Indian Journal of Chemical Technology Vol. 10. September 2003, pp. 447-453

Articles

A sensitive extractive spectrophotometric method for the determination of palladium(II) with 1, 3-bis(hydroxymethyl) benzimidazole-2-thione in catalysts

S H Gaikwad & M A A mise * Analytical Chemistry Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416 004 , India

Received 8 Apri/2003; revised received 7 July 2003; accepted 16 July 2003

A simple, rapid and highly sensitive extraction method has been developed for the spectrophotometric determination of palladium(ID with 1,3-bis(hydroxymethyl)benzimidazole-2-thione[BHMBT] in mEthyl isobutylketone. The optimum perchloric acid range for the complexation is 0.7-3.5M. The yellow complex has an absorption maximum at 370 nm. Beer's law is obeyed up to 6.0 ppm of palladium(ll). The optimum concentration range evaluated by Ringbom's method was found to be 1.3~-5.01 ppm. The method of continuous variations, molar ratio method, and slope-ratio method confirmed a palladium(II) to BHMBT ratio of 1:1 in the complex. The molar absorptivity and Sandell's sensitivity were 1.543x104 L mole"1 cm·1 and 0.0068 IJ.g cm·2

, respectively. The optimum conditions have been investigated by studying parameters like effect of medium, effect of acidity, reagent concentration, time period, solvent. and effect of diverse ions. The method has been applied successfully to the determination of palladium(II) in synthetic mixtures, corresponding to alloys and hydrogenation catalysts.

The determination of precious metals attracted the interest of analysts and methods were developed rapidly because these metals are valuable and rare, yet also very imp01tant for many industrial process and products. Their concentration level s are very low in many natural resources, metallurgical intermediates and environmental samples.

Platinum Group Metals (PGMs) especially palladium(JI) is very important in industry. The high prices of PMGs have stimulated investigation of their separation, concentration and purification by solvent

. h . 12 extraction tee mque · . However, the determination of palladium in real

samples has always been a cumbersome and expensive procedure usually involving fire assay followed by techniques such as, flame atomic absorption spectrometry, emission spectrometry or neutron activation analysis (NAA)3. It is also true that techniques like neutron activation analysis are available in very few laboratories, and the procedure is expensive. There is clearly a need for an alternative procedure with comparable precision, accuracy and sensitivity to NAA. Some of the difficulties encountered in the PGMs separation have been reviewed by Beamish and Van Loon4

• Sulphur containing ligands form very stable complexes with PGMs because, PGMs are soft acids and sulphur containing ligands are soft bases and both lead to the

*For correspondence (E-mail: maanuse@rediffmail.com; Fax: 0091-0231-2692333)

stable combination5. Some sulphur containing ligands

have also been employed for selective extracti on separation and spelltrophotometric determination of palladium(Il) from associated elements6·8 but. the methods suffer from lack of sensitivity and are time consuming. The complexing reagents showing low sensitivity and interference caused by foreign ions in spectrophotometric determination of palladium(ll) are, 3-phenoxy benzaldoxime9 (c=2.434x l0\ a­benzylmonoxime10 (c=l.Ox104), mercaptophenyl sali­cyladimine11 (c=9.2x10\ 1-phenyl-1-hydrazonyl-2-

oximino-1 ,2-ethanedione 12 (£ = 6.81 x i O\ isonitroso-4-methyl-2-pentanone13 (£=5 .37x l0\ a-benzoino­xime14 (E = 4xl03), 2-allylthiourea-thiocyanate15 (E =

2.62x 1 03), promazine hydrochloride 16 ( £=4 . l I x J0·1)

and l-phenyl-1,2-butanedione dioxime17 (c=3.74x 10\ Some other methods require higher equilibration or heating time for full complexation 18.24.

The proposed method _ for the extractive spectrophotometric determination of trace amount of palladium(II) offers advantages of simplicity, sensitivity, rapidity and stability. Colour development takes place at room temperature, instantly and extracted in a single stage.

