Anal Bioanal Chem (2002) 373 : 299–303 DOI 10.1007/s00216-002-1343-6
O R I G I N A L PA P E R
B. M. Nagabhushana · G. T. Chandrappa · B. Nagappa · N. H. Nagaraj
Diformylhydrazine as analytical reagent for spectrophotometric determination of iron(II) and iron(III)
Received: 16 November 2001 / Revised: 6 March 2002 / Accepted: 26 April 2002 / Published online: 7 June 2002 © Springer-Verlag 2002
Abstract The bidentate ligand diformylhydrazine (OHC– HN–NH–CHO), DFH, combines with iron(II) and iron(III) in alkaline media in the pH range 7.3–9.3 to form an intensely colored red–purple iron(III) complex with an absorption maximum at 470 nm. Beer’s law is obeyed for iron concentrations from 0.25 to 13 µg mL–1. The molar absorptivity was in the range 0.3258×104–0.3351×104 L mol–1 cm–1 and Sandell’s sensitivity was found to be 0.0168 µg cm–2. The method has been applied to the determination of iron in industrial waste, ground water, and pharmaceutical samples. Keywords Diformylhydrazine reagent · Iron(III)–diformylhydrazine complexes · Iron(II) determination · Iron(III) determination · Spectrophotometry.
Introduction
Fig. 1 Amido–imidol tautomerism of diformylhydrazine in solution: I (H2L) (0.016 mol L–1 NaOH or pH 7.6)
The tautomeric property (Fig. 1) of diformylhydrazine (DFH) in aqueous solution is the basis of its use as an analytical reagent for the spectrophotometric determination of iron. Diformylhydrazine (OHC–HN–NH–CHO) [1] is a potential ligand with O and N as electron-donor atoms [2]. DFH has not hitherto been used as a complexing agent in spectrophotometric studies. As a fuel DFH has been used extensively by Patil et al. in combustion syntheses of oxide materials [3], forsterite [4], mullite-zirco-
nia [5], and willemite powders [6]. DFH has the ability to forms a colored complex with iron in alkaline media. A variety of reagents has been proposed for the spectrophotometric determination of iron. Most are based on solvent extraction [8, 9, 10, 11, 12] – few reagents [13, 14, 15] can be used in aqueous media. DFH is water-soluble, does not require an oxidizing or reducing agent, readily forms a stable colored complex with both iron(II) and iron(III), and it is easy to perform the investigation at room temperature. An attempt is made to understand the behavior of the reagent in alkaline media and reaction mechanisms are proposed on the basis of the experimental observations.
B.M. Nagabhushana Department of Chemistry, M.S. Ramaiah Institute of Technology, Bangalore-560054, India
Experimental
G.T. Chandrappa (✉) Laboratoire de Chimie de la Matière Condensée, Université Pierre et Marie Curie (Paris VI – tour 54 E5), 4 place Jussieu, 75252 Paris, France e-mail:
[email protected] B. Nagappa · N.H. Nagaraj Karnataka State Pollution Control Board, Bangalore–560001, India
Apparatus and reagents Experiments were performed with a Varian DMS 100 S UV spectrophotometer equipped with 1-cm glass cells, a Systronics digital pH-meter (MKVI), and a century-cc 601 model digital conductivity meter. Fe(II) and Fe(III) solutions (2.792 g L–1 Fe) were prepared by dissolving FeSO4.(NH4)2SO4.6H2O (19.6 g) and FeCl3 (8.11 g) in
300 H2SO4 (0.36 mol L–1, 1 L) and HCl (0.005 mol L–1, 1 L), respectively. Both solutions were standardized [16] by use of potassium ferrocyanide as external indicator. Diformylhydrazine solution (0.2 mol L–1) was prepared by dissolving 17.6 g of the compound (synthesized in our laboratory) in 1 L water and its concentration was determined by iodometric titration [16]. Sodium hydroxide solution (0.05 mol L–1) was prepared by dissolving 2 g in 1 L water and was standardized. Buffer solution, pH 12.5, was prepared by dissolving 1.5 mL orthophosphoric acid (sp. gr. 1.7) in 500 mL water and adjusting to the required pH with 1 mol L–1 sodium hydroxide solution.
