Solvent extraction separation of rhodium(III) with N-n-octylaniline as an extractant

Talanta 58 (2002) 761
/771 www.elsevier.com/locate/talanta

Solvent extraction separation of rhodium(III) with N-noctylaniline as an extractant Sanjay S. Kolekar, Mansing A. Anuse * Analytical Chemistry Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416004, Maharashtra, India Received 7 September 2001; received in revised form 9 July 2002; accepted 11 July 2002

Abstract Solvent extraction separation method for the determination of rhodium(III) has been described. Selective and quantitative extraction of rhodium(III) by N -n -octylaniline, a high molecular weight amine (HMWA) into xylene takes place from aqueous sodium malonate medium. The effect of concentration of malonate, N -n -octylaniline, role of various diluents, stripping agents and foreign ions on the extraction of rhodium(III) has been studied. The procedure offers distinct improvements in need of real sample analysis and environmental safety as the extraction procedure carried out in weak organic acid media. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Solvent extraction; Rhodium(III) determination; N -n -octylaniline; Sodium malonate

1. Introduction Rhodium is present at about 0.001 ppm in the earth’s crust [1]. Rhodium metal is known for its stability in corrosive environments, physical beauty and unique physical and chemical properties. It commands a premium price because of its low abundance in nature. Rhodium is now widely used in combination with platinum, in addition to or instead of palladium for automobile-exhaust emission control catalysts. The platinum
/rhodium ratio was used about 11:1. These rhodium bearing

* Corresponding author. Fax: /91-231-692-333 E-mail address: [email protected] (M.A. Anuse).

catalysts are called ‘three-way’ catalysts, since they not only oxidise carbon monoxide and hydrocarbons but also remove various nitrous oxides (usually referred to as NOx ) [2]. Rhodium in combination with platinum finds the applications in the manufacture of nitric acid [3]. Rhodium is also used with iridium in high-temperature thermocouples. The value of rhodium as a precious metal has prompted several investigations into its separation and pre-concentration. The extraction and determination of rhodium(III) by using high molecular weight amine (HMWA) extractants is well known but little work is reported on the extraction of rhodium(III), particularly by secondary amines. The use of n -octylaniline in the extraction of noble

0039-9140/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 0 2 ) 0 0 3 6 5 - X

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metals has been described in number of papers [4
/ 10]. The comparison can be made of the merits of N -n -octylaniline relative to n-octylaniline as an extractant. These are (i) the method of preparation n -octylaniline [7]; (ii) more equilibrium time (30 min); (iii) higher reagent concentration [5,7]; (iv) requires multiple extraction for complete recovery of noble metals; (v) use of mineral acids which is not environmental friendly. The solution containing rhodium(III) was extracted with 4-octylaminpyridine [11] in chloroform from hydrochloric acid medium and determined by ICP spectrometry, however, the extract was not completely free from Fe, Co, Ni and Cu. The extraction of noble metals from hydrochloric acid solution with 0.1 M N -benzylaniline [12] in chloroform was investigated. The separation is carried out by using differential stripping method. The other commonly used extractants for rhodium(III) are tributylphosphate [13
/ 16], bis-(2-ethylhexyl) phosphoric acid (HDEHP) [17
/19], N ,N -dialkylN ?-benzoylthiourea, N -benoyl-N ,N ?-dihexylthiourea [20], N ?,N ? dihexyl and-phenyl and N ?-hexyl and -phenyl derivatives of N -benzoylthiourea [21], N ,N -dihexyl-N ?-benzoylthiourea [22], mercapto acetic acid and 2-mercaptobenzothiazole [23], thiocyanate [24] alkylaniline hydrochloride
/ petroleum sulphides [25,26] Calix(4)arenes [27], Kelex-100 [28,29] and 4-(non5-yl)pyridine [30]. The complex nature of solution chemistry of PGMs has contributed to the difficulty of developing methods for their separation from 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 chlorocomplexes with liquid ion exchangers or with oxygen containing solvents [31,32]. The study of the chloro-complexes of the PGMs clearly indicates that the strength of the interaction with ion exchangers is highly dependent on the charge of the complex and also depends on the age of the solution of rhodium. Therefore, the rhodium(III) chloro-complex is also poorly extracted which is due to the charge of the complex as well as its labile character toward aquation. Hence it is worthwhile to develop the solvent extraction