Experimental Procedure

Apparatus Absorbance measurements were made on a Elico

digital spectrophotometer model CL-27 with 1 em

Articles

quartz cells. Standard glasswares were used for volumetric measurements.

Standard palladium( II) solution A stock solution of palladium(II) was prepared by

dissolving lg of palladium chloride hydrate (Johnson Matthey, UK) in dilute analar HCI 1M and diluted to ?50 mL with double distilled water and standardized4

•

-\ working solution of 10 ).tg/mL was made from it by 1ilution of the stock solution with distilled water.

Synthesis of 1,3-bis(hydroxymethyl) benzimidazole-2-thione (BHMBT)

The BHMBT was synthesized in two steps, in the first step 2-mercaptobenzimidazole was synthesized from a-phenylenediamine and carbon disulphide. The second step involved formylation of 2-mercaptobenzimidazole in aqueous medium at reflux temperature.

Step-1:2-Mercaptobenzimidazole 25

a-Phenylenediamine (10.8 g; 1 M), carbon disul­phide (6 mL) and KOH (5 .6 g; 0.1 M) in 100 mL absolute ethanol were heated under reflux for 3h. Charcoal was added to this for decolourisation and filtered. To the filtrate , 100 mL of warm water was added and heated for 15 min . After the addition of acetic acid (8 mL in 16 mL water), colourless crystals were separated, and further cooled in ice bath to complete the crystallisation. Recrystallisation was carried out from absolute ethanol (yield 93%, m.p. 302°C).

a-Phenylenediamine

KOH, Ethanol

Reflux

~ N ,J-sH

- N

2-Mercaptobenzimidazole

Step-II: 1 ,3-Bis(hydroxymethyl) benzimidazole-2-thione 26

2-Mercaptobenzimidazole (15 g; 0.1 M) and 37% formalin (30 mL; 1 M) in 100 mL water were refluxed for 30 min. On cooling, colourless solid was separated which was repeatedly washed with water, filtered and dried. Recrystallisation was carried out from hot water (Yield 70%, m.p. 160-162°C). The purity of the compound was checked by thin layer chromatography (TLC) and elemental analysis.

448

Indian J. Chern. Techno!.. September 2003

~ N

,J -SH N

+ 2 HCHO H20 -Reflux

2-Mercaptobenzimidazole

Characterisation of BHMBT The absorption spectrum of BHMBT in ethy l

alcohol shows that the reagent exhibits sharp absorption maximum at Amax 330 nm, with the molar extinction coefficient 140 L mor 1 cm-1

•

IR spectra of BHMBT in KBr reveals the presence of >C=S peak at 1601 cm-1 and 3456, 3283 cm-1 peak for -OH groups. PMR spectra of the BHMBT in DMSO-d6 shows 8 values 5.67 (4H, s, 2xCH2), 7. 17-7.71 (4H, Ar-H) confirming the above structure of BHMBT.

A stock solution of 0.0 I M BHMBT was prepared in ethanol.

All the chemicals used were of AR grade. Doubl y distilled water was used throughout the work.

Recommended method To an aliquot of solution contall1ll1g 30 ).tg of

palladium(ll), adequate amount of perchloric acid was added to get the desired acidity of I M and 2 mL of O.OIM BHMBT in ethanol was added . The volume was made up to 25 mL with distilled water. The yellow coloured complex formed instantly was extracted into 10 mL methyl isobutyl ketone (MIBK). The organic extract was collected onto anhydrous sodium sulphate to remove traces of water. The organic extract was transferred in a 10 mL standard flask and made up to mark with MIBK, if required . The absorbance of the extracted yellow complex was measured at 370 nm against reagent blank . A calibration curve was prepared. Unknown amount of palladium(II) was determined from the calibration curve.

Results and Discussion

Spectral characteristics The absorption spectra of the extracted yel low

palladium (II)-BHMBT complex in MIBK was compared with that obtained with reagent blank in MIBK. It was found that the palladium complex has a Amax at 370 nm, while the reagent has relatively negligible absorbance at this wavelength(Fig. I ).

