A simple, efficient and inexpensive ligandless cloud point extraction method was developed for the preconcentration of trace amounts of iron from natural water samples, followed by flame atomic absorption spectrometry detection. The proposed method is based on the extraction of Fe(III) ions at pH 7.0 using the non-ionic surfactant Triton X-114 without the addition of any chelating ligand. The effect of parameters influencing the extraction efficiency such as sample pH, concentration of surfactant, incubation temperature and time, concentration of NaCl and sample volume were investigated and optimized. The effect of potentially interfering ions on the recovery of iron was also examined. Under optimum conditions, the detection limit (3σ) was 0.95 μg L−1 for Fe using a sample volume of 10 mL. A preconcentration factor of 20 was achieved. The accuracy of the method was checked through the analysis of certified reference materials (SLRS-5 river water, SPS-SW2 Batch 127 surface water) and spiked water samples. The percentage recovery values for spiked water samples were between 92% and 101%.

INTRODUCTION

Iron is widely distributed in nature as a variety of compounds and is one of the most important elements in environmental and biological systems (Khayatian et al. 2010). Iron plays many more biological roles in living systems than any other element. Among its vital functions, oxygen transfer to tissues is of primary importance. Iron is needed for the suitable functioning of many enzymes involved in synthesis of DNA, energy metabolism, and protection against free radicals and microbes (Khajeh & Dastafkan 2012). It is well known that an iron deficiency is the most common cause of anaemia (Kassem & Amin 2013). On the other hand, exposure to excess iron can cause several diseases. High levels of iron are associated with an increased risk for cancer, heart disease, and other illnesses such as endocrine problems, arthritis, diabetes, and liver disease (Duran et al. 2012; Kassem & Amin 2013). The taste of water changes when water contains a certain amount of iron, for example 40 μg/L of iron (as Fe2+) concentrations can be detected by taste in distilled water. In drinking-water supplies, iron(II) salts are unstable and are precipitated as insoluble iron(III) hydroxide. Anaerobic groundwaters may contain iron(II) at concentrations of up to several milligrams per litre without discoloration or turbidity in the water when directly pumped from a well, although turbidity and colour may develop in piped systems at iron levels above 0.05–0.1 mg/L (Department of National Health and Welfare (Canada) 1990; WHO 2003a). Although the maximum allowable limit for iron is 2.0 mg/L according to the World Health Organization (WHO) drinking water guidelines (WHO 2003b), much lower concentrations of iron than this value change the taste, turbidity and colour of the drinking water. The concentration of iron is often less than approximately 1 μg/L for seawater, which amount changes significantly and is different in the Atlantic and the Pacific Ocean, 0.05–0.5 mg/L for rivers, and 100 mg/L for groundwater. Iron levels may not be more than 200 μg/L in drinking water (Lenntech). Therefore, it is very important to develop rapid, sensitive and efficient preconcentration methods for the determination of iron in environmental water samples in order to ensure public health. It should also be noted that preconcentration/matrix separation techniques are required for many of these sample types, since interferences preclude the direct analysis of samples such as seawater.

