Full Paper

Voltammetric Determination of Platinum in Plant MaterialJoanna Kowalska,*a Sylwester Huszal,a Marek G. Sawicki,a Monika Asztemborska,a Ewa Stryjewska,a Elzbieta Szalacha,b

Jerzy Golimowski,a Stanislaw W.Gawronskib

a Warsaw University, Chemistry Department, 02-093 Warsaw, Pasteura St. 1, Polandb Warsaw Agricultural University, Department of Pomology and Basic Research in Horticulture, Faculty of Horticulture and

Landscape Architecture, 02-787, Warsaw, Nowoursynowska St. 166, Poland*e-mail: askow@chem.uw.edu.pl

Received: May 13, 2003Final version: July 10, 2003

AbstractA very sensitive procedure for the determination of platinum in hydroponically cultivated plant samples (Sinapis albaL and Lolium perenne) is presented. A microwave digestion with HNO3, HClO4 and HCl acids with additional UVirradiation is followed by the platinum determination by adsorptive stripping voltammetry at hanging mercury dropelectrode. The influence of HNO3 acid on voltammetric determination of platinum is discussed. For validation of theobtained results the recovery of Pt was examined and the Pt determination by ICP MS was carried out. The highestplatinum amounts were found in the roots of the plants, nevertheless the effective transport of platinum to the steamsand leaves was also observed.

Keywords: Platinum, Adsorptive voltammetry, Microwave digestion, Plants

1. Introduction

The interest in the determination of extremely low amountsof platinum in environmental samples has increased rapidlymainly as a consequence of the introduction of catalysts inmotor vehicles. The catalysts contain a catalytic activecoating, mainly consisting of metallic platinum on differentcarrier materials. Platinum as the major noble metal in thecatalysts (typically containing 0.08% Pt, 0.04% Pd and0.007% Rh) is of the greatest concern. Emission of catalyticmaterial caused by abrasion cannot be avoided, so theparticles containing nanocrystalline platinum attached toalumina are present in the environment. According to datafrom the UK, platinum concentrations were in the range0.3 ± 8 ng g�1for roadside samples and 0.42 ± 29.8 ng g�1 fordust [1]. It is estimated that between 0.5 ± 0.8 �g of platinumcan be emitted by one car per 1 km of the road [2], most ofthat is found in a distance of 1 to 3 m from aside the road.Increase of platinum deposition on the flora surroundingstreets and highways and high concentration of this elementin soils brought out some concern about its accumulation atpossible toxic levels, especially platinum(IV) oxide [3]. Dueto that the platinum uptake by plants is the subject of someinvestigations [4 ± 8]. Uptake of platinum group elements isdocumented for grass and cucumbers. Therefore, it can beassumed that, analogously to heavy metals, there are plantspecies that possess ability to accumulate noble metals. Suchplants might be considered as bioindicators or as speciesused for phytoremediation technology.

Environmental studies require the applying of reliableand efficient analytical methods for the platinum determi-nation at ng and sub-ng levels.

For this purpose electrothermal atomic absorption spec-trometry (ET AAS) [9 ± 12], mass spectrometry withinductively coupled plasma (ICP MS) [13 ± 16], neutronactivation analysis (NAA) [17 ± 19] and voltammetry [20 ±26] are more often used. Most of the methods, exceptvoltammetry, requires preconcentration of platinum. Byapplying adsorptive stripping voltammetry this step couldbe omitted. The procedure is based on the potentialsupported accumulation of a platinum complex at thesurface of a hanging mercury drop electrode. The electro-chemically active complex lowers the hydrogen overpoten-tial at the mercury electrode and thus produces a verysensitive catalytic current, which is measured in the differ-ential pulse mode.

The objective of this article was to apply an establishedmethod [20 ± 26], adsorptive stripping voltammetry, forplatinum determination in hydroponicaly cultivated plants:mustard (Sinapis alba L) and grass (Lolium perenne).During voltammetric determination formaldehyde hydra-zone (formazone) as a product of formaldehyde condensa-tion with hydrazine is a complexing agent of platinum.

In this presented work attention was focused on thedecomposition procedure, suitable for electrochemicaldeterminations. The influence of nitric acid, mentioned byother authors as an interference [23], on voltammetricdetermination of platinum was also examined. For valida-tion ICP MS was used as a reference method and recovery ofplatinum was studied.