Gaikwad & Anuse: Extractive spectrophotometric determination of palladium(II) in catalysts Articles

• u c

0.8.--- ------------------,

0.7

0.6

0.5

F BHMBT 0002Mt

!--Pd(ll) 1 ppm I

._..._ Pd(ll) 2 ppmLI __,._ Pd(ll) 3 ppm --Pd(ll) 4 ppm --Pd(ll) 5 ppm --------

i 0.4 0

11 ~

0.3

02

0.1

0~-~~~~-~-~-~--~-~~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

w.v ..... gth,nm

Fig. !- Absorbance spectrum of Pd(ll)-BHMBT complex and BHMBT in MIBK from I M perchloric acid medium.

Effects of experimento.l conditions The absorbance of the organic phase was measured

as a function of acidity of the aqueous phase. The data showed that, maximum and constant absorbance was obtained between 0.7-3.5 M perchloric acid. In less or more acidic solution, the absorbance decreases and solubility of MIBK increases above 3.5 M. The absorbance was found to be lower in hydrochloric, sulphuric and nitric acid media.

A molar ratio study demonstrated that, a fifty-fold molar excess of the ligand in the organic phase was necessary for maximum and reproducible absorbance. Addition of more reagent did not interfere with the formation and extraction of the complex.

The spectral characteristics of the complex depends on the solvent used. Methyl isobutyl ketone was chosen as the extracting solvent because, the highest absorbance of the complex was obtained in this solvent (Table 1). The absorbance value decreased in the order MIBK > isoamyl alcohol > 4-methyl-2-pentanol > I ,2-dichloroethane > butyl acetate > chloroform > toluene > xylene. There was no extraction of the complex in benzene and carbon tetrachloride.

Table !-Effect of sol vents on the absorbance

Pd(II) = 30 J.l.g HCI04 = I M BHMBT = 2 mL 0.01 Min ethanol Solvent= 10 mL

Solvent Dielectric Absorbance at A111a,

constant, £ 370 nm

Methyl isobutylketone i3 .ll 0.438

Isoamyl alcohol 13.90 0.430

Chloroform 4.40 0.330

Butyl acetate 5.00 0.393 4-Methyl-2-pentanol 0.411 Xylene 2.30 0.011 Toluene 2.38 0.119

1,2-Dichloroethane 10.50 0.398

The effect of equilibration time for yellow palladium(ll) complex from 1M perchloric acid media with 10 mL of MIBK was studied. The result showed that under the optimum conditions, the extraction is not dependent upon the equilibration time and an equilibration is achieved within I Os of contact between the two phases.

Stoichiometry The composition of the complex was determined by

Job's method of continuous variation and mole-rati o method using equimolar concentration of metal ion and BHMBT solutions. The results showed that the stoichiometric ratio of the metal to BHMBT in the complex is 1:1 (Figs 2 & 3). To confirm the number of ligand molecules co-ordinated to metal ion in the extracted species, log-log plot was also drawn by plotting logD[Pd(tl ll against logC1sHM BTJ at 0.4 M perchloric acid (Fig. 4 ), where D denotes the distribution ratio of palladium(Il) between the two phases and [BHMBT] is the equilibrium concentration of BHMBT in the organic phase. A linear plot with a slope of 0.89 suggests that the metal to ligand ratio is 1: I .

Conformance to Beer's law A calibration graph for the determination of

palladium(II) was prepared under the optimum conditions. Good linearity was obtained between the absorbance of the organic phase and the concentration of palladium(Il) up to about 5 ppm at 370 nm. The optimum concentration range for effecti ve spectrophotometric determination of palladium(II ) as evaluated by Ringbom's method is l.32-5.0I ppm of palladium (Fig. 5). According to Sandell's expression. the sensitivity of the reaction is 0.0068 J.!g cm·2 and the molar absorptivity is 1.543x l04 L mole·1 cm·1

• The

449

Articles

0.5,----------------------,

0.45

0.4

MI(M+l)

Fig. 2- Job's plot for determination of composition of complex. [M] = [L] =(I) 5.0 x 10'5 M, (II) 7.0 x 10·5 M, (III) 9.39 x 10·5 M

