Spectrophotometric determination of Iron(III)

Transcription

1 Spectrophotometric determination of Iron(III)

2 Section 1: Introduction The word ferric is derived from the Latin word Ferrus for iron. Ferric refers to iron containing materials or compounds. In chemistry, iron with an oxidation number of +3, also denoted iron (III) or Fe +3 which is usually the most stable from of iron in air. Iron is an abundant element with a Clarke number of 4.70, the fourth largest among the elements [1], and it is an essential component of almost every organism in the biosphere. It occurs in a variety of rocks and soil minerals of oxidation states 2 and 3; but it is only a trace element in biological system. Iron plays a central role in the biosphere. It is essential component or cofactor of numerous metabolic reactions and living cells including, both plants and animals. It is involved in oxygen transport and electron transfer and in enzymes including hydroxylayed peroxidases and dismutases [2]. Iron deficiency anemia is one of the world s most common nutritional deficiency diseases. Evidence has been presented that at low levels iron is an essential element in the diet whereas at higher concentrations it is toxic [3]. Excess of iron in body causes haemochromatosis. The toxicity of iron and in particular iron overload has are used considerable interest in recent year [4]. The average adult human body contains 4-6 grams of iron. In human beings, the majority of iron is found in the blood as a pigment called hemoglobin. The function of this is to transport oxygen from lungs to various tissues in the body where it is used to produce energy. One of the byproduct of this metabolism, carbon dioxide, is thus transported back to the lungs by hemoglobin. Both the oxygen and carbon dioxide molecules bind to the iron ion present in hemoglobin during transport. Humans obtain the iron necessary for the formation of hemoglobin from their diet in foods such as meat and leafy, green vegetables etc. 94

3 The bioavailability of iron is of great interest because all known forms of life require iron and ordinary iron (III) compounds are insoluble in an aerobic environment. The low bioavailability of iron affects all forms of life. The impact of increasing the bioavailability of iron was famously demonstrated by an experiment, where a large area of the ocean surface was sprayed with iron (III) salts. After several days, the phytoplankton within the treated area bloomed to such an extent that the effect was visible from outer space. This fertilizing process has been proposed as a means to mitigate the carbon dioxide content of the atmosphere [5]. Ferric iron is a d 5 center, means that the metal has fine valence electrons in the 3d orbital shell. The magnetism of ferric compounds is mainly determined by these five electrons, and their behavior depends on the number and type of ligands attached to iron, as described by ligand field theory usually ferric ions are surrounded by the ligands arranged in octahedron. Sometimes three and sometimes as many as seven ligands are observed. Due to its importance in the contest of clinical diagnosis, intoxication, environmental pollution monitoring etc., a number of sensitive analytical methods are available for the determination of iron. There are certain reagents applied for determination of iron by solvent extraction method [6-8]. Flame and graphite furnace atomic absorption spectroscopy (AAS) [9-13] are the most commonly used techniques for iron determination. But these methods are disadvantageous in terms of cost and instruments used in routine analysis. AAS is often lacking in sensitivity and affected by many conditions of samples such as salinity. Extraction methods [14-17] are highly sensitive but generally lack in simplicity. Spectrophotometry is essentially a trace analytical technique and is one of the most powerful tools in chemical analysis. A wide variety of reagents have been proposed for the spectrophotometric 95

4 determination of iron (III) [18-28]. Among them, 1-10 phenanthroline is considered as most selective reagent for the iron determination [29]. But this method suffers from interference of foreign ions, stability, simplicity and range of determination. Among existing methods for the spectrophotometric determination of Fe(III), some are extractive methods, some have low sensitivity and some have interference with other metal ions. Though large number of spectrophotometric methods are reported for the determination of Fe(III), still there is a demand for simple, highly sensitive and selective methods for its determination is complex material. Hence, the present method explores 2-hydroxy3-methoxy benzaldehyde thiosemicarbazone (HMBATSC) as a analytical reagent used for the spectrophotometric determination of Fe(III). The developed method can be employed for efficient determination of iron at microgram level. The proposed method is sensitive, rapid and free from limitations. The following section comprises the results obtained in the present investigations. In this thesis, spectrophotometric method for the determination of Fe(III) was developed by measuring the absorbance of greenish yellow colored complex solution (ph 6.0) of [Fe(III)-HMBATSC] at 385 nm. 96

