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Surfactant-mediated extraction of iron and its spectrophotometric determination in rocks, minerals, soils, stream sediments and water samples
Microchemical Journal 80 (2005) 39 – 43 www.elsevier.com/locate/microc
Surfactant-mediated extraction of iron and its spectrophotometric determination in rocks, minerals, soils, stream sediments and water samples Pranab K. Tarafder*, Raghbendra Thakur Analytical Chemistry Laboratory, Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Khasmahal, Jamshedpur-831002, India Received 22 June 2004; received in revised form 9 September 2004; accepted 12 September 2004 Available online 10 December 2004
Abstract An extraction spectrophotometric method for iron determination in rocks, minerals, soils, stream sediments and water samples has been developed. At pH 3-4, iron (III) forms a 1:2:1 ternary complex with thiocyanate and cetyltrimethylammonium bromide (Fe/SCN/CTAB) which is extracted into ethyl acetate. The readily formed purple complex is suitable for extraction spectrophotometric determination of iron in rocks and related materials from submicrogram to milligram levels. The method is free from any interference due to commonly associated ions present in the matrices of rock samples. The present method is at least fourfold more sensitive (e=3.2104 l mol 1 cm 1) than the conventional thiocyanate method and, in addition to the enhanced sensitivity and selectivity, it has got definite advantages over the corresponding binary thiocyanate system in terms of substantial improvement in the stability of the complex formed and broadening of Beer’s law adherence range (0–6.0 mg/l). The method has been applied to a number of geological and hydrogeochemical samples for the determination of iron and the results obtained have been found to be favourably comparable with those obtained from the standard methods. D 2005 Elsevier B.V. All rights reserved. Keywords: Extraction; Iron determination; Spectrophotometric method
1. Introduction There are many spectrophotometric methods for the determination of iron [1]. Amongst these, thiocyanate and 1,10-phenanthroline methods are generally used for the determination of iron in rocks and minerals [2,3]. Although ammonium thiocyanate has in the past been extensively used for the photometric determination of iron [4–6], presently it is not in much use because it is highly insensitive and suffers from a number of disadvantages, particularly when compared with the 1,10-phenanthroline method. The optical densities of ferric thiocyanate solutions depend upon the conditions used for the reaction (temperature, acidity, excess of reagent), the solutions may suffer
* Corresponding author. E-mail address: [email protected] (P.K. Tarafder). 0026-265X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2004.09.004
from some measure of fading, and do not completely follow the Beer–Lambert’s law [3]. Similarly, the 1,10phenanthroline method, which is even now widely used for the determination of iron in rocks and related samples, is also not advantageous when applied to samples having complex matrices and containing traces of iron, owing to its poor sensitivity and selectivity. Therefore, it is incumbent on us to develop a suitable method that can be successfully applied directly to samples having complex matrices and containing iron ranging from traces to percentage levels. In this endeavour, a robust method, overcoming most shortcomings associated with the existing thiocyanate method and meeting all the desired characteristics as expected from a standard method, has been developed by exploiting the reaction of iron (III) with thiocyanate and cetyltrimethylammonium bromide. Iron (III) reacts with thiocyanate and cetyltrimethylammonium bromide forming a bright purple hitherto unreported ternary
40
P.K. Tarafder, R. Thakur / Microchemical Journal 80 (2005) 39–43
complex, readily extracted into ethyl acetate. Unlike the binary thiocyanate complex, the present reaction is at least fourfold more sensitive. Besides, the method is highly selective because it is free from interference of most matrix elements present in the sample. Moreover, unlike the binary system, the colour developed is quite stable and interestingly enough, the results obtained are highly reproducible. The striking feature of this method is that, contrary to the existing thiocyanate method, the Beer’s law is obeyed over a wide range, i.e., from 0 to 6.0 mg/l of iron. It is pertinent to mention here that ternary complexes of Fe (III)–thiocyanate with other reagents [1,7] are known, but none has given as satisfactory results as given by this proposed method for iron determination in real samples. This is a new surfactant sensitized colour reaction of iron (III) with thiocyanate, and to the best of our knowledge, it is not reported in the literature so far. This prompted us to undertake a systematic study on the ternary complex formation of iron (III), thiocyanate and CTA, and to explore the possibility of its application to a host of diverse samples of complex matrices such as rocks, minerals, soils, stream sediments and waters for the accurate and rapid determination of iron. The present paper describes such systematic studies for the development of a suitable extraction spectrophotometric method for the simple and accurate determination of iron in a range of silicate rocks, minerals, soils, stream sediments and water samples, including geo-standard samples.
