chapter- iv rh iii solvent extraction studies of rhodium...

Chapter 4 – Solvent extraction studies of rhodium(III) using high molecular weight amine

Analytical Chemistry Laboratory, Dept. of Chemistry, Shivaji University, Kolhapur 68

CHAPTER-4

SOLVENT EXTRACTION STUDIES OF RHODIUM(III) USING HIGH MOLECULAR WEIGHT AMINE

4.1 Introduction

At present there is growing demand of platinum group metals, the name

platinum group metals (PGMs) include the six elements: ruthenium, rhodium,

platinum, palladium, osmium and iridium. In the past few decades these metals

have found new applications outside the jewellery and decorative industries

due to its excellent physical and chemical properties and are used extensively

for electronic devices, catalysis in the chemical and petroleum refining

industries, glass industries, pharmaceutical industries etc. Rhodium is one of

the most expensive platinum group metal and is indispensable for automotive

catalytic converters.

The high cost of recovery and limited resources of these metals make it

necessary to recover the metals from industrial waste. Considering the

difficulties related with the separation and purification of PGMs, it is important

to find an effective separation process to recover these metals with high purity

[1]. According to the published literatures, ion exchange and solvent extraction

have been widely employed to separate and recover them. Among Pt, Pd and

Rh, extraction of Rh is the most difficult owing to its intricate chemical

properties in chloride solution. Rhodium has seven existence forms of aqua-

chloro complexes from [Rh(H2O)6]3+ to [RhCl6]3-. The highly charged

octahedral complexes are difficult to extract owing to steric effects [2].

The extent to which a metal ion is extracted from an aqueous into an

organic phase is the result of many factors. One of these factors is the amount

of water which accompanies the metal complex. This water favors the

solubility of the metal complex in the aqueous phase and disfavors its solubility

in the organic phase. In the case where the metal ion is fully co-ordinated, i. e.,

all its co-ordination sites are occupied by ligand donor atoms, the water will

form the outer sphere of the complex by means of solvation (hydration).



Though, several sophisticated techniques are in use for the

determination of trace and ultra trace quantities of rhodium, spectrometric

technique still has the advantage in respect to simplicity and low operating

costs but suffers due to matrix effects. Hence, separation and preconcentration

of trace level quantities of rhodium is necessary prior to actual quantitative

analysis. Several extraction procedures are available for this purpose however

most of these are time consuming and costly, however liquid-liquid extraction

technique is one of the most suitable, selective, efficient and powerful

technique for the separation and purification of platinum group metals [3].

4.2 Review of literature for liquid-liquid extractive separation of

rhodium(III)

A variety of high molecular weight amine (HMWA) have been

explained extraction of rhodium(III) like almine 336 (A336) [4-7]. In order to

find an optimum condition to separate rhodium and iridium, solvent extraction

experiments were performed from chloride solution by using alamine 336 and

tri-n-butyl phosphate (TBP) as an extractant. The extraction depends on

concentration of hydrochloric acid [7]. Tri-octylamine [8-11], tri-iso-

octylamine (almine 308) [12], aliquat 336 [13, 14], N-n-octylaniline [15],

tetraoctyl and trialkylbenzylammonium chlorides [16], trioctyl-methyl-

ammonium chloride (TOMAC) [17,18], 4-(non-5-yl) pyridine [19],

4-octylamino pyridine [20], 2-ethylhexyl amino methyl pyridine [21] have been

tested for liquid-liquid extraction of rhodium(III).

Phosphorous containing extractant such as tri-n-butyl phosphate [22] has

been used for the quantitative extraction and separation of trace amounts of

rhodium from nitric acid and sodium trichloroacetate media has been

established based on the formation of an ion-association complex of

hexahydrated rhodium cation Rh(H2O)6 3+ and the trichloroacetate (TCA) anion

in tri-n-butyl phosphate (TBP). A systematic study on the solvent extraction of

rhodium with TBP in hydrochloric acid in presence of stannous chloride [23,

24] has been reported. Liquid-liquid extraction of rhodium(III) with



cyanex 921 from aqueous hydrochloric acid media [25], hydrobromic acid [26]

and chloride media [27] has been examined in the presence and in the absence

of stannous chloride. The extraction of rhodium(III) from bromide media [28]

with cyanex 923 and cyanex 471-X in toluene was studied. The extraction of

rhodium(III) from its chloride solution was carried out using cyanex 923,

cyanex 271 and cyanex 272 [27, 29-30] and distribution ratios for the metals

were determined under the different concentrations of H+ Cl- ions in the

aqueous phase. Separation of rhodium(III) from aqueous solution with tri-n-

octylphosphine oxide (TOPO) [31] was studied by using stannous chloride as

activating reagent. The formation of complex required 60oC temperature.

Percentage of stripping could reach 95 % only. Hexaaquarhodium(III),

(Rh(H2O)63+) was found to be extracted quantitatively within 5 min by shaking

with trichloroacetic acid (HTCA) and TOPO [32] in heptane with a pH range

of 3-5. The chemical species extracted is Rh3+.3TCA-.4TOPO and the

extaction constant (log K) for the equilibrium is 12.29 ± 0.06. Solvent

extraction behavior of hexaaqua Rh(III) was investigated in systems containing

picric acid and trioctylphosphine oxide (TOPO) [33] in heptane, benzene,

chloroform, 1,2-dichloroethane and nitrobenzene. The presence of picric acid

and TOPO was found to be effective in rapid extraction of Rh(III) at room

temprature. Solvent extraction of rhodium with bis-(2-ethyl) hexylphosphoric

acid HDEHP [34, 35] from thiourea chloride media was investigated. Under

optimum condition, rhodium(III) was extracted up to 88.3 %. Solvent

extraction of rhodium(III) from acidic media with D2EHPA [36] in toluene

was carried out after optimizing the conditions of extraction. The organic phase

containing extracted rhodium(III) was strriped with 4 M HCl.