Several analytical techniques, such as spectrophotometry (Silva et al. 2010), flame atomic absorption spectrometry (FAAS) (Shakerian et al. 2009; Silva et al. 2010), graphite furnace atomic absorption spectrometry (GFAAS) (Ohashi et al. 2005; Liang et al. 2006), inductively coupled plasma–optical emission spectrometry (ICP-OES) (Sereshti et al. 2011), inductively coupled plasma–mass spectrometry (ICP-MS) (Pu et al. 2005), voltammetry (Segura et al. 2008) and fluorescence (Du et al. 2013), have been used for the determination of iron species in various types of samples. However, each of these techniques have advantages/limitations regarding cost, interferences and especially limits of detection. In comparison with other instrumental analytical techniques, FAAS is less expensive, faster than some techniques such as voltammetry and fluorescence and is widely available in most laboratories. However, due to the high salinity in seawater, low levels of iron cannot be measured because of the salt effect in atomic spectroscopic techniques so its separation from the matrix is essential, and usually necessitates a preconcentration step prior to its determination. Cloud point extraction (CPE) is a separation and preconcentration method. This method is a relatively green extraction method since it does not use toxic organic solvents. It is simple, inexpensive and is a safe procedure with high efficiency (Pourreza & Zareian 2009). The CPE procedure consists of three simple steps: (1) solubilization of the analytes in the micellar aggregates; (2) clouding; (3) phase separation for analysis. When a micellar solution of a non-ionic or weakly polar surfactant is heated up, the polarity of the surfactant is decreased. Above a certain temperature, called the cloud point, the polarity is almost displaced and, hence, the surfactant molecules separate from the aqueous phase. As a result, the clear solution becomes turbid and phase separation occurs. Above the cloud point temperature, the micellar solution separates in a surfactant-rich phase of a small volume in a diluted aqueous phase, in which the surfactant concentration is close to the critical micellar concentration. The hydrophobic, amphiphilic or ionic components, which are initially present in the solution and bound to the micelles, will be favourably extracted to the surfactant-rich phase, and only a very small portion will remain in the aqueous phase. Centrifugation and decantation of the solution assist easy separation of the two phases (Kazemi et al. 2011; Gouda 2014). CPE has been used for the extraction and preconcentration of iron after chelation with N,N′-(2,2-(ethane-1,2-diylbis(oxy))bis(ethane-2,1-diyl))bis(2-chloroacetamide) (Duran et al. 2012), 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (Liang et al. 2006), 3-((indolin-3-yl) (phenyl)methyl) indoline (Ghaedi et al. 2009), and 8-hydroxy-7-iodoquinoline-5-sulfonic acid (Ferron) (Shakerian et al. 2009). In this work, a ligandless CPE method was developed for the preconcentration of Fe(III) prior to FAAS determination. Triton X-114 was chosen as the non-ionic surfactant for the work because of its low cloud point temperature and the high density of the surfactant-rich phase as well as its low cost, commercial availability and lower toxicity. The parameters that affect the extraction efficiency, such as sample pH, concentration of surfactant, incubation time and temperature, concentration of NaCl and sample volume were investigated and optimized. The developed method was successfully applied to the determination of total Fe in tap water, river water and seawater samples. The accuracy of the developed method was verified by analysing two certified reference materials (SLRS-5 river water and SPS-SW2 Batch 127 surface water).

EXPERIMENTAL

Instruments

A PerkinElmer model AAnalyst 200 (Shelton, CT, USA) flame atomic absorption spectrometer equipped with deuterium background correction and an air-acetylene burner was used for the determination of Fe in standard and sample solutions. An iron hollow-cathode lamp was the radiation source operating at a wavelength of 248.33 nm and with a lamp current of 30 mA. All absorbance measurements were carried out using an air-acetylene flame at flow rates of 10.0 and 2.5 L min−1, respectively. A Hanna Instruments model 221 (Cluj-Napoca, Romania) digital pH-meter with a combined glass electrode was used for all pH measurements. A Nuve ST 402 model thermostatic water bath (Ankara, Turkey) was used for controlling the temperature of the CPE experiments. Phase separation was assisted by a Hettich centrifuge model Rotafix 32 A (Germany).

Reagents and solutions

All the reagents used were analytical grade and water purified by a reverse osmosis system (AquaTurk Reverse Osmosis System, HSC ARITIM, Istanbul, Turkey) was used to prepare all the solutions. Nitric acid, acetic acid and ammonia were obtained from Riedel-de Haen (Sigma-Aldrich, St Louis, MO, USA), Triton X-114 was obtained from Fluka (Gillingham, Dorset, UK) and sodium dihydrogen phosphate, phosphoric acid, ammonium acetate, ammonium chloride, and sodium acetate were all purchased from Sigma-Aldrich (St Louis, MO, USA). The laboratory glassware used was kept in 10% (v/v) nitric acid overnight and rinsed with deionized water before use. A stock standard solution containing 1,000 mg L−1 Fe(III) was prepared by dissolving appropriate amounts of Fe(NO3)2.9H2O (Merck, Darmstadt, Germany) in 1% HNO3. The working standard solutions were prepared by appropriate stepwise dilution of the stock standard solution with deionized water. The following buffer solutions (0.5 M) were used to control the pH of the solutions: sodium dihydrogen phosphate/phosphoric acid (pH 2–3), sodium acetate/acetic acid (pH 4–5), ammonium acetate/acetic acid or ammonia (pH 6–8) and ammonium chloride/ammonia (pH 9). The certified reference materials surface water (SPS-SW2 level 2 Batch 127; Spectrapure Standards AS, Oslo, Norway) and river water (SRLS-5; National Research Council, Ottawa, Canada) were used in this study.

