Determination of palladium with thiocyanate and rhodamine b by a solvent-extraction method

Takmto, Vol. 33. No. 5, pp. 41 l-414. 1986 Printed III Great Britain. All rights reserved

~39-914~~86

%3.00-t 0.00 Ltd

Copyright ,!: 1986Pergamon Press

DETERMINATION OF PALLADIUM WITH THIOCYANATE AND RHODAMINE B BY A SOLVENT-EXTRACTION METHOD I. LOPEZ GARCIA*,

J. MARTINEZ AVILES and M. HERNANDEZ CORDOBA

Department of Analytical Chemistry, Faculty of Chemistry, University of Murcia. Murcia, Spain (Receiwd

19 February 1985. Revised 3 December

1985. Accepted 9 December

1985)

Summ~y-Sensitive s~ctrophotometric and s~ctrofluorimct~c procedures for the dete~i~ation of palladium have been developed, based on solvent-extraction of the ion-pair formed between Rhodamine B and the anionic complex of Pd(II) with thiocyanate. With an organic to aqueous phase-volume ratio of I : 5, the molar absorptivity is 9.0 x IO4I .moie-‘.cm-’ and the absorbance of the reagent blank is 0.026. S~ctrophotometricaiiy, palladium can be determined in the range 0. i-8.8 pg. ~pectrofluo~metricaiiy, it can be determined over the range 0.04-1.S pg. The spectrophotometric procedure has been applied to the determination of palladium in dental alloys, organopaiiadium compounds and hydrogenation catalysts.

Most of the s~ctrophotometri~ methods for palladium determination are based on the formation of chelates with organic reagents.‘” Conversely, few methods based on the formation of ion-pairs with basic dyes have been developed, e.g., the ion-

caily.‘J Working solutions (1 #g/ml) were prepared from the standard solution on the day of use. Procedures

association compounds of palladium with azide and Methylene Blue,’ iodide and pyronine G,‘” and with thiocyanate and Crystal Violet.” Some ionassociation compounds precipitate at the phase boundary or on the walls of the separatory funnel when the aqueous solution is shaken with an organic solvent (flotation methods), and Rhodamine 6G and Methylene Blue have been used for palladium determination in this way.j2.” The purpose of the present work was to find a suitable colour system for the determination of palladium after solvent extraction. We chose Rhodamine B as counter-ion for the anionic thiocyanate-palladium complex. The proposed method is sensitive, has a low reagent blank, and is easy to use. EXPERIMENTAL Apparatus

A Pye Unicam SPS-200 spectrophotometer was used for the absorbance measurements. Fiuorescence intensity mea3000 surements were made with a Perkin-Elmer spectrofiuorimeter; 5-nm band-widths were used in both the excitation and emission systems.

Spectrophofume~rjc method. To a volume of sample soiution, in a separatory funnel, containing between 0. I and 8.8 pg of paiiadium(II), add 5 ml of 4SM suiphuric acid, 1 ml of 0.025M potassium thiocyanate solution, 2.5 ml of Rhodamine B solution and dilute to 25 ml with water. Extract the mixture with 5 ml of butyl acetate (exactly measured) by shaking the funnel vigorously for I min, allow the phases to separate and discard the aqueous phase. Wash the organic layer with two IO-mi portions of 0.9M suiphuric acid, transfer it into a centrifuge tube and centrifuge it. Measure the absorbance of the organic layer at 55.5 nm against a reagent blank similarly prepared. Prepare a calibration graph, using different volumes of standard palladium(I1) solution. SpectroJlcorimerric method. In a separatory funnel place the sample solution (0.04-1.5 pg of palladium), add 5 ml of 4.5M suiphuric acid, I ml of O.OiSM potassium thiocyanate solution and 1.5 ml of Rhodamine B solution, dilute to 25 ml with water and extract the mixture with 8 ml of butyi acetate (accurately measured). Discard the aqueous phase and wash the organic layer as in the spectrophotometric method. Centrifuge the organic layer, take exactly 5 ml of it and add exactly 2 ml of ethanol. Measure the Auorescence at 573 nm with excitation at 550 nm. Prepare a calibration graph by the procedure described. Run a reagent blank in the same way and subtract its fluorescence from that of the sample.

Reagents

Ail inorganic chemicals used were of analytical reagent grade and doubly distilled water was used throughout. Rhodamine B solution (IO-‘M) was prepared from the commercial product (Merck) without further purification. Standard palladium solution (5 x iO-ZM) was prepared from paiiadium(l1) chloride and standardized gravimetri-

*Author to whom correspondence

should be addressed. 411

Procedure ,for dental alloys

Dissolve IO mg of sample in IO ml of aqua regia and evaporate to near dryness. Add about IO ml of water and heat again. Cooi, transfer to a separatory funnel and dilute to about 50 ml. Add IO ml of 1% sodium dimethyigiyoximate solution and mix. After 15 min, extract with two 5-ml portions of chloroform (shaking time 30 set). Evaporate the combined extracts to dryness on a water-bath Mineralize the residue by evaporation with a mixture of 3 ml of concentrated hydrochloric acid and 2 ml of concentrated nitric acid. Dissolve the residue in IM hydr~hioeic acid and make up to volume with water in a SOO-mlstandard flask. Take an aiiquot and determine palladium according to the methods given above.