Aneva et. al. investigated a scheme described for determination of noble

metals, combines solvent extraction procedures with atomic absorption

spectrometric [37] and graphite furnace atomic absorption spectrometric [38]

determination of rhodium in high-purity platinum matrix or material. The

metal ion was extracted into isoamyl alcohol-isobutyl methyl ketone (MIBK)

[39]. Extraction of rhodium(III) from hydrochloric acid solutions with dihexyl



sulfoxide (DHSO) [40] and with petroleum sulfoxides (PSOs) [40, 41] was

studied, and the optimal conditions for its recovery were found. Method

required 30 min phase contact time for extraction of rhodium(III) with

sulfoxides. Rhodium was extracted from an Rh-Sn-Cl system, with dialkyl

sulfoxide (DEHSO) [42] as the extractant and with stannous chloride as the

activator. The results indicate that the extraction rate of rhodium increases

with the molar ratio of Sn to Rh till the maximum of 99 % at a molar ratio of

6:1.

In comparison to these nitrogen-sulphur containing reagents, Schiff

bases showed promising effects in the field of analytical chemistry for the

separation and estimation of platinum group metals [43]. Rhodium being a soft

acid, can be selectively extracted with soft donor base extractants containing

‘N’ or ‘S’ atoms. Based upon this several extractants, namely, 4-(4-

ethoxybenzylideneamino)-5- methyl-4H-1, 2, 4-triazole-3-thiol (EBIMTT)[44],

N-mono-and N,N- di-substituted benzoylthiourea [43], N,N-di-n-hexyl-N'-

benzoylthiourea [45], N,N-Dialkyl-N′-benzoylthiourea [46], 4-(4-

methoxybenzylideneamino)-5-methyl-4H-1,2,4-triazole-3-thiol [47], Dibutyl,

dioctyl or dibenzyl sulphide [Thioethers (sulphides)] [48, 49] have been studied

or used for extraction of rhodium(III).

A method is proposed for the extraction and simultaneous

spectrophotometric determination of Rh(III) with benzil α-monoxime [50]. In

hot condition, Rh gives a yellow chelate at pH 1-6 with the reagent, which is

extractable into chloroform. The absorbance was measured at 400 nm. The

recovery and purification of rhodium from spent [51] castable refractory of

glass fiber industry are discussed. The rhodium was well extracted by a series

of treatments including aqua regia dissolution, ion exchange, solvent

extraction and levextrel resin separation. The results show that the recovery

rates of rhodium can be above 90 %.

N,N'-dimethyl-N,N'-diphenyltetradecylmalonamide (DMDPHTDMA)

[52] in 1,2-dichloroethane has been investigated as a solvent extraction reagent

to mainly perform the separation of rhodium from other PGMs and from some



commonly associated elements contained in concentrated hydrochloric acid

media. The developed separation scheme was applied to an automobile

catalytic converter leaching solution. The pH effect was studied on solvent

extraction of rhodium sulfates with N-octyl-, N,N-dioctyl aniline and N,N,N-

trioctyl anilinium O,O-di(iso-propyl)dithiophosphates [53]. Correlation

between pH values and partition coefficient were discussed. Solvent extraction

of rhodium was carried out from chloride media with Kelex 100 [54, 55] in the

absence and presence of a large amount of tin. Stripping of metal ion from the

loaded organic phase increases with increasing concentration of oxidizing

agent. A separation procedure for platinum-group metal from lead matrix were

presented after fire-assay preconcntration. The procedure included thermal

dissociation of platinum-group metal nitrates and subsequent extraction with

toluene solution contaning alkylaniline hydrochloride and petroleum sulphide

[56, 57]. Rhodium(III) solution react with stannous chloride to form a short-

lived yellow complex which is extracted by polyurethane foam [58]. Method

was applicable for separation of iridium(III) on the basis of stability of

complex. Rhodium(III) can be extracted from iodide solution of

2-aminobenzothiazole (ABT) [59] into hexane at pH 1.0-3.0 after heating in

boiling-water bath for 60 min for the formation of the extractable complex.

Separation of rhodium from iridium was possible by this method.

Rhodium(III) and iridium(III) were separated satisfactorily in Tween 80-

(NH4)2SO4-H2O [60] system by the liquid-solid extraction as complexes with

stannous chloride. The use of commercially available solid phase extraction

(SPE) anion exchange cartridges [61] for the separation and preconcentration

of rhodium(III) in chloride aqueous samples was described and characterized.

The method was based on adsorption and preconcentration phenomenon.

A liquid emulsion membrane (LEM) system [62] as a tool for process

intensification has been studied for rhodium recovery using di-2-

ethylhexylphosphoric acid (D2EHPA) as a metal carrier and Monemul 80 as a

surfactant, dissolved in liquid paraffin. The extraction and structural properties

of rhodium complexes with stannous chloride using N,N-dioctyl hexanamide



(DOHA) [63] were investigated by solvent extraction and Rh k-edge X-ray

absorption fine structure from a 1 M HCl solution. However, the Rhodium

extraction was enhanced with an increase in the Sn(II) concentration in the

initial aqueous phase. The various investigated systems are presented in tabular

form in Table 4.1 to review the literature in terms of various extractants used

and special characteristics regarding those systems.

