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Extraction-Spectrophotometric Study of the System Manganese(VII)—Methyl Violett—Water—Chloroform and its Application © 2011 г. D. Kostova
Agricultural University, Laboratory Complex, Plovdiv, Bulgaria E-mail: deny_kostova@yahoo.com Поступила в редакцию 10.02.2010 г.
The system manganese(VII)—methyl violet(MV)—water—chloroform has been studied. The composition of
the complex is established as MV : MnO4 = 1 : 1. The molar absorptivity of the complex is (2.67 ± 0.09) x
x 104 l mol-1 cm-1 at 300 nm, limit of detection is 0.027 p.g ml-1 and the system obeys Beer's law in the range 0.22-1.6 p.g ml-1 Mn(VII). The characteristic values for the extraction equilibrium and the equilibrium in the aqueous phase was determined. The influence of foreign ions on the accuracy of the results is investigated. Extraction investigations of the system discussed were carried out. A new rapid, selective, simple and sensitive extractive-spectrophotometric method for the determination of trace amounts of manganese(VII) in plants,soils and steels were developed.
INTRODUCTION
Various methods for determination ofmanganese have been published. Analyses with some reagents are used: 3,4,5,6-tetrafluoro-2-carboxyphenylfluorone [1], 2,3-dihydroxynaphthalen [2], 1-(2-pyridylazo)-2-naphthol [3], 1,3,3 - trimethyl- 2-[3-(1,3,3 -trimethyl-1, 3 - H -indol-2-ylidene)propenyl]-3H-indolium [4] N,N-bis(2-hy-droxy-5-bromobenzyl)1,2-diaminopropane [5], 4-(me-thylphenyl)-3-oxobutanamide [6], thiazolylblue tetra-zolium bromide [7], neotetrazolium chloride [8], 3,3'-(3,3'-dimethoxy-4,4'-biphenylen)-bis(2-n-nitrophenyl-tetrazol [9] and atomic absorption spectrophotometry [10-14].
The present paper aims at studying the system manga-nese(VTI)—methyl violet(MV)-water—chloroform. Here we report on the use of triphenylmethane dye Methyl Violett (MV) as counter-ion for formation of ion-association comples with the anion MnO4 and determination of manganese(VII). We propose an extremely simple and direct extraction for determination of microquantities of manganese and its application.
EXPERIMENTAL
Reagents, Solutions and Apparatus
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 ofdistilled water. After 10 days the exact concentration of manganese(VII) was checked titrimetri-cally using oxalate titration. Working solutions containing 1.98 x 10-4 mol l—1 Mn(VII) were prepared by dilution.
Methyl Violett (MV, Fluka). A 1 x 10-3 mol l-1 aqueous solution was prepared by dissolving 0.0394 g ofMV in 100 ml of distilled water. Other MV 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 glycocoll plus HCl; buffers of pH 4, 5 were of acetic acid glacial plus NaOH; buffers of pH 6, 7 were of KH2PO4 plus Na2HPO4.
Chloroform, p.a.; Hydrochloric, phosphoric, nitric, sulphuric and perchloric acids at concentrations of1.2, 2, 1.55, 2, 9 mol l-1, respectively, were used.
Absorbance measurements were made a spectropho-tometer UV-VIS with 1 cm quarts cuvette, 300 nm.
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-1 concentration of the ion of interest.