2. Materials and methods 2.1. Apparatus (1) (2)
UV–VIS. Spectrophotometer (CHEMITO-2500) pH-Meter (ELICO-LI-120)
2.2. Reagents All reagents used were of analytical grade. (1)
(2) (3)
(4)
Potassium thiocyanate, 10% w/v. Weigh out about 10 g of potassium thiocyanate and dissolve it in water in a 100 ml volumetric flask and make up to the mark. Ethyl acetate. Cetyltrimethylammonium bromide, 1% (w/v) aqueous solution. This reagent is readily soluble in water and no deterioration was observed over a period of at least 2–3 days. However, fresh solutions were prepared for this study. Standard solution of iron. Dissolve 0.7022 g ferrous ammonium sulphate in 100 ml water, add 5 ml of 1:5 sulphuric acid and run in cautiously a dilute solution of potassium permanganate(2.0 g l 1) until a slight pink coloration remains after shaking well. Dilute to 1 l and mix thoroughly (1 ml=0.1 mg of Fe). More
dilute solutions were prepared by diluting the stock solution. 2.3. Procedures 2.3.1. Dissolution of samples 2.3.1.1. For rocks, soils and stream sediments. Weigh out 0.5000 g of each sample(~ 200 mesh size) to a series of 90 ml platinum dishes. Moisten the powder with water and cautiously add 10 ml HF (48%). If a visible reaction occurs on first addition of the HF, cover the dishes and add the remainder of the HF when the reaction has diminished. Add 3 ml concentrated H2SO4 (98%), cover the dishes and place on a steam bath and allow for heating at least 6 h. Remove the dishes from the steam bath, wipe off the exterior and place it on a sand bath. Remove the cover and evaporate the contents to a volume of about 3 ml. Cautiously add 1 ml concentrated HNO3. Continue the evaporation until copious white fumes of sulfur trioxide are evolved. Cool, add 1 ml concentrated HNO3 and repeat the heating to fumes. Continue with the addition of nitric acid until all organic matter has been destroyed. Transfer quantitatively the contents of platinum dish to a 400-ml beaker, dilute to 100 ml and boil until the solution is clear. Cool and quantitatively transfer the solution to a 250-ml volumetric flask, dilute to volume at room temperature and mix thoroughly. 2.3.1.2. For minerals. Weigh a 0.2 g finely ground sample (~ 200 mesh) into a 50-ml silica crucible and fuse it with about 2 g potassium bisulphate. Transfer the fused mass quantitatively into a 250-ml beaker and add 5 ml 1:1 H2 SO4 followed by about 50 ml water. Boil the solution for a while in order to get a clear solution. Cool and transfer the solution into a 100-ml volumetric flask and dilute to volume. 2.3.2. Colour reaction, extraction and absorbance measurements Take a suitable aliquot of the sample in a 125-ml separating funnel, add 5 ml of distilled water and 5 ml of 10% potassium thiocyanate solution. Shake a while and adjust the pH in the range 3–4. Add 10 ml of ethyl acetate and 5 ml of 1% cetyltrimethylammonium bromide solution. Shake for 2 min. Set it aside for 5 min till the complete phase separation takes place. Drain the aqueous layer out, filter the ethyl acetate through Whatman 541 filter paper and measure the absorbance of the complex in ethyl acetate at 474 nm against the reagent blank prepared similarly but containing no iron.
