Spectrophotometric Determination of Iron(II) after Solid Phase Extraction of Its 2,2′ Bipyridine Complex on Silica Gel-Polyethylene Glycol

A new solid phase extraction procedure was developed for preconcentration of iron(II) using silica gel-polyethylene glycol (silicaPEG) as an adsorbent.emethod is based on retention of iron(II) as 2,2 bipyridine complex on silica-PEG.e retained complex is eluted by 1.0mol L of sulfuric acid-acetonemixture (1:2) and its absorbance ismeasured at 518 nm, spectrophotometrically.e effects of different parameters such as pH, concentration of the reagent, eluting reagent, sample volume, amount of adsorbent, and interfering ions were investigated. e calibration graph was linear in the range of 1–60 ng mL of iron(II). e limit of detection based on 3Sb was 0.57 ng mL −1 and relative standard deviations (R.S.D) for ten replicate measurements of 12 and 42 ng mL of iron(II) were 2.4 and 1.7%, respectively. e method was applied to the determination of of iron(II) in water, multivitamin tablet, and spinach samples.


Introduction
Nowadays, the extraction and determination of trace metal ions from different matrices have become of great importance and have received more attention.Among heavy metals, iron in small amounts is an essential element for most life on Earth, including humans and animals.It is well known that an iron de�ciency is the most common cause of anemia.On the other hand, too much iron can cause several health problems.High levels of iron are associated with an increased risk for cancer, heart, and liver diseases [1].erefore iron determination is of great interest and is crucial to develop simple, rapid, and efficient methods for monitoring iron in the environment.
Modern instrumental methods including ICP-MS, ICP-AES, and AAS [2][3][4][5][6][7][8] have been used for the determination of traces of metal ions in various media.However, spectrophotometry and �ame atomic absorption spectrometry (FAAS) are the most available instrumental techniques in laboratories but their sensitivity and selectivity are usually insufficient for direct determination of elements at a very low concentration levels in real samples.erefore, a separation/preconcentration step prior to the analysis is usually necessary for complex matrix environmental samples [7].
Solid phase extraction (SPE) has been used as a great tool for the separation and preconcentration of metal ions in different samples [8].e important advantages of solid phase extraction such as �exibility, simplicity, being economical, high enrichment factors, low cost because of lower consumption and, being environment friendly have made SPE as an attractive technique for separation/preconcentration of heavy metal ions [9][10][11].e choice of appropriate sorbent for solid phase extraction is a critical parameter in order to obtain full recovery and high enrichment factor [12].For this reason introducing new sorbent is still a challenge for analytical chemists.Various sorption materials, such as functionalized silica or alumina [13,14], carbon materials [15], naphthalene [16], and molecular-imprinted polymers (MIPs) [17] have been used for this purpose.
We have recently introduced silica gel-polyethylene glycol (silica-PEG) as an adsorbent for preconcentration of cobalt and nickel [18] followed by �ame atomic absorption spectrometric determination.In this paper a different aspect for using this adsorbent is presented and a new solid phase extraction method for preconcentration of trace iron(II) prior to its determination by spectrophotometery is established.Iron(II) as 2,2 � bipyridine complex was passed through a column containing silica-PEG.e retained complex was then eluted by sulfuric acid-acetone mixture (1 : 2) and its absorbance was measured at 518 nm by a spectrophotometer.

Instrumentation.
A GBC model Cintra 101, UV-Visible spectrophotometer (Sidney, Australia) was used for recording absorption spectra and absorbance measurements using 1 cm glass cells.A digital pH-Meter model 632, Metrohm (Herisau, Switzerland), with a combined glass electrode, was used for pH adjustments.A Tebazma vacuum pump (Tehran, Iran) was used for pumping the solutions.