.. u c

€ 0

.! <

0.35.------------:-----------,

0.3

0.25

0.2

0.15

0.1

0.05

0 0.2 0.4 0.6 0.6

' : I 1 - - - - -·- -

. -- - ~r-· · .. '' ' ' . '

MIL

1.2 1.4 1.6 1.8 2

Fig. 3-Mole-Ratio method for determination of composition of extracted complex [Pd(II)] = [BHMBT] = 7.0 x 10·5 M

450

Indian J. Chern. Technol.. September 2003

1.4 .---------------------,

1.2 Slope = 0.69 at 0.4 M HC104

0.8

0.6

f 0.4 c

~ 0.2

0

--4 -3.5 -3 -2.5

-0.2 log~

-0.4

-0 .6~------------------_j

Fig. 4-Log D1Pd(II)J-IogC18HMBn for determination of composition of extracted species in MffiK from 0.4 M perchloric acid medium.

log Pd(ll) ppm

Fig. 5-Ringbom's plot for Pd(II)-BHMBT system.

Gaikwad & An use: Extractive spectrophotometric determination of palladium(II) in catalysts Articles

Table 2-Effect of diverse ions on the determination of palladium(II)

Pd(Il) = 30 ~g BHMBT = 2 mL 0.01 M in ethanol

Amt. tolerated Foreign ions

HCI04 = I M MlBK= IOmL

in 'mg' 20.0 Mn(IT), Fe(UI), Ni(IT), Cu(Il), Zn(Il), Hg(II ),

U(VI), Pb(II), Sn(IV), Be(IT), Ca(II), Mg(II), Sr(IT), Ba(ll), Al(lll)

Fe(IT), Cd(U}, Cr(JII) 15.0

10.0 Sb(lll), Bi(lll), ln(lll), Ga(fll), TI(III ), Th(fV), W(VI}, Ce(IV}, V(V)

Ti(IV}, Cr(VI), Sn(IT), Te(fV )

Co(II)

5.0

3.0

1.0

0.5

0.2

0.1

200

Zr(IV), La(III ), Se(IV}, Ge(IV), Re(VII)

Mo(VI ), Ru(JII}, lr(lll), Au (III), Os(VIII)

Rh(lll )

Pt(IV)*

EDT A, fluoride, succi nate, oxalate, ci trate, malonate, acetate, tartrate

100 Bromide

*Absorbance is measured after one h.

Table 3-Analysis of synthetic mixtures containing palladium(ll) and other cations

Composition of synthetic mixture, Recovery of Relative ]..lg Pd*,]..lg error,%

Pd(ll). 30; Fe(JII), I 000; Cu(ll}, 29.8 0 .66 1000

Pd(ll ), 30; Ni (ll }, I 000; Co(II), 29.6 1.33 1000

Pd(ll ), 30; Fe(IIl), I 000; Cu(ll), 29.9 0 .33 1000: Ni( ll ), 1000; Co(ll}, 1000 Pd(ll ).30: Os(VIII}, I 00 29.5 1.66

Pd(ll), 30; Ru(ni), 100 29.7 1.0

Pd(ll). 30: Rh(III}, I 00 29.6 1.33

Pd(ll ). 30; lr(ni) I 00 29.6 1.33

Pd(ll) . 30; Pt(IV)" I 00 29 .9 0.33

*A,·erage for five determinations "Absorbance is measured after one h

standard deviation calculated from five determinations in a solution containing 3 ppm of palladium(II) is 0.088 and relative standard deviati on is 0.0014%.

Effect of diverse ions The extent of interference by diverse ions

associated with palladium in its alloys and catalysts was determined by measuring the absorbance of a solution containing 3 ppm of palladium(ll ), and varying amount of diverse ions. The tolerance limi t of an ion was fixed, as the maximum amount causing an error not greater than ±2% in the absorbance of the extract solution. The results are summari zed in Table 2. It was found that, many cations and anions have high tolerance limit however, the other PGM s have low tolerance limi t. The interference of platinum(IV) was eliminated by measuring the absorbance of complex after one h. The anions such as, thiocyanate, thiourea, thiosulphate interfered in the determination procedure of palladium(ll) because. they behave as soft bases. Iodide must be absent.