Development of new extraction system for extraction of rhodium(III)

especially in weak organic acid solutions is a topic of great interest.

n-Octylaniline with an exploring extracting capacity in malonate media belongs

to a kind of high molecular weight amine. Its potential advantages are as lower

cost, good physico-chemical properties, completely miscible with all common

hydrocarbon diluents, low aqueous solubility, good resistance to hydrolysis,

and high purity.

The present study represents experimental data on extraction of

rhodium(III) from malonate solutions and accesses n-octylaniline as an

extractant. The mechanism of extraction and nature of extracted complex was

sought. The key observation is that it is possible to separate rhodium(III)

directly from other associated rare earth and transition elements.



Table 4.1 Summary of methods for solvent extraction of rhodium(III)

System Aqueous phase

Organic phase Special features Ref.

No. Alamine 336 HCl Kerosene Method was able to

eliminate the base metal ions commonly associated with Rh.

Method applicable for extraction of Pt(IV) and Ir(III) group metals.

4

HCl

1-5 M

Adding SnCl2 to the mixed solutions increased the extraction percentage of Rh.

Rh(III) separates from Pt(IV), Pd(II).

5

HCl

The percentage of Rh(III) increases with increasing HCl concentration of up to 8 M.

Extraction was much higher than TBP.

6,7

HCl Rh(III) separated from Ir(III).

7

Tri-octylamine

(TOA)

1 M HCl Toluene Rh(III) separated from Pt(IV), Pd(II).

8

Rh(III) separated from Pt(IV).

9

Isoamyl

alcohol

Carbonyl-chloride complexes of rhodium extracted

10

HCl Toluene 87.6 % of Rh(III) recoverd from catalytic converters.

HCl and HNO3 used in stripping study.

11

Tri-iso-

octylamine

(Alamine 308)

HCl Kerosene The percentage extraction of rhodium(III) increased with increase in acid concentration up to 8 M.

12



Aliquat 336 SCN- Dodecane Quaternary amines were the most effective for metal transport. Alipatic organic diluents

were chosen to avoid the degradation of the polymeric support. 1-dodecanol was used as

a modifier to increase solubility of extractant. Back extraction by 1M

NaHSO3

13

HCl Kerosene Extraction of rhodium(III) carried out with organo- phosphorus which is used as synergists extractants.

Method is also applicable for Pt(IV)

14

N-n-octylaniline Sodium malonate

Xylene Method is also applicable for separation of rhodium(III) from other associated metals specially from Ir(III)

15

Tetraoctyl and Trialkylbenzyl- ammonium chlorides

HCl - Depolymerisation rate constants are determined.

16

Trioctyl-methyl-ammonium chloride (TOMAC)

HCl Nitro-benzene or benzene

Extraction efficiency promoted in chlorobenzene, nitrobenzene and 1,2-dichloroethane and suppressed in CCl4 and CHCl3. Separation factor about 16

times greater than that from the solution containing oligomer and chloro species. Distribution ratio

increases with increase in temperature.

17, 18

4-(non-5-yl)

pyridine

HCl Chloroform Extraction increased with increase in concentration of reagent and chloride. 95 % rhodium extracted in

30 min. Rhodium and iridium could not be separated.

19



4-octylamino

pyridine

HCl, 0.5-1 M

Chloroform Fe, Co, Ni did not interfere Platinum group metals

showed no mutual interference.

20

2-ethylhexyl

amino methyl

pyridine

(EHAP)

HCl - Excellent selectivity over base metals. Highly slective

extractant for Rh(III), Pd(II) and Pt(IV) Equilibration time 5 h.

21

Tri-n-butyl

phosphate

HNO3 and

trichloro-

acetate

- Method applied for determination of rhodium traces in chloroplatinic acid and palladium chloride.

22

- - SnCl2 added four times that of rhodium. 100 % stripping by 4 M

HCl. 60oC, aging for 2 h.

23

solution of HCl and NaClO3

- Rhodium(III) and Iridium(IV) separated from each other from low quantities.

24

Cyanex 921 HCl;

HBr

Toluene Method is also applicable for extraction and separate ion of Pt(IV) and Pd(II).

Rhodium(III) has been examined in the presence and in the absence of stannous chloride

25,

26

Chloride solution

- Also useful for extraction of Ir(III)

27

Cyanex 923 and Cyanex 471-X

bromide media

Toluene Method is useful for extraction and separation from other platinum metals.

Stoichiometric ratio of Rh(III) with both extractants was 1:1.

28

Cyanex 923, Cyanex 471, Cyanex 472

HCl Toluene Recovery from spent autocatalysts.

29



Chloride

solution Kerosene Distribution ratios for the

metals were determined under the different concentrations of H+ Cl- ions in the aqueous phase.

30

Tri-n-

octylphosphine

oxide (TOPO)

stannous

chloride

- Stannous chloride used as activating reagent and it was four times that of Rh(III).

Complex formation required 60oC temperature.

Percentage extraction was 98 % and percentage of stripping could reach 95 %.

31

Trioctylphos-phine oxide (TOPO)

pH, 3-5 Trichloroacetic acid (HTCA)

Heptane Equilibration time 5 min. 32

Picric acid

Heptane/ benzene/ chloro-form/1,2-dichloro ethane/ nitro-benzene

The presence of picric acid & TOPO found effective in rapid extraction of Rh(III) at room temperature

33

Bis-(2-ethyl-hexyl) hydrogen phosphate (HDEHP)

HCl, 0.2 M pH, 4.05 HCl, 0.5 M and thiourea 0.5 M pH,4.50

Isopar M 90.7 % rhodium extracted. 88.3 % rhodium extracted More than 95 % back

extraction into 1 M HCl

34, 35

Toluene Back extaction into 4 M HCl Separation from some

noble metals.