procedure in weak organic carboxylic acid media. One of the distinct advantages of the organic acid media is the facility of controlling the concentration of complexing ligand, the ease of adjustment of pH and wide difference in pH at which various metal form anionic complexes. The comparative ease of stripping of the complexes from the organic phase can be achieved by fully exploiting the differences in reactivity of various metals to backwash in the aqueous phase by mineral acid. It is known that organic acid media offer better separation of metals possibly due to high stability of metal organic acid complexes. This paper detail recent investigations that involve the formation of very stable complexes of rhodium(III) with weak organic acid like sodium salt of malonic acid prior to solvent extraction. In the novel separation method identified, aquochloro complexes of rhodium(III) converted into more stable rhodium malonate complexes, which undergo aquation to a lesser extent, making them more easily extracted. The use of weak carboxylic acids opens up new options for the separation of rhodium from iridium and other PGMs. N -n -octylaniline is a secondary amine, which acts as liquid anion exchanger. We have recently reported its use in solvent extraction of some noble metals from hydrochloric acid media [33
/37]. In this work, N -n-octylaniline-malonate system was studied to investigate the extraction of aqueous rhodium(III) solution as a function of various parameters. The proposed method is used for rapid and selective separation of rhodium(III) from associated elements in their binary and ternary mixture. It is also tested for the separation and determination of rhodium (III) from real samples as alloy.

2. Experimental 2.1. Apparatus An Elico digital spectrophotometer, model SL171 with 1 cm quartz cells was used for absorbance measurements; pH measurements were carried out using an Elico digital pH meter, model LI-120.

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2.2. Reagents

2.2.1. Standard rhodium(III) solution A stock solution of rhodium(III) was prepared by dissolving 1 g of rhodium trichloride hydrate (Johnson Matthey, UK) in dilute analar hydrochloric acid (1 M) and diluting to 250 ml with distilled water and standardised gravimetrically [38]. A working solution of 100 mg ml 1 was made from it by diluting the stock solution with distilled water. N -n-octylaniline was prepared by the method of Gardlund et al. [39] and its solutions (0.1 M) were prepared in xylene. All chemicals used were of AR grade. Doubly distilled water was used throughout.

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3. Results and discussion

3.1. Extraction as a function of pH Rhodium(III) was extracted in the pH range of 1
/11 in the presence of weak organic acids such as sodium salts of malonic acid (0.025 M) and salicylic acid (0.01 M). Quantitative extraction of rhodium(III) was observed in the pH range 9.0
/ 10.5 from 0.025 M sodium malonate media. There was incomplete extraction of rhodium(III) from sodium salicylate (75%) while no extraction from sodium succinate and sodium oxalate media (Fig. 1). This shows that the equilibrium in the pH range 9.0
/10.5 is favourable for the formation of ionpair complex from sodium malonate media.

2.3. General procedure for extraction and determination of rhodium(III) An aliquot of 2 ml rhodium(III) solution (100 mg ml1) was made 0.025 M with sodium malonate concentration, adjusted to pH 9.5 with dilute hydrochloric acid and sodium hydroxide solution in a total volume of 25 ml. The aqueous solution was shaken with 10 ml 0.1 M N -noctylaniline in xylene for 1 min. After the phase separation, the organic phase was stripped with two 10 ml portions of 1 M hydrochloric acid. The extracts were evaporated to moist dryness in order to remove excess of hydrochloric acid. The residue was dissolved in minimum amount of 1 M hydrochloric acid and transferred into 50 ml volumetric flask, 10 ml of 20% potassium iodide was added, the solution was mixed well and heated for 15 min in boiling water bath. To the cooled solution, 10 ml of 10% stannous chloride solution was added and diluted the solution up to the mark with distilled water containing 1 M hydrochloric acid in final concentration. Placed the unstoppered flask in the boiling water bath for 2 min. The solution was cooled and the absorbance of the reddish brown solution was measured at 460 nm against a reagent blank. The concentration of rhodium(III) was computed from the calibration curve in similar manner [40].

