,‘38 and explains the sorption of some osmium is reduced to an oxidation state lower than ruthenium on Dowex-50 cation-exchanger.” Since, the tervalent, and that this forms neither a stable like that of iridium, the chloro-complex of osmium is chloride complex with phosphine nor stable negakinetically inert toward aquation, the difference in the tively charged chloride complexes. tendency for formation of cationic complexes may be The presence of excess of chloride has a considused for separation by converting ruthenium into a erable effect on the separation of osmium and ruthenium, as noted’49 in the extraction from cationic form while keeping osmium as an anion, in 2-mercaptobenzothiazole medium into chloroform. the manner used for the separation of rhodium and iridium.13* Osmium can be quantitatively extracted from solution at pH 3.5-5.0 after 15 min standing at room Other effects used for separation of binary mixtures temperature. If the solutions are 3-6M in hydrochloric acid and boiled for even as little as 5 min, the Palladium andplatinum. The photosensitive nature extraction is reduced to only 0.4%;93 this may be due of platinum has been usedIs to enhance the forto some reduction and/or the competition between mation of the thiocyanate complex and the the ligand and chloride for osmium. On the other separation of palladium and platinum by extraction hand, the chloride accelerates the formation of the of the triphenylisopropylphosphonium salt into ethyl extractable ruthenium complex in comparison to the acetate. It would appear that in the absence of light aquated species. that the palladium complex should still be formed, Methods for the separation of osmium and ruthewhile the platinum remains as a chloro-complex, thus Table 6. Solvent extraction of osmium and ruthenium, based on kinetic effects Aqueous phase Anionic complexes 0.6M KSCN, 3M NH,Cl pH-3

Organic phase

Other information

Polyurethane foam

Reference

Heating the solution at 90” for 5 min was enough for maximum formation of Ru(SCN)i- while leaving OS as its unextractable chloro-complex

142

Presence of SnCl, accelerates formation of extractable Ru-triphenylphosphine complex and reduces OS(W) to lower states which form no stable complex with the ligand Heated at 100” for l&l5 min in presence of SnCl,, Ru forms an extractable red-violet complex, but OS does not react with the organic reagent Aqueous phase boiled for 5 min; Ru extracted quantitatively; less than 0.4% OS extracted

64

Neutral complexes -3M

HCl

Triphenylphosphine

1,2-dichloroethane

6M HCl, 1,4-diphenylthiosemicarbazide

Chloroform

6M HCI, 2-mercaptobenzothiazole

Chloroform

in

148

93

Separation of platinum metals permitting an extraction of Pd(SCN):by several organic solvents.52 The thiocyanate concentration has a considerable effect on the formation of Pt(SCN)i-: the rate of formation of the complex decreases up to O.lM thiocyanate concentration and then increases. The rate is also inversely proportional to pH and the reaction is completely inhibited at pH 7.0. Because Pd(SCN):is easily formed, at a rate relatively concentration, a independent of thiocyanate separation of these metals is possible, and extraction with polyurethane foam, which may be considered as an oxygen-containing solvent, has been reported.ls3 The importance of solution acidity can be seen in the separation of Pd(I1) and Pt(IV) from thiocyanate solution adjusted to pH 6.0 with pyridine, Pdpy2(SCN)z being extracted with methyl isobutyl ketone.” Platinum could only be extracted when the solution was adjusted to pH 2.0 with hydrochloric acid and heated at 90”. The separation of palladium and platinum by simultaneously controlling more than one of the effects of light, pH, thiocyanate concentration and use of different oxygen-containing solvents, needs more investigation than has been reported in the papers listed in Table 7. In addition, hydrophobic complexing anions other than thiocyanate, such as bromide and iodide, with which replacement reactions with hexachloroplatinate are reported to be strongly photosensitive,‘60.‘6’ also warrant study. Ruthenium and osmium. One of the characteristic features of ruthenium and osmium is their ability to form the volatile non-polar tetroxides, which are extractable with several organic solvents such as carbon tetrachloride,‘54~‘62-‘65 chloroform’54,‘66.‘67 and mepasine’68 (Table 7). Osmium can be selectively oxidized to the tetroxide and separated from rutheniumIs by extraction into carbon tetrachloride. Ruthenium and osmium can also be simultaneously oxidized and subsequently distilled as their tetroxides. A separation is possible because of the difference in the kinetics of reduction of the tetroxides. The reduction of 0~0, to [OSO~C~,]~ - and then to OsCliis slow and highly dependent on the hydrochloric acid concentration, temperature and reduction time, whereas that of RuO, to the corresponding anionic chloro-complexes is fast and takes place under much milder conditions.‘47 For example,lS5 ruthenium can be trapped by reduction in lOM, 4M or 3M hydrochloric acid while osmium is not, and instead is subsequently absorbed by 1: 1 sulphuric acid-phenyldimethylamine mixture. Several extraction-spectrophotometric determinations of osmium that start with the tetroxide are reported to suffer no interference from Ru(II1) chloride, e.g., the anionic osmium-thiocyanate complex’58~‘69 and the neutral 2-mercaptobenzothiazole’49 and 2-mercaptobenzimidazole”’ complexes. Alternatively,75~‘7’ ruthenium may be extracted quantitatively as the cationic thiourea complex,