36

MIBK HCl, HCl, 2-3 M, pH

Heating for 10-60 min

Method successfully applied to a converter matte. More than 95 % extraction.

37-

39

HCl, 6 M Cu-coextracts

Pre-equilibration with water to remove platinum. The method of standard

addition is used. Lower limit of

determination of rhodium 1 μg/g.



Dihexyl sulfoxide (DHSO) and Petroleum sulfoxides (PSOs)

HCl - Method required 30 min phase contact time for extraction of rhodium(III)

Species were detected by IR spectra.

40, 41

Dialkyl sulphoxide

HCl - Stannous chloride used as activating reagent.

The results indicate that the extraction rate of rhodium increases with the molar ratio of Sn to Rh till the maximum of 99 %.

42

N’N’-dihexyl and phenyl and N’-hexyl and phenyl derivatives of N-benzoyl thiourea

HCl, 1mM – 6 M

Toluene The dihexyl derivative showed the highest extracting power. Increase in ligand

concentration improves the extent of extraction. Heat is required at

25o C to 80oC

43

4-(4-ethoxybenzylideneamino)-5- methyl-4H-1, 2, 4-triazole-3-thiol (EBIMTT)

Sodium malonate medium

Chloroform Method is applicable for the analysis of binary mixtures, synthetic mixtures, alloys and real samples.

44

NN-dihexyl-N’-benzoyl-thiourea

- - Extraction of rhodium(III) was accelerated in the presence of stannous chloride with metal-ligand ratio 1:9

45

N-benzoyl-NN’-dihexyl thiourea

pH,3 Solvesso 150 Separation of Rh, Ru, Pd, Os, Ir, Pt, Cu, Fe from Mn, Co and Zn. Heat at 950C, Cu, Fe and

Ni interfere.

46

4-(4-methoxybenzylideneamino)-5-methyl-4H-1,2,4-triazole- 3-thiol in hydrochloric acid

HCl Chloroform Extraction of microgram level concentration of rhodium(III)

Rhodium(III) was stripped from organic phase with 1 M hydrochloric acid

47



Dibutyl, dioctyl or dibenzyl sulphide [Thioethers (sulphides)]

HNO3 Hexanol The extent of extraction of Rh(NO2)6

3- is much greater than that of Rh(H2O)6

3+ Separation of rhodium

from Pd, Ru and Tc

48, 49

Benzil α-monoxime

pH 1-6 Chloroform In hot condition, Rh gives a yellow chelate The absorbance was

measured at 400 nm. The results show that the

recovery rates of rhodium can be above 90 %.

50, 51

N,N'-dimethyl-N,N'-diphenyltetrade-cylmalonamide (DMDPHTDMA)

HCl 1,2-Dichloroetha-ne

Method was applicable for separation of rhodium from other PGMs

52

N,N,N-trioctyl anilinium O,O-di(iso-propyl) dithiopho-sphates

- - Method was applicable for platinum and Iridium.

53

Kelex 100 HCl - Addition of large amount of tin in the feed solution increases the Rh extraction and decreases the Pd and Pt extraction. Selective separation of

rhodium Stripping increases with

increase in concentration of oxidising agent.

54

Rh-chloro complexes suppresses the liquid-liquid extraction of rhodium.

55

Alkylaniline hydrochloride petroleum sulphide

HCl, 6 M Toluene

Determination by flame or electrothermal AAS. Shaking for 30 min.

56, 57

Polyurethane foam

- - Rhodium(III) react with SnCl2 to form short lived yellow complex which was extracted by extractant.

58



2-aminobenzothi-azole

HCl, 0.2 M KI, 5 %; sodium acetate 10 %; pH=2 Chloro- acetic acid

Hexane Separation of rhodium from iridium. Heating for 60 min.

69

Tween 80-(NH4)2SO4-H2O

Stannous chloride

- Separation of rhodium from iridium possible by this method.

60

Solid-Phase Extraction Cartridges (SPE)

- - Separation and preconcentration of rhodium(III) in chloride aqueous samples was described and characterized. The method was based on

adsorption and preconcentration phenomenon.

61

Di-2- ethylhexyl phosphoric acid (D2EHPA)

HCl Paraffin Perchloric acid was found to be a better stripping agent .

62

N,N-dioctyl hexanamide (DOHA)[

- - Rh(III) extraction was enhanced with an increase in the Sn(II) concentration

63



4.3 Experimental

4.3.1 Instruments

An Elico digital spectrophotometer model 12 Chemito 215D with 1 cm

quartz cells was used for absorbance measurements and pH measurements were

carried out using an Elico digital pH-meter model LI-127. All weighing

operations were carried out by using Tapson’s analytical single pan balance

model 200T having 0.001 g accuracy.

4.3.2 Chemicals and solutions

Standard rhodium(III) solution

A stock solution of rhodium(III) was prepared by dissolving 1 g of

rhodium trichoride hydrate (Johnson Matthey, UK) in dilute analar

hydrochloric acid (1M) and diluting to 25 mL with water and standardised

gravimetrically [64]. A working solution of 100 µg/mL was made from it by

diluting the stock solution with water.

n-octylaniline

The extractant n-octylaniline was prepared by the method of Pohlandt’s

[65] and its 0.1 M solution was prepared in xylene. All other solutions were

prepared from A. R. grade reagents and aqueous solutions were prepared using

water. Double distilled water was used throughout the experimental study.

Standard solution of diverse ions were prepared by dissolving AR grade

reagents in water or dil HCl. All the organic solvents were used after double

distillation. All chemicals used were of AR grade.

