ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2014, том 69, № 11, с. 1130-1136
ОРИГИНАЛЬНЫЕ СТАТЬИ
УДК 543
EXTRACTION OF CHROMIUM(VI) WITH BLUE TETRAZOLIUM CHLORIDE AND TETRANITROTETRAZOLIUM BLUE CHLORIDE
© 2014 D. Kostova
Agricultural University, Plovdiv, Laboratory Complex 12, Mendeleev Str., 4000 Plovdiv, Bulgaria
E-mail: deny_kostova@yahoo.com Received 12.07.2012; in final form 11.11.2013
The optimum conditions for extraction of microquantities of chromium(VI) as an ion-association complex with blue tetrazolium chloride (BTC) and tetranitrotetrazolium blue chloride (TNBT) has been determined. The extracted species was a 1 : 2 of the BTC and TNBT cation and the chlorochromate anion. Beer's law was obeyed in the range of 0.04-0.8 ^g/mL Cr(VI) for BTC and 0.1-1.6 ^g/mL Cr(VI) for TNBT. The molar absorptivities were s255 = 7.77 x 104 L/(mol cm) (for BTC) and s275 = 2.04 x 104 L/(mol cm) (for TNBT). Sandell's sensitivity of the systems were found to be 6.69 x 10-4 p.g/cm2 (for BTC) and 2.55 x 10-3 p.g/cm2 (for TNBT). Limit of detection (LOD) is 8.55 ng/mL and limit of quantitation (LOQ) is 0.028 ^g/mL Cr(VI) for BTC. For TNBT, LOD is 0.031 ^g/mL and LOQ is 0.103 ^g/mL. The characteristic values for the extraction equilibrium and the equilibrium in the aqueous phase have been determined. A sensitive method for determination of trace of chromium(VI) in plants has been developed.
Keywords: chromium, blue tetrazolium chloride, tetranitrotetrazolium blue chloride, plants.
DOI: 10.7868/S0044450214110097
The pollution of biospfere increases concentration of toxic elements in waters, soils and plants. Among these elements is chromium. But there are data showing the positive effect of chromium compounds upon various farm crops and participation of chromium in a number of oxidation—reduction processes. Chromium is one of the important trace elements for plants. The shortage of chromium causes diseases in plants. The biochemical and physiological function of this microelement is highly varied [1—5]. Obviously, the concentration and oxidation state of chromium, like those of other trace elements, are of great significance for its physiological action.
Various methods for determination of chromium have been published [6—14]. Spectral and chemical methods are constantly used for the determination of chromium. Therefore, new organic reagents for selective and sensitive determination of chromium are of particular interest. Numerous reagents have been suggested for the determination of chromium: 9-pyri-dine-2,3,7-trihydroxyfluoron [15] 1,5-diphenylcarba-zide [16, 17], fulvic acid [18], neotetrazolium chloride [19], nitrotetrazolium blue [20], triphenyltetrazolium chloride [21], iodnitrotetrazolium chloride [22], tetrazolium violet [22], leuco xylene cyanol FF [23], quercetin [24], a-benzoin oxime [25]. Some of the above mentioned methods for determination of chromium are characterized by a low sensitivity [17, 18],
low selectivity [15], long procedure [16, 18], low stability of the complexes obtained [18]. No data about determination of chromium with Blue tetrazolium chloride and Tetranitrotetrazolium blue chloride can be found in the literature.
Here we report on the use of BTC and TNBT as counter-ions for formation of ion-association complexes with the chlorochromate anion and determination of chromium(VI). We suggest the tetrazolium salts are new reagents for determination of traces of chro-mium(VI).This paper describes new rapid methods for the determination of chromium in plants.
EXPERIMENTAL
Reagents and apparatus. All chemicals used were of analytical-reagent grade, and distilled water was used throughout. 3,3'-Dianisole-4,4'-bis-(3,5-diphe-nyltetrazolium chloride) — blue tetrazolium chloride (Fluka), 1 x 10-3 M aqueous solution, was prepared by dissolving 0.0364 g BTC in 50 mL of water. 3,3'-(3,3'-Dimethoxy-4,4'-biphenylene)-bis-[2,5-bis-(p-nitro-phenyl)]-2H-tetrazolium chloride — tetranitrotetrazolium blue chloride (Chemapol), 1 x 10-4 M aqueous solution, was prepared by dissolving 0.0091 g TNBT in 100 mL ofwater. Standard chromium(VI) solution (1 x x 10-3 M) was prepared by dissolving 0.0194 g of potassium chromate in 100 mL of water. Solutions of
lower concentrations were prepared from the stock solution by dilution. Buffer compositions were as follows: buffers of pH 1, 2, 3 were of glycocol plus HCl; buffers of pH 4, 5 were of glacial acetic acid plus NaOH; buffers of pH 6, 7 were of KH2PO4 plus Na2HPO4. Other reagents were 1,2-dichloroethane, 1.2 M hydrochloric acid, 2 M sulphuric acid, 1.55 M nitric acid, 9 M perchloric acid and 2 M phosphoric acid. Solutions of diverse ions for interference studies were prepared by dissolving the amount of each compound needed to give 10 mg/mL concentration of the ion of interest.
Absorbance measurements were made using a spectrophotometer UV-VIS (Carl Zeiss, Germany) with 1 cm quartz cuvette.