Moreover, the ability of mustard and grass for platinumuptake and translocation to the above ground organs, inorder to characterize these species for platinum biomoni-toring and phytoextraction purposes, is discussed.

1266

Electroanalysis 2004, 16, No. 15 ¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/elan.200302907

2. Experimental

2.1. Reagents

HNO3 (d� 1.40 g mL�1), HCl (d� 1.15 g mL�1), H2SO4

(d� 1.84 g mL�1), HClO4 (d� 1.67 g mL�1), Suprapur(Merck).

Standard solution of Pt(II) containing 1 mg mL�1 wasprepared from ampules AAS standard (Merck); formalde-hyde, hydrazine sulfate (POCh Gliwice, Poland).

All solutions were prepared using deionized water fromMilli-Q-Water-System, Millipore, (USA).

2.2. Apparatus

Polarographic analyzer �AUTOLAB (The Netherlands)and an electrode system: HMDE as a working electrode,saturated Ag/AgCl as a reference electrode and glassy-carbon as an auxiliary electrode. Determinations werecarried out in a quartz voltammetric vessel.

Inductively Coupled Plasma Mass Spectrometer SCIEX™Elan 6100 DRC∫ Perkin Elmer (USA).

Domestic microwave oven Sharp, R-5A51 (850 W) (Ja-pan).

Hg-lamp 125 W (Poland).Low-pressure vessels type P/N 323000, CEM (USA)

(max. pressure 13.8 bar).Laboratory mill, Fritsch (Germany).

2.3. Procedure

2.3.1. Plant Cultivation

Plant cultivation was carried out in growth chamber attemperature 20 �C during days and 16 �C during nights and14-hour photoperiod. Seeds of mustard and grass were sowninto sand and seedlings at cotyledon stage were placed inglass jars containing 1 L of nutrient solution:

Ca(NO3)2: 1003 mg, KNO3: 583 mg, MgSO4: 513 mgKH2PO4: 263 mg, NH4NO3: 488 mg, MnSO4: 6.1 mg, H3BO3:1.7 mg, Na2MnO4 ¥ 2H2O: 0.37 mg, FeNaEDTA: 79.0 mg,CuCl2 ¥ 2H2O: 0.39 mg, ZnSO4: 0.44 mg.

Solutions were aerated every second hour for one hour.Hydroponicaly grown plants at a proper stage (14 ± 20 days)were exposed to platinum for 7 days. Platinum in a form of[Pt(NH3)4](NO3)2 was added in amount 50 or 500 �g L�1.

Simultaneously some plants were cultivated in nutrientsolutions without Pt addition. The presence of platinum innutrient solutions did not influence plant biomass or theirgrowth.

2.2.2. Samples Pretreatment

When the hydroponic cultivation was finished plants werecollected and roots were washed three times with deionizedwater. Then the plants were divided into roots, leaves and

stems. Plant material was oven dried at 105 �C for 3 hoursfollowed by 24 hours at 75 �C. Thus the obtained drymaterial was ground in an agate mill. Homogenized sampleswere stored in 50 mL PE bottles.

2.2.3. Microwave Decomposition of the Samples

About 250 mg of sample was weighed into CEM Teflonvessel and a mixture of 1 mL HNO3, 1 mL HClO4 and0.5 mL HCl was added. The vessel was screwed up tightlyand placed in the microwave oven. Heating of the samplewas carried out in several cycles increasing time and powerof microwave energy with 2 min waiting time between thecycles (3 min 255 W, 4 min 425 W, 5 min 595 W, 6 min850 W). After cooling down it was quantitatively transferredinto volumetric flask (25 mL) and diluted to the mark withdeionized water. Then the sample solution was placed inquartz tube, 50 �L H2O2 (30%) was added, and the UVirradiation was done for 6 hours. For voltammetric deter-mination, except control samples, solutions were diluted 10or 100 times.

2.2.4. Voltammetric Determination

In the quartz crucible 10 mL deionized water, 200 �L conc.H2SO4, 100 �L HCHO (2 mol L�1), 100 �L N2H2 ¥ HCl(15 mmol L�1) were placed.