4.3.3 General extraction and determination procedure for rhodium(III)

An aliquot of 200 μg rhodium(III) solution was mixed with a sufficient

quantity of sodium malonate to make its concentration 0.03 M in a total volume

of 25 mL of the solution. The pH of the aqueous solution was adjusted to 9.0

by dilute sodium hydroxide and hydrochloric acid solution. The solution was

then transferred to a 125 mL separating funnel and shaken with 10 mL of 0.1 M

n-octylaniline in xylene for 3 min. After separating the two phases, the aqueous



phase was discarded and the organic phase was stripped with two 10 mL

portions of 1 M hydrochloric acid solution. The stripped aqueous phase was

evaporated to moist dryness and extracted into water. 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 upto the mark with water containing 1 M hydrochloric acid

in final concentration. The unstoppred flask was kept in the boiling water bath

for development of reddish brown solution which was measured at 445 nm

against a reagent blank. The concentration of rhodium(III) was computed from

the calibration curve in similar manner [66].

4.4 Results and discussion

4.4.1 Extraction as a function of pH

The extraction studies of rhodium(III) was performed at fixed

concentration of 0.03 M sodium malonate and between pH 1-10 with a 0.01 M

solution of n-octylaniline in xylene (Table 4.2). The pH range observed for the

quantitative extraction was 7.5-9.5 with n-octylaniline. Hence, the extractions

of rhodium(III) were carried out at pH 9.0 for all extraction experiments

(Fig. 4.1).

4.4.2 Effect of n-octylaniline concentration

Extraction of rhodium(III) was carried out with various concentrations

of n-octylaniline in xylene (Table 4.3). To optimize the extraction condition,

other parameters like pH, period of equilibration and diluent were kept

constant. The extraction was found to be increased with increasing reagent

concentration. The extraction of rhodium(III) was quantitative in the range

0.07 M to 0.15 M of n-octylaniline in xylene. However, 10 mL of 0.1 M

n-octylaniline in xylene was recommended for general extraction procedure

(Fig. 4.2).



4.4.3 Effect of weak organic acid concentration

The extraction of rhodium(III) was examined at pH 9.0 with 0.1 M

n-octylaniline in xylene in presence of varying concentrations from

0.001 - 0.1 M of various weak organic acids (Table 4.4). The extraction of

ion-pair complex of rhodium(III) was found to be quantitative in the range of

0.025 – 0.035 M sodium malonate. Hence, 0.03 M concentration of sodium

malonate was used for further studies while incomplete extraction of

rhodium(III) was found to be in sodium salicylate and no extraction from

sodium succinate (Fig. 4.3).

4.4.4. Effect of diluents

The studies were then performed to find out the most suitable solvent for

the extraction of the ion-pair complex of rhodium(III). It was found that a

0.1 M solution of n-octylaniline in benzene, toluene, xylene provides

quantitative extraction of rhodium(III). The extraction of rhodium(III) was

incomplete if n-octylaniline is dissolved in chloroform (41.1 %), methyl

isobutyl ketone (42.7 %) while no extraction in amyl alcohol,

1,2-dichloroethane, n-butyl alcohol, amyl acetate was observed (Table 4.5). On

safety ground, xylene was preferred to other solvents.

4.4.5 Effect of equilibration time

The extraction of rhodium(III) was studied for various time intervals in

the range of 10 sec - 30 min with 0.1 M n-octylaniline (Table 4.6). It was

observed that, under the optimized experimental conditions a minimum 1 min

time interval was required for attaining equilibrium in the sense to extract

rhodium(III) quantitatively. But with prolonged shaking over 10 min there was

decrease in the percentage extraction of rhodium(III) due to the dissociation of

ion-pair complex Hence, in all further studies both the phases were equilibrated

for 3 min (Fig 4.4).



4.4.6 Effect of stripping agent

Rhodium(III) from organic phase was stripped with two 10 mL

portions of various stripping agents at different concentrations of mineral acids,

buffer solutions and some bases. Rhodium(III) was quantitatively stripped with

hydrochloric acid (1.0 M to 3.0 M), nitric acid (1.0 M to 3.0 M), sulphuric acid

(1.0 M to 3.0 M) and hydrobromic acid (1.0 M to 3.0 M) from the organic

phase (Table 4.7). However, percentage recovery of rhodium(III) from organic

phase was found to be incomplete with water and no extraction in ammonia

buffer (pH 10), ammonia and sodium chloride. In recommended procedure,

two 10 mL portions of 1.0 M hydrochloric acid were used for the complete

stripping of loaded organic phase.

4.4.7 Effect of aqueous to organic volume ratio

The extraction of rhodium(III) was carried out in different aqueous

volumes in the range 150-10 mL from 0.03 M sodium malonate medium with

10 mL 0.1 M n-octylaniline in xylene (Table 4.8). There was quantitative

extraction of rhodium(III), when phase ratio A/O varied from 10:10 to 50:10.

Therefore in the recommended procedure the phase ratio 2.5:1 was maintained

throughout the experimental study.

4.4.8 Metal loading capacity of extractant

The influence of the initial rhodium(III) concentration 50-2500 µg on

the extraction by 0.1 M n-octylaniline in xylene was studied. It was observed

that, varying the initial rhodium(III) concentration in the range of 50-1500 µg

has no significant influence on rhodium(III) extraction with the 10 mL of 0.1 M

extractant (Table 4.9). The maximum loading capacity of 10 mL 0.1 M solution

of n-octylaniline in xylene was found to be 1500 µg rhodium(III).

4.4.9 Nature of extracted species

Attempts were made to ascertain the nature of extracted species of

rhodium(III) with the extractant using conventional slope analysis method.