3. Results and discussion Twenty micrograms of Fe (III) per 10 ml of ethyl acetate was used for subsequent experiments.
P.K. Tarafder, R. Thakur / Microchemical Journal 80 (2005) 39–43
3.1. Choice of solvents Different solvents were tried for extraction studies. These were carbon tetrachloride, chloroform, benzene, n-butyl alcohol, ethyl acetate, methyl-iso-butyl ketone (MIBK), etc. Only ethyl acetate gave the most desired results. 3.2. Effect of pH The effect of pH on complex formation and its subsequent extraction in ethyl acetate, as evident by having obtained maximum absorbance was studied by varying it over a wide range. As shown in Fig. 1, the most effective range of the pH was found to be 3–4. An increase in acidity particularly due to nitric acid resulted into the instability of the colour. Hence, a pH of 3–4 was chosen to be the ideal as the stability of the complex was found to be the best here. 3.3. Effect of reagent concentration 3.3.1. Thiocyanate The effect of thiocyanate concentration on the optimum colour formation was studied by varying its concentration from 1% to 20% (w/v) aqueous solution. It was observed that a 5 ml of 10% thiocyanate solution gave the most satisfactory results as evident by having obtained the maximum absorbance for the extracted species. 3.4. Cetyltrimethylammonium bromide The effect of this reagent, too, in the formation of the ternary complex suitable for the extractive determination of
41
iron, was also studied by varying its concentration from 0.01% to 5% (w/v) solution. A 5 ml of 0.1% (w/v) aqueous solution of this reagent was found to be sufficient in order to obtain the desired result. 3.5. Effect of temperature Unlike the Fe (III)–SCN binary system (the prevailing thiocyanate method), the present method has been found to be independent of temperature in the range 20–40 8C. 3.6. Composition of the complex The stoichiometry of the complex formed was found out by curve fitting method [8], i.e., by plotting log D vs. log [thiocyanate] as well as log [CTAB]. A slope of 1.8, which is close to the integer 2, is obtained for thiocyanate. This shows that 2 molecules of thiocyanate take part in the complex formation. Similarly, a slope of one is obtained for CTAB, confirming that one molecule of CTAB takes part in the formation of the ternary complex. Thus, the stoichiometry of the ternary complex may be presumed as 1:2:1 (Fe/SCN /CTAB). 3.7. Spectral characteristics, Beer’s law, sensitivity and precision of the method The complex formed absorbs maximum at 474 nm (see Fig. 2). The absorbance due to the reagent blank at this wavelength was in the range 0.010–0.015. The method obeyed Beer’s law over a wide range, i.e., 0–6 mg/l. The molar absorptivity of the method is 3.2104 l mol 1 cm 1. The relative standard deviation (RSD) of the method at a level of 1 mg/l iron is 1.2%. 3.8. Effect of diverse ions
Fig. 1. Effect of pH on the extraction of iron from aqueous to organic phase.
In order to assess the efficacy of the method under development, the effect of many ions on the determination of 1 mg/l iron by this technique was studied. The results are shown in Table 1. From the results obtained for this study, it was found that most ions which generally accompany iron in rocks of diverse matrices did not interfere. Unlike in the tiron, 1,10-phenanthroline and thiocyanate (binary system) methods, the presence of large quantities of Cu, Ni, Zn, Co, V, Mo, U, Mn, Nb, Ta, Ti, Pb, Hg, etc. did not pose any problem in the estimation of iron using the present method. It shows that the method developed is quite selective for iron determinations. This fact was further attested by applying the present method to a number of samples, including geo-standards of diverse matrices for iron determination. The results obtained thereof were found to be in excellent agreement with the certified values and those obtained from standard methods.
42
P.K. Tarafder, R. Thakur / Microchemical Journal 80 (2005) 39–43 Table 2 Determination of Fe2O3 in diverse samples by the proposed method and its comparison with 1,10-phenanthroline method (n=5) Sample
Fig. 2. (A) Absorption spectrum of the ternary complex of iron (2.0 mg/l) against reagent blank and (B) absorption spectrum of reagent blank against ethyl acetate.