Reagents. Analytical reagent-grade chemicals were used.
A stock solution of 1000 g mL −1 Fe(II) was prepared by dissolving 0.7020 g of (NH 4 ) 2 Fe(SO 4 ) 2 .6H 2 O (Merck, Darmstadt, Germany) in water and diluting to 100 mL in a volumetric �ask.6.4 × 10 −4 mol L −1 of 2,2 � bipyridine (Merck) was prepared by dissolving 0.01 g in water and diluting to 100 mL in a volumetric �ask.A phosphate buffer pH 8 was prepared by diluting 4.8 mL phosphoric acid (Merck) to 250 mL in a volumetric �ask and then adjusting its pH to 8 by adding 1.0 mol L −1 NaOH (Merck) and using a pH meter.Silica gel (Aldrich, USA, 70-270 mesh, 60 Å, surface area > 500 m 2 /g) was activated by treatment with 5 mom L −1 HCl (Merck) and dried in vacuum at 120 ∘ C. PEG was heated at 80 ∘ C under vacuum for 30 min before use to remove traces of moisture.

Adsorbent Preparation by Immobilization of Polyethylene
Glycol on Silica Gel.An oven dried 250 mL �ask equipped with a magnetic stirrer was charged with dried activated silica gel (20 g) under nitrogen atmosphere.en freshly distilled SOCl 2 (45 mL) was added slowly to the �ask through an addition funnel and the reaction mixture was stirred at room temperature.Evolution of copious amounts of HCl and SO 2 occurred instantaneously.Aer stirring for 4 h, the excess unreacted thionyl chloride was distilled off and the resulting grayish silica chloride was �ame dried and stored in airtight container before use.
To a well-stirred silica chloride (20 g) in dry CH 2 Cl 2 (40 mL) PEG (10 g) was added dropwise under nitrogen atmosphere and at room temperature.HCl was instantaneously evaluated.Aer stirring for 2 h, the obtained silicagel-supported PEG was removed by �ltration.For elimination of any additional PEG, the PTC was washed several times by acetone (3-30 mL).Silica-gel-supported PEG was dried in a vacuum oven overnight [19] and was used as an adsorbent in this work.
2.4.Recommended Procedure.0.1 g of silica-PEG adsorbent was placed in a glass tube (7 cm length and 7 mm i.d) with a very �ne bore and used as a preconcentration column.250 mL of the solution containing 1-60 ng mL −1 of iron(II), 2 mL of 6.4 × 10 −4 mol L −1 2,2′ bipyridine, and 5 mL of phosphate buffer pH 8 was passed through the mini column at a �ow rate of 10 mL min −1 .e adsorbed iron complex was eluted with 2.0 mL of 1.0 mol L −1 of sulfuric acid-acetone mixture (1 : 2).e absorbance of the color complex was measured at 518 nm.A blank solution was also run under the same analytical conditions without adding any Fe(II).
2.5.Sample Pretreatments.e tap water sample was collected, acidi�ed, boiled, and �ltered.An aliquot of water sample was then treated under recommended procedure.
Fresh spinach sample was purchased from local market Ahvaz, Iran.It was washed thoroughly with water and dried in an oven at 100 ∘ C for 7 h.It was then grinded to produce a �ne powder.12 g of this powder was placed in a porcelain crucible and burned on a �ame until there was no fume.It was then placed in a muffle furnace at 600 ∘ C for 5 h; 2 mL of hydrochloric acid was added and boiled for 30 min.It was then �ltered and diluted to 50 mL in a volumetric �ask.An aliquot of this solution was taken and treated under the recommended procedure.
e content of a multivitamin tablet was dissolved in water; 5 mL of hydrochloric acid 10% was added, �ltered, and diluted to 250 mL in a volumetric �ask.An aliquot of this solution was taken and treated under the recommended procedure.