The distribution ratio of rhodium(III) was evaluated at different concentrations



in molar of sodium malonate at fixed n-octylaniline concentration at pH 6.0

and pH 7.0. A graph of log D[Rh(III)] versus log C[malonate] gave a slope of 1.75

and 1.80, respectively (Fig. 4.5). Similarly, a plot of log-log D[Rh(III)] versus

log C[n-octylaniline] concentrations at a fixed pH 6.0 and pH 7.0 with 0.03 M

malonate gave slope of 1.1 and 1.0, respectively (Fig. 4.6). This indicates a

mole ratio of rhodium(III) to sodium malonate as 1:2 and that of n-octyaniline

as 1:1. Thus, the extracted species was calculated to be an ion association

complex with the probable composition 1:2:1 (metal: acid: extractant). The

probable mechanism of extracted species is as follows,

CH3(CH2)7C6H4 NH2(org) + H + (aq) [CH3(CH2)7C6H4NH3]+

(org) ........(4.1)

Rh3+(aq) + 2C3H2O4

-(aq) Rh(C3H2O4)2

-(aq) .......(4.2)

CH3(CH2)7C6H4NH3+

(org) + Rh(C3H2O4)-2(aq)

[CH3 (CH2)7C6H4NH3+Rh(C3H2O4)2

-](org). .....(4.3)

4.4.10 Effect of diverse ions

The effect of various cations and anions on recovery of rhodium(III)

was investigated. The tolerance limit was set as the amount of foreign ion

causing a change ± 2 % error in the recovery of rhodium(III). It was observed

that the method is free from interference from a large number of cations and

anions. Initially, the foreign ion was added to the rhodium(III) solution in large

excess; 100 mg for anions and 25 mg for cations. When interference was found

to be intensive, the tests were repeated with successively smaller amount of

foreign ion. The only species showing interference of Ir(III) was eliminated by

masking with oxalate. The anionic species showing interference in the

procedure were EDTA, succinate, thiocyanate, acetate, tartrate, and bromide

due to formation of strong metal complexes (Table 4.10).

4.5 Applications

4.5.1 Separation and determination of rhodium(III) from binary mixture

The separation of rhodium(III) from some commonly associated metal

ions like Pt(IV), Pd(II), Ru(III), Au(III), Os(VIII), Se(IV), Te(IV),

Fe(III),Co(II), Ni(II) and Cu(II) using n-octylaniline can be achieved by taking



advantage of the difference in the extraction conditions of metal such as pH of

the aqueous phase, reagent concentration and use of masking agent (Table

4.11).

Rhodium(III) was separated from these associated metal ions, under the

optimum extraction conditions of rhodium(III) where, all the added metal ions

were remained quantitatively in aqueous phase from which they were

determined spectrophotometrically by standard methods [66-71]. Rhodium(III)

from organic phase was stripped and estimated spectrophotometrically by KI+

SnCl2 method.

The proposed method was also extended for separation of rhodium(III)

from Ir(III) by masking with 25 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.

4.5.2 Separation of rhodium(III) from ternary mixtures

The method was extended to the determination of rhodium(III) in some

synthetic mixtures of associated metal ions. The rhodium(III) was extracted

using the proposed method, the results are presented in Table 4.12.

4.5.3 Sepration of rhodium(III) from synthetic mixtures

A solution containing 200 μg of rhodium(III) was taken and known

amount of other metals were added which are commonly associated with

rhodium(III). Extraction of rhodium(III) was carried out using the method

developed. The results obtained were in good agreement with the amounts

added (Table 4.13).



4.5.4 Analysis of rhodium(III) in synthetic mixtures corresponding to

alloys

The proposed method was applied for analysis of synthetic mixture

corresponding to alloys such as pseudo palladium (equal amount of Rh(III) and

Ag(I)), iron-rhodium alloy, platinum-rhodium alloy, rhodium- platinum

catalyst. The real samples of these alloys were not available at working place,

which forced us to use synthetic mixture with corresponding composition to

alloys, the rhodium(III) was extracted under its optimum extraction conditions

and determined spectrophotometrically, the results of analysis are reported in

Table 4.14.

4.6 Conclusion

Quantitative extraction of rhodium(III) was achieved in 3 min with

0.1 M n-octyaniline in xylene at pH 9.0.

Trace level of rhodium(III) extracted using low concentration of

n-octylaniline.

Extraction reaction occurred through anion-exchange mechanism.

Developed method is efficient for quantitative separation of

rhodium(III) in presence of various interfering cations and anions.

The proposed extractive separation method is simple, rapid, selective

reproducible and suitable for separation and determination of

rhodium(III) from associated metal ions and synthetic mixtures.



Table 4.2 Extraction of rhodium(III) as a function of pH

Rh(III) = 200 μg Aq: Org = 2.5: 1

Sodium malonate = 0.03 M n-octylaniline = 0.1 M in xylene

Equilibrium time = 3 min Strippant = 1 M Hydrochloric acid

(2×10 mL)

pH Percentage extraction, (% E)

Distribution ratio, (D)

1.0 45.3 2.07

2.0 57.6 3.39

3.0 67.8 5.26

4.0 72.3 6.52

5.0 80.5 10.32

6.0 84.5 13.62

7.0 88.1 18.50

7.5 100 ∞

8.0 100 ∞

8.5 100 ∞

9.0* 100 ∞

9.5 100 ∞

10.0 84.2 13.32

* Recommended for general extraction procedure



Table 4.3 Extraction behaviour of rhodium(III) as a function of

n-octylaniline concentration Rh(III) = 200 μg pH = 9.0

Aq: Org = 2.5: 1 Sodium malonate = 0.03 M

Equilibrium time = 3 min Strippant = 1 M Hydrochloric acid

(2×10 mL)

n-octylaniline, (M)

Percentage extraction, (% E)