The solution was purged with argon gas for 15 min. Thepreconcentration was carried out at the potential of 0.0 V ina stirred solution for 15 s. After the resting time of 10 s thevoltammetric curve, base line, was recorded in the potentialrange of � 0.57 to � 1.0 Vusing differential pulse techniquewith scan rate 20 mV s�1 and amplitude 50 mV.

Then an aliquot of the sample solution was added andvoltammetric determination was continued. The catalyticcurrent of the hydrogen reduction was recorded at about �0.9 V.

For quantitative determinations the double standardaddition method was used.

In all cases the automatic subtraction of base line wasperformed.

3. Results and Discussion

Before platinum determination in plant materials someexperiments with standard solutions were carried out. Toestablished optimal conditions for electrochemical deter-mination the influence of deposition potential, depositiontime, scan rate, concentration of formaldehyde, hydrazine,H2SO4 on the peak height was examined.

In the first step of the experiment the optimal content ofthe supporting electrolyte was established. To choose theoptimal concentrations of formaldehyde and hydrazine themeasurements were carried out in the solution containing2 pg mL�1 Pt in the presence of 0.4 mol L�1 H2SO4. It hasbeen found that peak height increased with the increase offormaldehyde and hydrazine concentrations up to 2.0� 10�2

1267Voltammetric Determination of Platinum in Plant Material

Electroanalysis 2004, 16, No. 15 ¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

mol L�1 and 1.5� 10�3 mol L�1, respectively, and thenremained unchanged. For further experiments 2.0� 10�2

mol L�1 formaldehyde and 2.0� 10�3 mol L�1 hydrazinewere applied.

The voltammetric determinations of platinum werecarried out in acidic solution so the influence of H2SO4

concentration on the peak height was examined within therange 0.1 to 1.1 mol L�1 H2SO4 . The best peaks wereachieved in 0.5 ± 0.6 mol L�1 H2SO4.

The effect of deposition potential on the catalytic currentfor different Pt contents in the supporting electrolyte wasexamined over the range 0.0 V to � 1.1 V (Fig. 1). Theoptimal preconcentration potential chosen to give themaximum peak height is slightly different from those ofother scientists. We obtained the highest well-defined peaksat � 0.9 V after preconcentration at the potential 0.0 V,while the most authors usually applied deposition potentialmore negative than � 0.3 V [23, 24, 25].

Similar situation was observed during optimization ofdeposition time. Based on our studies the peak heightincreases rapidly with the increase of preconcentration timeup to 100 s and further increase of preconcentration timedoes not influence the peak height (Fig. 2). So during theexperiments deposition time did not exceed 60 s. WhileWang et al. [25] applied deposition time of 30 min toobtained detection limit equal 0.24 pg mL�1 .

Studies of the effect of scan rate have shown that the bestresults could be obtained with scan rate 10 mV s�1. Al-though scan rate 20 mV s�1 showed well-defined peakswithin the shorter time. The linearity of the method was alsoexamined. Well-defined stripping peaks were recorded afterincreasing of Pt concentration. Highly linear calibration plotwas observed within the range 0.2 ± 120 pg mL�1 of plati-num. The detection limit of the method calculated as Pt

content in blank solution� 3-fold value of standard devia-tion was found to be 0.1 pg mL�1.

As described above, the very sensitive voltammetricmethod was applied for Pt determination in plant material.

The digestion of the analyzed plant material requiresusage of nitric, perchloric and hydrochloric acids. As,according to some authors [24, 27], this is likely to causeinterferences in the platinum determination by AdSV, theinfluence of the presence of acids mentioned above had tobe investigated. Voltammetric determinations of platinumwere carried out in the presence of different amounts ofnitric acid within the range 0 ± 150 mmol L�1 and addition-ally in the presence of different amounts of potassiumnitrate up to 960 mmol L�1 . It was observed that nitric acidand nitrate ions in supporting electrolyte caused thedecrease of method sensitivity and in a consequence madethe determination impossible, when nitric acid concentra-tion exceeded 100 mmol L�1. Similar situation was observedin case of HCl and HClO4 acids. Nevertheless during Ptdeterminations in plants an aliquot of the digested samplesolutions did not exceed 0.25 mL and no problems caused byHNO3, HCl and HClO4 acid were observed. In other case,interferences should be removed from the solution (forexample by heating an aliquot in the presence of concen-trated H2SO4 ).