Fig. 1. Extraction behaviour of Rhodium(III) as a function of pH from 0.025 M sodium malonate and 0.01 M sodium salicylate solution with 0.1 M N -n -octylaniline in xylene.

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3.2. Extraction as a function of N -n -octylaniline concentration In order to optimise the conditions for extraction of rhodium(III), xylene solutions of N -noctylaniline with varying concentration (0.01
/0.20 M) were employed. It was found that 10 ml of 0.06 M N -n -octylaniline was sufficient for quantitative extraction of 200 mg rhodium(III) from 0.025 M sodium malonate, but in recommended procedure 0.1 M N -n -octylaniline in xylene was used to ensure the complete extraction of metal ion. There was no adverse effect if one can use excess of N -noctylaniline (0.2 M). However, a decrease in concentration of extractant resulted in lower distribution ratio, D values for rhodium(III). 3.3. Extraction as a function of weak organic acid concentration The extraction of rhodium(III) was carried out at pH 9.5 with 0.1 M N -n -octylaniline in xylene in the presence of varying concentration of sodium malonate, sodium salicylate, sodium succinate and sodium oxalate as weak acid media. The extraction of ion-pair complex of rhodium(III) was found to be quantitative in the range of 0.02
/0.03 M sodium malonate concentration. With increased concentration of sodium malonate there is decrease in the extraction of rhodium(III). The decrease in the extraction at high acid concentration is presumably due to preferential formation of the malonate of the N -n -octylaniline. Therefore, a 0.025 M concentration of sodium malonate was used throughout this work. While extraction was found to be incomplete in sodium salicylate (Table 1) and no extraction from sodium succinate and sodium oxalate media. 3.4. Extraction with various diluents It is well known that diluents are played an important role in the solvent extraction of metals. In the field of solvent extraction technology the diluent constitutes a rather important part in the economy of the process. The diluent has to be cheap, give practically negligible losses and give good solubility to the extractant and its various

compounds, it should be less toxic. The extraction of rhodium(III) was quantitative with hydrocarbon solvents such as benzene, toluene and xylene because the ion-pair complex has high value of distribution ratio in them, where as chloroform (65%) and methyl isobutyl ketone (MIBK) (57%) were found to be very poor solvents, while there was no extraction of the complex, in carbon tetrachloride, amyl alcohol, n -butyl alcohol, 4methyl-2-pentanol, amyl acetate. However, benzene and toluene are too toxic than xylene. Hence xylene was preferred as a diluent for further study. 3.5. Nature of the extracted species Attempts were made to ascertain the nature of the extracted complex species using log D/log C plots. The graphs of log D [Rh(III)] against log C[N -n -octylaniline] at fixed sodium malonate concentration (0.025 M) were found to be linear and having slopes of 1.15 and 1.07 values at pH 8.0 and 8.5, respectively (Fig. 2). Also plots of log D[Rh(III)] against log C[malonate] at fixed N -n -octylaniline concentration (0.1 M) were linear and slope values are found to be 1.99 and 2.06 at pH 8.0 and 8.5, respectively (Fig. 3). The probable composition of extracted species is calculated to be 1:2:1 (metal: acid:extractant). The possible mechanism of extracted species appears to be protonated N -n octylaniline which forms cationic species as RR?NH2, while malonate combines with rhodium(III) to form anionic species as Rh(C3H2O4)2(H2O)2 [31] and both of them associate to form ion-pair of the type [RR?NH2 Rh (C3H2O4)2(H2O)2]org and being neutral constitutes extractable species. 3.6. Stripping Stripping is the reverse of extraction, so it should be promoted by those factors that affect extraction negatively, such as acidic and salt media. Alkalies were unsuccessful because the anion complex adhered in the organic medium under these conditions. The stripping percentage was calculated relative to the initial amount of rhodium(III) in the pregnant organic solutions. The rhodium(III) was stripped with two 10 ml

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Table 1 Extraction behaviour of rhodium(III) as a function of concentration of weak acids at pH 9.5 Acid concentration (M)

0.000 0.001 0.005 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.20

Sodium malonate

Sodium salicylate

E (%)

D

E (%)