0.01 35.1 1.35

0.02 40.4 1.69

0.03 61.1 3.92

0.04 72.6 6.62

0.05 78.9 9.34

0.06 92.1 29.14

0.07 100 ∞

0.08 100 ∞

0.09 100 ∞

0.10* 100 ∞

0.12 100 ∞

0.15 100 ∞




Table 4.4 Extraction behavior of rhodium(III) as a function of weak

organic acid concentration

Rh(III) = 200 μg pH = 9.0

Aq: Org = 2.5: 1 n-octylaniline = 0.1 M in xylene

Equilibrium time = 3 min Strippant = 1 M Hydrochloric acid (2×10 mL)

Acid concentration (M)

Sodium malonate Sodium salicylate

% Ea Db % Ea Db

0.001 13.8 0.39 6.9 0.18

0.005 69.9 5.80 12.5 0.35

0.010 77.6 8.66 62.5 4.16

0.020 93.4 35.37 64.4 4.52

0.025 100 ∞ 65.7 4.78

0.030* 100 ∞ 61.5 3.99

0.035 100 ∞ 59.5 3.67

0.040 83.8 12.93 58.5 3.52

0.050 60.7 3.86 49.3 2.43

0.070 51.9 2.69 41.1 1.74

0.080 46.7 2.19 35.5 1.37

0.090 40.0 1.66 30.9 1.11

0.10 34.8 1.33 28.0 0.97 * Recommended for general extraction procedure a = Percentage extraction b= Distribution ratio



Table 4.5 Extraction behaviour of rhodium(III) as a function

of diluents

Rh(III) = 200 μg pH = 9.0

Aq: Org = 2.5: 1 Sodium malonate = 0.03 M

Equilibrium time = 3 min Strippant = 1 M Hydrochloric acid (2×10 mL)

Solvent Dielectric constant,

(ε)

Percentage extraction,

(% E)


Benzene 2.27 100 ∞

Xylene* 2.30 100 ∞

Toluene 2.38 100 ∞

Chloroform 4.80 41.1 1.74

Methyl isobutyl ketone

13.10 42.7 1.86

n-Butyl alcohol 17.80 No extraction -

Amyl alcohol 13.90 No extraction -

Amyl aceate 13.90 No extraction -

1, 2-Dichloroethane

10.50 No extraction -




Table 4.6 Extraction behavior of rhodium(III) as a function of

equilibrium time

Rh(III) = 200 μg pH = 9.0


Strippant = 1 M Hydrochloric acid (2×10 mL) Aq: Org = 2.5: 1

Time in min Percentage extraction,

(% E )

Distribution ratio,

( D )

10 sec 60.5 3.82

30 sec 72.6 6.62

1 100 ∞

2 100 ∞

3* 100 ∞

4 100 ∞

5 100 ∞

6 100 ∞

7 100 ∞

8 100 ∞

9 100 ∞

10 100 ∞

15 90.1 22.75

30 62.1 4.09




Table 4.7 Extraction behavior of rhodium(III) as a function of

stripping agents

Rh(III) = 200 μg pH = 9.0


Aq: Org = 2.5: 1 Equilibrium time = 3 min

Strippant M / pH Percentage extraction, (% E )

Ammonia 1-10

No stripping

HCl* 1-3

100

H2SO4 1-3

100

HNO3 1-3

100

HBr

1-3

100

Water

-

31.9

NaCl

1-5 %

No stripping

Ammonia buffer pH-10 No stripping




Table 4.8 Extraction of rhodium(III) as a function of aqueous to

organic volume ratio

Rh(III) = 200 μg pH = 9.0


Strippant = 1 M hydrochloric acid Equilibrium time = 3 min (2×10 mL)


Aqueous to organic volume ratio

Percentage extraction,

( % E )

Distribution ratio, ( D )

10:10 100 ∞

20:10 100 ∞

25:10* 100 ∞

30:10 100 ∞

35:10 100 ∞

40:10 100 ∞

50:10 100 ∞

70:10 92.7 31.74

100:10 62.5 4.16

150:10 45.3 2.07



Table 4.9 Metal loading capacity of n-octylaniline

pH = 9.0 Aq: Org = 2.5: 1


Strippant = 1 M hydrochloric acid Equilibrium time = 3 min (2×10 mL)

Rh(III), ( μg ) Percentage extraction,

(% E )

Distribution ratio,

( D)

50 100 ∞

100 100 ∞

200* 100 ∞

300 100 ∞

400 100 ∞

600 100 ∞

800 100 ∞

1000 100 ∞

1500 100 ∞

2000 81.5 11.01

2500 39.5 1.63




Table 4.10 Effect of foreign ions on the extraction of 200 μg rhodium(III) at pH 9.0 in 0.03 M sodium malonate with 0.1 M n-octyaniline in xylene

Ratio of ions

Rhodium: ion Mass tolerated, mg Foreign ion

2:500 50 Iodide

2:250 25 Zn(II), oxalate.

2:150

15

Ni(II), Te(IV), Tl(III),

Mo(VI), Se(IV), Ba (II),

Ce(IV)

2:100

.

10

Mg(II), Cd(II),

Sb(III), V(V), Pb(II),

Sn(II), Bi(III), fluoride,

thiourea, Nitrate.

2:50 5 Cu(II), Co(II), Fe(III)

2:20 2 Fe (II), Hg (II), Cr(VI)

2:1 1 Ag (I), Pt(IV), OS(VIII),

Au(III), Pd(II)

2:0.5 0.5 Ru(III), Ir(III)a

a masked with oxalate



Table 4.11 Separation of rhodium(III) from binary mixtures

Amount of metal ion, (μg)

Mass taken, µg

Average (%) Recovery*

Chromogenic ligand

Ref. No.

