KLAUS R KOCH+ and DEREK AUER. Department of Chemistry, University of ... around a fixed 15 mm diameter plastic rod. A. 2 mm path length U-shaped quartz ...
Talanta.Vol.40,No. I2,pp. 1975-1980, 1993 Printed in GreatBritain.All rightsreserved
0039-9140/93sfxlO+0.00 Copyright0 1993F’ergamon PressLtd
DETERMINATION OF PLATINUM AND PALLADIUM IN STRONGLY ACID SOLUTION BY MEANS OF FLOW INJECTION ANALYSIS KLAUS R KOCH+ and DEREKAUER Department of Chemistry, University of Cape Town, Private Bag Rondebosch, Cape Town, 7700, South Africa (Received 12 April 1993. Accepted 12 June 1993) Rntnmar-Microamounts of Pt(II/IV) (0.25400 &ml) and Pd(I1) (5-600 @ml) in z-O.5 M hydrochloric acid can readily be determined by means of a simple FIA method baaed on the selective reaction of tin(II)chlo with these metals. The FIA method has a high linear dynamic range, and is relatively free from interferences of many transition metals, with the exception of Au and W, small amounts of other PGMs can be tolerated. Determination of Pt on a hydrogenation catalyst by this method compares well with that found by flame atomic absorption spectroscopy. By monitoring at two or more wavelengths, Pt and Pd can be determined in mixtures by this means, to yield a simple, cost-effective FIA method for possible on-line determinations and quality control of, in particular, Pt containing acidic refinery and other process streams.
The platinum group metals (PGMs), widely used in many important chemical processes as efficient catalysts, are of importance in the electronics industry, while the significance of, particularly, Pt is growing in cancer chemotherapy.’ The control of automobile exhaust-gas emissions using Pt and Rh rich catalytic converters is currently utilizing the largest share of the world production of these metals,2 while use in this application is set to grow substantially in the next decade as ever more stringent exhaust emission standards are being introduced by legislation in the developed world.3 Against this background, the need for the rapid and accurate determination of the PGMs in process and quality control is obvious. A recent survey of methods for the determination of these metals attests their importance, and shows that overall the predominant analytical methods in use are based on atomic absorption and/or emission spectrosc~py.~ While the power of such instrumental methods is indesputable, these methods are usually laboratory based, rather expensive and not readily suited for automation and online analysis. The development of the family of flow-injection based techniques has recently been reviewed,’ and it is clear that such methods offer many advantages in the performance of *Author for correspondence.
process control and on-line analysis. Surprisingly however, a survey of the literature reveals only very few flow-injection analysis (FIA) based determinations of the PGMs, although recent reports of the determination of Pd” and Ptg by FIA method8 suggest a growing interest in this area. In this paper we report the first part of the development of a simple, cost effective method for the determination of the potentially simultaneous photometric determination of Pt(II/IV) and Pd(I1) in hydrochloric acid solution by means of FIA. Our method is based on the long known,” but until recently poorly understood”v’2 reaction of tin(II)hal with Pt(II/IV) and Pd(I1) in aqueous acid solution. This reaction formed the basis of a highly selective batchwise spectrophotometric determination of Pt(II/IV) some four decades ago.‘2-‘s In the batchmode however, the latter method suffers from substantial disadvantages, principally the need for accurate timing, careful control of reaction conditions and working under anaerobic atmospheres, so that often erratic results are obtained. We have found that this chemistry is ideally suited for the flow-injection determination of Pt and Pd, making it possible to exploit the high selectivity of the reaction of tin(II)chlo with these metals in hydrochloric acid solution.