3.9. Analytical applications The method finds an excellent application in silicate rocks, minerals, soil, stream sediments and water samples for the determination of iron from submicrogram to percentage level. For the estimation of iron in water samples, pretreatment of these samples is a must, preferably just after sampling. A few drops of HNO3 are added in order to prevent precipitation of iron because iron starts precipitating above pH 4. Hence, it is advisable that the water should have pHb4. The method is not only suitable for iron determination in water samples (where the iron content may be in ppm level) but also in the rock samples which contain iron up to 20%. The method was applied to a number of certified geo-standards, in-house standards, soils, stream sediments as well as different water samples. The determination of iron was made by 1,10-phenanthroline as well as thiocyanate (binary complex) methods, and the results obtained were compared with those obtained by the proposed one (Table 2). In the case of samples containing low as well as high iron, the conventional thiocyanate method (binary system) failed to give accurate and reliable results due to its poor sensitivity, instability of the colour and narrow range of Beer’s law adherence. As per our findings, the 1,10phenanthroline method, too, was not able to yield satisfactory results for iron determination in samples containing iron in traces. This may be attributed to the low sensitivity of the method. The 1,10-phenanthroline method, which has
Table 1 Effect of foreign ions on the determination of 1 mg/l of Fe Foreign ions
Tolerance limits (mg/l)a
Li, Na, K, Rb, Cl, Br, NO3 and SO4 Ca, Mg, Ba and Sr Al, Be, La, Y, Pb, Hg, Bi and PO34 As and Sb Cu, Ni, Mn and Sn Co and Zn Ti, V, Nb, Ta, Zr, Cr, Mo, Ce, W and U
N2000 100 50 30 20 15 10
a
For an error less than 2%.
Soil-1 Soil-2 Soil-3 Stream sediment-1 Stream sediment-2 Rock-1 Rock-2 Rock-3 Rock-4 Rock-5 Geo-standard-1 (SY-3)b Geo-standard-2 (MRG-1)b Stream water-1 Stream water-2
Fe2O3, % Proposed methoda
1,10-Phenanthroline method
10.10F0.12 9.95F0.10 13.10F0.15 9.47F0.10
9.90 9.90 13.62 9.70
13.27F0.14
13.41
9.48F0.11 12.10F0.16 1.64F0.03 1.41F0.02 8.60F0.09 6.42F0.08
9.70 11.76 1.55 1.43 8.65 6.44
6.40
17.20F0.15
17.33
17.30
0.15 (ppm) 0.65 (ppm)
0.10 (ppm) 0.70 (ppm)
Certified value
a
FStandard deviation. Reference materials—rock samples [SY-3 (syenite) and MRG-1 (gabbro)] from Canada Centre for Mineral and Energy Technology (CANMET Report 79-35 by Sydney Abbey, 1979), Energy, Mines and Resources Canada, 555 Booth St., Ottawa, Canada K1A 0G1. b
been recommended by Shapiro and Brannock for the silicate-rock analysis [2], is also found to be not very suitable for samples containing very low to very high iron, because of the inherent limitations of the method, i.e., the absorbance measurements have been suggested to be made at 560 nm as opposed to its k max (510 nm). At this wavelength, the sensitivity as well as the range of Beer’s law adherence reduced substantially although the selectivity of the method is claimed to be improved.
4. Conclusion The present method developed is highly effective for iron determination in rocks, minerals, soils, stream sediments and water samples. It has got many advantages over the thiocyanate (binary system) and even 1,10-phenanthroline method in terms of high sensitivity, excellent selectivity, reproducibility of results and accuracy. The results obtained from the application of this improved method on a number of diverse samples cause us to recommend this method for routine determination of iron in these samples.
Acknowledgement The authors are thankful to Dr. P. Krishnamurthy, Regional Director, AMD, ER, Jamshedpur, and Dr. H.C. Arora, Associate Director (Chem), AMD, Hyderabad for
P.K. Tarafder, R. Thakur / Microchemical Journal 80 (2005) 39–43
providing necessary facilities. Authors are also thankful to Director, AMD, for his kind permission to publish this work.
References [1] Z. Markzenko, Separation and Spectrophotometric Determination of Elements, Horwood, Chichester, 1986, p. 327.
43
[2] J.A. Maxwell, Rock and Mineral Analysis, Wiley, New York, 1968, p. 424. [3] P.J. Jeffery, Chemical Methods of Rock and Mineral Analysis, Pargamon, Oxford, 1981, p. 204. [4] E. Cerrai, G. Ghersini, Analyst 91 (1966) 662. [5] C.L. Luke, Anal. Chim. Acta 36 (1966) 122. [6] J.J. McCown, D.E. Kudera, Anal. Chem. 34 (1962) 870. [7] A.R. Jha, R.K. Mishra, Analyst 106 (1981) 1150. [8] L.G. Sillen, Acta Chem. Scand. 10 (1956) 185.