D

0.00 25.0 66.0 96.5 100.0 100.0 95.5 80.0 70.0 59.0 50.0 36.0 33.5 22.5


/ 0.83 4.85 68.93

53.05 10.00 5.83 3.59 2.50 1.41 1.26 0.73

0.00 22.0 59.0 75.0 70.5 62.0 58.5 55.0 47.0 43.0 39.5 28.0 21.0 0.00


/ 0.71 3.59 7.5 5.97 4.08 3.52 3.05 2.22 1.89 1.63 0.97 0.66
/

Rh(III)/200 mg, pH 9.5, N -n -octylaniline/0.1 M in xylene, Aqueous: Org. ratio/2.5:1, Strippant/1 M hydrochloric acid (2/ 10 ml), E (%)/Percentage extraction, D /Distribution ratio.

Fig. 2. log
/log plot of log D[Rh(III)] against log C[N -n -octylaniline] at fixed sodium malonate concentration (0.025 M).

portions of various stripping agents at different concentrations. The stripping was found to be complete with hydrochloric acid (1
/5 M), nitric acid (1
/3 M), perchloric acid (0.5
/5 M) and

Fig. 3. log
/log plot of log D[Rh(III)] against log C[Sodium malonte] at fixed N -n -octylaniline concentration (0.1 M).

sulphuric acid (1
/3 M) media while acetic acid, sodium nitrate, acetate buffer (pH 4.5) and water become unsuccessful for the recovery of rhodium(III) from the organic solution. In actual proce-

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dure, two 10 ml, portion of 1 M hydrochloric acid was used as a strippant.

3.10. Effect of various foreign ions on percentage extraction of rhodium(III)

3.7. Extraction of rhodium(III) as a function of aqueous to organic volume ratio

The effect of large number of foreign ions on the extraction of 200 mg of rhodium(III) with the proposed reagent was investigated following the recommended procedure. Initially the foreign ion was added to the rhodium(III) solution in large excess; 100 mg for an ions and 25 mg for cations. When interference was found to be intensive, the tests were repeated with successively smaller amounts of foreign ion. The tolerance was set at the amount of the foreign ion that could be present to give an error less than 9/2% in the recovery of rhodium(III) (Table 2). It was observed that the method is free from interference from a large number of cations and anions. The only species showing interference in the procedure was Ir(III). However, the interference of Ir(III) was eliminated by masking with oxalate. The an ionic species showing interference in the procedure were tartrate, citrate, ascorbate and EDTA due to formation of strong metal chelates, while thiosulphate and thiocyanate form very strong complexes with rhodium(III) because PGMs belong to the soft acids which possess a strong affinity to ligands containing donating type sulphur atoms which act as soft bases [41,42].

The results of contacting different volume ratios of organic to aqueous phase have been studied. The results indicate that a preferred aqueous/ organic (A/O) phase ratio in this study was found to be 5:1 or less. This is evident from the sharp increase in the separation efficiency as well as the distribution ratio of rhodium(III) when phase ratio (A/O) changed from 20:1 to 5:1, This may simply be due to the unavailability of reagent for metal extraction and so a crowding effect occurs at low phase ratio. However, in the recommended procedure the phase ratio is maintained as 2.5:1 so as to avoid the large consumption of the sodium malonate. 3.8. Effect of time of equilibrium When two immiscible phases were equilibrated for a period 10 s to 30 min. The shaking period showed that a 15 s equilibration time was adequate for quantitative extraction of rhodium(III) from malonate media. But in our work, 1 min equilibration time was recommended in order to ensure the complete extraction of rhodium(III). However, a prolonged equilibration period (/10 min) was found to have an adverse effect on the extraction and should be avoided. Variation of the stripping period showed that a 30 s time was sufficient for quantitative recovery of rhodium(III) from organic phase. 3.9. Loading capacity of N -n -octylaniline The loading capacity of the extractant was determined by the repeated contact of organic phase with a fresh feed solution of the metal of same concentration. For a 10 ml, 0.1 M solution of N -n -octylaniline in xylene at 0.025 M sodium malonate concentration and a A/O of 2.5:1, the maximum loading capacity for rhodium(III) was found to be 2.0 mg at 300 K.