827

because in concentrated hydrobromic acid medium, the formation of an osmium-thiourea complex is inhibited. The tetroxides of ruthenium’57,‘72 and osmium’44~‘58~‘69~‘70~‘7~‘75 have been used for their preconcentration, and the difference in their tendency to form extractable species can play a major role in their separation by organic solvents (Table 7). The thiocyanate concentration also appears important in the formation of the extractable complexes.‘57 After distillation of the tetroxides and collection in 0.2M ammonium thiocyanate in 0.4M hydrochloric acid, osmium can be extracted quantitatively with diethyl ether containing hydrogen peroxide. If the conditions are then changed to 0.3M thiocyanate in 1M hydrochloric acid, ruthenium can then be quantitatively transferred into methyl isobutyl ketone. In the presence of 0.04M thiocyanate, the extraction of osmium does not require addition of hydrogen peroxide,‘69 and although this was not reported, may in fact give a good separation from ruthenium. The presence of chelating agent and excess of chloride ion increases the difference in the rate of formation of the extractable neutral species of osmium and ruthenium. For example, ruthenium can be separated from osmium by extraction into chloroform after the formation of a rutheniumdiphenylthiourea74 or 2,4_thiosemicarbazide complex’59 at high hydrochloric acid concentration. The effect of neutral organic ligands and hydrophobic anions such as Br-, SnCl;, SnBr;, etc. on the labile nature of the complexes and its contribution to the extraction of ruthenium and osmium with oxygen-containing solvents and liquid anionexchangers still needs investigation. The tendency of osmium tetroxide to form more than one extractable species with thiocyanate,‘57.‘58*‘69,‘7G’79whereas ruthenium forms only one, may also be potentially useful. Other binary and multicomponent

systems

Although Pd-Pt, Rh-Ir, and Ru-OS are the common pairs of platinum metals which often exhibit some difficulty in their separation, there are several other combinations which have been investigated, including those with multiple components. These separations use the electrostatic effects, kinetic effects, and selective oxidation, often in conjunction with one another for complex systems. Tetroxides. Ruthenium and osmium are commonly separated first from any of the other platinum metals because of their easy removal from the system as their tetroxides. The selective oxidation of ruthenium and osmium to the tetroxides and subsequent extraction by chloroform or carbon tetrachloride has been utilized for the separation of these metals from the rest of the platinum metals. Furthermore, the differences in the kinetic behaviour of these tetroxides toward reduction in acidic solutions has been used for the separation of ruthenium and osmium as discussed

-

phase

OSM HCI,

-

6M HCI, 2,4-thiosemicarbazide

Chloroform

.. - ._ .

Chloroform

HCI, diphenylthiourea

5-7M

Peroxide-containing diethyl ether

ketone

foam

isobutyl

Chloroform

Methyl

Polyurethane

-

7. Solvent

Hexamethylphosphoramide in chloroform Chloroform

0.4M HCI

phase

Table

Ethyl acetate and methyl isobutyl ketone

Organic

0.034X06M NH,SCN, 1.25-1.75M HCI HCIO,, 1,5-diphenylcarbohydrazide in ethanol

0.2M NH,SCN,

HCIO,

HClO,

Ruthenium ,and osmium HNO,

H,O, KSCN

0.15M KSCN,

Palladium and platinum NaSCN

Aqueous

Other

information

based on other effects

-

-

_^ - - _^

- -

_. -

.- _”