CPE procedure

An aliquot of Fe(III) working standard solution was transferred to a 50 mL polyethylene centrifuge tube. Ammonium acetate buffer (1.0 mL, pH 7.0) and 0.5 mL of 1% (w/v) Triton X-114 solution were added. The mixture was diluted to 10 mL with deionized water. The mixture was manually shaken for 5–6 sec and left to stand for 10 min in a thermostated water bath set at 40 °C. The resulting solution was centrifuged at 4,000 rpm for 7 min to obtain phase separation. It was then cooled at +4 °C in a refrigerator for 10 min to increase the viscosity of the surfactant-rich phase. The aqueous phase was removed with a Pasteur pipette and, to decrease its viscosity, the surfactant-rich phase was diluted to 0.5 mL with 1.0 mol L−1 HNO3. The final solution was aspirated directly into the FAAS instrument.

The CPE procedure described above was also applied to the blank and calibration standards.

Applications to real samples

The certified reference materials, SPS-SW2 level 2 Batch 127 and SLRS-5, were analysed to evaluate the accuracy of the developed method. The pH of the certified water samples was adjusted to 7.0 with ammonium acetate buffer solution and 10% (w/v) sodium hydroxide solution. Afterwards, the analytical procedure given above was applied to the samples.

The proposed method was also applied to five water samples including a sample of tap water (Balıkesir University), three samples of seawater (from the Aegean Sea near the Edremit Coast, and the Marmara Sea close to the İzmit and Yalova Coasts) and a sample of river water (Küçük Bostancı, Balıkesir). The accuracy of the method was also checked by measuring the recovery of Fe(III) in spiked water samples. The river water and seawater samples were filtered through a cellulose membrane filter of 0.45 μm pore size, acidified to pH 2 with nitric acid and stored in pre-cleaned polyethylene bottles. For the determination of total Fe in the water samples, a 10 mL aliquot of each sample was oxidized by addition of 0.2 mL concentrated HNO3. The beaker was covered with a watch glass and boiled on a hot plate for 15 min. After cooling to room temperature, the pH was adjusted to pH 7.0 using ammonium acetate buffer solution and 10% (w/v) sodium hydroxide solution. Then, the sample was analysed according to the proposed method.

RESULTS AND DISCUSSION

Optimization of the experimental variables

In order to obtain maximum extraction efficiency for Fe(III) ions, several analytical parameters affecting the CPE efficiency were optimized. The analytical parameters (sample pH, concentration of surfactant, incubation temperature and time, concentration of NaCl and centrifugation time) were optimized using a 10 mL standard solution containing 50 μg L−1 of Fe(III) ions. The optimization procedure was undertaken by varying one parameter at a time while the others were kept constant. All experiments were performed in triplicate.

The effect of pH on the extraction system was investigated over the range of 2.0–9.0. The Triton X-114 concentration (0.05%), incubation temperature (60 °C), incubation time (50 min) and centrifugation time (7 min) were used to investigate the effect of pH on the extraction of Fe(III). The effect of the sample solution pH on the recovery of Fe(III) is shown in Figure 1. Quantitative recoveries were obtained over the pH range of 7.0–8.0. Therefore, pH 7.0 was selected for all subsequent experiments.
Figure 1

Effect of pH on the extraction efficiency of Fe(III).

Figure 1

Effect of pH on the extraction efficiency of Fe(III).

The aqueous chemistry of Fe is complex. However, in brief, Fe(OH)3 (aq) may be present as part of the dissolved iron in natural water at alkaline pH.

A reaction for the formation of Fe(OH)2+ is given as 
formula

K for the equilibrium can be computed from this value for total solubility, assuming a neutral pH, and amounts to 2.5 × 10−8.

The little bit of Fe(OH)3 (s) dissolves if a small amount of H+ is available in pure water at pH 7 (Nolan 1962): 
formula

The effects of temperature and ionic strength on the solubility at low and high pH have been attributed to the effects on the solubility product and the formation of Fe(OH)2+ and (Liu & Millero 1999).