Surfactant-mediated extraction of iron and its spectrophotometric determination in rocks, minerals, soils, stream sediments and water samples Pranab K. Tarafder*, Raghbendra Thakur Analytical Chemistry Laboratory, Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Khasmahal, Jamshedpur-831002, India Received 22 June 2004; received in revised form 9 September 2004; accepted 12 September 2004 Available online 10 December 2004
Abstract An extraction spectrophotometric method for iron determination in rocks, minerals, soils, stream sediments and water samples has been developed. At pH 3-4, iron (III) forms a 1:2:1 ternary complex with thiocyanate and cetyltrimethylammonium bromide (Fe/SCN/CTAB) which is extracted into ethyl acetate. The readily formed purple complex is suitable for extraction spectrophotometric determination of iron in rocks and related materials from submicrogram to milligram levels. The method is free from any interference due to commonly associated ions present in the matrices of rock samples. The present method is at least fourfold more sensitive (e=3.2104 l mol 1 cm 1) than the conventional thiocyanate method and, in addition to the enhanced sensitivity and selectivity, it has got definite advantages over the corresponding binary thiocyanate system in terms of substantial improvement in the stability of the complex formed and broadening of Beer’s law adherence range (0–6.0 mg/l). The method has been applied to a number of geological and hydrogeochemical samples for the determination of iron and the results obtained have been found to be favourably comparable with those obtained from the standard methods. D 2005 Elsevier B.V. All rights reserved. Keywords: Extraction; Iron determination; Spectrophotometric method
1. Introduction There are many spectrophotometric methods for the determination of iron [1]. Amongst these, thiocyanate and 1,10-phenanthroline methods are generally used for the determination of iron in rocks and minerals [2,3]. Although ammonium thiocyanate has in the past been extensively used for the photometric determination of iron [4–6], presently it is not in much use because it is highly insensitive and suffers from a number of disadvantages, particularly when compared with the 1,10-phenanthroline method. The optical densities of ferric thiocyanate solutions depend upon the conditions used for the reaction (temperature, acidity, excess of reagent), the solutions may suffer
* Corresponding author. E-mail address: [email protected] (P.K. Tarafder). 0026-265X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2004.09.004
from some measure of fading, and do not completely follow the Beer–Lambert’s law [3]. Similarly, the 1,10phenanthroline method, which is even now widely used for the determination of iron in rocks and related samples, is also not advantageous when applied to samples having complex matrices and containing traces of iron, owing to its poor sensitivity and selectivity. Therefore, it is incumbent on us to develop a suitable method that can be successfully applied directly to samples having complex matrices and containing iron ranging from traces to percentage levels. In this endeavour, a robust method, overcoming most shortcomings associated with the existing thiocyanate method and meeting all the desired characteristics as expected from a standard method, has been developed by exploiting the reaction of iron (III) with thiocyanate and cetyltrimethylammonium bromide. Iron (III) reacts with thiocyanate and cetyltrimethylammonium bromide forming a bright purple hitherto unreported ternary
40
P.K. Tarafder, R. Thakur / Microchemical Journal 80 (2005) 39–43
complex, readily extracted into ethyl acetate. Unlike the binary thiocyanate complex, the present reaction is at least fourfold more sensitive. Besides, the method is highly selective because it is free from interference of most matrix elements present in the sample. Moreover, unlike the binary system, the colour developed is quite stable and interestingly enough, the results obtained are highly reproducible. The striking feature of this method is that, contrary to the existing thiocyanate method, the Beer’s law is obeyed over a wide range, i.e., from 0 to 6.0 mg/l of iron. It is pertinent to mention here that ternary complexes of Fe (III)–thiocyanate with other reagents [1,7] are known, but none has given as satisfactory results as given by this proposed method for iron determination in real samples. This is a new surfactant sensitized colour reaction of iron (III) with thiocyanate, and to the best of our knowledge, it is not reported in the literature so far. This prompted us to undertake a systematic study on the ternary complex formation of iron (III), thiocyanate and CTA, and to explore the possibility of its application to a host of diverse samples of complex matrices such as rocks, minerals, soils, stream sediments and waters for the accurate and rapid determination of iron. The present paper describes such systematic studies for the development of a suitable extraction spectrophotometric method for the simple and accurate determination of iron in a range of silicate rocks, minerals, soils, stream sediments and water samples, including geo-standard samples.