_^

Tetroxides of Ru and OS reduced with (NH),),Fe(SO,),; OS selectively oxidized with 5M HNO, and extracted Ru and OS oxidized with HClO, and distilled; RuO, reduced in IOM, 4M, 3M HCl; 0~0, reduced with 1: 1 sulphanilic acid~phenyldimethylamine mixture The tetroxide complexes passed into five flasks; RuO, reduced with 1: 1, 1:2, I:3 HCI; 0~0, reduced with 10% NaOH and alcohol 5 min in a boiling water-bath then OS extracted; Ru was extracted with methyl isobutyl ketone from solution made 0.3M in NH,SCN, IM in HCI No interference in determination of the blue OS-SCN complex suggests a method for its separation from Ru With excess of chloride 0~0, is reduced to OsClz- which prevents its reaction with the ligand; Ru(II1) is highly extractable and can be separated from OS Extraction of 30 pg/ml Ru as diphenylthiourea complex was not affected by 10 mg of OS Heating at 100” for 10-15 min produced extractable Ru complex; extraction not interfered with by OS

Solution irradiated for 15 min with 500-W incandescent bulb; Pt extracted as ion-association complex [(C,H,),C,H,P+],.[Pt(SCN)i-] with ethyl acetate then Pd as [(C,H,),C,H,P+],.[Pd(SCN):-Iwith methyl isobutyl ketone Under these conditions Pd is present as Pd(SCN):and Pt as PtCIi-; 95% Pd extracted, but less than 2% Pt Pd extracted as Pdpy,(SCN), at pH 6 fixed by pyridine; Pt extracted by adjusting the pH to 2 with HCI and heating the solution at 90 for 4 min

extraction

.,, -

_.. _,. .,. - - _..

159

74

144

158

157

156

155

154

87

153

152

Reference

-

Separation of platinum metals above. Some of these procedures are summarized in Table 8. Electrostatic effects. The differences in the strength of interaction of the doubly and triply-charged chloride complexes of platinum metals towards extractants, either through the anion-exchange or hydration-solvation mechanism, have played an important role in the separation of binary and multiple mixtures of these metals. Examples in which liquid anion-exchangers, including amines, quaternary ammonium salts and other nitrogen-containing as well as oxygen-containing extractants, are used, are summarized in Table 9. Kinetic eflects. The differences in the kinetic behaviour of platinum metals in forming extractable species with hydrophobic anions and neutral ligands have also been very important in the separation of these metals. The labile character of the platinum metals decreases in the order Pd > Pt > Ru > Rh > Ir, with osmium between ruthenium and iridium, depending on the solution conditions. Several of the studies listed in Table 10 used these differences in the rate of formation of anionic and neutral species, and the tendency of rhodium for Rh(H,O)i+ formation. Combination eficts. The separation of multicomponent sytems of platinum metals often uses a combination of more than one point of difference in the behaviour of their chloride complexes, such as the tendency to volatile oxides, the strength of interaction with anion-exchangers as well as oxygen-containing solvents, and lability in the formation of extractable anionic or neutral species. If osmium and ruthenium are present, they are commonly removed first by oxidation to their tetroxides. Several examples are tabulated in Table 11. More extensive investigations of the separation of anionic chloride complexes of uncommon binary and multicomponent systems of platinum metals are needed, utilizing the differences in their strength of

Table Metals separated*

8. Solvent

Oxidizing

extraction

agent

Ru-Rh

Ru-OS in a mixture of Pt-metals Ru-OS in a mixture of Pt-metals *Notation

of other

Organic

platinum

829

interaction with quarternary ammonium salts and taking into account the effect of acidity, the oxidation-reduction nature of the aqueous phase, and the tendency to form cationic ammonium complexes. In addition, combinations of these effects in the back-extraction of platinum metals may provide more efficient methods. Several hydrophobic anions such as Br-, I-, SCN-, SnCl;, SnBr;, etc. and neutral organic compounds containing N, P, S, As, Sb, etc. may alter the lability of the platinum metals in formation of extractable species. A combination of such studies should generate more efficient methods for the separation of platinum metals in any combination. In the years since the review by Beamish,‘” many investigators have provided new methods for isolating and separating the platinum metals. As he predicted, knowledge of the solution chemistry of these metals has allowed a somewhat more systematic approach to development in this area. However, there are still many reports which do not treat the platinum metal solution chemistry involved in the separations, or include methods where it is not clear. As suggested throughout this review, there still remain several areas of separation which need considerable investigation and many where the chemistry of the method requires clarification. We expect that the use of newer instrumental techniques in conjunction with the standard methods will facilitate these studies and that the growing knowledge of the solution chemistry of these metals will continue to offer new and exciting answers for their separation and isolation. This expectation seems justified by the recent report of the use of systematic solvent-extraction separation for production of high-purity platinum metals.*“’ Acknowledgement-This the Natural Canada.