In one research paper, ligandless CPE of lead was used for the preconcentration of lead from waters. In this paper, the extraction of lead between pH 8 and 10 was attributed to the formation of a cationic complex with Triton X-114 and [Pb(OH)]+ through the polyoxyethylene groups (Suzuki et al. 1980; Pramauro & Pelezetti 1996; Rahnama et al. 2014). In our study, and species form complexes with Triton X-114 through their polyoxyethylene groups. Even though the concentrations of these species are very low in pure water at pH 7, on the media of Triton X-114, the complexation effect occurs, so Triton X-114 works as chelating agent as well as a surfactant agent.

The concentration of surfactant used in CPE is a critical factor. Its concentration affects not only the extraction efficiency but also the volume of surfactant-rich phase. The effect of Triton X-114 concentration on the extraction efficiency of Fe(III) was investigated over the range 0.005–0.375 (w/v) %. The sample pH (7.0), incubation temperature (60 °C), incubation time (50 min) and centrifugation time (7 min) were kept constant whilst investigating the effect of the surfactant concentration. The results are shown in Figure 2 and indicate that the recoveries of the analyte increased up to 0.05% Triton X-114 and then decreased. This can be attributed to an increase in volume and viscosity of the surfactant-rich phase. The highest signal was obtained using 0.05% (w/v) Triton X-114. Therefore, 0.05% Triton X-114 was used in order to minimize the phase volume ratio and to achieve the highest extraction efficiency.
Figure 2

Effect of Triton X-114 concentration on the extraction efficiency of Fe(III).

Figure 2

Effect of Triton X-114 concentration on the extraction efficiency of Fe(III).

To achieve easy phase separation and efficient preconcentration, it is imperative to optimize the incubation temperature and time. The dependence of extraction efficiency upon incubation temperature was studied over the range 25–70 °C. The sample pH (7.0), Triton X-114 concentration (0.05%), incubation time (50 min) and centrifugation time (7 min) were kept during the optimization of the incubation temperature. The results are given in Figure 3. Quantitative extraction (98%–104%) was obtained between 30 and 70 °C. An incubation temperature of 40 °C was selected for all subsequent experiments. The effect of incubation time was evaluated over the range 5–50 min. The other experimental variables were kept constant at their optimal values. The results are given in Figure 4. Quantitative extraction (101%–105%) was obtained between 10 and 50 min. Therefore, a time of 10 min was selected as optimum.
Figure 3

Effect of incubation temperature on the recovery of Fe(III).

Figure 3

Effect of incubation temperature on the recovery of Fe(III).

Figure 4

Effect of incubation time on the recovery of Fe(III).

Figure 4

Effect of incubation time on the recovery of Fe(III).

Usually, centrifugation time hardly has an effect on micelle formation but does accelerate phase separation. The effect of centrifugation time upon analytical signal was also studied over the range of 1–10 min at 4,000 rpm. The results are given in Figure 5. Quantitative extraction (94%–102%) was obtained between 5 and 10 min. For complete phase separation, a centrifugation time of 7 min at 4,000 rpm was selected for subsequent experiments.
Figure 5

Effect of centrifugation time on the recovery of Fe(III).

Figure 5

Effect of centrifugation time on the recovery of Fe(III).

The effect of NaCl concentration on the extraction efficiency was investigated over the range of 0.0–0.8 mol L−1 using the same CPE procedure. Quantitative extraction (98%–102%) was obtained for all of the NaCl concentrations studied. Therefore, all the extraction experiments were performed without the addition of salt.

Effect of the volume of sample solutions

In order to obtain a high preconcentration factor, the sample volume is a key consideration. Under optimized conditions, the effect of sample volume on the extraction of Fe(III) was examined. To evaluate the effect of sample volume on the recovery of Fe(III), 10, 20, 30 and 40 mL of sample solutions containing 100 and 200 ng Fe(III) were used. The results obtained are given in Table 1. Quantitative recoveries (95.6%–110.0%) were obtained for the sample volumes studied. The preconcentration factors obtained were 20, 40, 60 and 80 for 10, 20, 30 and 40 mL sample solutions, respectively.

Table 1

The effect of sample volume on the preconcentration factor

Sample volume (mL)Fe(III) mass (ng)Recovery (%)aPreconcentration factor
10 100 99.3 ± 6.8 20 
10 200 95.6 ± 5.9 20 
20 100 97.8 ± 4.4 40 
20 200 100.0 ± 5.9 40 
30 100 110.0 ± 3.2 60 
30 200 109.1 ± 3.4 60 
40 100 103.7 ± 6.8 80 
40 200 105.9 ± 3.4 80 
Sample volume (mL)Fe(III) mass (ng)Recovery (%)aPreconcentration factor
10 100 99.3 ± 6.8 20 
10 200 95.6 ± 5.9 20 
20 100 97.8 ± 4.4 40 
20 200 100.0 ± 5.9 40 
30 100 110.0 ± 3.2 60 
30 200 109.1 ± 3.4 60 
40 100 103.7 ± 6.8 80 
40 200 105.9 ± 3.4 80 

aMean ± standard deviation based on three replicate determinations.

Effect of matrix ions

The effects of representative potentially interfering species were tested. The effect of foreign ions was evaluated by analysing 10 mL of 50 μg L−1 Fe(III) solution containing concomitant ions at a concentration of 10 mg L−1 (except at 1 mg L−1 for Cr3+), and following the recommended extraction procedure. The results are given in Table 2. The presence of the tested ions does not affect the recovery of Fe(III) ions. In addition, the effects of some alkali and alkaline earth metal cations and some anions found as major components in natural water samples were investigated. This was accomplished using a synthetic seawater sample (1,270 mg L−1 Mg2+, 400 mg L−1 Ca2+, 10,800 mg L−1 Na+, 390 mg L−1 K+, 5,100 mg L−1, 600 mg L−1, 16,600 mg L−1 Cl and 620 mg L−1). A 10 mL solution containing seawater matrix ions spiked with 50 μg L−1 of Fe(III) was analysed according to the recommended procedure. The results are given in Table 2. It can be seen that the seawater matrix ions have no significant effect on the recovery of Fe(III). The proposed method can therefore be used successfully to extract Fe(III) from high salinity water samples prior to its determination using FAAS.

Table 2

Effect of interfering ions on the recovery of Fe(III)

Interfering ionaAdded asRecovery of Fe(III) (%)b
Pb2+ Pb(NO3)2 96.3 ± 2.2 
Cr3+ Cr(NO3)3.9H296.9 ± 2.6 
Ni2+ Ni(NO3)2.6H298.7 ± 1.8 
Ba2+ Ba(NO3)2 100.4 ± 0.6 
Cd2+ Cd(NO3)2.4H294.5 ± 0.7 
Mn2+ Mn(NO3)2.4H2101.0 ± 5.7 
Zn2+ Zn(NO3)2.6H2111.5 ± 3.2 
Cu2+ Cu(NO3)2.3H299.5 ± 9.2 
Al3+ Al(NO3)3.9H297.4 ± 7.3 
Co2+ Co(NO3)2.6H295.0 ± 1.4 
Sr2+ Sr(NO3)2 107.3 ± 3.1 
Mo6+ (NH4)6Mo7O24.H294.3 ± 6.6 
Sn2+ SnCl2 95.3 ± 4.2 
Ag+ AgNO3 102.1 ± 3.7 
Hg2+ Hg(NO3)2.H2101.6 ± 8.4 
As3+ As2O3 97.3 ± 4.8 
Synthetic seawater KNO3, NaCl, MgSO4.7H2O, CaCO3 105.4 ± 8.6 
Interfering ionaAdded asRecovery of Fe(III) (%)b
Pb2+ Pb(NO3)2 96.3 ± 2.2 
Cr3+ Cr(NO3)3.9H296.9 ± 2.6 
Ni2+ Ni(NO3)2.6H298.7 ± 1.8 
Ba2+ Ba(NO3)2 100.4 ± 0.6 
Cd2+ Cd(NO3)2.4H294.5 ± 0.7 
Mn2+ Mn(NO3)2.4H2101.0 ± 5.7 
Zn2+ Zn(NO3)2.6H2111.5 ± 3.2 
Cu2+ Cu(NO3)2.3H299.5 ± 9.2 
Al3+ Al(NO3)3.9H297.4 ± 7.3 
Co2+ Co(NO3)2.6H295.0 ± 1.4 
Sr2+ Sr(NO3)2 107.3 ± 3.1 
Mo6+ (NH4)6Mo7O24.H294.3 ± 6.6 
Sn2+ SnCl2 95.3 ± 4.2 
Ag+ AgNO3 102.1 ± 3.7 
Hg2+ Hg(NO3)2.H2101.6 ± 8.4 
As3+ As2O3 97.3 ± 4.8 
Synthetic seawater KNO3, NaCl, MgSO4.7H2O, CaCO3 105.4 ± 8.6 

aConcentration of Cr3+ = 1 mg L−1, concentration of other interfering ions = 10 mg L−1.

bMean ± standard deviation based on three replicate determinations.

Analytical performance of the method and comparison with other methods

The analytical performance of the proposed method was evaluated under the optimized conditions. A calibration graph was constructed using 10 mL of the standard solutions buffered at pH 7.0 and containing known amounts of Fe(III) in the concentration range of 5–200 μg L−1. The standard solutions were processed by the optimized CPE method. The calibration equation was A = 2.35 × 10−3(±9.71 × 10−5)C − 4.30 × 10−3(±8.78 × 10−3) with a correlation coefficient of 0.9993, where A is absorbance and C is the concentration of Fe(III) in the solution (μg L−1). The limit of detection, defined as LOD = 3Sb/m where Sb is the standard deviation of 10 replicate blank signals and m is the slope of the calibration curve after preconcentration, was found to be 0.95 μg L−1. The calibration equation obtained using direct aspiration to FAAS without the preconcentration procedure was A = 1.07 × 10−4(±3.09 × 10−6)C − 6.40 × 10−5(±5.52 × 10−3) over the concentration range 100–4,000 μg L−1. The enrichment factor was calculated as the ratio of the slopes of the calibration graphs obtained using the preconcentration method and direct aspiration. The enrichment factor was found to be 22 for a 10 mL sample solution.

A comparison of the developed method with the other reported preconcentration methods for iron determination is given in Table 3. As seen from the table, the LOD of the method is better than or comparable to those obtained with other methods. Although the preconcentration factor of the proposed method is already lower than most of the other methods, it can be improved further by using larger sample volumes. As shown in Table 1, the proposed method yielded very good recovery values and a preconcentration factor of 80 when 40 mL of the sample was used.

Table 3

Comparisons of the analytical performance of the method with those reported in the literature with FAAS detection techniques

MethodaSample typeDetection limit (LOD) (μg L−1)bPreconcentration factorSample volume (mL)Reference
CPE Water samples 0.95 20 10 This work 
IL-DLLME Serum samples 1.29 50 10 Arain et al. (2015)  
SPE Cosmetic products, hair brilliantine and gel, water, soil, and food samples 2.2 200 1,000 Unsal et al. (2015)  
Flotation Spinach, soil, water samples 0.7 93 750 Karimi et al. (2008)  
SPE Water samples 0.63 166 2,500 Khayatian & Pouzesh (2007)  
CPE Tobacco, black tea and water samples 1.22 10 50 Duran et al. (2012)  
CPE Biological, soil and blood samples 2.8 30 15 Ghaedi et al. (2009)  
MethodaSample typeDetection limit (LOD) (μg L−1)bPreconcentration factorSample volume (mL)Reference
CPE Water samples 0.95 20 10 This work 
IL-DLLME Serum samples 1.29 50 10 Arain et al. (2015)  
SPE Cosmetic products, hair brilliantine and gel, water, soil, and food samples 2.2 200 1,000 Unsal et al. (2015)  
Flotation Spinach, soil, water samples 0.7 93 750 Karimi et al. (2008)  
SPE Water samples 0.63 166 2,500 Khayatian & Pouzesh (2007)  
CPE Tobacco, black tea and water samples 1.22 10 50 Duran et al. (2012)  
CPE Biological, soil and blood samples 2.8 30 15 Ghaedi et al. (2009)  

aCPE: cloud point extraction, IL-DLLME: ionic liquid dispersive liquid–liquid microextraction, SPE: solid phase extraction.

bLimits of detection were calculated based on 3Sb/m.

Analysis of certified sample

In order to evaluate the accuracy of the developed method, two certified reference materials, SPS-SW2 level 2 Batch 127 (surface water) and SRLS-5 (river water), were analysed. The analytical results are given in Table 4. Experimental data for these samples were in good agreement with the certified values. The Student's t-test was applied to results of the proposed method and certified values. For two degrees of freedom at the 95% confidence level, the critical t value is 4.30. As can be seen, t values are lower than the critical value, indicating that there is no evidence of systematic error in the proposed method. The results show that the proposed method is suitable for determination of Fe in water samples.