412

I.LOPEZ GARCIA et al.

1 .o

-

0.8 -

$j 0.6x

fluorescence intensity of the palladium complex. Figure 3 shows the effect of dilution with ethanol. Although the dilution process decreases the final concentration of palladium, the fluorescence intensity is maximal in a 65:35 v/v mixture of butyl acetate and ethanol. Therefore, a dilution of 5 ml of extract with 2 ml of ethanol was used (equivalent to _ 70 : 30 v/v ratio).

Sulphuric

acid concentrurion.

With

fixed concen-

P 9

trations of the other reagents, the absorbance and fluorescence for palladium samples measured against 0.4 the reagent blank were maximal and constant for the acidity range 0.1-1&V. The value of the reagent blank decreases gradually with increase in sulphuric acid concentration, but almost constant in the acid range stated. A sul~huric acid concentration of about 0.9M was therefore selected as optimal. Rhodamine B co~t~e~ttruti~~~. As can be seen in Fig. 4 the absorbance for palladium (measured 0 against the reagent blank) is maximal and constant 480 530 580 630 with a dyestuff concentration 26 x 10-5M. A 10m4M Wavelength (nm) Rhodamine B concentration was chosen for the Fig. I. Absorption spectra. (A) reagent blank (reference, spectrophotometric method. When the spectroextracting solvent); (B) and (C) with 5.87 and 2.68 pg of fluorimetric procedure is used, a 4 x 10VSM Rhopalladium, respectively (reference extracting solvent); aqueous phase, 25 ml, organic phase, 5 ml. damine B concentration is recommended because the lower the amount of Rhodam~ne B used, the lower the reagent blank. Thiocyanate concentration. Figure 5 shows the concentration on effect of thiocyanate the Dissolve the sample as above, filter if necessary and dilute fluorescence. Similar results were found for the specto known volume. Take an aliquot containing 0.04-8.8 ~.cg trophotometric technique. Consequently, 6 x 1O‘-J of palladium and apply one of the recommended proand lO-~‘M concentrations were chosen for the cedures. fluorimetric and spectrophotometric procedures, respectively. RESULTS AND DISCUSSION In a preliminary study we found that palIadjum forms an ion-association complex with Rhodamine B

in the presence of an excess of thiocyanate. This compound is extractable into organic solvents. The use of very polar solvents leads to high values for the reagent blank whereas if the polarity is too low the colourless form of the dye predominates in the organic layer. The best results are obtained with butyl acetate as solvent. The complex extracted is very stabte and washing with two lo-ml portions of 0.9.M sulphuric acid gives a reagent blank absorbance of about 0.026 at 555 nm. Figure 1 shows the absorption spectra of the complex in butyl acetate and of the reagent blank obtained as described in the procedure. The absorption spectrum of the extracted ion-association compound is almost identical in shape to the spectrum of an aqueous solution of Rhodamine B, with a slight red-shift of the absorption maximum. The excitation and emission spectra are shown in Fig. 2. As can be seen, maximum fluorescence intensity is obtained at 573 nm, with excitation at 5.50 nm. Addition of ethanol to the organic phase greatly increases the

Wavelength (nm) Fig, 2. Excitation and emission f’) spectra of reagent blank (A) and the ion-association compound (B) in 5:2 v/v butyl acetatc-ethanoi mixture; aqueous phase, 1.17 pg of palladium.

Determination

of palladium

0

20

40

I

I

I

I

73

58.4

43.8

29.2

413

60

8o 14’6

Ethanol

(%l

PdUIl(~g/I.I

Fig. 3. Effect of dilution with ethanol on fluorescence. (A) reagent blank: (B) and (C) with palladium: (B) reference solvent mixture. (C) reference reagent blank.

Characteristics of‘ the compound ertracted

The extraction efficiency under the selected conditions was calculated from the results for two successive extractions of the same sample. A value of about 99% was obtained. The palladium: Rhodamine B and palladium: thiocyanate molar ratios were calculated by the continuous-variations method, 1: 2 and I :4 ratios respectively being found. Therefore, the extracted compound is [Rhodamine B], [Pd(SCN):-1. The rate of extraction is high. Shaking times ranging from 40 set to 3 min did not produce any change in absorbance. so a I-min shaking time was selected. Both the absorbance and fluorescence values of the butyl acetate extract remain constant for at li?dSt 8 hr. Calibration graph

Under the recommended conditions, the spectrophotometric calibration graph is linear over the

range 0.1-8.8 pg of palladium. The molar absorptivity at 555 nm calculated from the slope of the graph is 9.0 x IO4 l.mole-‘.cm-‘. The precision is shown in Table 1. The standard deviation for the absorbance of the reagent blank was 0.0011. The spectrofiuorimetric calibration graph was iinear over the range 0.04-1.5 pg of palladium. The relative standard deviation of the fluorescence intensity for 10 determinations of 0.5 pg of palladium was 1.3%.