Results and Discussion
e PEG-graed silica particles are special type of branched inorganic-organic copolymer which has polyether functional groups in the side chains that have been found to be stable to acid, base, high temperature, and oxygen.Its FT-IR spectrum shows the presence of OH and ethylene oxide groups on the surface of the adsorbent.Our preliminary experiments showed that iron(II) as 2,2 � bipyridine complex is retained by silica-PEG.e 2,2 � bipyridine complex is probably adsorbed through interaction with OH groups of the PEG.Since the amounts of the complex retained on the adsorbent is very small, the FT-IR spectrum aer loading the complex shows very slight shis.e absorption spectra of eluted complex from the column by sulfuric acid-acetone mixture (Figure 1) showed that maximum absorption takes place at 518 nm.erefore absorbance measurements were conducted at this wavelength.In order to achieve the highest performance for the system, the effect of different parameters such as pH, concentration of the reagent, type and concentration of eluent �ow, rates and amount of adsorbent on the solid phase extraction were studied.ions as 2,2 � bipyridine complex formation and hence its adsorption on the silica-PEG adsorbent were studied.For this purpose, the pH value of a set of solutions (100 mL) each containing 50 ng mL −1 of iron(II) was adjusted in the range of 4.0-10.0by adding 0.1 mol L −1 of HCl or NaOH.e obtained solutions were passed through the column, eluted by sulfuric acid-acetone mixture (1 : 2), and the absorbance of the color complex was measured at 518 nm.According to the results shown in Figure 2, the absorbance is constant over the pH range of 7.0-9.0.erefore, pH 8 was selected as optimum for further work and 5 mL of phosphate buffer pH 8 was added to 100 mL solutions to maintain this pH.

Effect of the Ligand Concentration
. e effect of 2,2 � bipyridine concentrations on the absorbance of iron(II)-2,2 � bipyridine complex eluted from the column was investigated.e results indicated that the absorbance was increased up to a ligand concentration of 0.96 × 10 −5 mol L −1 and became almost constant above that (Figure 3).For this reason, the present study was carried out with a ligand �nal concentration of 1.28 × 10 −5 mol L −1 as optimum value.us 2.0 mL of 6.40 × 10 −4 mol L −1 of 2,2 � bipyridine was added to 100 mL solutions to maintain this concentration.

Choice of Eluent.
e other important factors which could affect the solid phase extraction procedure are the type, volume, and concentration of the eluent used for desorption of metal complex from the adsorbent.In order to choose a proper eluent for the retained complex aer its extraction, the analyte was eluted with a series of selected eluents such as various inorganic acids, organic solvents, and their mixtures.e results of this investigation presented in Table 1 show that among different eluents used sulfuric acid-acetone mixture provided the highest absorbance for the eluted complex.erefore, this solution was chosen as an eluent for further investigations.e sulfuric acid concentration and H 2 SO 4acetone ratio were also optimized.According to the obtained results H 2 SO 4 concentration of 1.0 mol L −1 with a ratio of 1 : 2 (H 2 SO 4 -acetone) was most suitable solvent for eluting the complex from the column.e effect of eluent volume was also investigated and 2 mL of 1: 2 (H 2 SO 4 -acetone) was found to be adequate for the elution of retained iron(II)-2,2 � bipyridine complex from the column.

e Effect of the Mass of Adsorbent.
In order to test the effect of the mass of adsorbent on quantitative retention of iron(II)-2,2′ bipyridine complex different amounts of silica-PEG adsorbent were chosen and the experimental method was applied.e results showed that quantitative adsorption for the complex was obtained in the range of 0.05-0.25 g of adsorbent.Quantitative adsorption was not obtained when the mass of extractant was smaller than 0.05 g. us 0.10 g of silica-PEG was selected for further studies.

e Effect of Flow Rate.
In a solid phase extraction procedure, the �ow rate of sample solution not only affects the recoveries of the analytes, but also controls the analysis time.erefore, the effect of sample solution �ow rate was examined under the optimum conditions by passing 100 mL of sample solution through the minicolumn at different �ow rates.It was found that the retention of iron(II)-2,2 � bipyridine complex was independent of �ow rate up to 10 mL min −1 and the absorbance of the solution was decreased slightly when the �ow rate was over 10 mL min −1 .erefore, a �ow rate of 10 mL min −1 was employed in this work.