work was financially supported by Sciences and Engineering Research Council of

metal-mixtures,

phase

Other

based on tetroxide

extraction

information

Reference

Carbon tetrachloride

After RuO, extraction, Rh co-precipitated with Fe(OH),. redissolved in 6M HCI and senarated the’ anion-exchanger Deacidite FF (Cl-)-

WSO&

Carbon tetrachloride

From H,SO, extracted

NaOCl

Carbon tetrachloride

Afier ‘03Ru0, extraction, the solution was boiled to eliminate the contamination resulting from droplets of CCI, containing ‘03Ru0, and possibly some nonextractable tetroxide

182

-

After simultaneous distillation of Ru and OS tetroxides, RuO, reduced selectively in HCI medium and 0~0, redistilled and absorbed in ethanohc NaOH After simultaneous extraction of tetroxides at pH S-7, Ru recovered with 6M HCl saturated with SO, and OS recovered with 6M NaOH

183

NaClO,

+ NaBrO,

Na,O, , NaOCl

used: (-)means

“separated

Carbon tetrachloride from”.

solution,

180 bv _

96.2% Ru and 6% Rh were

181

184

separated*

“separated

*Notation

HBr

from”;

“and”;

ketone

Methyl

4.38M

Pt(IVkPd(II)Rh(II1)

(,) means

in

Tri-n-octylamine benzene

HCI

3.5-6.5M

Pd(II),/Pt(IVk Rh(II1)

(/) means

isobutyl

“or”.

amounts;

Pd

190

Rh extracted in presence by same extractant

Countercurrent extraction and recovery with 25% HNO, saturated with NaNO,; 45 stages requried for 95% separation

After Pd and Pt extraction, SnClz as RhhSnCl, complex

61

189 of

extraction; Rh and Ir separation not to reduction of Ir(IV) by the extractant

Countercurrent feasible, owing

phosphate

Tri-n-butyl

3M HCI saturated with NaCl

Pt(IVtRh(II1)

for milligram

34

Six extraction stages required and Pt recovered with HNO,

Diantipyrylpropylmethane in dichloroethane

1M HCI

Pd(IIkRh(III) Pt(IV)-Rh(II1)

in dichlo-

188

187

Poor extraction of Rh(II1) and Ru(II1); their separation from Pt(IV) or Pd(I1) may also be possible

acid (5-10x)

from organic phase solution of PtCIi-

Ru extracted

Displacement of [Rh(H,O)CI,]‘~ takes place easily with aqueous

<0.05%

(CH,),(PhCH,)RNCI (R = C,,-C,,) in 5% isoamyl alcohol in dichloroethane

quantitatively;

Caprylic roethane

means

186

chloride

185

Pt removed

Reference

Tetraoctylammonium in dichloroethane

Extraction twice to remove Pt quantitatively and Rh partially; Pt and Rh recovered with cont. HCl and extracted twice again; 99.8% Rh in aqueous phase; 99.8% Pt extracted

2M HCl, tetraoctylammonium bromide

used:(-)

effects

information

111

Other

on electrostatic

Pt determined in the organic phase as an orange Pt(IIkSn(I1) complex; amount of Rh should not exceed that of Pt

based

Primene, tribenzylamine, tri-nhexylamine or tri-n-octylamine in chloroform or benzene

metal mixtures,

O.lM HCI

platinum

2M HCl

of other

in

extraction

Tri-n-octylamine toluene

phase

phase

Table 9. Solvent Organic

Aqueous

Pd(IIkIr(III) Pt(IV)-Ir(II1)

Pt(IV)-Rh(II1)

Pt(IF+Rh(III)

Metals

4 8

Pd-Ir

Neutral complexes Pd/Ru/Os-Rh, Ir Pd-Ru

H, SO.,

pH 3.15, 3-phenyl-5(2-furyl)pyrazoline dithiocarbamate 2M HCI, di-noctylsulphoxide

NaSCN,

1M HBr-1.5M HCIO,, 0.15M SnBr, a-Nitroso-Pnaphthol, 2-3M HCI

Pd/Pt/Rh-Ir

Pd-PtJRh

0.05-o.

