Rapid simultaneous quantitative determination of ferric and ferrous ions in geological solution sample with new synthesized âdiketone

 Iron is the fourth most abundant element, by weight, making up the crust of the earth [1].Concerning its biological activity, iron is a highly versatile element, serving as active center of proteins responsible for oxygen and electrons transference in metalloenzymes such as oxidizes and dehydrates [2]. Iron is an essential element for living organisms ,being considered as the transition metal is more important for the biological system ,about 70% of active iron in mammals is encountered as prophyrinic complexes in hemoglobins,myoglobins and cytochromes.The colorimetric determination of iron ions is regarded as the most precise and simple method.

 Among the most popular colorimetric reagents such as 1,10-phenanthroline[3],bathophenanthroline(4,7-diphenyl-1,10phenanthrolin)[4],ferrozine(3-(2-pyridyl)-5,6-bis(4-phenylsulfonic acid)-1,2,4-triazine)[5] and 2,4,6-tris (2-pyridyl)-1,3,5-triazine(TPTZ)[6] are the

common chelating agent for Fe(II).Triton(4,5-dihydroxy-1,3-benzene disulfonic acid[7] and 1-(2-pyridylazo)-2-naphthol(pan)[8] are the chelating reagent for Fe(III).

Atomic absorption spectrophotometry is useful for the determination of iron, but it measures only total iron, the method requires also an expensive spectrophotometer. The main problem in the above methods is that only one of the oxidation state of the iron (either Fe (II), Fe(III) or total Fe)can be measured at a time. This requires an additional reduction or oxidation step in order to find concentration of the second oxidation state of the iron ions. Therefore, quantitative determination of both oxidation states of iron is relatively complicated and sometimes inaccurate. The new synthesized diketone forms a red colored complex with Fe(III) in acidic media. The â-diketone forms chelates with various metallic ions and has enol-keto forms in a solution by a tautomerism.After a proton is dissociated from enol-form; the anionic group more strongly coordinates with a metal ion to form a complex. The acetyl acetone is one derivative of â-diketone that is known to form various complexes with about 60 metallic elements [9].

2 Methoxy Benzene Thiol Acetyl Acetone (2MBTAA) was used as a chelating agent in this work. It was one of â-diketone derivatives in which hydrogen were substituted with 2 Methoxy Benzene Thiol (Fig.1)

2. Materials and Methods:

All spectrophotometric measurements were made with a shimadzu 3100S spectrophotometer, coupled with a color plotter model 20091027, at 25.0^oC.Stoppered quartz cells of 1.00 cm optical path length were always utilized. A Varian model AA-20 atomic absorption spectrophotometer is used for comparison of results.pH measurements were made with Metrohm Herisau E-603 pH –meter, solutions were prepared with deionized water purified with an advantec Millipore F2 HN 6496SC system. The studied â-diketone (2MBTAA) was synthesized and was checked with HNMR and IR.

All chemicals [Merck, Darmestadt,Germany] were of analytical reagent grade and were used without further purification. ligand solution [10^-2M] was prepared in Ethanol.Fe(III) in different concentration was prepared from Fe(NO₃)₃ standard solution and Fe(II) solution was prepared from (NH4)₂Fe(SO₄)₂.6H₂O.HClO₄ and NH₃ were used for pH adjustment. Interference studies were made using different salt solutions, preferentially Nitrate form (for cations) and Sodium form (for anions).

Volumetric flask of 5, 10 and 25 ml were used to prepare all samples analyzed by spectrophotometer. Aqueous solution of ferric nitrate and ferrous were prepared containing between 0.5 and 25 ppm.

3. Results and Discussion

3.1. Effect of the pH on complexation

The pH of Fe (III) - (2MBTAA) mixtures were varied between 1-10 by the use of HClO₄ or NH₃ solutions, since maximum absorbance was obtained at about pH 2.0.This value was selected as the working pH.

3.2. Effect of the light wavelength

The colored complexes of Fe(III) with (2MBTAA) was formed immediately after mixing the solutions and the color formed was stable for at list 10 h.For this reason absorbance was measured 10 min after preparing the complex. (2MBTAA) forms a red colored complex with Fe (III) in Ethanol/Water (50%v/v).Our first goal was to find the wavelength of the maximal optical absorbance of this complex using a solution containing 2.5 ml aqueous solution of Fe⁺³(11mg/L) as the nitrate, 2.5 ml â-diketone (6x10^-3M).With a final concentration of 5.5 mg/L ferric ions. This solution was red-colored due to the Ferric- â-diketone complex. Blank was used containing 2.5 ml Fe⁺³(11mg/L) mixed with 2.5 ml Ethanol. The effect of the wavelength on the light absorbance by these solutions is shown in (Fig. 2).                  

It can be seen that the maximal absorbance was observed at a wavelength of 464 nm in solution. A richly colored red complex with all the iron ions is formed when H₂O₂ is added to the solution of iron-(2MBTAA) complex. We studied the effect of the light wavelength on the optical absorbance of solution containing this complex. The complex was prepared by adding 50 µl of 30% aqueous solution of H₂O₂ to the solution of red-colored complex obtained in the previous experiment.

