ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2011, том 66, № 4, с. 394-398
^=ОРИГИНАЛЬНЫЕ СТАТЬИ
УДК 543
TRIPHENYLTETRAZOLIUM CHLORIDE AS A NEW ANALYTICAL REAGENT FOR MOLYBDENUM(VI): APPLICATION TO PLANT ANALYSIS
© 2011 г. D. Kostova
Agricultural University — Plovdiv Laboratory Complex 4000 Plovdiv, Bulgaria, Mendeleev Str., 12 Received 13.02.2009; in final form 05.08.2010
Solvent extraction of molybdenum(VI) ion associate with triphenyltetrazolium chloride (TTC) has been studied. TTC was proposed as reagent for the spectrophotometric determination of micro amounts of molybdenum(VI) at ^max 250 nm. The optimum conditions for extraction of molybdenum(VI) as an ion-association complex with TTC has been determined. Beer's law is obeyed in the range of 0.5—10 (g/mL molybdenum(VI). The molar absorptivity of the ion-pair is 1 x 106 L/mol cm. The sensitivity of the method is 9.6 x 10-5 (ag/cm2. The characteristic values for the extraction equilibrium and the equilibrium in the aqueous phase are: distribution constant KD = = 32.64, extraction constant Kex = 2.19 x 1010, association constant P = 6.71 x 108. The interferences of different cations, anions on molybdenum(VI) determination were also investigated. A sensitive and selective method for the determination of microquantities of molybdenum(VI) has been developed. The determination was carried out without preliminary separation of molybdenum. A novel procedure of molybdenum(VI) extraction and spectro-photometric determination in different plant samples was examined.
Keywords: molybdenum(VI), triphenyltetrazolium chloride, spectrophotometry, liquid — liquid extraction, plants.
In the conditions of the intensive chemization in agriculture the effect of fertilization on the system of soil-plant becomes an object of a careful study. The studies of the variation the content of the microelements are of a particular interest. The concentration of certain trace elements in the soil is an important factor for their entry into plants. Low levels cause some functional diseases in the plants, as well as low yields and low quality.
Molybdenum is one of the microelements which are actively absorbed by plants. Complete information about the molybdenum content in plants is required to establish the need for molybdenum in fertilizers. Molybdenum 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. The formation of plant mass depends on molybdenum supply. Molybdenum is important for the synthesis of the organic substance in plants and the metabolism of a number of nutrient elements in a plant organism.
Spectral and chemical methods are constantly used for the determination of molybdenum. Therefore, new organic reagents for selective and sensitive determination of molybdenum are of particular interest.
Numerous reagents have been suggested for the determination of molybdenum: 2-hydroxyacetophenone ben-zoylhydrazone [1], sodium pentamethylene ditiocarbam-ate [2], 4-(2-pyridylazo)-resorcin [4], 2,4-diaminophenol [5], 2,4-dihydroxyachetophenon [6], 8-hydroxyhinolyn [7], 9-(2,4-dihydroxyphenyl)-2,3,7-trihydroxyl-6-fluor-
one [8], 3,5-dibromo-4-hydroxyphenylfluoron [9], 2,3,7-trihydroxyphenylfluorone [10], bythyltriphenyl fosphoniev bromid [1l].
Some ofthe methods proposed [1, 2] are not very sensitive, other methods [5—7] are of low selectivity, and the rest [4, 11] are time consuming.
The chromogenic reagent triphenyltetrazolium chloride, that we suggest, is a new reagent for the determination of microquantities ofmolybdenum(VI) without preliminary separation. This paper describes a new sensitive, selective and raapid method for the determination ofmo-lybdenum in diverse plants.
EXPERIMENTAL
Apparatus. The spectrophotometric measurements were made with UV-VIS spectrophotometer utilizing 1-cm quartz cells.
Reagents and solutions. All the reagents used were of analytical reagent grade.
A stock solution of molybdenum(VI) was prepared by dissolving 0.618 g of ammonium heptamolybdate tetrahydrate in 50 mL of distilled water. More dilute solutions were prepared by diluting suitable aliquots of the standard solution to known volumes with distilled water as required.
2,3,5-Triphenyltetrazolium chloride (TTC), Fluka. A 1 x 10-3 M aqueous solution was prepared by dissolving 0.0167 g of TTC in 50 mL of distilled water. Other TTC
concentrations were prepared by appropriate dilution. The solutions were stable for months.
Phosphoric, hydrochloric, nitric and sulfuric acids at concentrations of 0.2, 0.12, 0.15 and 0.2 M, respectively, and 1,2-dichloroethane were used.
Solutions of ions for interference studies were prepared by dissolving the amount of each compound needed to give 10 mg/mL of the ion concerned.