2. Materials and methods 2.1. Apparatus (1) (2)
UV–VIS. Spectrophotometer (CHEMITO-2500) pH-Meter (ELICO-LI-120)
2.2. Reagents All reagents used were of analytical grade. (1)
(2) (3)
(4)
Potassium thiocyanate, 10% w/v. Weigh out about 10 g of potassium thiocyanate and dissolve it in water in a 100 ml volumetric flask and make up to the mark. Ethyl acetate. Cetyltrimethylammonium bromide, 1% (w/v) aqueous solution. This reagent is readily soluble in water and no deterioration was observed over a period of at least 2–3 days. However, fresh solutions were prepared for this study. Standard solution of iron. Dissolve 0.7022 g ferrous ammonium sulphate in 100 ml water, add 5 ml of 1:5 sulphuric acid and run in cautiously a dilute solution of potassium permanganate(2.0 g l 1) until a slight pink coloration remains after shaking well. Dilute to 1 l and mix thoroughly (1 ml=0.1 mg of Fe). More
dilute solutions were prepared by diluting the stock solution. 2.3. Procedures 2.3.1. Dissolution of samples 2.3.1.1. For rocks, soils and stream sediments. Weigh out 0.5000 g of each sample(~ 200 mesh size) to a series of 90 ml platinum dishes. Moisten the powder with water and cautiously add 10 ml HF (48%). If a visible reaction occurs on first addition of the HF, cover the dishes and add the remainder of the HF when the reaction has diminished. Add 3 ml concentrated H2SO4 (98%), cover the dishes and place on a steam bath and allow for heating at least 6 h. Remove the dishes from the steam bath, wipe off the exterior and place it on a sand bath. Remove the cover and evaporate the contents to a volume of about 3 ml. Cautiously add 1 ml concentrated HNO3. Continue the evaporation until copious white fumes of sulfur trioxide are evolved. Cool, add 1 ml concentrated HNO3 and repeat the heating to fumes. Continue with the addition of nitric acid until all organic matter has been destroyed. Transfer quantitatively the contents of platinum dish to a 400-ml beaker, dilute to 100 ml and boil until the solution is clear. Cool and quantitatively transfer the solution to a 250-ml volumetric flask, dilute to volume at room temperature and mix thoroughly. 2.3.1.2. For minerals. Weigh a 0.2 g finely ground sample (~ 200 mesh) into a 50-ml silica crucible and fuse it with about 2 g potassium bisulphate. Transfer the fused mass quantitatively into a 250-ml beaker and add 5 ml 1:1 H2 SO4 followed by about 50 ml water. Boil the solution for a while in order to get a clear solution. Cool and transfer the solution into a 100-ml volumetric flask and dilute to volume. 2.3.2. Colour reaction, extraction and absorbance measurements Take a suitable aliquot of the sample in a 125-ml separating funnel, add 5 ml of distilled water and 5 ml of 10% potassium thiocyanate solution. Shake a while and adjust the pH in the range 3–4. Add 10 ml of ethyl acetate and 5 ml of 1% cetyltrimethylammonium bromide solution. Shake for 2 min. Set it aside for 5 min till the complete phase separation takes place. Drain the aqueous layer out, filter the ethyl acetate through Whatman 541 filter paper and measure the absorbance of the complex in ethyl acetate at 474 nm against the reagent blank prepared similarly but containing no iron.