5 Section 2: Zero order spectrophotometric determination of Fe(III) The reaction between Fe(III) and 2-hydroxy-3-methoxy benzaldehyde thiosemicarbazone (HMBATSC) in the ph range was resulted in a greenish yellow colored water soluble complex. The color formation was instantaneous, and found to be maximum and constant in the ph range 5.5 to 6.5. a) Absorption Spectra The absorption spectra of greenish yellow colored [Fe(III) HMBATSC] complex and yellow colored HMBATSC solutions were measured in a wavelength region of nm using the general procedure 3.a, and corresponding absorption spectra were presented in Fig The Fe(III) metal complex shows a absorption maximum at 385 nm where the reagent has considerably low absorbance. Therefore, Fe(III) was determined by measuring the absorbance at 385 nm using reagent blank as reference solution. b) Effect of ph The greenish yellow color formation between Fe(III) and HMBATSC was ph dependent and occurs only in acidic buffer medium. Hence, the optimum ph range in which maximum and constant color intensity was determined by measuring the absorbance of Fe(III) complex solution at different ph values employing the procedure given in 3.b. The results presented in the form of a plot in Fig which reveals that maximum sensitivity can be obtained in the ph range 5.5 to 6.5. Therefore, ph 6.0 was chosen as the optimum ph to get maximum sensitivity for the determination of Fe(III). 97

6 Fig Absorption Spectra of (a) [Fe (III) HMBATSC] Vs HMBATSC blank (b) HMBATSC Vs buffer blank [Fe (III)] = M; HMBATSC = M ph = 6.0 Fig Effect of ph on absorbance of [Fe (III) HMBATSC] [Fe(III)] = M; [HMBATSC] = M Wavelength = 385 nm 98

7 c) Effect of reagent concentration The absorbance data presented in Table for solutions containing different molar ratios of metal to reagent concentrations at ph 6.0 has been confirmed that a 10-fold excess of reagent compared to the metal ion concentration was necessary to get maximum and constant coloration. Table Reagent effect [Fe(III)] = M [HMBATSC] = M ph = 6.0 Wavelength = 385 nm [Fe(III)] : [HMBATSC] Absorbance 1 : : : : : : : d) Calibration and Precision The absorbance values of experimental solutions containing variable amounts of Fe(III) and fixed amounts of HMBATSC and buffer ph 6.0 measured at 385 nm as described in 3.d were plotted against the amount of Fe(III) and presented in Fig A straight line obtained as shown in the Fig , indicated that Beer s law was obeyed over the range of µg ml -1 of Fe(III). The straight line corresponds to the equation A 385 = C with a correlation coefficient of The molar absorptivity of the proposed method calculated from the slope of 99

8 the calibration graph was x 10 4 l mol -1 cm -1 at 385 nm. The Sandell s sensitivity was calculated to be µg cm -2. e) Effect of foreign ions Amount of Fe(III) (µg ml 1 ) Fig Calibration plot [HMBATSC] = M Wavelength = 385 nm ph = 6.0 The selectivity of a spectrophotometric method can be determined from the tolerance limits of other associated ions with the analyte ion evaluated using the procedure described in 3.e. The tolerance limits of various diverse ions in the present method are placed in Table The amount of diverse ion which causes a change in the absorbance value by +2% was taken as its tolerance limit. 100

9 Diverse ion Table Tolerance limit of foreign ions Amount of Fe(III) = µg ml -1 Tolerance limit (µg ml -1 ) Diverse ion Tolerance limit (µg ml -1 ) Thiosulphate 1220 Te(IV) 1050 Thiourea 1060 U(VI) 1020 Thiocyanate 890 Na(I) 880 Fluoride 810 K(I) 790 Sulphate 780 Ti(IV) 700 Phosphate 740 Zn (II) 650 Iodide 720 W(VI) 555 Bromide 600 Y(III) 510 Ascorbate 560 Ce(IV) 340 Nitrate 520 Cd(II) 280 Bromate 440 Ag(I) 150 Tartrate 400 Mn(II) 120 Acetate 380 Ru(III) 100 Chloride 365 Mo(VI) 90 Formate 350 Ga(III) 90 Oxalate 280 Pd(II) 75 Citrate 210 Cr(VI) 45 In(III) 40 V(V) 30 Ni(II) 25 Co(II) 20 Cu(II) Fe(II) V(IV) Au(III) 10, 110 a 8, 105 a 10, 100 b 10, 120 c In the presence of (a) 1220 µg of thiosulphate; (b) 810 µg of fluoride; (c) 720 µg of iodide. 101