Rh(III) Ir(III)a

200 100

99.0 97.4

stannous chloride-hydrobromic acid

[67]

Rh(III) Pt(IV)

200 300

99.0 99.4

stannous chloride-hydrochloric acid

[67]

Rh(III) Pd(II)

200 200

98.8 98.4

4’-ChloroPTPT

[68]

Rh(III) Au(III)

200 200

98.8 97.1

SnCl2

[67]

Rh(III) Se(IV)

200 200

99.0 97.0

4’-BromoPTPT

[69]

Rh(III) Te(IV)

200 200

98.8 98.9

4’-BromoPTPT

[70]

Rh(III) Os(VIII)

200 200

98.0 99.3

Thiourea

[67]

Rh(III) Fe(III)

200 500

99.0 98.8

Thiocynate

[67]

Rh(III) Co(II)

200 500

99.0 98.7

Thiocynate

[67]

Rh(III) Ni(II)

200 1000

99.0 98.9

DMG

[66]

Rh(III) Ru(III)

200 200

99.2 98.9

4’-ChloroPTPT

[68]

Rh(III) Cu(II)

200 1500

99.0 98.7

4’-ChloroPTPT

[71]

*average of five determinations a = masked by 20 mg oxalate



Table 4.12 Separation of rhodium(III) from ternary mixtures

Metal ion

Amount taken,

μg

Average recovery of rhodium(III),*

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

200 100 100

99.0

Rh(III) Fe(III) Ni(II)

200 150 100

98.6

Rh(III) Cu(II) Ni(II)

200 150 100

98.6

Rh(III) Pd(II) Cu(II)

200 200 150

99.0

Rh(III) Pd(II)

Au(III)

200 200 100

99.0

Rh(III) Ir(III)a

Pd(II)

200 100 200

98.6

* Average of five determinations a = masked by oxalate



Table 4.13 Separation of rhodium(III) from synthetic mixtures

Composition, μg Rhodium(III)

found, μg Recovery*, % RSD,%

Rh(III),200;Pd(II),200;

Pt(IV),200;Ir(III)a,100

197.2

98.6

1.4

Rh(III),200;Au(III),100;

Pd(II),200;Pt(IV),100

197.0

98.5

1.5

Rh(III),200;Pd(II),200;

Pt(IV),100;Ru(III),200

198.0

99.0

1.0

Rh(III),200;Pd(II),200;

Pt(IV),100;Ru(III),200;

Ir(III)a,100

197.2

98.6

1.4

Rh(III),200;Pd(II),200;

Pt(IV),100;Ru(III),200;

Ir(III)a,100;Os(VIII),100

197.0 98.5 1.5

* Average of five determinations a = masked by 20 mg oxalate



Table 4.14 Determination of rhodium(III) from synthetic mixture

corresponding to alloys

Alloys Metal

mass, (µg)

Rhodium(III)

mass found

/(µg)

R*

(%)

RSD,

%

Pseudo palladium (equal

amount of rhodium(III) and

Ag(I)

Rh(III),50 %; Ag(I),50 %

200

198.0

99.0

1.0

Iron- rhodium alloy

Rh(III),74.98 %; Fe(III),

25 %

148

145.9

98.6

1.4

Rh(III),67.71 %; Fe(III)

32.28 %

135

133.1

98.6

1.4

Platinum-rhodium alloy

Pt(IV) 87 % Rh(III)13 %

174 172.2 99.0 1.0

* Average of five determinations R = % Recovery of rhodium(III)



0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10pH

Perc

enta

ge E

xtra

ctio

n (%

E)

Fig. 4.1 Plot of pH versus percentage extraction of rhodium(III)

(200 μg/mL) from malonate medium (0.03 M) by using

n-octylaniline (0.1 M) as an extractant in toluene with 3 min

shaking time.



0

10

20

30

40

50

60

70

80

90

100

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13 0.14 0.15

n-octylaniline (M)

Perc

enta

ge E

xtra

ctio

n (%

E)

Fig. 4.2 Extraction of rhodium(III) (200 µg/mL) at pH 9.0 from

0.03 M sodium malonate as a function of n-octylaniline

concentration.



0

10

20

30

40

50

60

70

80

90

100

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

Weak acid (M)

Perc

enta

ge E

xtra

ctio

n (%

E)

Sodium malonate

Sodium salicylate

Fig. 4.3 Extraction behavior of rhodium(III) (200 µg/mL)as a function of

weak organic acid concentration.



0

10

20

30

40

50

60

70

80

90

100

0.1 0.3 1 2 3 4 5 6 7 8 9 10 15 30Time in Min.

Perc

enta

ge E

xtra

ctio

n (%

E)

Fig. 4.4 Extraction of rhodium (III) (200 µg) with 0.1 M n-octylaniline

at pH 9.0 as a function of equibration period.



▲ Slope at pH 6= 1.75

■ Slope at pH 7= 1.80

-1.5

-1

-0.5

0

0.5

1

1.5

2

-3.1 -2.9 -2.7 -2.5 -2.3 -2.1

Log C [malonate]

Log

D[R

h(II

I)]

Fig. 4.5 Log-log plot of distribution ratio LogD[Rh(III)] versus

Log C[malonate] at fixed n-octylaniline concentration.



▲ Slope at pH 6 = 1.1

■ Slope at pH 7 = 1.0

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

-2.3 -1.8 -1.3 -0.8

Log C [n-octylaniline]

Log

D[R

h(II

I)]

Fig. 4.6 Log-log plot of distribution ratio Log D[Rh(III)] versus

Log C[n-octylaniline] at fixed malonate concentration.



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