ЖУРНАЛ НЕОРГАНИЧЕСКОЙ ХИМИИ, 2012, том 57, № 12, с. 1692-1697


УДК 541.49



Agricultural University — Plovdiv Laboratory Complex, Bulgaria E-mail: deny_kostova@yahoo.com Поступила в редакцию 07.04.2011 г.

The system manganese(VII)—3,7-bis(Dimethylamino)-phenothiazin-5-ium chloride (MB)—water—1,2-dichloroethane has been studied using UV-spectrophotometry. The molar absorptivity of the complex is (3.86 ± ± 0.06) x 104 L mol—1 cm-1 at 290 nm and the system obeys Beer's law in the range 0.1—0.99 p.g mL—1 Mn(Vll). The detection limit (DL) and quantitation limit (QL) of Mn(VII) determination were found to be

0.0146 and 0.049 ^g mL—1, respectively. The composition of the complex is established as MB : MnO- = 1 : 1. Extraction investigations of the system discussed were carried out. The characteristic values for the extraction equilibrium and the equilibrium in the aqueous phase was determined: extraction constant Kex = (1.12 ± 0.05) x x 105, distribution constant KD = 75.61 ± 0.1 and association constant P = (1.48 ± 0.08) x 103. A new method has been developed for the microdetermination of manganese(VII) in plants and steels.

Manganese takes part in a number of important physiological and biological processes — in the nitrogen metabolism, photosynthesis, breathing the needed oxidation-reduction conditions in the cell Manganese is important for the synthesis of the organic substance in plants and the metabolism of a number of nutrient elements in a plant organism [1—4].

Spectral and chemical methods are constantly used for determination of manganese. The reagents suggested for spectrophotometric determination of manganese include: 2',4'-dihidroxi-azo-benzene-4-sul-phonic acid [5], 3,4,5,6-tetrafluoro-2-carboxyphe-nyl-fluorone [6]; 1-(2-pyridylazo)-2-naphthol [7]; triphenylphosphonium [8]; isophtaldihydroxamic acid [9]; 1,10-phenantroline [10, 20]; 8-hydroxyquinoline [11]; 4-(2-pyridylazo) resorcinol [12]; 1-(2-pyridyla-zo)-2-naphthol [13, 14]; N-diphenylbenzamidine [15]; 2,3-dihydroxinaphthalene [19]; triphenyltetrazolium chloride [21]; thiazolylblue tetrazolium bromide [22]. Some of the above mentioned extraction methods are characterized by a long procedure, are not very selective [10—14], low stability of the complexes obtained [8, 9], low sensitivity [9, 19, 21, 22].

The present paper aims at studying the system man-ganese(VII)—3,7-bis(Dimethylamino)-phenothiazin-5-ium chloride—water— 1,2-dichloroethane using UV-spectrophotometry. Manganese(VII) forms stable ion-association complex with 3,7-bis(Dimethylami-no)-phenothiazin-5-ium chloride (thiazine dyestuff Methylene Blue, MB). Helping the practice to control the manganese in different objects, the present study aims to determine the contents of manhanese, using UV-spectrophotometry as oppose to different tech-

niques, applied until now. This is an extremely simple and direct extraction for determination of microquan-tities of manganese(VII).



Absorbance measurements were made a spectrophotometer UV-VIS, Germany, with 1 cm quartz cuvette, 290 nm.


All reagents were of analytical-reagent grade. All solutions were prepared with distilled demineralized water.

Stock manganese(VII) solution. A 1 x 10-2 mol L-1 aqueous solution was prepared by dissolving 0.395 g of KMnO4 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 x 10-4 mol L-1 Mn(VII) were prepared by dilution.

3,7-bis(Dimethylamino)-phenothiazin-5-ium-chloride; C16H18N3ClS • 3H2O (Reachim). A 1 x x 10-3 mol L-1 aqueous solution was prepared by dissolving 0.0373 g of MB in 100 mL of distilled water. Other MB concentrations were prepared by appropriate dilution. The solutions were stable for months.

Buffer compositions were as follows: buffers of pH 1, 2, 3 were of aminoacetic acid plus HCl; buffers of pH 4, 5 were of acetic acid glacial plus NaOH; buffers of pH 6, 7 were of KH2PO4 plus Na2HPO4.


Solutions of diverse ions for interference studies were prepared by dissolving the amount of each compound needed to give 10 mg mL-1 concentration of the ion of interest.

1,2-Dichloroethane, p.a.; Nitric, perchloric, hydrochloric, phosphoric and sulphuric acids at concentrations of 1.55, 9, 1.2, 2 and 2 mol L-1, respectively, were used.