3. Results and discussion Twenty micrograms of Fe (III) per 10 ml of ethyl acetate was used for subsequent experiments.
P.K. Tarafder, R. Thakur / Microchemical Journal 80 (2005) 39–43
3.1. Choice of solvents Different solvents were tried for extraction studies. These were carbon tetrachloride, chloroform, benzene, n-butyl alcohol, ethyl acetate, methyl-iso-butyl ketone (MIBK), etc. Only ethyl acetate gave the most desired results. 3.2. Effect of pH The effect of pH on complex formation and its subsequent extraction in ethyl acetate, as evident by having obtained maximum absorbance was studied by varying it over a wide range. As shown in Fig. 1, the most effective range of the pH was found to be 3–4. An increase in acidity particularly due to nitric acid resulted into the instability of the colour. Hence, a pH of 3–4 was chosen to be the ideal as the stability of the complex was found to be the best here. 3.3. Effect of reagent concentration 3.3.1. Thiocyanate The effect of thiocyanate concentration on the optimum colour formation was studied by varying its concentration from 1% to 20% (w/v) aqueous solution. It was observed that a 5 ml of 10% thiocyanate solution gave the most satisfactory results as evident by having obtained the maximum absorbance for the extracted species. 3.4. Cetyltrimethylammonium bromide The effect of this reagent, too, in the formation of the ternary complex suitable for the extractive determination of
41
iron, was also studied by varying its concentration from 0.01% to 5% (w/v) solution. A 5 ml of 0.1% (w/v) aqueous solution of this reagent was found to be sufficient in order to obtain the desired result. 3.5. Effect of temperature Unlike the Fe (III)–SCN binary system (the prevailing thiocyanate method), the present method has been found to be independent of temperature in the range 20–40 8C. 3.6. Composition of the complex The stoichiometry of the complex formed was found out by curve fitting method [8], i.e., by plotting log D vs. log [thiocyanate] as well as log [CTAB]. A slope of 1.8, which is close to the integer 2, is obtained for thiocyanate. This shows that 2 molecules of thiocyanate take part in the complex formation. Similarly, a slope of one is obtained for CTAB, confirming that one molecule of CTAB takes part in the formation of the ternary complex. Thus, the stoichiometry of the ternary complex may be presumed as 1:2:1 (Fe/SCN /CTAB). 3.7. Spectral characteristics, Beer’s law, sensitivity and precision of the method The complex formed absorbs maximum at 474 nm (see Fig. 2). The absorbance due to the reagent blank at this wavelength was in the range 0.010–0.015. The method obeyed Beer’s law over a wide range, i.e., 0–6 mg/l. The molar absorptivity of the method is 3.2104 l mol 1 cm 1. The relative standard deviation (RSD) of the method at a level of 1 mg/l iron is 1.2%. 3.8. Effect of diverse ions
Fig. 1. Effect of pH on the extraction of iron from aqueous to organic phase.
In order to assess the efficacy of the method under development, the effect of many ions on the determination of 1 mg/l iron by this technique was studied. The results are shown in Table 1. From the results obtained for this study, it was found that most ions which generally accompany iron in rocks of diverse matrices did not interfere. Unlike in the tiron, 1,10-phenanthroline and thiocyanate (binary system) methods, the presence of large quantities of Cu, Ni, Zn, Co, V, Mo, U, Mn, Nb, Ta, Ti, Pb, Hg, etc. did not pose any problem in the estimation of iron using the present method. It shows that the method developed is quite selective for iron determinations. This fact was further attested by applying the present method to a number of samples, including geo-standards of diverse matrices for iron determination. The results obtained thereof were found to be in excellent agreement with the certified values and those obtained from standard methods.
42
P.K. Tarafder, R. Thakur / Microchemical Journal 80 (2005) 39–43 Table 2 Determination of Fe2O3 in diverse samples by the proposed method and its comparison with 1,10-phenanthroline method (n=5) Sample
Fig. 2. (A) Absorption spectrum of the ternary complex of iron (2.0 mg/l) against reagent blank and (B) absorption spectrum of reagent blank against ethyl acetate.