Applications

Determination of palladium in syntheic mixtures The present method was applied to the

determination of palladium(ll ) (3 ppm) in various synthetic mixtures containing PGMs and base metals which are commonly associated wi th palladi um (Table 3).

Determination of palladium in synthetic mixture corresponding to alloys

The method was applied to the determ ination of palladium in various alloys (Table 4). However. as analysed samples of these al loys were not available. synthetic mixtures corresponding to the compositi on

Alloy

Table 4--Analysis of sy nthetic mixture corresponding to alloys

Composition,% Amount of Pd(ll) Relati ve

Low melting dental alloy

Stibio palladinite mineral

Oakay alloy"

Jewelry alloy ( Pd-Au alloy)

Solder alloy

Pd-Cu alloy

Golden colored silver alloy resistant to tarn ishing

*Average for five determinations. "Absorbance is measured after one h

Pd, 34: Au , I 0; Co. 22; Ni , 34

Pd, 75;Sb, 25

Pd, 18.2: Pt, 18.2 Ni, 54.2; V, 9.1

Pd, 50: Au. 50

Pd. 30: Pt. I 0: Au, 60

Pd, 60; Cu, 40

Pd, 25.5: ln. 21: Cu, 18; Ag, 35

Taken. ]..lg

30

30

30

30

30

30

30

Found *, )..lg

29 .5

29.9

29.9

29.8

29 .5

29.8

29.5

e rror. (>lc

1.66

0 .33

0.33

0.66

1.66

0.66

1.66

451

Articles Indian J. Chern. Techno!., September 2003

Table 5-Analysis of catalysts for palladium content

Catalyst sample

Hydrogenation catalyst (Pd on BaS04 10%) (Merck)

Lindlar hydrogenation catalyst (Pd on CaC03 5%) (Merck)

Amount of Pd(II) Taken, 11g Found*,!lg

30 29.6

30 29.8

Relative error. %

Lindlar hydrogenation catalyst (Pd on CaC03 10%) (Merck)

Hydrogenation catalyst (Pd on asbestos 5%) (S.D. Fine Chern. Ltd.)

Hydrogenation catalyst (Pd on asbestos 10%) (S.D. Fine Chern. Ltd.)

30

30

30

29.9

29.5

29.8

1.33

0.66

0.33

1.66

0.66

*Average for five determinations

of these alloys were prepared and palladium content was determined following the recommended procedure.

Analysis of real samples In order to confirm the usefulness of the proposed

method, it was applied to the determination of palladium in the palladium catalysts. The results listed in Table 5 showed that, the amount obtained by the proposed method is in good agreement with certified values.

Dissolution of the catalyst sampl/ 7·27

A sample (0.1 g) of palladium catalyst was dissolved in aqua-regia. The solution was evaporated to near dryness with the addition of three 5 mL portions of concentrated hydrochloric acid to remove the oxides of nitrogen and then extracted with I 0 mL (1M) hydrochloric acid. The solution was filtered, if necessary, and di luted to 100 mL with double distilled water. An appropriate aliquot of the solution was taken for the analysis of palladium content.

Conclusions The newly synthesized heterocyclic thione,

BHMBT, which acts as a monodentate ligand, is highly sensitive and reasonably selective reagent for the determination of palladium(II) . The sensitivity of the determination is 0.0068 J..l.g cm·2. On the basis of the extractability of the palladium(II)-BHMBT complex into MIBK, a simple, rapid and reproducible method for the spectrophotometric determination of trace amount of palladium has been devised and applied successfully to the analysis of palladium catalyst samples.

452

Acknowledgement One of the authors (S .H.G.) is grateful to the

University Grants Commission, New Delhi, for providing financial assistance. The authors express their gratitude to Prof. M.B. Chavan for his valuable discussion and kind encouragement.

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