1975
EXPERIMENTAL
A 1000 &ml BDH atomic absorption standard solution se;rved as independent calibrant and Apparatus source of Pt(IV) solution. Tin(II)chlo solA single line FIA manifold using 0.8 mm ID utions were prepared from Analar grade PTFE tubing, equipped with a Spetec Ferimax SnC& ~~~~U,~~~~o~~~~ the desired amount 12 four charmel pump and a abbe 5020 of salt in the required amount of cone. HCl and &port PTFE rotary injection valve with a 180 id1 allowing the initially cloudy solution to stand sample loop, was used for preliminary expcrfor 15-20 min in a warm water bath until available Omnifiit completely clear, followed by dilution to volume iments. Commercially PTFElfEEK connectors and mixing tees, and with water, These solutions were standardized reaction coils of variable Iengths were coiled by titration with ~~ssi~ iodate, after which around a fixed 15 mm diameter plastic rod. A a few small pieces of meta& tin were added to 2 mm path length U-shaped quartz flow ceil and the tin(II)chIoride solution to preserve them a Varian SuperscanI-N/visible spectrophofrom oxidation by atmospheric oxygen, It tometer operation in fixed wavelength ab- should be noted that the hydrochloric acid sorbance mode with the abscissa in a time drive con~ntration of these tin(II~~o~de solutions ~~~at~o~ served as a detector, For determishould be kept above 0.5 M to prevent hydroiynations of trace amounts of Ft (x5 ~g/rnl~ a sis of the stannous chloride, Platinum solutions modified computer controlled manifold consistwere standardized where necessary by means of ing of a carrier (1 M HCl) and reagent stream flame atomic absorption spectroscopy using a (SnC&) merged into a single line with a mixing Varian Techtron 1000 AAS with a strongly tee after the sampie injection valve, was found oxidizing air-acetylene flame at 265.9 nm, slit. to be ~v~~~u~ (Fig. 1). width 0.2 nm and 10 mA lamp current. All Pt containing solutions were made up to contain O.~(W/V)% La3+ (with La(NO,), . (iH@) as Materials culd methodr interference suppressant, All solutions were prepared with glass distilied water and concentrated AWar grade HCI, using glassware previously soaked in nitric acid. Stock solutions containing IO00 pg[rnl each of Pt(I1) and Pd(I1) were prepared from I$ PtC& The reaction between SnCl, and PtCG- pr and Kz PdCl, obtained fram Johnson Matthey, PtCli- in >0.5 M hydrochloric acid solutions the salts being used without further purification. rapidly leads to the development of an intensely cherry red colour, the origia of which is now known to result from the rapid reduction of any R(IV) species,16 followed by or concomitant with, complex formation of PtCli- with the SnCl; spcies to yield a series of anionic Cpt(SnC13),Cl,&J~- (3~= llr) complexes, as we11as the relatively stable p(SnCl,),]‘complex anion.‘f**2 The latter complex has been characterized by X-ray crystallography in the solid stateI and has been shown to predominate in aqueous HCl containing sufficient SnC&. 12*1s These solutions show characteristic ~/visibl~ a~~~~n spectra with a~o~~on maxima at *_....: : ‘---_________---7 r----2 = 310 nm (molar absorptivity -3.95 x 104)+ 400 mn (absorptivity ~7.8 x lo*), and 475 nm (shoulder) as shown in Fig. 2. Although the overall rate of the complex formation is not accurately known, in the presence of excess &Cl,, steady state is reached within a few minutes, Fig. I. Manifakh used for the cietmninatim of Ft and P& Preliminary experiments to establish opti(A) carrier O,1 M SnCl,, 1.0 M NC1 (R) carrier 1.0 M HCI, mum conditions for the determination of Pt by reagent 0.3 M SnCI,.
Determination of Pt and Pd in acid
400
5oo
nm
600
700
Fig. 2. Absorption spectra of R(W) and Pd(II) in 1.0 M HCI containing excess SnCI, at steady state.
means of FIA were undertaken with a simple single line manifold equipped with a 2 m reaction coil and monitoring at 400 nm. Using a carrier stream with fixed [SnCl,] = 0.057 A4, the hydrochloric acid concentration was varied from 0.3 to 3.9 A4 and peak height monitored for separate injections containing 20 pg/ml Pt(IV) and Pt(I1). Within experimental error there was no difference between the heights of the Pt(I1) and Pt(IV) signals, confirming rapid reduction of the latter to Pt(I1) followed by complex formation. The variable HCI concentration of the carrier stream does affect the peak height in a non-linear way, relatively higher values being obtained in the 0.3-0.8 h4 and >2.1 M HCl range, reaching a plateau in the 1J-2.1 M range, where peak heights were effectively constant. We thus arbitrarily chose to fix the HCl concentration of the carrier stream at 1.0 M throughout; variations of kO.2 M do not significantly affect the peak heights other factors being equal. The optimum SnCl, concentration was then found by varying its concentration in the carrier in the range 0.025-0.5 M. For the more concentrated solutions substantial viscosity differences between the carrier stream and sample injections result in undesirable effects, so that for this and for reasons of economy, the SnCl, concentration was kept as low as practical and set at 0.1 M SnCI, using the single line manifold. The flow rates and reaction coil lengths were also varied taking into account other variables, which led to a set of near optimum conditions: 1.0 M HCl, 0.1 M SnCl,, flow rate 1.1 ml/min,
1977
reaction coil length 2 m and sample injection volume 180 ~1. With these conditions 20 repeat injections of 20 hg/ml Pt(IV) gave highly reproducible peak heights (absorbance -0.1 AU, RSD = 0.6%) and excellent day-to-day reproducibility (RSD < l.l%), so that 30 injections per hr could comfortably be achieved with negligible carryover between successive high (100 pg/ml) and low (10 ,ug/ml) Pt(IV) injections. With the above conditions, a rapid method for the determination of Pt(II/IV) obtains which has a high dynamic range, the analytical sensitivity of the response depending only on the selected wavelength. At 1 = 460 nm completely linear calibration plots are obtained without dilution for a Pt(IV) range of 10-800 pg/ml, as shown in Fig. 3a. Such high dynamic ranges are generally not achievable in flame atomic absorption (FAAS) or atomic/plasma emission spectroscopic (AES/PES) methods without sample dilution.4 High dynamic ranges can be a substantial advantage in methods designed for on-line applications which may have to deal with highly variable Pt(II/IV) concentrations. On the otherhand, using the single line manifold and monitoring at rZ= 400 nm in the absence of interferents, Pt concentrations in the l-200 pg/ml range can readily be determined. We found that the sensitivity of the determination could be further improved using a slightly modified two-line manifold with 1.0 M HCI as carrier stream into which the sample is injected followed by merging this zone with a 0.3 M SnCl, reagent stream prior to the detector (Fig. lb). With this manifold monitoring at A = 400 nm. the sensitivity of the determination
Fig. 3. Typical calibration data using a single-line manifold monitoring at 460 nm for (a) Pt(IV) and (b) W(U). Numerals refer to concentrations in pg/ml.