Procedures. Determination of chromium(VI) with BTC. In a 100 mL separatory funnel, place 2 mL of1.2 M HCl, 1 mL of 1 x 10-4 M BTC, and a known volume of Cr(VI) solution containing 0.4—8 ^g of chromium. Dilute to 10 mL with water and shake for 1 min with 3 mL of 1,2-dichloroethane. Allow the phases to separate. Transfer the organic layer through a dry filter paper into a 1 cm cell and measure the absorbance of the extract at 255 nm against a blank solution prepared under the same conditions.
Determination of chromium(VI) with TNBT. In a 100 mL separatory funnel, place 4 mL of 1.2 M HCl, 1 mL of 1 x 10-4 M TNBT, and a known volume of Cr(VI) solution containing 1—16 ^g of chromium. Dilute to 10 mL with water and shake for 15 s with 3 mL of 1,2-dichloroethane. Allow the phases to separate. Transfer the organic layer through a dry filter paper into a 1 cm cell and measure the absorbance of the extract at 275 nm against a blank solution prepared under the same conditions.
Determination of the distribution constant. The following solutions are introduced into 100 mL separating funnels: 0.4 mL of1 x 10-3 M BTC , 2 mL of1.2 M HCl (or 1 mL of 1 x 10-4 M TNBT, 4 mL of 1.2 M HCl), and the corresponding amounts of chromi-um(VI). The volume of the aqueous phase is brought to 10 mL with distilled water. It is extracted with 3 mL of1,2-dichloroethane for 1 min for BTC and 15 sec for TNBT. After the separation of the two phases the organic phase is transferred through filter paper into a standard flask of 25 mL and diluted to the mark with 1,2-dichloroethane. The light absorption of the organic phase is measured at 255 nm (for BTC) and 275 nm (for TNBT) with a 1 cm light path cuvette. The absorbance (A) was measured against a reagent blank prepared in the same way (without chromium present).
Determination of chromium in plants. 1 g of plant material was reduced to ashes in an oven at 450— 500°C. The dry residuum was dissolved in a dilute HCl (1 : 1). Obtained solution was transferred into a volumetric flask of 50 mL and diluted to the mark with distilled water. Aliquot parts of this solution were taken for analysis.
The oxidation of Cr(III) to Cr(VI) is performed with potassium permanganate in a sulphuric acid medium [26]. In separatory funnel of 100 mL were introduced the solutions: 2 mL 1.2 M HCl, 1 mL 1 x 10-4 M BTC (or 1 mL of 1 x 10-4 M TNBT, 4 mL of 1.2 M HCl), aliquote of the prepared solution of plant sample. It was diluted up to a volume of10 mL with distilled water and extracted with 3 mL of 1,2-dichloroethane 1 min for BTC and 15 sec for TNBT. The organic phase was filtered through a dry paper into a 1 cm cuvette and the absorbance was measured at 255 nm (BTC) or 275 nm (TNBT). A blank, containing the same solutions without plant sample, was prepared. The concentration of Cr(VI) was determined using a standard curve.
RESULTS AND DISCUSSION
Absorption spectrum. With chromium(VI), in hydrochloric acid medium, blue tetrazolium chloride and tetranitrotetrazolium blue chloride form ion-associates with compositions BTC2+ [CrO3Cl-]2 and TNBT2+ [CrO3Cl-]2, respectively. This composition is in agreement with the earlier reports on the nature of chromium(VI) complexes in hydrochloric acid media [27-29].
The solubility of the ion associate of Cr(VI) with BTC and TNBT in the following organic solvents were tested: benzene, nitrobenzene, tetrachloromethane, 1,2-dichlo-roethane, chloroform and toluene. Only 1,2-dichloroethane gave quantitative extraction of the ion associate of chromium(VI). The analysis of the absorption spectra of the ion associate obtained from the interaction of Cr(VI) and BTC showed that the compound had maximum light absorption at 255 nm (Fig. 1). The TNBT ion-associate has an absorption maximum at 275 nm (Fig. 2).
Composition and characteristics of the complexes.
The composition of the ion-association complexes between chromium and BTC or TNBT was determined by the isomolar series method [30] and found to be 2 : 1. Under the optimal conditions the molar absorptivities determined by the Beer's law were s255 = 7.77 x 104 L/(mol cm) (for BTC) and s275 = 2.04 x 104 L/(mol cm) (for TNBT), which demonstrates the high sensitivity of the reaction. Sandell's sensitivity [14] of the system were found to be 6.69 x 10-4 |g/cm2 (for BTC) and 2.55 x x 10-3 |g/cm2 (for TNBT). LOD is 8.55 ng/mL Cr(VI) and limit of quantitation LOQ is 0.028 |g/mL Cr(VI) for BTC. For TNBT, LOD is 0.03 |g/mL Cr(VI) and LOQ is 0.10 |g/mL Cr(VI).
Calibration graph. Beer's law is obeyed over the Cr(VI) concentration ranges 0.04-0.8 |g/mL (for BTC) and 0.1-1.6 |g/mL (for TNBT) in the aqueous phase. The least squares equation describing the calibration graph was ^Cr(VI) = 1.4785x for BTC and ACr(VI) =
A
1.2
0
225 230 235 240 245 250 255 260 265 270 275
X, nm
Fig. 1. Absorption spectra of ion-association complex of Cr(VI) with BTC (1), BTC (2) and CrO3Cl- (3) in 1,2-
dichloroethane; CQ-ryi) cBTC = 1 x 10-5 M.
= 1 x 10-5 M, cHCl = 0.24 M,
A 0.40 г
0.35 -
0.30 - 1
0.25 -
0.20 -
0.15 -
0
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