VI. simpozij HDZZ, Stubičke Toplic HR0500091
DETERMINATION OF CHROMIUM(III), CHROMIUM(VI), MANGANESE(II) AND MANGANESE(VII) BY EDXRF METHOD
Luka Mikelić1, Višnja Oreščanin', Stipe Lulić' and Mirta Rubčić2 'Ruđer Bošković Institute, Bijenička c. 54, HR-10000 Zagreb, Croatia 2Faculty of Science, Zvonimirova 8, HR-10000 Zagreb, Croatia e-mail: lmikelic(S>irb.hr
INTRODUCTION It is well known that metal exists in several forms in nature, each one with a specific bio-toxicological activity. For this reason, it is necessary to use a sensitive method for determination different metal species. In solutions chromium may exist in Cr(III) and Cr(VI) oxidation state and manganese dominantly in Mn(II) and Mn(VII) oxidation state. In literature are various techniques for their determination recommended. The most employed techniques include UV-VIS spectrophotometry [1], high-performance liquid chromatography (HPLC) [2,3], capillary electrophoresis (CE) [4], catalytic cathodic stripping voltammetry [5], flame and furnace atomic adsorption spectroscopy (AAS) [6-8] and mass spectrometry [9]. In the spectrophotometric methods several conditions such as temperature and amount of reagent must be kept strictly constant in order to achieve good reproducibility. Mayor disadvantage of these methods including AAS is in handling complex sample matrices which have to be combined with a conversion step of Cr(III) to Cr(VI) or Mn(VII) to Mn(II). Separation of chromium and manganese species using chromatographic techniques often results in inadequate sensitivity for trace concentration of Cr and Mn present in real samples because of low sample loading. In all flow injection techniques the objective is to separate a single analyt or group of analytes from interfering components or matrices, often simultaneously achieving some degree of preconcentration and therefore gaining sensitivity at the expense of separation power. Electrochemical methods were highly sensitive, but disadvantage of these methods is in poor selectivity for chromium determination especially in the presence of other metals and also in small linear dynamic ranges. Disadvantage of capillary electrophoresis is in optical detection because the optical path length (diameter of the capillary) is generally less than 100 um in order to favour better dissipation of Joule heat during the separation process. We report here a method for the determination of chromium and manganese valence states in liquid samples. The method is based on the preconcentration of Cr(III), Cr(VI), Mn(II) and Mn(VII) with APDC followed by the energy dispersive X-ray fluorescence (EDXRF) analysis.
433 VI. simpozij HDZZ, Stubičke Toplice, 2005.
EXPERIMENTAL All solutions were prepared using analytical reagents grade chemicals and distilled water. APDC solution was prepared daily by dissolving APDC (Aldrich) in distilled water to produce 1% (w/v) solution. Cr(III), Cr(VI), Mn(II) and Mn(VII)
(1 ppm) stock solutions were prepared from Cr(NO3)3, K2Cr207, Mn(NO3)2 and
KMnO4 (Merck). One thousand micrograms per liter solutions of EDTA was prepared from KOMPLEKSAL II (Kemika). One hundred microgram per litre standard solutions of Fe2+, Mn2+, V4+ were prepared from Merck standard solutions for AAS. One hundred millilitres of a solutions containing 1000 ug/L of Cr(III) or Cr(VI) or Mn(II) or Mn(VII) was adjusted to pH values 3-11 by adding hydrochloride acid and ammonium hydroxide to estimate the effect of pH on the recovery of chromium and manganese species. All pH measurements were made with Mettler Toledo digital pH meter. The influence of organic meter, carbonate or metal ions on the recovery of each specie over whole pH range was also tested. This was carried out to stimulate conditions occurring in natural waters. After pH adjustment 2 mL of 1% APDC solution was added into each flask. After complexation lasted for 20 minutes, the suspension was filtered through Millipore HAWP (pore size 0.45 um, diameter 25 mm). A Millipore micro filtration system was used for that purpose. Prepared thin targets were air dried, protected by thin mylar foil, inserted into a plastic carrier and placed above the X-ray source of the X-ray spectrometer. All targets were analysed with energy dispersive X-ray fluorescence (EDXRF) [10]. Instrumental and measurement conditions were presented in Table 1. Spectrums were collected by Genie 2000 software (Canberra). Spectral data were analysed by a WinAxil software (Canberra). The IR spectra of all compounds were recorded using Perkin-Elmer FT Spectrum RX1 as KBr pellets in the region of 4000-450 cm"1.
434 VI. simpozij HDZZ, Stubičke Toplice, 2005.
Table 1. Excitation, detection and geometry condition used in conducted EDXRF measurement 109 c Source Cd o Half live 464 days 'o Calibration date 12/03/2001 1X UJ Calibrated Activity 594.00 MBq Crystal Si Crystal thickness 3.00 mm Crystal density 2.34 g/cm3 Crystal dead layer 0.300 urn Contact Au io n Contact thickness 20.000 nm tec t
RESULTS AND DISCUSSION The effect of pH between 3 and 11 on the recovery of Cr(III)/Cr(VI) and Mn(II)/Mn(VII) in the absence of other substances is presented in Figure 1. The maximum recovery of Cr(VI) was obtained at pH 4. The complexation with APDC was irregular in the pH range 5-9 while at pH 10 and 11 no complex formation occurred at all. On the contrary, the maximum recovery of Cr(III) was obtained exactly at pH 10 (98%). The maximum recovery of Mn(II) was obtained at pH 10 while maximum recovery of Mn(VII) was obtained at pH 8 (97,5%). Spectra of products obtained via preconcentration with APDC at pH 10 from solutions containing mixture of Cr(III) and Cr(VI) salts and Cr(III) salt only are identical. The characteristic feature of spectra of both compounds is very strong and broad band in the region of 3400-3500 cm"' that can be assigned to O-H stretching and very poor 'finger print' region which indicate that no organic mater is bonded to Cr(III). Therefore, it can be concluded that at pH 10 dominant specimen is Cr(OH)3 and that no Cr(III)-PDC complex is formed, which is in accordance with calculated distribution of inorganic Cr(III) species at different pH values [11].
