ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2010, том 65, № 2, с. 163-167
ОРИГИНАЛЬНЫЕ СТАТЬИ
УДК 543
DETERMINATION OF MANGANESE BY A NEW SPECTROPHOTOMETRIC
METHOD USING TOLUIDINE BLUE
© 2010 г. D. Kostova
Agricultural University — Plovdiv Laboratory Complex 12, Mendeleev Str., 4000 Plovdiv, Bulgaria Received 11.06.2008; in final form 16.06.2009
The formation of the ion associate of manganese(VII) with Toluidine Blue is studied spectrophotometrically in water-organic media. The composition of the complex is established as TB: Mn O4 = 1 : 1. The molar absorptivity
s290 = (2.46 ± 0.09) x 104 L/mol cm is determined. The selectivity of the reaction is studied and the method for determination of 0.1—2.9 ^g/mL manganese(VII) is developed. Investigations of extraction in the system discussed were carried out. The characteristic values for the extraction equilibrium and the equilibrium in the aqueous phase were determined: extraction constant Kex = 1 x 104, distribution constant KD = 26.08, and association constant
в = 3.83 x 102. A rapid and selective extractive-photometric method for the determination of manganese(VII) in plants was proposed.
The determination of microquantities of manganese in plants and other naturally occurring materials is ofpar-ticular importantce in connection with the growing interest in environmental problems. Manganese takes part in a number of important physiological and biological processes: in the nitrogen metabolism, photosynthesis, breathing and maintaining the needed oxidation/reduction conditions in the cell. Various methods for the determination ofmanganese have been reported [1, 7]. Atomic absorption spectrophotometry [10—15] and spectropho-tometric analyses with some reagents can be used: isoph-thaldihydroxamic acid [2], 1,10-phenanthroline [3], 8-hydroxyquinoline [4], 1-(2-pyridylazo)2-naphthol [5], 4-(2-pyridylazo) resorcinol [6], 3-brombenzohydroxam-ic acid [8], pentamethilendithiocarbaminat [9].
No data about determination of manganese(VII) with Toluidine Blue can be found in the literature. The triphe-nylmethane dye that we suggest is a new reagent for the determination of microquantities of manganese(VII). Here we report the use of triphenylmethane dye, Toluidine Blue (TB), as counter-ion for formation ofion-asso-
ciation comples with the anion Mn O-, and determination of manganese(VII). This can be used in developing an extraction-photometric method for the determination of manganese. The present paper represents the conditions needed for the determination of manganese with Toluidine Blue. This is an extremely simple and direct extraction for the determination of microquantities of man-ganese(VII).
The present work examines Toluidine Blue for the first time for the determination of microquantities of manganese(VII). The goal is to develop an extraction-spectrophotometric method using TB for determination
of manganese(VII), superior, in some respects, to the existing methods. For example, the organic extract needs to be stable for a long time, and not only for a few hours as it is in the method of Zhang and Zhou [7]. Some of the above mentioned methods are characterized by a low selectivity [3, 4, 8] and long procedure [5, 6, 8, 9].
EXPERIMENTAL
Apparatus. Absorbance measurements were made on a spectrophotometer UV-VIS with 1 cm quarts cuvette, at 290 nm.
Reagents and solutions. All reagents were of analytical reagent grade. All solutions were prepared with distilled demineralized water.
Stock manganese (VII) solution. A 1 x 10-2 M aqueous solution was prepared by dissolving 0.395 g ofKMnO4 in 250 mL of distilled water. After 10 days the exact concentration of manganese(VII) was checked titrimetrically using oxalate titration. Working solutions containing 1.98 x x 10-4 M Mn(VII) were prepared by dilution.
Toluidine Blue C15H16ClN3S (TB, Chemapol). A0.5 x 10-3 M aqueous solution was prepared by dissolving 0.0306 g of TB in 200 mL of distilled water. Other TB concentrations were prepared by appropriate dilution. The solutions were stable for months.
Foreign ion solutions. 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.
1,2-Dichloroethane, p.a., hydrochloric, phosphoric, nitric, sulphuric and perchloric acids at concentrations of 1.2, 2, 1.55, 2, 9 M, respectively, were used.
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KOSTOVA
250 260 270 280 290 Wavelength, nm
300
310
0.5
0.4
e
о
S 0.3 a
•8 £0.2
^ 0.1
Fig. 1. Absorption spectra in dichloroethane: Toluidine Blue (1), ion-association complex (2) and MnO- (3); cMn(yii) = = 1.98 x 10-5 M, cHClO4 = 9 x 10-2 M, cTB = 3 x 10-4 M.