Preparation of the calibration graph. Transfer an aliquot of the stock solution containing 1-9 ^g of manga-nese(VII) to a 100 mL separatory funnel. Add 1 mL of MB 1 x 10-5 mol L-1, 1 mL of nitric acid 15.5 mol L-1. Dilute to 10 mL with distilled water and shake with 3 mL of 1,2-dichloroethane for 20 s. 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 samples was carried out and a mixture of sulfuric and nitric acids were used for the oxidation of the organic substance. A portion of 2 g of air dry plant material was placed into a Kjeldahl flask and moistened with 4 mL distilled water. 5 mL conc. sulfuric acid and 10 mL conc. nitric acid was added. The flask was slightly heated to avoid splashing of the solution, decomposition and fuming away of nitric acid. When all the organic material was oxidized, the solution was heated at a higher temperature for 10 min [16]. 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). It was heated to boiling point and the temperature was maintained for 10 min. After cooling the solution was diluted with water. 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 are introduced the solutions: 1 mL of nitric acid 15.5 mol L-1, 1 mL of MB 1 x 10-5 mol L-1 aliquote of the prepared solution of plant sample. It is diluted up to a volume of the aqueous phase of 10 mL with distilled water and extracted with 3 mL of 1,2-dichloroethane for 20 s. The organic phase is filtered through a dry paper into a 1 cm cuvette and the absorbance 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.

Determination of manganese in steels. Dissolve a 0.1 g sample of the steel in 30 mL of a 1 : 3 v/v mixture of 1 : 1 v/v nitric acid and 1 : 1 v/v hydrochloric acid. Cool, add 10 mL of concentrated sulfuric acid and evaporate until SO3 fumes appear. Cool, take up the




о n 0.8



о и 0.6






250 260

270 280 290 Wavelength, nm

300 310

Fig. 1. Absorption spectra of the complexes: 1 — MB, 2 —

Mn(VII) with MB, 3 - MnO- extracted in 1,2-dichloro-ethane; CMn = 7.9 x 10-6 mol L-1, C HNO3 = 0.31 mol L-1,

CMB = 1 x 10-5 mol L-1.

salts in water, and dilute to volume in a 100 mL standard flask. To a suitable aliquot of the resultant solution add 1 mL of MB 1 x 10-5 mol L-1, 1 mL of nitric acid 15.5 mol L-1 and dilute to 10 mL with distilled water. Extract the ion associate and complete the determination as for plant analysis.


Choice of organic solvent and determination of the wavelength of maximum absorption

Manganese(VII) forms an ion-pair with MB. The solubility of the ion-associate in various solvents was investigated. Several organic solvents such as 1,2-dichloroethane, chloroform, benzene, chlorobenzene and toluene were tested as solvents for the extraction of the complex. It is not extractable into benzene, chlo-robenzene, toluene, partially extractable into chloroform. The extraction was complete and quantitative with 1,2-dichloroethane and a clear separation was obtained. The absorption spectrum of the ion-pair in 1,2-dichloroethane is shown in Fig. 1. We chose 290 nm as a working wavelength because of the greater absorptivity at this wavelength.

Stability, composition and effect of time of the ion-association complex

The aqueous phase containing 9.9 x 10-6 mol L-1 manganese(VII) was extracted as described under "Procedures". 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 18 days after extraction.




0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05

20 40 60 Time, s


Fig. 2. Dependence of the absorbance on the extraction

time; CMn (VII) = 9.9 x 10 6 mol L 1, CMB = 1 x 10

L—1 — П

' mol

, CHNO3 = 0.15 mol L

Molar absorptivity, s [L mol-1 cm-1] (3.86 ± 0.06) x 104

Sandell's sensitivity, WS [p.g cm-1] 1.42 x 10-3

Beer's law, [p.g mL-1] 0.1-0.9

Regression equation AMn(Vii) = 0.6958X -- 0.0033

Correlation coefficient r = 0.997

Detection limit (DL), [^g mL-1] 0.0146

Quantitation limit (QL), [^g mL-1] 0.049

Extraction time, [s] 15

Wavelength, X [nm] 290

nitric acid 0.93-3.1 mol L-1

Organic solvent 1,2-Dichloroethane

0.6 0.5




H 0.3 s

< 0.2


5 10

MB x 10-6 mol L-1


The Bent—French [17] method showed that the extracted species was a 1 : 1 ion-associate of the dye cation MB+ and the MnO4 anion.

The effect of reaction time was studied in the range 5 s — 1 min. Equilibrium between the two phases, aqueous and organic, was reached in 15 s (Fig. 2).

Photometric characteristics and Precision

A plot of the concentration of aqueous phase man-ganese(VII) vs. absorbance of the organic layer showed good linearity in

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