435 VI. simpozij HDZZ, Stubičke Toplice, 2005.
Spectra of compounds prepared via preconcentration with APDC at pH 4 from solutions containing mixture of Cr(III) and Cr(VI) salts and Cr(VI) salt only were compared mutually as well as with spectra of free APDC. IR spectra of prepared compounds obtained in both cases are equivalent. Very strong band, corresponding to N-H stretching positioned at 2956 cm"1 in the spectra of free APDC, in the spectra of Cr(VI)-products disappears and suggests that complex Cr(VI)-PDC is formed. Furthermore, strong doublet observed at 1001, 990 cm"1 in the spectra of free APDC assigned to C-S stretching becomes strong single band positioned at 1005 cm"1 in the Cr(VI)-PDC spectra along with shift of the band corresponding to C-N stretching from 1413 (APDC) to 1485 cm"1 (Cr(VI)-PDC) indicating that ligand (PDC) is bidentate [12]. Spectars of manganese products obtained via preconcentration with APDC shows same results as spectars of chromium compounds. Also, same method was used to investigate preconcentration of manganese and chromium species with DTPA. Results for the recoveries are lower then for APDC method. A characteristic of Cr(III)/Cr(VI) and Mn(II)/Mn(VII) to create complexes with APDC at different pH ranges makes possible to separate the species. Results presented here show that complicated and time consuming methods could be successfully replaced by a simple as well as rapid method based on preconcetration of different species with non-specific chelating agent ammonium- pyrrolidinedithiocarbamate. The major advantage of EDXRF method over commonly used methods is simultaneous analysis of wide ranges of metals in the complex environmental samples without separation of species or any kind of pretreatment.
—»—Cr(lll)*APDC 100% • 90% • \ -*-Cr(VI)+APDC \ /\ 70% • 2 60% • \ A/ • 50%- \ / \/ Š 40% • \\/ / AY " 30% • 20% • v A /\ 10% • /\ J \ / \ /•* 0% • / v/ \ 5 7 9 11 PH Figure I. Percentage of recovery of Cr(III)/Cr(VI) and Mn(II)/Mn(VII) by APDC for different pH values
436 VI. simpozij HDZZ, Stubičke Toplice, 2005.
REFERENCES [I] Llobat-Estelles M, Mauri-Aucejo AR, Lopez-Catalan MD. J Anal Chem 2001; 371: 358. [2] Ou-Yang GL, Jen JF. Anal Chim Acta 1993; 279: 329. [3] Collins CH, Pezzin SH, Rivera JFL, Bonato PS, Windmeller CC, Archundia C, Collins KE. J Chromatogr A 1997; 789: 469. [4] Himeno S, Nakashima Y, Sano K. Anal Sci 1998; 14: 369. [5] Li Y,XueH. Anal Chim Acta 2001; 448: 121-134. [6] M.J. Marques, A. Morales-Rubio, A. Salvador, M. De la Guardia, Talanta 2001; 53:1229. [7] Gaspar A, Posta J. Anal Chim Acta 1997; 354: 151. [8] Pasullean B, Davidson CM, Littlejohn D. J Anal Atom Spectrom. 1995; 10: 241. [9] Andrle CM, Jakubowski N, Broekaert JAC. Spectrochim Acta B 1997;52: 189. [10] Oreščanin V, Mikelić L, Lulić S, Rubčić M.Anal Chim Acta 2004;527: 125-129. [II] Sperling M, Xu S, Welz B. Anal Chem 1992; 64: 3105. [12] Bernal C, Almeida Neves E, Cavalheiro ETG. Thermochim Acta 2001; 370: 50.
ABSTRACT This paper describes EDXRF, a quick, sensitive and selective method for determining Cr(III), Cr(VI), Mn(II) and Mn(VII) in environmental and industrial liquid samples via preconcentration with ammonium pyrrolidinedithiocarbamate (APDC) and diethylenetriaminepentaacetic acid (DTPA). The effect of pH in the range of 3-11 on the recovery of Cr(III), Cr(VI), Mn(II) and Mn(VII) was investigated separately and in combination. The influence of organic matter, carbonate species and elements V and Fe was also tested on the recovery of each chromium and manganese species (separately/in combination) over the whole pH range in order to simulate conditions in natural waters that usually contain a certain amount of dissolved organic matter and carbonate ions. Characteristic of different species to create complexes with APDC and DTPA in the different pH ranges makes possible to separate those two species. All created complexes of Cr(III), Cr(VI), Mn(II) and Mn(VIII) with APDC and DTPA were characterised by IR spectroscopy.
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