Preparation of sample solutions Sludge samples collected from industry were dried at 120 °C for 6 h. A finely powdered sludge sample (6 g) was digested with 10 mL concentrated nitric acid and excess acid was expelled by repeated evaporation. The solution was cooled and diluted to 250 mL with water in a volumetric flask. Standard solutions were obtained by appropriate dilution of the stock solution with water. Raricap-haematinic tablet weighing 1.157 g containing 25 mg iron as ferrous calcium citrate was dissolved in 1 mL concentrated nitric acid by heating for 3 min. The solution was mixed with 10 mL water and boiled to remove excess acid. The resulting solution was diluted to 500 mL in a volumetric flask. Standard solutions were prepared by appropriate dilution of the stock solution with water. A ground water sample, 3000 mL, collected from the catchment area of the river Vrushabhavathi, near Bangalore, India, was reduced to 250 mL by evaporation. Concentrated nitric acid (20 mL) was added and the resulting mixture was digested for 2 h. After removal of excess nitric acid by evaporation the small volume remaining was cooled to room temperature and diluted to 50 mL in a volumetric flask.
Fig. 2 Absorption spectra of the iron(III)–diformylhydrazine complex: (1) iron(II) ammonium sulfate, 5 µg; 0.2 mol L–1 DFH, 6 mL; 0.05 mol L–1 NaOH, 3 mL; (2) iron(II) ammonium sulfate, 5 µg; 0.2 mol L–1 DFH, 6 mL; buffer of pH 12.5, 2.5 mL; (3) iron(III) chloride, 40 µg; 0.2 mol L–1 DFH, 15 mL; no NaOH or buffer (the final volume was always 25 mL)
Solutions containing 0.25–13 µg mL–1 iron(II) or iron(III) were transferred into 25-mL volumetric flasks, DFH solution (0.2 mol L–1, 6 mL) and sodium hydroxide solution (0.05 mol L–1, 3 mL) were added successively and the mixture was diluted to volume with water. When buffer was used solutions containing 0.5–12.5 µg mL–1 iron(II) were transferred into 25-mL volumetric flasks, DFH solution (0.2 mol L–1, 6 mL) and buffer solution (pH 12.5, 2.5 mL) were added, and the mixture was diluted to volume with water. The absorbance was measured at 470 nm with a 1-cm cell against reagent blank solution prepared under the same conditions but without iron solution.
state combines with DFH. The iron(III)–DFH complex has an absorption peak at 470 nm; subsequent measurements were therefore made at this wavelength. In aqueous solution the imidol form of DFH is present in smaller amounts than the amido form (Fig. 1). Deprotonated DFH in the imidol form reacts readily with iron(III) to form an iron(III)–DFH complex. Formation of the complex is promoted by sodium hydroxide or buffer, owing to increased conversion of the amido form to the imidol form; this results in intense red–purple color formation. The accessibility of the imidol form of the reagent and oxidation of iron(II) to iron(III) depend solely on the strength of the alkali in the reaction medium.
Results and discussion
Effect of reagents and foreign ions
Absorption spectra
Effect of sodium hydroxide concentration
The absorption spectra (Fig. 2) of iron(III)–DFH complex containing 5 µg mL–1 iron(II) ammonium sulfate and 6 mL 0.2 mol L–1 DFH solution in sodium hydroxide medium (curve 1) and in buffer medium (curve 2) were recorded in the region 350–750 nm. The difference between curves 1 and 2 is, under these experimental conditions, very small. Significant absorbance was observed (curve 3) for an alkali-free sample containing a large amount of iron(III) chloride (40 µg mL–1) and DFH (0.2 mol L–1, 15 mL) but no such color development was observed when iron(II) ammonium sulfate (40 µg mL–1) was mixed with DFH (0.2 mol L–1, 15 mL) in the absence of alkaline solution. This observation clearly reveals that iron in its trivalent
The concentration of sodium hydroxide has a considerable effect on the absorbance of the colored complex (Fig. 3). The amount of sodium hydroxide required (4.8×10–3 mol L–1 to 8.8×10–3 mol L–1) in the iron(III) solution for development of maximum color intensity is more than the amount of sodium hydroxide (2.0×10–3 to 6.0×10–3 mol L–1) used for iron(II), despite of the same concentrations of iron(II) and iron(III) (4 µg mL–1). As stated earlier, the absorbance spectrum of the complex is the same whether iron is added as iron(II) or as iron(III). The oxidation of iron(II) to iron(III) occurs in the presence of reagent and sodium hydroxide or buffer solution but the colored complex was not formed when iron(III) solution was added to a solu-