Procedure. Preparation of the calibration graph. Transfer an aliquot ofthe stock solution containing 1—29 ^g ofman-ganese (VII) to a 100 mL separatory funnel. Add 1.8 mL of 0.5 x 10-3 M Toluidine Blue, 0.5 mL of 9 M perchloric acid. Dilute to 10 mL with distilled water and shake with 3 mL of 1,2-dichloroethane for 1 min. Transfer the organic layer through a dry filter paper into a 1 cm cuvette and measure the absorbance at 290 nm against a reagent blank similarly prepared. Plot the amount of manganese in the sample solution against absorbance to obtain the calibration graph.
Determination of manganese in plant samples. A wet burning of the plant sample was carried out in which a mixture of sulphuric and nitric acids was used for the oxidation of the organic matter. A portion of 2 g of air-dry plant material was placed into a Kjeldal flask and moistened with 4 mL distilled water. 5 mL conc. sulphuric acid and 10 mL conc. nitric acid were added. The flask was slightly heated to avoid splashing of the solution decomposition and fuming away of HNO3. If the oxidation of the organic substance was not completed, HNO3 was added and heated again. When all the organic material was oxidized, the solution was heated at a higher temperature for 10 min [19]. After cooling, the solution was diluted with water and filtered. Portions of 3 mL conc. H2SO4, 2 mL conc. H3PO4 and 0.1 g potassium periodate were added for oxidation Mn(II) to Mn(VII). The mixture was heated to boiling point and the temperature was maintained for 10 min [17]. After cooling, the solution was diluted with water and filtered. It 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.
In separatory funnel of 100 mL, the solutions are introduced: 1.8 mL of 0.5 x 10-3 M Toluidine Blue, 0.5 mL of 9 M perchloric acid, aliquote of the prepared solution of plant sample and 1 mL of saturated solution of EDTA (masking reagent). The mixture is diluted up to a volume ofthe aqueous phase of10 mL with distilled water and ex-
Time, min
Fig. 2. Dependence of the absorbance on the extraction time; cMn(vn) =1.98 x 10-5 M, chcio4 = 0.45 M, ctb = 3 x 10-5 M.
tracted with 3 mL of 1,2-dichloroethane for 1 min. The organic phase is filtered through a dry paper into a 1 cm cuvette and the absorbance is measured at 290 nm. A blank is run in parallel in the absence of plant sample. A calibration graph is constructed with standards similarly treated.
RESULTS AND DISCUSSION
Choice of solvent and determination of the wavelength of maximum absorption. Manganese(VII) forms an ion-pair with triphenylmethane dye Toluidine Blue (TB). To clear out the possibility of extracting the ion associate a number of different types of organic solvents were tested: benzene, toluene, ketones, alcohols, chloroform and dichloroethane. The studies showed that the associate is better dissolved in 1,2-dichloroethane. It is partially ex-tractable into chloroform, not extractable into benzene, toluene, amyl alcohol. The absorption spectra of triphe-nylmethane dye and ion-association complex in perchloric acid show that the absorption maxima are around 290 nm (Fig. 1). We have chosen 290 nm as a working wavelength because of greater absorptivity at this wavelength. The absorption additivity, without a significant shift in the absorption maximum, is an indication of a formation of an ion-association complex.
Effect of time, stability and stoichiometry of the ion-association complex. The effect of reaction time was studied in the range 30 s-2 min. The ion-associate of Mn(vII) was completely extracted in a single extraction of only 1 min (Fig. 2).
The absorption was measured at 290 nm against time in 1 cm cells with 1,2-dichloroethane in the reference cell. It was established that the ion-pair had high stability. The absorption did not change for over 7 days after extraction.
The stoichiometric coefficients of the associate formation were established by various methods [16].
The ratio TB+: Mn O- was proved by the method of Bent-French. The results for the light absorption of
DETERMINATION OF MANGANESE BY A NEW SPECTROPHOTOMETRIC METHOD
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the extracts at 290 nm showed that the associate was formed at a ratio TB+: Mn O- = 1 : 1.
Photometric characteristics and Precision. A plot of the concentration of aqueous phase manganese (VII) vs. absorbance of the organic layer showed good linearity in the range 0.1-2.9 ^g of manganese (VII) per 1 mL of aqueous phase. The obtained regression equation is: ^Mn(VII) = 0.4479 X- 0.0083, r = 0.999. Molar absorptivity calculated using the Beer's law is s290 = (2.46 ± 0.09) x x 104 L/mol cm. The relative standard deviation (8 determinations with 2 ^g/mL of manganese (VII), 95% confidence level) of the method is ±1.5%. The Sandell sensitivity index [17] is 2.23 x 10-3 ^g/cm.
Effect of reagents and conditions on absorbance measurements. To find the optimum conditions for extraction of the ion-associate, the influence of pH and the concentration of Toluidine Blue was studied.
The acidity of the aqueous phase is a major factor in the determination. The influence of hydrochloric, perchloric, nitric, sulphuric and phosphoric acids on the extraction equilibrium was
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