1M KI

pH 2.5, 0.6M KSCN, 3M NH&I KSCN, pyridine isobutyl

alcohol

alcohol

Benzene

Diantipyrylmethane in isobutyl alcohol-benzene Chloroform

Isoamyl

Isoamyl

(90%)

ketone

foam

Diantipyrylpropylmethane in chloroform

Methyl

Polyurethane

Tri-n-butylphosphate in benzene

phosphate

phase

of other olatinum

4.38M HBr

Organic

extraction

Tri-n-butyl

phase

10. Solvent

pH 1, KSCN

Aqueous

Pt-Ir

Pd-Ru

Ru-Rh

Anionic complexes Pd-Rh Pd-Ir I+Rh Pt-Ir Pd-Rh Pt-Rb

Metals separated*

Table Other

effects

information

based on kinetic

Extraction

difference

of 105; separation

should

be possible

Over 95% Pd, Ru, OS extracted; Rh and Ir did not form extractable thiocyanate complexes Pd extracted quantitatively; < 1% Ru extracted

108

194

193

128

128

68

192

141

190

191

Reference

continued overleaf

Pd extracted as Pdpy,(SCN), at pH 11 adjusted with NaOH; Ru then extracted as thiocyanate complex from 2M HCI after heating to 90” Forms extractable PtI:- whereas IrCli- is reduced to unextractable IrCIi-; subsequent extraction of Ir by the same reagent in dichloroethane achieved after removing iodide and oxidizing to Ir(IV) Pd. Pt react immediately at room temperature with SnBr,; Rh requires l-2 hr; Ir does not react Pd forms complex with the ligand preferentially to Pt and Rh

Metals extracted as bromo-complexes recovered with 2.5M HNO, saturated with NaNO,; 10 equilibration stages separated 98.5% Pt-99.5% Rh, 98.5% Pd-98.2% Rh Aqueous phase heated at 90” for 5 min; extracted for 1 hr

Solutions heated for 10 min before extraction; effective separation of 95% of each metal with less than 10 equilibration stages

metal mixtures,

Rh, Ir

*Notation it”.

Table

phase

0.05M HCI

from”;

effects

information

(/) means “or”;

temperature,

Pt

separation

of these metals after the preliminary

138

113

112

197

196

104

195

101

Reference

step shown above

Rh, then acidic with from Ru

Rhdiphenylthiourea

Rh complex

Solution made basic with NaOH to precipitate HCI to dissolve the precipitate; poor separation

[ ] indicates “a subsequent

Dinonylnaphthalenesulphonic acid in heptane

Separates Pd and Pt from Rh and Ir SnC12 accelerates formation of extractable complex

Chloroform Chloroform

of extractable

Pd separated from Pt SnClz accelerates formation

Chloroform Chloroform

Chloroform

polar

Weakly

solvents

tetrachloride

Carbon

Chloroform

(,) means “and”;

HCI, KI, 2-mercaptobenzothiazole Dimethylglyoxime HCI, SnClz, 2-mercaptobenzothiazole HCI, KI, diphenylthiourea HCI, SnC&, diphenylthiourea

Other

based on kinetic

Conversion of chloro-complexes of Pd, Pd into PdIj-, PtI:- accelerates the reaction with the ligand Pd extraction after evaporation of extractant to dryness and mineralization of residue by HNO, and Hz02 Pd completely extracted; less than I”/, Pt, Rh, Ir extracted; used for Pd in Pt-wire Differences in distribution at different temperatures and rate of attaining equilibrium can be utilized for separation Separates Pd and Pt from Rh and Ir

metal mixtures,

Chloroform

phase

of other platinum

Pd forms extractable species with sulphide at room requires SnCI,, and Ir must be heated for 1.5 hr Selective extraction of Pd

Organic

extraction

Dichloroethane

10. Solvent

Organic petroleum sulphide Di-n-octyl sulphide, decylmethyl sulphide or dihexyl sulphide Diethyldithiocarbamate, KI, HCL pH 2-5, p-nitroso dimethylaniline pH 5.6, 2-mercaptobenzothiazole 2-Mercaptobenzothiazole

Aqueous

used: (-) means “separated

Cationic complexes Rh-Pd Rh-Pt RI-Ir Rh-Ru

Pd-Pt-Rh-Ir [Rh-Ir]

[Pd-Pt] [Rh-Ir]

Pd-Pt-RhIr

Pd-Pt-Rh

Pd-Pt,

[Pd-Pt]

Pd from the rest of Ptmetals Pd, Pt-Rh, Ir

PddPt-Ir

Metals separated*

E 8

?

Separation of platinum metals

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834 REFERENCES

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Separation

of platinum

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108.

109. 110. 111. 112. 113. 114.

115. 116.

117.

118.

119. 120. 121. 122.

123. 124. 125. 126. 127. 128. 129. 130. 13 1. 132.

133. 134. 135. 136. 137. 138.

metals

835

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