The color immediately turned richly colored red .The maximal absorbance in solutions was observed at a wavelength of 464 nm (Fig. 3).

3.3. Effect of the reagent concentration

The effect of volumes of H₂O₂ (30%v/v) and (2MBTAA) concentration, used in the analysis, was studied next. The volumes and concentration of these solutions were varied between 0.01 and 1 ml H₂O₂, 1 and 10 mole ratio (2MBTAA)/Fe⁺³.Our results showed that the optimal volume 50 µl

H₂O₂ and 5 fold molar excess of reagent to that of metal ion leads to the maximum absorbance.

The following procedure was adopted in the next experiment:

1.        For the measurement of ferric iron: 2.5 ml of aqueous ferric nitrate solution in pH=2 with 2.5 ml of â-diketone .the light absorbance was measured at 464 nm.

2.        For the measurement of total iron: 50 µl of 30% H₂O₂ solution was added to the above solution. The spectrometric measurement was performed at 464 nm.

3.4. The effect of ferric and total iron concentrations on light absorbance

 The dependence between the ferric iron concentration and the light absorbance was studied next. The Fe⁺³ concentration in the solution was varied between 05-22ppm after addition of â-diketone .The relationship was linear in the entire range of concentrations studied (Fig. 4) and obeys the

Beer's law with the  linear regression equation was  of

A= 0.024x10⁵C _Fe+3+0.044     

Where A is the absorbance and C _Fe+3 is the iron concentration in Mx10⁵.The correlation coefficient was equal to 0.999 for absorbance of up to 1 ,corresponding to Fe⁺³ concentration of 22 ppm in the analyzed solution.

The relation between total iron concentration in the initial solution (contain constant ferrous concentration and different ferric concentration) (C_{Fe (total)}) and Absorbance (Fig. 5) is linear and also obeys Beer's law. The following linear regression equation was obtained again:

For a curve: A= 0.024x10⁵C_{Fe (total)} +0.2861

For b curve: A=0.024x10⁵C_{Fe (total)} +0.5296

The correlation coefficient was 0.999.This relationship is valid for A<1.0

3.5. The ratio between ferrous and ferric ions concentrations

The effect of ferrous ion concentration on determination of Fe⁺³ was studied next. The concentration of Fe⁺³ was measured. In solutions containing ferrous and ferric ions in ratios between 0 and 20 .The effect of ferrous iron on the measurement of Fe⁺³ is shown in (Fig. 6).

It can be seen that ferrous iron has practically no effect on the ferric iron determination, only when Fe⁺² increased, the intensity of the red color increased, which is probably due to the oxidation of ferrous iron by atmospheric oxygen.

The ferrous ion concentration was calculated as a difference between the total and ferric ion.

3.6. Stability of color complex in time and stoichiometry of the complex

The stability of color complex in time was also studied .Solution containing ferric iron was used, and colored solution was kept at a temperature of 25⁰C.our results showed that the absorbance of red complex did not change significantly during 10 h after their preparation, the maximum change observed was below 1.3% of the initial value.Stoichiometry of complex was found 3/1 ligand/metal by contineous method.

3.7. Interference effects in iron determination

The effect of cations and anions, usually found along with iron in solutions, on the precision of the new analytical method, was studied. the following cations (as nitrate) were used:Mg⁺²,K⁺¹,Na⁺¹,Ca⁺²,Al⁺³,Si⁺² didn't affect the ferric iron determination with ratios of at least 50,50,50,,40,10,10 g/g Fe respectively. The following anions (as Na) were used:NO₃^-,ClO₄^-,Cl^-,SO₄^-2 didn't affect the ferric iron determination with ratio upper 50 g/g Fe.

The effect of the above-mentioned ions on the total iron determination was also studied, the presence of H₂O₂ did not affect on the above result for total iron.

Bauxite (Dominican B 103) was used as reference material [10].That containing 61.34 ppm Fe⁺³ which was spiked with 33.52 ppm of Fe⁺² and determined with the preposed method. It is presented in (Table. 1.), very good concordance was obtained between theoretical and experimental methods for the material tested.

3. Conclusion:

A rapid and precise spectrophotometric method for quantitative measurement of both ferric and ferrous in solution is reported. Both forms of iron are measured in the same single sample which significantly reduces the error in measuring the ratio between the both oxidation states of iron ions. The most appropriate conditions for the analytical procedure were determined. The concentration of total iron in the analyzed solution should be below 22 ppm. It has been shown that the color complexes are stable in time, no significant change in absorption was detected at least 10h after their preparation. The stoichiometry of complex is 3/1: ligand/metal. The effect of the cations and anions usually present in solution was studied.

Acknowledgements:

This work was supported by Ferdowsi Mashhad university of Iran and the Geological survey of Iran .

Reference

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