Additionally the effects of various metal ions (Cd(II),Ni(II), Zn(II), Cu(II), Cd(II), Pb(II), Cr(III,VI), Al(III),Co(II), Ti(II), Fe(II,III), Pd(II)) on voltammetric peakheight were investigated. Serious interferences caused byPd(II) were observed. 100-fold excess of Pd concentrationcaused approx. 20% increase of Pt peak height. Thepresence of other ions could cause a decrease of Pt peakheight if their concentration exceeded 105 times theplatinum concentration.

Adsorptive stripping peaks are seriously affected by thepresence of organic compounds. So before voltammetricdetermination of platinum in plants complete digestion is

Fig. 1. Influence of the deposition potential on the peak heightof Pt (2.0� 10�2 mol L�1 formaldehyde, 2.0� 10�3 mol L�1 hydra-zine, 0.4 mol L�1 H2SO4; td� 60 s).

Fig. 2. Influence of the deposition time on the peak height(10 pg mL�1 Pt, 2.0� 10�2 mol L�1 formaldehyde, 2.0� 10�3

mol L�1 hydrazine, 0.4 mol L�1 H2SO4; Ed� 0.0 V).

1268 J. Kowalska et al.

Electroanalysis 2004, 16, No. 15 ¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

required. To shorten the duration of this analysis stagepressurized digestion with microwave energy source wasused. Complete decomposition of samples needed addi-tional UV irradiation of sample solutions obtained aftermicrowave digestion with the mixture of HNO3, HClO4 andHCl acids according to the previously described temper-ature program. UV irradiation was carried out for 6 hoursafter addition of 50 �L of H2O2 (30%) per 25 mL of samplesolution.

To control this analysis stage a series of synthetic solutionscontaining 10.0 ng Pt was digested using procedure descri-bed above. The voltammetric determinations of platinum inobtained solutions showed recovery close to 100% (10.0 ngadded; 10.2� 0.2 ng found).

According to the same digestion and determinationprocedures the detection limit, calculated as Pt content inblank solution� 3-fold value of standard deviation wasfound to be 1 ng g�1 dry mass.

The whole described procedure was then applied forplatinum determinations in mustard and grass samples. ICPMS method was used as a reference one. The obtainedresults are presented in Table 1.

Based on the results it can be concluded that both plantspecies ± mustard and grass ± have taken up considerableamount of platinum. Platinum was translocated to all plantorgans but the highest amounts of this element were foundin roots.

Because platinum, taken up by roots of plants, wasefficiently translocated to above ground organs, it suggeststhat mustard plant can be considered for phytoextractionpurpose while grass, commonly grown along roads andhighways, as a Pt bioindicator.

Moreover the results of this study showed the usefulnessof adsorptive stripping voltammetry for platinum determi-nation in plant material. Comparison of the results obtainedby electrochemical method and ICP MS and the recovery ofstandard solution close to 100% confirmed the accuracy ofthe propose procedure.

4. References

[1] M. E. Farago, P. Kavanagh, R. Blanks, J. Kelly, G. Kazantzis,I. Thornton, P. R. Simpson, J. M. Cook, S. Parry, G. M. Hall,Fresenius J. Anal. Chem. 1996, 354, 660.

[2] E. Helmers, ESPR ± Environ. Sci. Poll. Res. 1997, 4, 100.[3] W. Wegscheider, M. Zischka, Fresenius J. Anal. Chem. 1993,

346, 525.[4] F. Alt, J. Messerschmidt, G. Weber, Anal. Chim. Acta 1998,

359, 65.[5] F. Alt, H. R. Eschnauer, B. Mergler, J. Messerschmidt, G.

Tˆolg, Fresenius J. Anal. Chem. 1997, 357, 1013.[6] H. J. Ballach, R. Wittig, Environ. Sci. Pollut. Res. 1996, 3, 3.[7] J. Schafer, D. Hannker, J.-D. Eckhardt, D. Stuben, Sci. Total

Environ. 1998, 215, 59.

Table 1. Results of platinum determination in plants, n� 4.Results are presented as a mean� t95% ¥ SD (SD means SD of the mean).