Procedures Preparation of the calibration graph
Transfer an aliquot of the stock solution containing 2.2—16 ^g of manganese(VII) to a 100 ml separatory funnel. Add 0.3 ml ofMethyl Violett 1 x 10-3 mol l-1, 5 ml of buffer pH 4. Dilute to 10 ml with distilled water and shake with 3 ml ofchloroform for 30 s. Transfer the organic layer through a dry filter paper into a 1 cm cuvette and measure the absorbance at 300 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 [15]. After cooling, the solution was diluted with water and filtered. It was transferred into a volumetric flask of50 ml and diluted up 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: aliquote ofthe prepared solution ofplant sample, 5 ml ofbuffer pH 4, 0.3 ml of Methyl Violett 1 x 10-3 mol l-1. It is diluted up to a volume of the aqueous phase of 10 ml with distilled water and extracted with 3 ml of chloroform for 30 s. The organic phase is filtered through a dry paper into a 1 cm cuvette and the absorbance measured at 300 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 soils
4 g of the air-dried soil sample is weighed and introduced into a platinum crucible, then heated for 3 hrs in a muffle furnace at 500-550° C to digest the organic matter. After cooling, the sample is transferred into a platinum bowl and 5 ml of concentrated H2SO4, 5 ml of concentrated HNO3 and 2 ml H2O2 are added. The sample was heated until the white fumes of SO3 appeared. Next, 1 ml ofH2SO4 and 3 ml of HNO3 was added and evaporated to dryness. 5 ml of concentrated HCl and 2 ml of concentrated HClO4 were added to the dry residue, then heated to dryness evaporating chromium as chromylchloride with distraction of the organic compounds [15]. The dry residue is then dissolved in distilled water and acidified with hydrochloric acid. The solution obtained is then neutralized with concentrated ammonia solution to precipitate the iron(III) as the hydroside. The precipitate is filtered through filter paper and the filtrate is evaporated until the volume is about 30 ml. 3 ml of concentrated H2SO4, 2 ml of concentrated H3PO4 and 0.1 g ofpotassi-um periodate are added, the solution is heated to the boiling point and the temperature maintained for 10 min [16]. Then the solution was cooled and transferred into a 50 ml volumetric flask, and diluted to the mark with distilled water. To mask the interfering ions 0.3 ml of Methyl Violett 1 x 10-3 mol l-1, 5 ml of buffer pH 4 and 2 ml of saturated tartrate solution were added to 3 ml of soil solution. The solution was diluted to 12 ml with distilled water, then shaken for 30 s with 3 ml chloroform. The organic phase was filtered through a dry paper into a 1-cm cuvette and the absorbance was measured at 300 nm. A blank was run
1.2 1.0
8 0.8
I 0.6
O ca
% 0.4
0.2
0
3
270 280
290 300 310 Wavelength, nm
320 330
Fig. 1. Absorption spectra in chloroform of Methyl Violett (1), ion-association complex (2) and MnO- (3); CMn(vii) = = 1.98 x 10-5 mol l-1, CHCl = 2.4 x 10-2 mol l-1, CMV = = 3 x 10-5 mol l-1.
in parallel in the absence of the soil sample. A calibration graph was constructed with standards similarly treated.
Determination of manganese in steels
Dissolve a 0.1 g sample ofthe steel in 30 ml ofa 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 salts in water, and dilute to volume in a 100 ml standard flask. Oxidize a suitable aliquot as described above for soil samples. To a suitable aliquot of the resultant solution add 0.3 ml of Methyl Violett 1 x 10-3 mol l-1, 5 ml of buffer pH 4, 2 ml of saturated tartrate solution (to mask any interfering ions) and dilute to 12 ml with distilled water. Extract the ion associate and complete the determination as for soil analysis.
RESULTS AND DISCUSSION
Choice of organic solvent and determination ofthe wavelength of maximum absorption
A number of different types ofsolvents were tested: hydrocarbons (benzene and toluene), ethers, ketones, alcohols, chloroform and dichlorethane. The extraction was complete and quantitative with chloroform and a clear separation was obtained (Fig. 1).
The associate is partially extractable into 1,2-dichloro-ethane (Fig. 2), not extractable into benzene, toluene, chlorobenzene, amyl alcohol. Chloroform appears to be a better extractant for manganese(VII). The absorption spectra of solutions of triphenylmethane dye and the ion-association complex in hydrochloric acid show that the absorption maxima are around 300 nm (Fig. 1). We chose 300 nm as a working wavelength because of the greater absorptivity at this wavelength. The absorption additivity, without a significant shift in the absorption maximum, is
1
0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05
0
100 200 Wavelength, nm
300
400
Fig. 2. Absorption spectra in 1,2-dichloroethane of Methyl Violett (1), ion—association complex (2) and MnO— (3); CMn(VII) = I-98 x I0
-5
mol l-1, CHCl = 2.4 x 10-2 mol l
-1
CMV = 3 x 10-5 mol l
an indication of a formation of an ion-association complex.
-1
0.25
« 0.20
O
M 0.15
ÎH
i^o.io
0.05 0
0.5
1.0 1.5
Time, min
2.0
2.5
Fig. 3. Dependence of the absorbance on the extraction time; CMn(VII) = 1.98 x 10-5 mol l-1, CHNO3 = 0.62 mol l-1, CMV = 3 x 10-5 mol l-1.
of the method is ±1.2%. The Sandell sensitivity index [16] is 2.06 x 10-3 ^g cm-2. The characteristics of the extrac-tion-spectrophotometric study of manganese(VII) are indicated on Table 1.
1
2
3
Photometric characteristics and precision
A plot of t
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