e Effect of Breakthrough Volume.
In order to achieve a high preconcentration factor, the breakthrough volume of sample solution should be established.e effect of sample volume on the adsorption of iron(II)-2,2 � bipyridine complex was studied in the range of 100-500 mL.Each solution contained same amount of Fe 2+ (5 g) and the adsorption and desorption processes were performed under the optimum conditions.e results showed that the Fe-2,2 � bipyridine present in various volumes of solution was completely and quantitatively adsorbed on silica-PEG.e adsorption decreased at higher volumes.erefore, for determination of trace quantities of Fe 2+ in samples, a sample volume up to 500 mL could be selected in order to increase the preconcentration factor to 250.However, in order to decrease the analysis time we used 250 mL for the preparation of calibration curves for which a preconcentration factor of 125 was employed.
3.7.Analytical Performance.e analytical characteristics of the developed method such as the limit of detection, reproducibility, linear range, and correlation coefficient were obtained by processing standard solutions under optimum conditions.A linear calibration graph was obtained for the determination of iron(II) under the proposed experimental conditions.e data yielded a good linearity in the range of 1-60 ng mL −1 with the equation of   1 + 2(  8) where A is the absorbance and C is the concentration of iron(II) in ng mL −1 .e limit of detection, de�ned as the concentration of the analyte giving signals equivalent to three times the blank standard deviation, was 0.57 ng mL −1 and relative standard deviations (R.S.D) for ten replicate measurements of 12 and 42 ng mL −1 of iron(II) were 2.4 and 1.7%, respectively.

Interference Studies.
In order to investigate the selectivity of the solid phase extraction procedure, the in�uence of different ions which could interfere with the determination of iron(II) in various environmental samples was studied.For this purpose constant amounts of iron(II) were taken with different amounts of diverse ions and the general procedure was followed.Any deviation of ±5% or more from the absorbance value of the standard solution was selected as interference.Results given in Table 2 show that the presence of ma�or cations and anions has no signi�cant effect on the determination of iron(II) under selected conditions.

Applications
e developed method was applied to the determination of iron in tap water, multivitamin tablet, and spinach samples, with satisfactory results (Table 3).e concentration of iron(II) could not be determined in water due to its very low concentration.ere was a good agreement between the added and the recovered concentrations of the analyte.e obtained mount of iron per tablet was 9.7 mg and the reported value was 10.0 mg which shows that there is good agreement between the two results.

Conclusion
Silica-PEG is a promising adsorbent which could be used in different ways and this paper presents one of them.e method presented in this paper is highly sensitive and selective for the determination of trace amounts of Fe(II) by spectrophotometry.e detection limits achieved are better than or comparable to some of the previously reported works.A comparison of the proposed method with some of the methods reported in literature is given in Table 4.Moreover, the adsorbent is ecofriendly and stable for at least six month.

3. 1 .F 1 :F 2 :
e Effect of pH.It has been mentioned in the literature that Fe(NN) chelates such as iron(II)-2,2 � bipyridine complex are stable in the pH range of3-10 [20].erefore, the in�uence of the pH of the sample solution on the iron(II) Absorption spectra of (a) blank and (b) the eluted iron(II) as 2,2 � bipyridine complex from the column.Effect of pH on the absorbance of 50 ng mL −1 iron(II) as 2,2 � bipyridine complex aer desorption from the column.
T 2: e effect of different ions on the determination of 50 ng mL −1 of iron(II).
a ± √ at 95 % con�dence (  5).b Amount of iron per tablet was 9.7 mg.c Amount of iron was 40.6 g g −1 of spinach.ND: not detected.
T 4: Comparison of the proposed method with some of the methods reported in literature.
a Preconcentration factor.