1978
KLAUSR KOCH and DEREKAUER
of Pt can be improved by a factor of 10 to cover the range of 0.1-2 pg/ml with a limit of quantification of cu 0.05 pg/ml. In principle by monitoring at 1 = 310 nm, a further sensitivity improvement of between 2 and 4 times is possible, but some background effects complicate this and we are investigating ways of circumventing these. A comparative study of the determination of Pt by the proposed FIA method and FAAS using a sample of hydrogenation catalyst of platinized asbestos yielded satisfactory agreement. Dissolution of the Pt using aqua regia followed by volatilization of excess HN03 with cont. HCl and suitable dilution yielded 12.0% (w/w) Pt as found by FIA compared to 12.2% (w/w) Pt obtained by FAAS for triplicate determinations. Determination of palladium and platimum simultaneously The reaction of PdCc- with SnCl, is much more rapid and apparently more complicated than that with PtCl$-, resulting in a series of rapid colour changes, from yelloworange to red (A = 355,420 nm) through blue and finally to green (A = 635 nm). The origin of these changes is not well understood, although it has been established that several complex species are involved the nature of which depends on inter alia the Pd: Sn mole ratio and the HCl concentration.lg*M Under conditions used in this work the PdCl:- reacts with the SnCl, to rapidly yield yellow-red species, which convert into an olive-green form, which is stable for cu 1 hr. Figure 2 shows the absorption spectrum of the Pd-Sn species at steady state. The latter form shows absorption maxima at A = 635, 460 and 380 nm which approximately follow the Beer-Lambert law over a reasonable range of Pd concentrations (10-200 pg/ml). Since above 600 nm the absorption of any Pt-Sn species is negligible, it is possible to determine Pd(I1) by monitoring at 635 nm with no interference from Pt(II/IV). On the other hand, since the Pd-Sn species also absorbs light in the 300-500 nm range, the presence of Pd(I1) may constitute a serious interference in the determination of Pt by the proposed method and vice versa. However, in view of the absorbance of the Pd-Sn species at 635 nm, this apparent disadvantage may be used to determine Pt and Pd in the same solution by sequential injection of two identical samples and monitoring at the two wavelengths separately, so that from the peak height at 635
Table I. Recovery experiments for sequential determination of Pt and Pd in synthetic mixtures monitoring at 460 and 635, respectively using a single-line manifold (see text) Taken (pg/ml) Pt
80.0 Z:8 20.0 10.0
Pd 20.0 40.0 60.0 80.0 100.0
Found @g/ml) Pt Pd 79.8 59.9 39.4 19.5 9.9
20.1 40.0 60.8 81.4 102.6
nm the Pd concentration may be estimated, which then allows the Pt concentration to be obtained from the peak height at 460 nm. Alternatively the use of a diode-array photometric detector will allow for rapid multiwavelength monitoring*’ over a range of wavelengths in the absorption spectrum, from which both Pt and Pd can in principle be determined simultaneously. We did not have access to such a detector at the time of undertaking this work and thus carried out simple sequential injections and manual monitoring at 460 and 635 nm in order to determine both Pt and Pd in HCl solutions using the single-line manifold. As may be seen from Fig. 3b, it is readily possible to determine only Pd(I1) in the absence of Pt(II/IV) in an exactly analogous way to Pt(II/IV) using similar conditions, monitoring in the 400-500 nm range. (In this wavelength range the sensitivity for Pd is considerably higher than when monitoring at 635 nm.) Furthermore we found that the absorbance (peak height) of solutions containing mixtures of Pt-Sn and Pd-Sn species are additive at selected wavelengths within the concentration ranges studied in this paper, so that for solutions known to contain both metals, two sequential injections of the same sample, monitoring at 460 and 635 nm allows the determination of both Pt and Pd, as shown in Table 1. These data were obtained by determining the [pd(II)] at 635 nm from one injection, followed by a second injection of the same sample, monitoring at 460 nm which allows the calculation of the pt(II/IV)] from the total peak height by subtraction of the Pd-Sn contribution at this wavelength. A series of calibration curves for pure Pt and Pd standard solutions at the selected wavelengths can be setup to assist in this task. We monitor at 460 nm in mixed metal solutions since the difference in molar absorptivity between the Pt-Sn and Pd-Sn species is a maximum at this wavelength,