4. Applications 4.1. Separation and determination of rhodium(III) from binary and ternary mixture The suitability of the above developed method was examined by applying it to the separation and determination of rhodium(III) in a variety of binary mixtures which are commonly associated with it. It was found that Pd(II), Pt(IV), Ru(III), Os(VIII), Fe(III), Co(II), Ni(II) and Cu(II) remained unextracted under the basic conditions with rhodium(III) using 0.025 M sodium malonate with 10 ml 0.1 M N -n -octylaniline in xylene. The pregnant organic phase was stripped with 1 M hydrochloric acid (2 /10 ml) and determined spectrophotometrically as recommended in the

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Table 2 Effect of foreign ions on the extraction of 200 mg rhodium(III) at pH 9.5 in 0.025 M sodium malonate with 0.1 M N -n -octylaniline in xylene Foreign ion

Addes as

Tolerance limit (mg)

Foreign ion

Addes as

Tolerance limit (mg)

Ti(IV) V(V) Cr(III) Cr(VI) Mn(II) Fe(II) Fe(III) Co(II) Ni(II) Cu(II) Zn(II) Mo(VI) Re(VII) lr(III)a Pd(II) Pt(IV) Ru(III) Os(VIII)

K2 ×/ TiF6 ×/ H2O NH4 ×/ VO3 ×/ H2O CrCl3 K2Cr2O7 MnCl2 ×/ 6H2O FeSO4 ×/ 7H2O NH4 ×/ Fe(SO4)2 ×/ 12H2O CoCl2 ×/ 6H2O NiCl2 ×/ 6H2O CuSO4 ×/ 5H2O ZnSO4 ×/ 7H2O (NH4)6Mo7O24 ×/ 2H2O KReO4 IrCl3 ×/ x H2O PdCl2 ×/ x H2O H2PtCl6 RuCl3 ×/ x H2O OsO4

5 5 10 2 5 5 10 10 10 10 10 5 4 2 4 3 4 3

Au(III) Se(IV) Te(IV) Ag(I) Cd(II) Hg(II) Be(II) Mg(II) Sn(II) Sb(III) Pb(II) Bi(III) U(VI) Iodide Oxalate Thiourea Nitrate Fluoride

HAuCl4 ×/ H2O SeO2 Na2TeO3 AgNO3 1 /CdCl2 × 2 = H2 O/ 2 HgC12 BeSO4 ×/ 4H2O MgCl2 ×/ 6H2O SnCl2 ×/ 2H2O Sb2O3 Pb(NO3)2 Bi(NO3)3 ×/ 5H2O UO2(NO3)2 ×/ 6H2O KI (COOH)2 ×/ 2H2O Thiourea KNO3 NaF

3 5 5 5 10 5 5 5 10 5 10 15 10 15 20 15 15 20

a

Masked with oxalate.

procedure. The raffinate containing added metal ion was estimated by standard procedure (Table 3). The proposed method was also extended for separation of rhodium(III) from Ir(III) by masking with 20 mg of oxalate. The masked Ir(III) remained in the aqueous phase quantitatively under the optimum extraction conditions of rhodium(III). After demasking Ir(III) with 5 ml concentrated hydrochloric acid with little boiling the solution, it was estimated spectrophotometrically with stannous chloride-hydrobromic acid method. Rhodium(III) was stripped with 1 M hydrochloric acid and determined as described above. Alternatively, Ir(III) was separated from rhodium(III) by prior extraction with N -n -octylaniline in amyl alcohol (0.1 M) at pH9.5. At this condition rhodium(III) remained in the aqueous phase. Iridium(III) was stripped with 2 M hydrochloric acid (2 /10 ml) and estimated as above. Rhodium(III) was extracted from the aqueous phase and determined as per the procedure given (Table 3). The separation of rhodium(III) from Au(III) was based on the use of different pH values.