10 The clear investigation on interference studies (Table 5.2.2) indicates that all the anions did not interfere even when present in more than 200-fold excess. Majority of cations did not interfere when they were present in more than 50-fold excess. Cr(VI) and In(III) were tolerable up to 40-fold excess. V(V), Ni(II) and Co(II) interfered when present in between fold excess. The metal ions, Cu(II), Fe(II), V(IV) and Au(III) are interfered under 10-fold excess. However, Cu(II) and Fe(II), in presence 1220 µg of thiosulphate, V(IV) in presence of 810 µg of fluoride and Au(III) in presence of 720 µg of iodide were tolerable up to 100-fold excess. f) Composition and stability constant The stoichiometry of the greenish yellow colored [Fe(III)-HMBATSC] complex solution was determined by Job s continuous variation method (Fig ) and molar ratio method (Fig ). Both the methods gave a 1:2 (metal : ligand) stoichimetry. The stability constant of the complex was calculated from the Job s curve and obtained as

11 Fig Jobs Curve [Fe(III)] = [HMBATSC] = M Wavelength = 385 nm; ph = 6.0 Fig Molar ratio plot [Fe(III)] = [HMBATSC] = M Wavelength = 385 nm; ph =

12 g) Applications The proposed method was applied for the determination of Fe(III) in steel alloys and environmental water samples. (i) Determination of Fe(III) in Stainless steel and Chromium-nickel type steel: A known amount of the samples (~1.0 g) were placed in a 100 ml beaker, added 10 ml of 20% (v/v) sulfuric acid and carefully covered with a watch glass until the brisk reaction subsided. The solution was heated and simmered gently after addition 5 ml of 14 M HNO 3 until all carbides were decomposed. Then, 2 ml solution of H 2 SO 4 (1:1) was added and the mixture was evaporated carefully until the dense white fumes dried off the oxides of nitrogen and then cooled at room temperature. After appropriate dilution with water, the contents of the beaker were warmed to dissolve the soluble salts. The solution was then cooled, neutralized with NH 4 OH solution and filtered through a Whatman 41 filter paper into a calibrated flask of known volume. The residue was washed with a small volume of hot 1% H 2 SO 4 followed by water and the volume was made up to the mark with water. The analytical results are shown in Table The amount of Fe(III) in these sample solutions could be determined from a predetermined calibration plot. (ii) Determination of Fe(III) in environmental water samples The water samples collected in a clean 1 liter beakers from different place of Anantapur and Kurnool districts (Andhra Pradesh, India) were filtered through 0.45 µm pore size membrane filters immediately after sampling. The water samples were slowly evaporated to about 25 ml. 5 ml of H 2 O 2 was added and evaporated to dryness [30]. It was then dissolved in 2 ml of water and filtered to remove insoluble substance. The filtrate was collected in 100 ml volumetric flask quantitatively and 104

13 diluted to the mark with distilled water. The filtered water samples were analyzed using the proposed to determine iron(iii) using zero order method. A known amount of Fe(III) was added to the water samples and the recovery was evaluated as an average of five determinations. The results were presented in Table and indicate the recoveries were in the acceptable range. Table Determination of Iron(III) in Steel alloys Alloy sample Composition (%) Amount of Fe(III) (%) Taken Found* Relative error (%) Cr (11-13), Ni (10), C ( ), Fe (77) Cr (16), Mn (14), Ni (1), Fe (69) Mn (0.81), Cr (0.66), Mo (0.58), Ni (2.55), Cu(0.088), Sn (0.011), Fe (95.0) * Average of five determinations. Table Determination of Fe(III) in environmental water samples Sample Amount of Au(III) ( µg ml -1 ) Added Found * Recovery (%) Ground water Rain water Drain water Lake water * Average of five determinations. 105

14 Section 3: Derivative spectrophotometric determination of Fe(III) In the proposed zero order method, serious interference from certain ions was observed in the determination of Fe(III). To overcome these interferences and to improve the selectivity of the method, an attempt has been made to develop some derivative spectrophotometric methods for the determination of Fe(III). a) Derivative Spectra The second and third order derivative spectra were recorded for known aliquots of experimental solutions containing variable amounts of Fe(III) at ph 6.0 in the wavelength region nm and presented in Fig and 5.3.3, respectively. In second order spectra a trough at 400 nm and an intensified crust at 425 nm, and in the third order spectra a crust at 385 nm a trough at 395 nm and another crust at 405 nm were noticed and the curve showed zero amplitude at 389 nm and 402 nm. b) Determination of Fe(III) By adopting the general procedure as described in 3.d. linear plots were constructed between the amount of iron(iii) and measured derivative amplitudes at 400 nm and 425 nm for 2 nd order (Fig ), and at 395 and 405 nm for 3 rd order (Fig ). The linear plots show that, the calibration plots are perfectly linear in the concentration range, λ 400 = and λ 425 = µg ml -1 at 400 nm (2 st order) and λ 395 = and λ 405 = µg ml -1 at 405 nm for third order derivative method. The analytical results of the present investigations are compared and presented in the Table