Procedure. Preparation of the calibration graph. Equal molar amounts of the solution containing molybdenum, 0.5 mL of 2 x 10-2 M phosphoric acid and 0.5 mL of1 x X 10-3 M TTC are introduced into separatory funnels. The mixture is diluted with distilled water to an aqueous phase volume of 10 mL and extracted with 3 mL of1,2-dichloroethane for 30 s. The organic layer is then transferred through filter paper into a 1-cm cell and measured at 250 nm. A blank solution containing all reagents except molybdenum was prepared and treated in the same way.
Determination of molybdenum in plant samples. A wet burning of the plant samples was carried out and a mixture of sulfuric and nitric acids 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 were 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 [12]. After cooling, the solution was diluted with water and filtered. It was transferred into a volumetric flask of 50 mL and diluted up to the mark with distilled water. Aliquote parts of this solution were taken for analysis.
In separatory funnel of 100 mL the following solutions were introduced: 0.5 mL phosphoric acid, 2 x 10-2 M; 0.5 mL triphenyltetrazolium chloride, 1 x 10-3 M, aliquote of the prepared solution of plant sample. The mixture was diluted up to a volume of the aqueous phase of 10 mL with distilled water and extracted with 3 mL of1,2-dichloroethane for 30 sec. The organic phase was filtered through a dry paper into a 1 cm cuvette and the absorbance was measured at 250 nm. A blank was run in parallel in the absence of plant sample. A calibration graph was constructed with standards similarly treated.
RESULTS AND DISCUSSION
Extraction of the complex into organic solvents and determination of the wavelength of maximum absorption.
Molybdenum(VI) forms an ion-pair with triphenyltetrazolium chloride. The solubility ofthe ion-associate in various solvents was investigated. Several organic solvents such as benzene, toluene, chlorobenzene, chloroform, dichloroethane and amyl alcohol were tested as solvents for the extraction of the complex. It is not extractable into benzene, toluene, chlorobenzene, amyl alcohol, partially extractable into chloroform, and completely extractable
0.8 0.7 0.6
<D
S 0.5
CS
H 0.4
|0.3 0.2 0.1
0
235 240 245 250 255 260 265 270 Wavelength, nm
Fig. 1. Electronic spectra of the TTC — Mo(VI) ion associate in 1,2-dichloroethane: 1 — Mo — TTC and 2 — TTC.
cMo(VI) = °-8 x 10 5 M, cTTC = 1 x 10
-5
M.
into 1,2-dichloroethane. The absorption spectrum of the ion-pair in 1,2-dichloroethane is shown in Fig. 1. We chose 250 nm as a working wavelength because of greater absorptivity at this wavelength.
Composition, stability and effect of time of the ion-association complex. The isomolar series method [13] showed that the extracted species was a 2 : 1 ion-associate
of the TTC cation and the MoO4 anion.
The organic phase containing 0.8 x 10-5 M molybde-num(VI) was extracted as described under "Procedure". The absorption was measured at 250 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 25 days after extraction.
The effect of reaction time was studied in the range 5 s—1 min. The optimum reaction time was 30 s.
Photometric characteristics. A plot of the concentration of molybdenum(VI) in aqueous phase vs. absorbance ofthe organic layer showed good linearity in the range 0.5— 10 ^g of molybdenum (VI) per 1 mL of aqueous phase. The molar absorptivity ofthe complex is 1 x 106 L/mol cm. This demonstrates the high sensitivity of the given reaction. The sensitivity of the method is 9.6 x 10-5 ^g/cm2. The reproducibility of the method was checked by analysing two series of solutions (10 determinations for each series) having Mo(VI) concentration of 3 or 4 ^g/mL. The relative standard deviation is found to be 0.8 and 0.5%, respectively.
Effect of acidity. The effect of the type of mineral acid and its concentration upon the interaction of molybde-num(VI) and TTC was studied. The extraction of molyb-denum(VI) was carried out over the concentration range of 0.2 x 10-3 to 6 x 10-3 M phosphoric acid. The results indicate that the quantitative extraction of molybdenum(VI) is optimum in the concentration range of 0.6 x 10-3 to 1 x x 10-3 M phosphoric acid (Fig. 2). There is incomplete extraction in hydrochloric acid medium and no extraction in
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KOSTOVA
0.8 0.7 0.6
О ce
•8 0.4
о
0.3 A 0.2
0.1
4
[H3P04]x10-3, M
Fig. 2. Effect of phosphoric acid concentration on the extraction ofthe TTC — Mo(VI) complex into 1,2-dichloro-ethane. cMo(VI) = 0.8 x 10-5 M, cTTC = 3 x 10-5 M.
1.0 0.9 0.8 0.7 8 0.6 J 0.5 s0.4
^0.3 0.2 0.1
0
10
15
20
25
[TTC]x10 , M
Fig. 3. Dependence of the ab
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