Gold(III) has been extracted quantitatively with N -n-octylaniline (0.1 M) in xylene from 0.03 M sodium malonate media in the pH range 0.5
/2.0 [37]. It was found that there was zero extraction of Au(III) at pH 9.5. Hence, rhodium(III) can be extracted quantitatively leaving behind Au(III) in the aqueous phase. Gold(III) in the aqueous phase was estimated by stannous chloride method. The proposed method was also used for the separation of rhodium(III) from ternary mixtures. When a ternary mixture containing Pd(II), Pt(IV) and Rh(III) was extracted with 10 ml, 0.1 M N -n octylaniline in xylene with 0.025 M sodium malonate in the aqueous phase it was found that rhodium(III) was extracted whereas Pd(II) and Pt(IV) remained unextracted, Rhodium(III) was stripped with 1 M hydrochloric acid (2 /10 ml) and estimated as per the recommended procedure. The aqueous phase containing Pd(II) and Pt(IV) was evaporated to moist dryness in order to decompose the malonate with the addition of perchloric acid. Then the solution was adjusted to 0.025 M with sodium salicylate and Pd(II) was extracted quantitatively with 10 ml of 1 M N -n octylaniline in xylene at pH 3.0. Palladium(II)

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Table 3 Separation and determination of rhodium(III) from binary mixtures Amount of metal ion (mg)

Average (%) Recoveryb

Rh(III) 200 Pd(II) 100 Rh(III) 200 Pt(lV) 200 Rh(III) 200 Os(VIII) 300 Rh(III)200 Ru(III) 300 Rh(III) 200 Ira (III) 50 Rh(III) 200 Au (III) 100 Rh(III) 200 Cu(II) 10 000 Rh(III) 200 Fe(III) 10 000 Rh(III) 200 Co(II) 10 000 Rh(III) 200 Ni(II) 10 000

99.9 99.9 99.9 99.9 99.9 99.8 99.9 99.9 99.8 99.8 99.9 99.7 99.9 99.8 99.9 99.9 99.9 99.8 99.8 99.8

a b

Chromogenic ligand

Reference number

4?-chloro PTPT

[43]

SnCl2

[44]

Thiourea

[44]

Thiourea

[44]

SnCl2/HBr

[44]

SnCl2

[44]

4?-chloro PTPT

[45]

Thiocyanate

[44]

Thiocyanate

[40]

DMG

[40]

Prior extraction with N -n -octylaniline in amyl alcohol or masked with oxalate (20 mg). Average of six determinations.

from organic phase was stripped with 7 M ammonia (2 /10 ml) and estimated spectrophotometrically with 4?-chloroPTPT [43] with prior evaporation of excess of ammonia, Whereas Pt(IV) remained unextracted and estimated with stannous chloride. This separation scheme is applicable for the separation of rhodium(III) from platinum
/rhodium thermocouple wire. Similarly, in a mixture of Pd(II), Au(III) and Rh(III), only rhodium(III) is extracted, whereas Pd(II) and Au(III) remained in the aqueous phase. The aqueous phase containing sodium malonate was decomposed with concentrated perchloric acid and the residue was extracted with dilute hydrochloric acid. Palladium(II) was extracted from 0.025 M sodium salicylate with 0.1 M N -n octylaniline in xylene at pH 3.0 whereas in salicylate media there is no extraction of Au(III) [49] and hence remained in the aqueous phase quantitatively. Palladium(II) was stripped with 7 M ammonia and estimated as mentioned above. The aqueous phase was evaporated to remove

sodium salicylate by addition of concentrated perchloric acid. The residue was treated with 5 ml aqua-regia and then with dilute hydrochloric acid and again evaporated to moist dryness. The solution was adjusted with 0.03 M sodium malonate and Au(III) was extracted with 10 ml of 0.1 M N -n -octylaniline in xylene at pH 1.0 [37]. Gold(III) was stripped with 7 M ammonia solution (2 /10 ml) and estimated spectrophotometrically with stannous chloride method (Table 4).

4.2. Determination of rhodium(III) in a synthetic mixture A solution containing 200 mg of rhodium(III) was taken and known amounts of other metals were added. Extraction of rhodium(III) was carried out using the method developed here. The results obtained were in good agreement with the amounts added (Table 5).