1979
Determination of Pt and Pd in acid
resulting in the most accurate calculation of the respective Pt and Pd concentrations, by this method. The feasibility of potentially simultaneous determinations of Pt(II/IV) and Pd(I1) in hydrochloric acid solution has thus been demonstrated monitoring at two wavelengths, so that the use of a diode-array detector should facilitate the accurate and rapid determination of these two metals over a relatively large concentration range. Interferences
Table 2. Interference tolerance at 20 &ml Pt and Pd level monitoring at the given wavelengths using the singlaline manifold
WV)
Interferent
4OOnm
46Onm
M92’ CrNIl
200 100
-
,,&2+’
400
-
Fe’+
960
Ni2+ cl,?+ Al’+
coz+
Pd(II) 4OOnm 635nm 400 200
-
200 100 200 400
2 2
= 1
200 ;g
200 (a)
400 200 10 -2 -2 10
10 -2 10 50
200 200
z 400 (a) 200 10 120
l
+ Rh3+ An extensive evaluation of potential interferRub/3+ -2 ences is essential if the proposed method is to be IPA+ (a) of practical value. Consideration of the chem- *Not tested but likely to interfere at low levels. istry of the reaction of tin(II)chlo with the PGMs involved, allows one to roughly group potential interferences into three categories: (i) oxidative interferences, leading to competitive On the other hand, the interferences from consumption of Sn(I1); (ii) absorptive interferAu(III), Rh(II1) and other PGMs can be ences, having high molar absorptivities in the significant in the determination of Pt and Pd. wavelength region of interest; and (iii) non-seNevertheless, with the exception of Au, it is in lective reactions of SnCI, with substances other principle possible to determine other members than the PGMs. In general the established high of the PGMs in the same way as Pt and Pd selectivity of the reaction of SnCI, with the using a multiwavelength detection approach PGMs’“‘~ makes interferences of (iii) unlikely alluded to above. Preliminary studies suggest but implies that interferences from other PGMs that it is indeed possible to determine Pt in and Au(I/III) may be expected in the determithe presence of substantial amounts of Ru and nation of Pt and Pd. Since the carrier stream is Ir, while we are examining ways to determine 1 A4 in HCl, substantial interferences of type (i) Rh simultaneously with Pt and Pd by this and (ii) from first row transition metals are means. unlikely, with the possible exception of Fe(III) In conclusion we have found that the deand Cr(V1) (as Cr2 e-) in relatively high con- scribed FIA method monitoring at a single centration. Anions such as SO:-, NO;, CIO; wavelength is suitable for the rapid and costand PO:- do not interfere at relatively high effective determination of Pt(II/IV) and/or levels (~0.1 M), although Br- and I- ions Pd(II) in relatively pure hydrochloric acid should be kept to a minimum. medium as may be encountered in process We find that the determination of Pt and Pd streams, using only small sample volumes. The by the proposed method tolerates fairly large presence of many transition metals in reasonamounts of many transition metals remarkably able amounts does not interfere, although well in keeping with our expectations above, as Rh(III), Ru(IV/III), Au(U), and to a lesser is evident from data in Table 2. The table shows extent Ir(IV) interferes with the determination the maximum amount of interferent tolerated, at low levels. Nevertheless with the aid of a resulting in a peak height of 20 pg/ml Pt(IV) (at photodiode-array detector it appears to be poss400 nm) and Pd(I1) (at 400 and 635 nm) to not ible to circumvent these interferences as has differ from that obtained from pure standard been shown in the determination of Pt and Pd solutions, by more than 1%. It is clear that in this work. We are currently studying the Pt(II/IV) and Pd(I1) can be determined in the potential multielement determination of the presence of relatively large amounts of the first PGMs using multiwavelength detection. row transition metals, particularly Fe(II1) and Acknowledgements-We are grateful to the University of Cr(VI), monitoring at 400 nm. In reasonably pure Pt and Pd containing concentrates in Cape Town, the FRD and MINTEK for generous financial assistance and a MINTEK studentshin to D.A. The loan of HCl, the interference of many first row tranPGM salts from Johnson Matthey is also acknowledged with thanks. sition metals thus appears to be insignificant.
KLAUSR KOCH and DEREKAuna
1980 REF’ERENCES
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