Sample Pt in nutrientsolution [�g L�1]

Pt in plant samples [�g g�1]

Series 1 Series 2 Series 3

AdSV AdSV AdSV ICP MS

mustardleaves 50 1.13� 0.05 1.72� 0.12 1.09� 0.20 1.11� 0.01steams 50 2.43� 0.03 3.70 � 1.03 2.27� 0.14 2.07� 0.16roots 50 7.37� 0.31 9.76� 0.51 18.9� 0.9 18.5� 1.43leaves 500 5.41� 0.90 8.70� 1.27 5.90� 0.60 6.50� 0.06steams 500 15.6� 1.1 21.3� 2.1 9.23� 1.15 9.44� 0.18roots 500 72.6� 1.4 85.� 4.3 65.3� 1.9 65.3� 2.2grassleaves 50 0.71� 0.02 0.35� 0.02 0.32� 0.01roots 50 13.3� 0.8 9.69� 0.56 10.9� 0.2leaves 500 8.42� 0.7 5.41� 0.19 6.07� 0.11roots 500 50.8� 1.5 72.0� 3.6 90.3� 3.0

Fig. 3. Quantification of platinum in mustard leaves sample bydouble standard addition (20 ± 40 pg Pt).

1269Voltammetric Determination of Platinum in Plant Material

Electroanalysis 2004, 16, No. 15 ¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

[8] D. Verstraete, J. Riondato, J. Vercauteren, F. Vanhaecke, L.Moens, R. Dams, M. Verloo, Sci. Total Environ. 1998, 218,153.

[9] J. Begerow, M. Turfeld, L. Dunemann, Anal. Chim. Acta1997, 340, 277.

[10] E. Beinrohr, M. L. Lee, P. Tschopel, G. Tˆolg, Fresenius J.Anal. Chem. 1993, 346, 689.

[11] R. Eller, F. Alt, G. Tˆolg, H. J. , Tobschall, Fresenius Z. Anal.Chem. 1989, 334, 723.

[12] M. L. Lee, G. Tˆolg, E. Beinrohr, P. Tschopel, Anal. Chim.Acta 1993, 272, 193.

[13] C. Ludke, E. Hoffmann, J. Skole, S. Artelt, Fresenius J. Anal.Chem. 1996, 355, 261.

[14] M. Parent, H. Vanhoe, L. Moens, R. Dams, Fresenius J. Anal.Chem. 1996, 354, 664.

[15] J. Schafer, J. D. Eckhardt, Z. A. Berner, D. Stuben, Environ.Sci. Technol. 1999, 33, 3166.

[16] J. Schafer, H. Puchelt, J. Geochem. Explor. 1998, 64, 307.[17] Z. B. Alfassi, T. U. Probst, B. Rietz, Anal. Chim. Acta 1998,

360, 243.

[18] A. M. G. Figueiredo, J. Enzweiller, J. E. S. Sarkis, A. P. S.Jorge, E. K. Shibuya, J. Radioanal. Nucl. Chem. 2000, 244,632.

[19] G. S. Reddi, C. R. M. Rao, Analyst 1999, 124, 1531.[20] O. Nygren, C. Lundgren, Int. Arch. Occup. Environ. Health

1997, 70, 209.[21] O. Nygren, G. T. Vaughan, T. M. Florence, G. P. M. Morrison,

I. M. Warner, L. S. Dale, Anal. Chem. 1990, 62, 1637.[22] J. Messerschmidt, F. Alt, G. Tˆolg, J. Angerer, K. H. Schaller,

Fresenius J. Anal. Chem. 1992, 343, 391.[23] K. Hoppstock, F. Alt, K. Cammann, G. Weber, Fresenius J.

Anal. Chem. 1989, 335, 813.[24] C. M. G. Van den Berg, G. S. Jacinto, Anal. Chim. Acta 1988,

211, 129.[25] J. Wang, J. Zadeii, M. S. Lin, J. Electroanal. Chem. 1987, 237,

281.[26] C. Leon, H. Emons, P. Ostapczuk, K. Hoppstock, Anal.

Chim. Acta 1997, 356, 99.[27] Z. Zhao, H. Freiser, Anal. Chem. 1986, 58, 1498.

1270 J. Kowalska et al.

Electroanalysis 2004, 16, No. 15 ¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim