Journal of the Chinese Chemical Society, 2004, 51,

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1 Journal of the Chinese Chemical Society, 2004, 51, An Extractive-Spectrophotometric Method for Determination of Fluoride Ions in Natural Waters Based on its Bleaching Effect on the Iron (III)-Thiocyanate Complex M. A. Farajzadeh Department of Chemistry, Faculty of Science, Urmia University, P.O. Box , Urmia, Iran An extractive-spectrophotometric method based on the bleaching effect of F - ions on the iron (III)-thiocyanate complex extracted into methyl iso-butyl keton (MIBK) is proposed for the determination of fluoride ions in natural waters. This method is a simple and rapid method and there is no need for special and expensive reagents. The experimental conditions such as SCN - concentration, ph and kind of solvent were optimized, and we found that 0.15 M SCN -, ph = 5 and MIBK are the best selections. Limit of detection is 0.1 mg F - /L with a linear dynamic range mg/l which covers optimum concentration of F - ions in drinking water ( mg/l). Interference of Fe 3+ and Al 3+ ions was easily eliminated by the extractive procedure using a solution of oxine in chloroform. Finally, the proposed method was used in determination of fluoride content of some real water samples and the obtained results compared with those obtained from the standard method. No significant differences were observed between them. Keywords: Fluoride ions; Extractive-Spectrophotometry; Bleaching effect; Methyl iso-butyl keton; Natural water. INTRODUCTION Fluoride has been shown to have beneficial effects on teeth and on bone structure in animals, including humans, at low levels of fluoride. Moderate or high levels of fluoride in water also have caused fluorine toxicity in animals and mottled teeth in humans. Fluoride containing substances show toxicity in both animals and plants when they cause exposure to fumes and dust from industrial facilities as well as natural emissions from the eruption of volcanoes. However, in trace amounts, fluoride is a natural diet ingredient essential for the formation of healthy bones. Fluoridation is often considered to be desirable in balancing the supply of this essential nutrient in fluoride-deficient drinking water. As a concentration of fluoride in excess of 1.5 mg/l may cause mottling of tooth enamel, it is mandatory to exercise very careful control to maintain the desirable 1.0 mg/l dosage. Water supply samples are frequently tested by municipal authorities. 1,2 Numerous methodologies have been applied to the measurement of fluoride in various sample matrices. These include potentiometry with fluoride ion selective electrodes (ISE), 3-7 atomic absorption spectrometry, 8 inductively coupled plasma emission spectrometry, 9 molecular emission cavity spectrometry, 10 UV-VIS spectrophotometry, 11,12 fluorimetry, 13,14 chromatography, 15 electrochemical methods, 16 and most recently capillary electrophoresis. 17 The use of ISE has been the preferred technique for this determination. Some analytical methods such as SPADNS and Alizarin colorimetric methods undergo some errors, which are caused by the presence of interfering ions. Distillation of fluoride as HF from the sample should be performed before determination procedure. 18 In previous work 19 we described a sensitive technique with which fluoride content of natural waters was determined by interaction of fluoride ions on the aluminum - oxine complex adsorbed onto an octyl solid phase extraction cartridge. In continuing studies another simple and expensive extractive - spectrophotometric method based on the bleaching effect of fluoride ions on the highly colored charge transfer complex, Fe 3+ -SCN -, is described. In this case organic phase containing complex is mixed with aqueous phase having fluoride ions. A portion of complex is destroyed due to higher interaction of F - and Fe 3+ with respect to interaction between SCN - and Fe 3+, and the remaining complex in organic phase is * Corresponding author. farajzade@yahoo.com

2 304 J. Chin. Chem. Soc., Vol. 51, No. 2, 2004 Farajzadeh monitored by spectrophotometry. It needs no distillation process prior to measurement. thiocyanate complex (the above solution) was added and well mixed. The absorbance of organic phase was read at 497 nm against the blank solution in which fluoride ions were absent. EXPERIMENTAL Apparatus An LKB UV-visible spectrophotometer model 4054 equipped with an automated cell driver, spectrum recorder and 1 cm quartz cell was used for absorbance measurements. Chemicals Different solvents all from E. Merck (Darmstadt, Germany) such as methyl iso-butyl keton (MIBK), ethyl acetate, cyclo hexanol, diethyl ether, butanol, chloroform and diethyl amine were used. Inorganic salts such as Al(NO 3 ) 3,Na 2 HPO 4, NaCl, Fe(NO 3 ) 3, Ce(NO 3 ) 3,NH 4 SCN, Ca(NO 3 ) 2, La(NO 3 ) 3 and Mg(NO 3 ) 2 with analytical purity were also purchased from E. Merck. Solutions M iron(iii) nitrate, 1 M ammonium thiocyanate and 20 mg/l aluminum nitrate were prepared in double distilled water M 8-hydroxyquinoline (oxine) in chloroform was used. Procedure 10 ml M iron(iii) nitrate was added to 10 ml 1 M ammonium thiocyanate, and the produced iron(iii)- thiocyanate complex was extracted into 10 ml methyl isobutyl keton. For determining fluoride ions concentration and optimizing parameters, in a 250 ml separatory funnel, 100 ml solution containing fluoride ions was mixed with 10 ml 1.65 M ammonium thiocyanate; ph was adjusted to by HCl or NaOH 0.1 M and 10 ml MIBK containing iron(iii) RESULTS AND DISCUSSION Selecting extraction solvent Due to the anionic nature of iron(iii)-thiocyanate complex and its relatively small size, the type of solvent used has an important role. Here, solvent must have two characteristics: 1) distribution coefficient of iron(iii)-thiocyanate complex between water and the selected solvent should be high and 2) it must be a solvating solvent or an inert solvent in which a solvating agent was dissolved. For selecting optimum solvent, different solvents were tested. For this purpose a 10 ml iron(iii)-thiocyanate complex in water was mixed with 10 ml different organic phases and after equilibration the absorptive spectrum of organic phase was recorded in the range of nm using pure solvent as a blank. The obtained results are given in Table 1. max of complex is 500 nm with a little shift in different solvents. The absorbances of organic phases show that MIBK has preference over other solvents used. The extractive properties of solvents arises from three parameters: type of functional group on the solvating solvent, molecular dipole, and steric strain. The short alkyl groups lead to the lipophilicity of MIBK being decreased, thus its extractive power towards anionic complex of Fe 3+ - SCN - is enhanced. On the other hand, MIBK has an asymmetric structure and high dipole moment, which increase electron density around the carbonyl group. By this means the affinity of MIBK for formation of covalent bond or solvation of complex is increased. This is in agreement with the previous data. 20 The absorptivity of iron(iii)-thiocyanate in MIBK is Table 1. max of Fe 3+ -SCN - Complex in Different Solvents. A 1,A 2 and A 3 are Absorbance of Fe 3+ -SCN - Complex Extracted into MIBK at 1 st,2 nd and 3 rd Extraction Performed on the Same Aqueous Solution Solvent MIBK Ethyl acetate 5% Butanol in CCl 4 5% Cyclo hexanol in CCl 4 Diethyl ether 10% Diethyl amine in CCl 4 max (nm) A A A

3 Determination of Fluoride Ions in Natural Waters J. Chin. Chem. Soc., Vol. 51, No. 2, about L.mol -1.cm -1 in comparison with that of the same complex in water which is L.mol -1.cm -1.Asa consequence, using MIBK instead of water as a solvent improves sensitivity of the method two fold. By use of a higher volume of aqueous phase relative to organic phase, the limit of detection (LOD) of the presence method is also decreased. Effect of thiocyanate ions concentration Thiocyanate ions have two important roles in this study. Firstly, SCN - acts as a complexing agent for Fe 3+ ions to produce iron(iii)-thiocyanate which is extracted into MIBK. Secondly, when Fe 3+ -SCN - complex comes into contact with an aqueous phase, it dissociates. Dissociation of complex is decreased in aqueous phase in the presence of SCN - ions. However, high concentration of SCN - ions in aqueous phase is not favored. SCN - in high concentration competes with F - and decreases its bleaching effect on the Fe 3+ -SCN - complex. For optimizing SCN - concentration, after extraction of Fe 3+ -SCN - complex into MIBK, organic phase comes into contact with 100 ml aqueous phase containing F - ions with a constant concentration (3 mg/l) and variable concentration of SCN - ions. Blank solution for each instance is organic phase containing Fe 3+ -SCN - complex ( M) mixed with 100 ml aqueous phase having the same concentration of SCN - ions in the absence of F - ions. Differences of absorbance of each solution with respect to absorbance of related blank were recorded and are shown in Fig. 1. The absorbance difference reaches the maximum value at SCN - concentration of 0.15 M. Data in Fig. 1 show that Fe 3+ -SCN - complex is not quantitatively produced in a lower concentration of SCN - ions. On the other hand, the 0.25 bleaching effect of F - ions on the Fe 3+ -SCN - complex is completely suppressed in a higher concentration of SCN -. The formations of successive complexes between SCN - and Fe 3+ leads to the two maximums observed in Fig. 1. It is mentioned that they have different dissociation constants and absorptivities. However adjusting SCN - concentration to 0.15 M is favored in this study. Effect of ph ph of aqueous phase is an effective parameter on the absorbance of Fe 3+ -SCN - complex extracted in MIBK. In higher ph hydroxide ions form hydroxide complexes or precipitate with Fe 3+, hence Fe 3+ -SCN - complex is destroyed and its absorbance decreased. Moreover, in lower ph fluoride ions in aqueous solution are protonated to form HF which cannot react with Fe 3+ ions. However it is necessary to obtain optimum ph in this study. To determine the optimum ph of aqueous phase, 100 ml solution of 0.15 M SCN - was selected, and its ph was adjusted in different values in the presence of F - (3 mg/l). Another solution, in which F - ions are absent, is used as a blank solution. The absorbance differences between two solutions were plotted as a function of ph (Fig. 2). Because of interaction between buffer constitutents and Fe 3+ -SCN - complex, HCl or NH 3 was used in ph adjustment. From the results in Fig. 2, ph = 5 was selected as an optimum ph. Interferences studies on the determination of fluoride ions in natural waters Different cations and anions were studied. Those were 0.35 Absorbances difference Thiocyanate concentration (M) Absorbances difference ph Fig. 1. Absorbances difference of Fe 3+ -SCN - complex extracted into MIBK in contact with aqueous phase in the presence (3 mg/l) and absence of fluoride ions as a function of SCN - concentration in aqueous phase. Fig. 2. Difference of absorbances in the presence and absence of F - ions as a function of ph of aqueous phase. Volume of aqueous phase, 100 ml; F - concentration, 3 mg/l; SCN - concentration, 0.15 M. ph was adjusted by HCl or NH 3.

4 306 J. Chin. Chem. Soc., Vol. 51, No. 2, 2004 Farajzadeh Table 2. Interferences Studies. Data are Absorbance of Fe 3+ -SCN - Complex Extracted into MIBK in the Presence of Different Cations and Anions in Different s Interfering species (ppm) Cl - Ca SO 4 Mg PO 4 La 3+ Ce 3+ Al 3+ Fe ions, which are present in natural waters, or those which react with F -,Fe 3+ or SCN - ions. Cations react with fluoride ions and compete with Fe 3+ ions. This causes the absorbance of Fe 3+ -SCN - complex to not be decreased by F - ions. Al 3+ and Ca 2+ ions can be classified in this category. On the other hand, cations can react with SCN - and produce complex, which absorb radiation at about 500 nm. Moreover, anions compete with F - ions to react with Fe 3+ ions in complex and absorbance is decreased. The studied cations and anions were as follows: La 3+,Fe 3+,Al 3+,Ca 2+,Mg 2+,Ce 3+,PO 4 3-,Cl - and SO For this purpose 100 ml aqueous phase containing 3 mg/l F - and 0.15 M SCN - (ph = 5) and different amounts of interferences come into contact with 10 ml MIBK in which Fe 3+ -SCN - complex has been extracted. After establishment of equilibrium, organic phase was separated and its absorbance recorded at 497 nm against pure MIBK. The obtained results are given in Table 2. In this study less than 5% deviation from initial value in which interferences are absent was selected as a tolerable amount. Ce 3+,La 3+,Mg 2+,Ca 2+,Cl -,SO 4 2- and PO 4 3- are ineffective in the determination of F - by the proposed method up to 5, 15, 80, > 10 4,>10 4, 400 and 18 mg/l, respectively. The only interfering ions are Fe 3+ and Al 3+. Both cations cause positive error in absorbance reading. By increasing Fe 3+ concentration in aqueous phase more Fe 3+ -SCN - complex is produced and extracted in organic phase and its absorbance is increased. When Al 3+ concentration is increased in aqueous phase, it prevents complex formation between F - and Fe 3+. This also leads to absorbance of organic phase being increased. In the case of real samples, the formation of complex between Al 3+ and F - prevents interaction between F - and Fe 3+ -SCN - complex and hence increasing F - concentration does not decrease absorbance of organic phase. Quantitative characteristics of the proposed method To plot a calibration curve, 100 ml aqueous solutions (ph = 5) having 0.15 M SCN - and variable concentrations of fluoride ions are prepared and come into contact with 10 ml MIBK containing Fe 3+ -SCN - complex. The absorbance of organic phase is recorded at 497 nm against pure MIBK and plotted vs fluoride concentration. The calibration curve equation is A = C (A = absorbance of organic phase and C = concentration of fluoride ions in mg/l in aqueous phase) with a linear dynamic range mg/l. Correlation coefficient and limit of detection are and 0.1 mg/l, respectively. Repeatability of the method is accessed by determination of fluoride content of similar solutions (n = 6, C=1mg F - /L). The relative standard deviation (RSD%) is obtained to 1.5%. Application in real samples To evaluate the efficiency of the proposed method, it was used in determination of fluoride content of natural waters. For elimination of Fe 3+ and Al 3+ interferences, extraction by 5 ml 0.01 M oxine in chloroform was performed as a clean-up process. Five natural water samples were selected from local sources and their fluoride contents were analyzed by the proposed method and zirconium - eriochrome cyanine R standard method. The obtained results are given in Table 3. Both data are in agreement and no significant differences are

5 Determination of Fluoride Ions in Natural Waters J. Chin. Chem. Soc., Vol. 51, No. 2, Table 3. Comparison of Results Obtained by the Proposed Method and Standard Method (Zirconium - Eriochrome Cyanine R). Data are Fluoride (mg/l) Sample number Method Proposed method < < 0.10 Standard method < < 0.03 Table 4. Comparison of the Proposed Method (Fe 3+ -SCN - Method) with Other Fluoride Ions Determination Methods. Data are of Ions in which 10% is Observed in Fluoride (1 mg/l) Fluoride ISE SPADNS method Alizarin method Fe 3+ -SCN - method Interfering ion Al 3+ 3 Negative 0.1 Negative 0.25 Negative 0.05 Negative Cl Positive 2000 Negative > Fe Negative 10 Negative 2 Positive 0.05 Negative 3- PO Positive 5 Positive 18 Positive 2- SO Negative 200 Negative 300 Positive 400 Positive observed. Comparison of the proposed method with other methods used in F - determination For this purpose three methods, ion selective electrode, SPADNS, and Alizarin were selected and the tolerable amounts of interfering species are shown in Table 4. Except for the ion selective method, the proposed method (Fe 3+ - SCN - method) is comparable to the two other methods. SPADNS and Alizarin methods need distillation of sample in the presence of concentrated sulfuric acid prior to determination of F - ions. Whereas in the Fe 3+ -SCN - method interferences are eliminated with a simple and rapid extraction procedure by oxine solution in chloroform. CONCLUSION In this study, an extractive-spectrophotometric method was presented for the determination of fluoride ions in natural waters. This method needs no special reagents, and an inorganic ligand, thiocyanate, was used as a chromogenic agent. Bleaching effect of F - ions on the Fe 3+ -SCN - complex extracted in MIBK is the basis of this indirect spectrophotometric method. Fe 3+ and Al 3+ are two serious interferences; their removal from samples was carried out by the extraction prior to analysis of fluoride ions. The proposed method is comparable with most previously presented methods in the view of quantitative characteristics, but it is very simple and rapid. Received April 7, REFERENCES 1. Purves, D. Trace Element Contamination of the Environment, Elsevier, Amsterdam, 1977, pp (Chapter 3). 2. Douglas, M.-C. Van Nostrand s Scientific Encyclopedia, 8th Edition, Van Nostrand Reinhold, New York, 1995, pp Me todos Normalizados para el Ana lisis de Aguas Potablesy Residuales, American Public Health Association, Ed. D az Santos S. A., 1992, pp Kahama, R.-W.; Damen, J.-J.; Cate, J.-M. Analyst 1997, 122, Perdikaki, K.; Tsagkatakis, I.; Chaniotakis, N.-A.; Altmann, R.; Jurkschat, K.; Reeske, G. Anal. Chim. Acta 2002, 467, Dressler, V.-L.; Pozebon, D.; Flores, E.-L.; Paniz, J.-N.; Erico, J.-N.; Flores, M.-M. Anal. Chim. Acta 2002, 466, Chang, F.-C.; Tsai, H.-T.; Wu, S.-C. J. Chin. Chem. Soc. 1975, 22, Gutsche, B.; Kleinoeder, H.; Herrmann, R. Analyst 1975, 100, 192.

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