Table 4

Results for the certified reference materials

Certified reference materialCertified value (μg L−1)Found valuea (μg L−1)tb
SPS-SW2 Batch 127 100 ± 1.0 97.8 ± 3.9 1.0 
SLRS-5 91.2 ± 5.8 94.8 ± 4.1 1.5 
Certified reference materialCertified value (μg L−1)Found valuea (μg L−1)tb
SPS-SW2 Batch 127 100 ± 1.0 97.8 ± 3.9 1.0 
SLRS-5 91.2 ± 5.8 94.8 ± 4.1 1.5 

aMean value ± standard deviation based on three replicate determinations.

b, where t is the statistical value (for two degrees of freedom, the critical value of t at the 95% confidence level is 4.30), s is the standard deviation, N is the number of independent determinations, x is the experimental mean value, and μ is the certified value.

Application of the method to real samples

In order to evaluate the applicability of the proposed method, it was applied to the determination of Fe in tap water, river water and seawater samples. The accuracy of the proposed method was checked by spiking the samples with different concentrations of Fe(III). The results are given in Table 5. As can be seen from Table 5, the recoveries of the spiked water samples were in the range of 92%–101%, indicating the good reliability and accuracy of the proposed method.

Table 5

The results of the water samples

SampleAdded (μg L−1)Founda (μg L−1)Recovery (%)RSD (%)
Tap water – 212.6 ± 2.3 – 1.1 
10 222.6 ± 1.6 100 0.7 
River water – 6.1 ± 0.4 – 6.5 
10 15.9 ± 0.9 98 5.7 
Seawater (Edremit) – 49.3 ± 1.3 – 2.6 
40 88.2 ± 3.5 97 4.0 
Seawater (İzmit) – 6.0 ± 0.5 – 8.3 
20 24.4 ± 1.4 92 5.7 
Seawater (Yalova) – 6.6 ± 0.9 – 13.6 
20 26.8 ± 1.8 101 6.7 
SampleAdded (μg L−1)Founda (μg L−1)Recovery (%)RSD (%)
Tap water – 212.6 ± 2.3 – 1.1 
10 222.6 ± 1.6 100 0.7 
River water – 6.1 ± 0.4 – 6.5 
10 15.9 ± 0.9 98 5.7 
Seawater (Edremit) – 49.3 ± 1.3 – 2.6 
40 88.2 ± 3.5 97 4.0 
Seawater (İzmit) – 6.0 ± 0.5 – 8.3 
20 24.4 ± 1.4 92 5.7 
Seawater (Yalova) – 6.6 ± 0.9 – 13.6 
20 26.8 ± 1.8 101 6.7 

aMean value ± standard deviation based on three replicate determinations.

CONCLUSIONS

In this work, a ligandless CPE method was developed for the preconcentration of Fe(III) prior to FAAS determination. The method is simple, accurate, precise, sensitive, safe, environmentally friendly, easy to use and economic. Only Triton X-114 and ammonium acetate are used and these are readily available in most laboratories. Triton X-114 is inexpensive and of low toxicity. In addition, only 0.5 mL of 1% (w/v) Triton X-114 solution is used per sample. The amount of Triton X-114 used per sample is therefore extremely low. Consequently, the cost of this preconcentration method is also very low. The method requires nearly 30 minutes of sample preparation time per sample. However, eight samples in 15 or 50 mL tubes can be prepared for analysis simultaneously yielding a preconcentration factor of 20 or 80, respectively. The surfactant-rich phase can easily be introduced into the flame after dilution with 1.0 mol L−1 HNO3 and determined directly. The method can be applied to the determination of trace amounts of Fe in various water samples. The developed method can be considered to be an alternative to other more sensitive analytical techniques such as GFAAS, ICP-OES and ICP-MS, of which the latter two instruments, especially ICP-MS, are not found in many laboratories because of their price. However, if the proposed method could be used in conjunction with these techniques, then a much lower detection limit could be obtained enabling the determination of much lower concentrations of iron found in ocean waters. In addition, it also has the benefit of separating the analyte from the sample matrix, facilitating interference-free determination. The presence of salt in the seawater is extremely problematic for both GFAAS and ICP-MS analyses because of light absorption and blockage of torch injectors and cones, respectively.

ACKNOWLEDGEMENTS

This work was supported by Balιkesir University Research Grant No. 2016/65. The authors are thankful for the financial support from the Unit of the Scientific Research Projects of Balιkesir University.

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