Determination of iron in sodium hydroxide and buffer media
301 Table 1 Tolerance limits for the determination of 5 µg mL–1 iron(II) and iron(III)
Fig. 3 Effect of sodium hydroxide concentration: (1) iron(II) ammonium sulfate, 4 µg; 0.2 mol L–1 DFH, 6 mL; 0.05 mol L–1 NaOH; (2) iron(III) chloride, 4 µg; 0.2 mol L–1 DFH, 6 mL; 0.05 mol L–1 NaOH (final volume, 25 mL)
tion of DFH and buffer. The presence of PO43– ions in the buffer acts as masking agent and prevents iron(III) ions combining with DFH reagent. This different reactivity enables differentiation of iron solutions. The same buffer provides conditions enabling deprotonation of the DFH reagent and oxidation of iron(II) to iron(III) to form a complex. Iron(II) and iron(III) have their maximum, and almost constant, absorbance in sodium hydroxide media in the pH range 7.3 to 8.7 and 8.2 to 9.1, respectively. The pH of the solution mixture containing iron(II) in the presence of buffer media seemed to be maximum and constant from 8.3 to 9.3. On the basis of these observations probable reactions can be proposed. From Eqs. (1) (iron(III) in the presence of sodium hydroxide solution) and (2) (iron(II) in the presence of sodium hydroxide or buffer solution) it is evident that twice the amount of sodium hydroxide is required for iron(III) than for iron(II) (as observed in Fig. 3). 2H2 L DFH 3+
Fe + 2HL− 2H+ + 2OH− Fe3+ + 2H2 L + 2OH− 2H2 L DFH 2+
+
Fe + 2H Fe3+ + 2HL− Fe2+ + 2H2 L + 2OH−
→
2H+ + 2HL− + Fe (HL)2 2H 2O + Fe (HL)2 + 2H2 O
(1)
2H+ + 2HL− Fe3+ + H2 O + 1/2H2 + (2) Fe (HL)2 + Fe (HL)2 + 2H2 O + 1/2H2
Effect of DFH concentration The effect of DFH concentration was studied at pH 8.5 by measuring the absorbance at 470 nm of solution containing a fixed amount (5 µg mL–1) of iron and different amounts of reagent. The results revealed that constant, maximum absorbance is obtained if excess DFH is used.
Foreign ion
Added as
Amount tolerated (µg mL–1)
Ni2+ Hg2+ Mn2+ Zn2+ Cu2+ Al3+ EDTA Co2+ Sn2+ Ce2+ NO3– Zr4+ Pb2+ Na+ C2O42– Bi3+ Mg2+
Sulfate Chloride Chloride Sulfate Sulfate Sulfate EDTA disodium Chloride Chloride Sulfate Sodium salt Oxychloride Acetate Carbonate Sodium salt Nitrate Nitrate
10 60 2 2 80 1 6 2 4 2 600 2 2 12 100 20 500
Effect of foreign ions The effect of foreign ions on the determination of iron(II) and iron(III) was investigated by adding known amounts of each foreign ion to a solution mixture containing 5 µg mL–1 iron(II) or iron(III), DFH (0.2 mol L–1, 6 mL), and sodium hydroxide (0.05 mol L–1, 3 mL) in a 25-mL volumetric flask. The mixture was diluted to volume with water and absorbance was recorded at 470 nm. The results are presented in Table 1. Stability of the complex The absorbance of the iron(III)–DFH complex is maximum in the presence of alkali. This colored complex is stable for 4 days over a temperature range 0–40 °C. Initially absorbance in alkaline media increases and then remains constant. The reagent DFH occurs as the HL– form when the concentration of alkali is in the range discussed above. When the concentration of sodium hydroxide is increased further the absorbance of the system tends to decrease, possibly because of conversion of HL– to the L2– form. The L2– form of DFH is unstable, which causes dissociation of the iron(III)–DFH complex. Dissociation of H2L to HL– and HL– to L2– is readily demonstrated by conductometric and pH-measurement studies. The first and second equivalence points in Fig. 4, correspond to H2L→HL–+H+ and HL–→L2–+H+, respectively. Addition of reagent to a solution containing iron and alkali does not result in the development of a colored complex, possibly because of strong and preferential bonding of OH– ions to iron(III) before the DFH reagent is added. It is, therefore, necessary to add an alkaline solution to the mixture containing reagent and iron solution.
302
tion, this method is simple, rapid, stable, non-extractive and does not require additional reducing or oxidizing agent. It is proposed that reaction mechanisms could be studied by use of this novel method.