The effect of foreign ions on the spectrophotometric determination of 3 ,ug of palladium is presented in Table 2. An error of &2% in the absorbance value was considered tolerable. The most serious interferents were Pt(IV), AgfI), Au(W), Rh(III) and Hg(I1). The interference of silver can be avoided by adding bromide and filtering. If Pt(IV), Au(III), Rh(II1) or Hg(II) is present a preliminary

0.8 r

800

__2*-*-•.

0.6

/s

_

_

_

_

-

4

0

i-

““‘-7 -*-•-

A

-*8

Rhodamine

12

B concentratjon log [SCN-]

(1 d5Ml Fig. 4. Effect ofconcentration of Rhodamine B. (A) reagent blank (reference extracting solvent); (B) and (C) with 4.5 fig of pailadium, (B) reference extracting solvent and (C) . . . reterenCe

reagent

blank.

Fig. 5. Effect of the concentration of thiocyanate. (A) reagent blank (reference solvent mixture): (B) and (C) with 0.7 pug of palladiums (B) reference solvent mixture and (C) reference

reagent

blank

414

f. LOPEZ GARCIA et al.

Table 1. Precision of the s~ctr~photometrj~ determination of palladium with thiocvanate and Rhodamine B (10 replicates) Palladium added, 11~

Palladium found, fig

Std. devn., !JR

Confidence limits (probability level 0.95), fig

I.25 3.75 6.25

1.25 3.74 6.26

0.012 0.029 0.034

1.25 & 0.09 3.74 * 0.02 6.26 + 0.02

Table 2. Effect of foreign ions on the spectrophotometric of 3 tia of nalladium

determi~tion

Limiting molar ratio [Ionl/tPd(II)~

Ion

1oooo* 5000

F-, Cl, phosphate, Co(H) Zn(II), Ru(II1) Br-, nitrate, Cu(II), Ni(II), Al(IfI), Mn(II), Fe(III)$ Cr(VI), Bi(III), FetIII) Perchlorate, V(V), Ag(I)t Mo(VI), W(VI) lr(III) I-, Os(VII1) Pt(IV), Ag(I), Au(III), Hg(II)+ Rh(II1)

2000 500 100 50 20 tl

*Maximum molar ratio tested. $In the presence of O.OIM fluoride. ?With pretreatment. Table 3. Palladium dete~ination Sample Dental alloy I Dental alloy 2 Catalyst (Al&$) Catalyst (BaSQ) Organopalladium compound I Or~anopalladium compound 2 *Average of three determinations. iNominal content. §Palladium found s~ctrophotometri~ally dimethyl~lyoxime and chloroform.

separation of palladium from the matrix is necessary. Extraction of palladium dimethyl~iyoximate with chloroform can be used for this purpose. Applications

The method was applied to the determination of ~dlladium in dental alloys, catalysts and organopalladium compounds. Sampies were analysed according to the recommended spectrophotometric procedure. Table 3 gives the results compared with those obtained by another recommended solvent~xtraction method.”

in samples

Pd content, %

Pd found, %*

30t 22§ 5t 5t 1.5.7§ 1g.e

29.8 22.1 5.0 5.1 15.6 18.8

by solvent extraction with

REFERENCES

Botisova, M. Koeva and E. Topalova, Talantu, 1975, 22, 791. 2. T. Kotsuyanagi, H. Hoshino and K. Aomura, Anal. 1. R.

Chim. Acta, 1974, 71, 349.

3. S. Banerjee and R. K. Dutta, Zh. AnaNf. Khim., i9’75, 277, 379.

4. B. Morelli, Analyst, 1984, 109, 47. 5. E. L. Kothny, Mikrochim. Acta, 1978 I, 425. 6. F. Corigliano, S. Di Pascuale and A. Rainieri, Anulyst, 1977, 102, 25.

7. A. E. Mahgoub, N. A. Darwish and M. M. Shoukry, ibid., 1978, 103, 879.

8. P, W. Beaupre and W. J. Holland, M~krach~m. Acta, 1978 II, 327.

Conclusion

The methods proposed are simple, rapid and sensitive. Both compare very favourably with most published methods for the determination of palladium by use of ion-association compounds, and they can certainly be classed amongst the most sensitive. A~k~ouliedgement-~~~ authors thank Dr. G. Lopez (Inorganic Chemistry Department. University of Murcia) for the kind gift of organopalladium samples.

9. R. Kuroda, N. Yoshikuni and Y. Kamimura. Anal. Chim. Acta, 1972, 60, 71.

IO. S. Jaya, T. Rao and T. V. Ramakrishna,

J. tiss~ornrn~~ Met., 1983, 91, 261. 11. Z. M. Khvatkova and U. U. Golovina, Zh. Analit. Khim., 1979, 34, 2035.

12. Z. Marczenko and M. Jarosz, Talanta, 1981, 28, 561. 13. idem, Analyst, 1981, 106,751. 14. I. M. Kolthoff and P. J. Elving (eds.) Treatise on Analytical Chemistry, Part II, Vol. 8, p. 453. Interscience, New York, 1963. IS. R. S. Young, Analyst, 1951, 76, 49.