3.9. Analytical applications The method finds an excellent application in silicate rocks, minerals, soil, stream sediments and water samples for the determination of iron from submicrogram to percentage level. For the estimation of iron in water samples, pretreatment of these samples is a must, preferably just after sampling. A few drops of HNO3 are added in order to prevent precipitation of iron because iron starts precipitating above pH 4. Hence, it is advisable that the water should have pHb4. The method is not only suitable for iron determination in water samples (where the iron content may be in ppm level) but also in the rock samples which contain iron up to 20%. The method was applied to a number of certified geo-standards, in-house standards, soils, stream sediments as well as different water samples. The determination of iron was made by 1,10-phenanthroline as well as thiocyanate (binary complex) methods, and the results obtained were compared with those obtained by the proposed one (Table 2). In the case of samples containing low as well as high iron, the conventional thiocyanate method (binary system) failed to give accurate and reliable results due to its poor sensitivity, instability of the colour and narrow range of Beer’s law adherence. As per our findings, the 1,10phenanthroline method, too, was not able to yield satisfactory results for iron determination in samples containing iron in traces. This may be attributed to the low sensitivity of the method. The 1,10-phenanthroline method, which has
Table 1 Effect of foreign ions on the determination of 1 mg/l of Fe Foreign ions
Tolerance limits (mg/l)a
Li, Na, K, Rb, Cl, Br, NO3 and SO4 Ca, Mg, Ba and Sr Al, Be, La, Y, Pb, Hg, Bi and PO34 As and Sb Cu, Ni, Mn and Sn Co and Zn Ti, V, Nb, Ta, Zr, Cr, Mo, Ce, W and U
N2000 100 50 30 20 15 10
a
For an error less than 2%.
Soil-1 Soil-2 Soil-3 Stream sediment-1 Stream sediment-2 Rock-1 Rock-2 Rock-3 Rock-4 Rock-5 Geo-standard-1 (SY-3)b Geo-standard-2 (MRG-1)b Stream water-1 Stream water-2
Fe2O3, % Proposed methoda
1,10-Phenanthroline method
10.10F0.12 9.95F0.10 13.10F0.15 9.47F0.10
9.90 9.90 13.62 9.70
13.27F0.14
13.41
9.48F0.11 12.10F0.16 1.64F0.03 1.41F0.02 8.60F0.09 6.42F0.08
9.70 11.76 1.55 1.43 8.65 6.44
6.40
17.20F0.15
17.33
17.30
0.15 (ppm) 0.65 (ppm)
0.10 (ppm) 0.70 (ppm)
Certified value
a
FStandard deviation. Reference materials—rock samples [SY-3 (syenite) and MRG-1 (gabbro)] from Canada Centre for Mineral and Energy Technology (CANMET Report 79-35 by Sydney Abbey, 1979), Energy, Mines and Resources Canada, 555 Booth St., Ottawa, Canada K1A 0G1. b
been recommended by Shapiro and Brannock for the silicate-rock analysis [2], is also found to be not very suitable for samples containing very low to very high iron, because of the inherent limitations of the method, i.e., the absorbance measurements have been suggested to be made at 560 nm as opposed to its k max (510 nm). At this wavelength, the sensitivity as well as the range of Beer’s law adherence reduced substantially although the selectivity of the method is claimed to be improved.
4. Conclusion The present method developed is highly effective for iron determination in rocks, minerals, soils, stream sediments and water samples. It has got many advantages over the thiocyanate (binary system) and even 1,10-phenanthroline method in terms of high sensitivity, excellent selectivity, reproducibility of results and accuracy. The results obtained from the application of this improved method on a number of diverse samples cause us to recommend this method for routine determination of iron in these samples.
Acknowledgement The authors are thankful to Dr. P. Krishnamurthy, Regional Director, AMD, ER, Jamshedpur, and Dr. H.C. Arora, Associate Director (Chem), AMD, Hyderabad for
P.K. Tarafder, R. Thakur / Microchemical Journal 80 (2005) 39–43
providing necessary facilities. Authors are also thankful to Director, AMD, for his kind permission to publish this work.
References [1] Z. Markzenko, Separation and Spectrophotometric Determination of Elements, Horwood, Chichester, 1986, p. 327.
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[2] J.A. Maxwell, Rock and Mineral Analysis, Wiley, New York, 1968, p. 424. [3] P.J. Jeffery, Chemical Methods of Rock and Mineral Analysis, Pargamon, Oxford, 1981, p. 204. [4] E. Cerrai, G. Ghersini, Analyst 91 (1966) 662. [5] C.L. Luke, Anal. Chim. Acta 36 (1966) 122. [6] J.J. McCown, D.E. Kudera, Anal. Chem. 34 (1962) 870. [7] A.R. Jha, R.K. Mishra, Analyst 106 (1981) 1150. [8] L.G. Sillen, Acta Chem. Scand. 10 (1956) 185.