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Table 4 Separation of Rh(III) from ternary mixtures Metal ion Amount taken (mg)

Aqueous phase, (25 ml)

Stripping agent (2/10 ml)

Determination method

Recoverya (%)

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

100 200 200

0.025 M salicylate pH 3.0
/ 0.025 M malonate pH 9.5

7.0 M ammonia Unextracted 1.0 MHCl

4?-chloro PTPT SnCl2 SnCl2/Kl

99.8 99.9 99.9

Pd (II) Au (III) Rh (III)

100 200 200

0.025 M salicylate pH 3.0 0.03 M malonate pH 1.0 0.025 M malonate pH 9.5

7.0 M ammonia 7.0 M ammonia 1.0 MHC1

4?-chloro PTPT SnCl2 SnCl2/Kl

99.8 99.7 99.9

distilled water to 10 ml in a standard volumetric flask. An aliquot of the sample solution was taken and rhodium(III) was determined using the procedure described above. The results of analysis are given in Table 6.

4.3. Determination of rhodium(III) in platinum
/ rhodium thermocouple wire The platinum base alloy containing more than 10% of iridium and rhodium. These alloys are insoluble in aqua-regia at atmospheric pressure but can be dissolved in it at elevated temperature in high pressure systems [46]. More convenient is preliminary fusion of the sample with zinc and dissolution of the melt in hydrochloric acid [47]. A black powder remains, containing the platinum metals in elemental form, ready for an attack by acids or by other appropriate means, prior to distillation (Os or Ru), chlorination (Rh, Ir) or chemical separation such as by bromate hydrolysis (Pt from Ir and Rh). A known weight (0.100 g) of Pt
/Rh thermocouple wire [48] was preliminary fused with zinc powder and the melt was dissolved in hydrochloric acid. The black powder remained was treated with 5 ml aqua-regia. After the reaction was over the whole solution was heated with two 5 ml portions of concentrated hydrochloric acid until complete removal of oxides of nitrogen and diluted with

5. Conclusion The proposed extractive separation procedure is simple, rapid, selective and suitable for the separation of rhodium(III) from other PGMs, gold(III) and base metals. The extraction mechanism corresponds to an anion exchange, in which a complex of stoichiometric formula [RR?NH2 Rh (C3H2O4)2(H2O)2]org is formed in the organic phase. The use of non-toxic solvents like xylene was favourable. N -n-octylaniline can be synthesised at low cost, with high yield, in the best purity and recovered for reuse without loss of extraction efficiency. The time needed for equilibration is very short (15 s) as compared with other reported HMWA. Another important feature of the pro-

Table 5 Separation of rhodium(III) from synthetic mixtures Composition (mg)

Rhodium(III) found (mg)

Rh, Rh, Rh, Rh, Rh, Rh,

199.9; 199.8; 199.9; 199.7; 199.9; 199.9;

a b

200: 200: 200: 200: 200: 200;

Pd, 100; Pt, 100 Pd, 100; Pt, 100; Ira, 50 Pd, 100; Pt, 100; Ru, 100 Pd, 100; Pt, 100; Ru, 100; Ira, 50 Pd, 100; Pt, 100; Ru, 100; Ira, 50; Os, 100 Cu, 10 000, Ni, 10 000

199.9; 199.8; 199.9; 199.8; 199.9; 199.9;

199.9; 199.7; 199.9; 199.8; 199.8; 199.8;

199.8; 199.7; 199.9; 199.7; 199.8; 199.9;

199.9 199.8 199.9 199.8 199.9 199.9

Prior extraction with N -n -octylaniline in amyl alcohol or masked by oxalate (20 mg). RSD (%)/(standard amount/amount found/standard amount)/100.

Mean (mg)

Recovery (%)

RSDb (%)

199.88 199.76 199.9 199.76 199.86 199.88

99.94 99.88 99.95 99.88 99.93 99.94

0.06 0.12 0.05 0.12 0.07 0.06

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Table 6 Analysis of rhodium(III) in Pt
/Rh thermocouple wire Sample

Manufacturer

Composition (%) Amount of Rh(III) found by proposed method a (%)

RSDb (%)

Platinum-Rhodium thermocouple wire

Ruia Resistance wire Pvt. Ltd., Mumbai

Pt, 87, Rh, 13

0.76

a b

12.9

Average of five determinations. RSD (%)/(standard amount/amount found/standard amount)/100.

posed method is that a large number of foreign ions were tolerated in high ratios. The developed method is reliable, as can be seen from the complete agreement of the results observed for the analysis of various practical samples with added ions or certified values.

Acknowledgements The authors thank Professor K.J. Patil, Head, Department of Chemistry for his help and inspiration in the research work.

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