Structure and charge of the complex
Fig. 4 Measurement of the pH and conductivity of diformylhydrazine: (1) pH measurement, 0.2 mol L–1 DFH, 5 mL, + distilled water, 15 mL, in 100-mL beaker; 0.05 mol L–1 NaOH added from burette; (2) Conductivity measurement, 0.2 mol L–1 DFH, 5 mL, + distilled water, 15 mL, in 100-mL beaker; 0.05 mol L–1 NaOH, added from burette
Figures of merit The molar absorptivity was found to be in the range 0.3258×104–0.3351×104 L mol–1 cm–1 and Sandell’s [17] sensitivity index was 0.0168 µg cm–2 iron. The overall relative standard deviation (RSD) for 20 determinations was 1.23% and the standard analytical error (SD/√n) did not exceed 0.029. The limit of detection and linear dynamic range values calculated by use of Beer’s law were found to be 0.25–13 µg mL–1 for iron(II) and 0.5–12.5 µg mL–1 for iron(III). The sensitivities of different spectrophotometric methods in which highly sensitive iron chromogenic reagents have been used are presented in Table 2, with results from this method. Methods based on solvent extraction were usually more sensitive than methods employing aqueous media. This method has the advantage of determining iron in both its oxidation states either in a binary mixture, or individually, in aqueous media. In addi-
Excess reagent in solution is necessary to maintain the stability of the complex. The composition of the complex was determined at pH 8.5 by the equilibrium-shift [18] method. A plot of the logarithm of the distribution ratio against log [DFH] had a slope of 1.9, indicating the presence of two DFH molecules in the complex. The formation-constant [19] method enables determination of the amount (mol) of Fe and DFH by maintaining all concentrations constant except that of the component of interest. Variation of [Fe] at constant excess [DFH] resulted in a linear plot of slope 0.92 for the dependence of log A on log [Fe]. These results show that the composition of the iron(III)–DFH complex is 1:2. The electrolytic nature of the complex was determined by ion-exchange experiments. When a solution (5 µg mL–1, 25 mL) of the complex was passed through a Dowex 50 W-X8 cation-exchange resin column (30×1.4 cm3) the red color was completely absorbed and the eluate was colorless. When Dowex 1-X8 anion exchange resin was used, the eluate was red-purple in color. This indicates the complex was cationic.
Analytical application The proposed method is suitable for the determination of iron in a variety of samples. Sample solutions were prepared as described in the experimental section. The absorbance was measured at 470 nm, by use of 1-cm glass cells, against a reagent blank. The concentration of iron was calculated by use of Beer’s law (calibration graph). Owing to the low iron content (~0.3 µg mL–1) of the ground water sample, 10 mL of the sample solution was used in each determination. The iron content of Raricap-
Table 2 Comparison of spectrophotometric reagents for analysis of iron(II) and iron(III) Reagent
Molar absorptivity (L mol–1 cm–1×104)
Sandell’s sensitivity (µg cm–2)
Ref.
2,2′-Dipyridyl-2-benzothiazolyl hydrazone Pyridine-2-carbaldehyde-2-hydroxybenzoyl hydrazone 2,2′-Dipyridyltetraphenylborate 1,10-Phenanthroline picrate 2,2′-Dipyridyl ketone picolinohydrazone Cyclohexylthioglycolate 5,5-Dimethylcyclohexane-1 2,3-trione-1,2-dioxane-3-thiosemicarbazone 5,5-Dimethyl-1,2,3-cyclohexanetrione-1,2-dioxime-3-thiosemicarbazone 2-Chloroquinoline-3-carbaldehydethiosemicarbazone Diformylhydrazine
3.41 0.364 0.889 13 0.664 0.7 0.89 0.89 0.35 0.33
0.016 0.15 0.063 0.043 0.016 0.008 0.05 0.05 0.016 0.017
[7] [8] [9] [10] [11] [12] [13] [14] [15] This work
303 Table 3 Determination of the concentration (µg mL–1) of iron(III) in different samples by use of the proposed method (with diformylhydrazine), and comparison with the thiocyanate method and atomic absorption spectrometry (AAS) Sample
1 Sludge sample 2 Iron tablet
3 Water sample
Source
Industrial effluent Raricaphaematinic tablet Ground water
Spectrophotometry
AAS [21]
KSCN [20]
Diformylhydrazine
4.1
3.8
4
3.3
3.5
3.5
0.3
0.31
0.32
Each result is the mean from five determinations
haematinic tablet (25 mg) and ground water samples (~ 0.3 µg mL–1) determined by this method was in good agreement with data obtained by other methods [20, 21] commonly used for determination of iron (Table 3). Acknowledgements The authors wish to express thanks to Dr Thierry AZAIS and Dr Galo Soler-Illia, Laboratoire de Chimie de la Matière Condensée, Universite Pierre et Marie Curie, Paris, France, for their valuable suggestions and help in the preparation of this manuscript.
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