15 Fig Second derivative spectra of [Fe (III) HMBATSC] Vs reagent blank [Fe(III)] (µg ml -1 ) = (a) ; (b) ; (c) ; (d) λ 425 = C Amplitude λ 400 = C Amount of Fe(III) (µg ml -1 ) Fig Beer s Law plot of Second order derivative data [HMBATSC] = M; ph =

16 Fig Third order spectra of [Fe (III) HMBATSC] Vs Reagent Blank [Fe(III)] (µg ml -1 ) = (a) ; (b) ; (c) ; (d) λ 395 = C Amplitude 0.08 λ 405 = C Amount of Fe(III) (µg ml -1 ) Fig Beer s Law plot of third derivative data [HMBATSC] = M; ph =

17 c) Effect of diverse ions The effect of various cations and anions on the derivative amplitude was studied by following the general procedure in chapter 3.2.e. It was noticed that all the ions that did not interfere in the zero order determinations of Fe(III) (cf. Table 5.2.2) also did not interfere in both second and third order derivative methods. In the zero order, the metal ions Cu(II), V(IV) and Au(III) were interfered in 10-fold excess, but in all the derivative methods they were tolerable up to 25 fold excess. Further, Fe(II) was interfered 8-fold excess in zero order, also intereferes in the derivative methods. From the interference study it can be observed that the metal ions which interfere in zero order method are greatly enhanced in the derivative methods indicating the greater selectivity of derivative methods than the direct method. d) Application Based on the results, the proposed second and third order derivative methods were applied for the determination of Fe(III) in different water samples. Procedure A known aliquot of the sample solution was taken in a 10 ml volumetric flask containing 5 ml of buffer solution (ph 6.0) and 1 ml of the HMBATSC solution (1 x 10-3 M). The contents were made up to the mark with distilled water. The derivative amplitudes at 425 nm and 405 nm were measured for 2 nd and 3 rd order derivative methods, respectively. The amounts of Fe(III) present in the sample solution was determined from the predetermined calibration plots. The results are presented in Table

18 Table Determination of Fe(III) in water samples Water Samples Drinking water Natural water Bore well water Polluted water 2 nd order derivative Amount of Fe(III) (µg ml -1 ) Recovery (%) 3 rd order derivative Amount of Fe(III) (µg ml -1 ) Added Found * Added Found * Recovery (%) * Average of five determinations Table Analytical Characteristics of [Fe(III) HMBATSC] Parameter Zero Order Second Derivative Third Derivative Analytical Wavelength (nm) and and 405 Molar absorptivity, ε (lmol -1 cm -1 ) Beer s law range (µg ml -1 ) λ 400 = λ 425 = λ 395 = λ 405 = Sandell s sensitivity (µg cm -2 ) Angular coefficient (m) Y-intercept (b) & & Correlation coefficient (r) Standard deviation (%) Complex ratio 1 : Stability constant Detection limit (µg ml -1 ) Determination limit (µg ml -1 )

19 Discussion The developed direct and derivative spectrophotometric methods for the determination of iron(iii) are simple, fast, less cumbersome, sensitive and reasonably selective. The reaction between the Fe(III) ion and the reagent was quite fast and the resultant light green colour was stable for more than 24 hours. The ph dependence of the color formation was also not critical (Fig ). The molar absorptivity ( l mol -1 cm -1 ), the detection limit (0.022 µg ml -1 ) and determination limit (0.066 µg ml -1 ) indicate the sensitivity of the proposed direct method. The relative standard deviation (0.57%) and correlation coefficient (0.9998) show the precision and linearity of the calibration plot of the present method respectively. The zero order method was successfully applied for the analysis of alloy-steels and water samples. The results indicate the acceptability of the proposed method. The derivative method developed was found to be more sensitive and at some wavelengths more selective than the direct method (Table 5.3.3). The determination and detection limits as well as the RSD values were calculated for the derivative method. The derivative method was applied for the determination of iron (III) in environmental water samples with good acceptability. The acid dissociation constants P K and P 1 K 2 of HMBATSC were found to be 4.0 and 7.0 which correspond to the dissociation of hydroxyl proton and SH proton respectively. At ph 6.0 where the analytical studies are carried out, the hydroxyl proton undergoes dissociation to give a mono anionic ligand. The composition of the resultant Fe(III)-HMBATSC complex was determined by Job s method and molar ratio method and obtained as 1:2 complex [(Fe(III): HMBATSC)]. 111

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