Analyst, July, 1979, Vol. 104, pp. 613-619 613 Pyrid i ne-2-ca r ba Ide h yde 2-Hyd roxy benzoy I hyd razone as a Selective Reagent for the Extraction and Spectrophotometric Determination of Iron( I I) M. Gallego and M. Garcia-Vargas Department of Analytical Chemistry, Faculty of Sciences, University of Seville, Seville, Spain and M. Valcarcel Depadmen.t of Analytical Chemistry, Faculty of Sciences, University of Cdvdoba, Cdvdoba, Spain Pyridine-2-carbaldehyde 2-hydroxybenzoylhydrazone reacts with iron (11) to produce a green 2: 1 complex (Amax. = 620 nm, 4 = 3.64 x lo3 1 mol-l cm-1 in aqueous ethanolic solution, and A,,,. = 640nm, E = 3.67 x lo3 1 mol-l cm-l in chloroform). The green complex, extracted into chloro- form, has been used for the spectrophotometric determination of trace amounts of iron.The method has a high selectivity and has been applied to the determination of iron in different samples, such as industrial waste water, non-ferrous materials and minerals. Keywords Iron determination ; spectroplzotometry Hydrazones have been used widely for the spectrophotometric determination of metal ions and several papers have dealt with the use of aroylhydrazones as analytical reagents. The most studied examples have been derived from 2-hydroxybenzaldehyde,lp2 4-dimethyl- aminobenzaldehyde, 2-hydroxynapht halene-1 -carbaldehyde*s5 and pyridine-2-carbaldehyde.' Aroylhydrazones behave as bidentate,798 tridentate,gs10 or tetradentatell ligands, forming coloured complexes with transition metal ions. In moderately acidic media or alkaline solution the hydrogen atom of the -CONH- group can split off and neutral metal complexes are formed.gJ0 Pyridine-2-carbaldehyde 2-hydroxybenzoylhydrazone (PAHB) has been used in spectro- photometric determinations of nickel and zinc.10 In this paper, the characteristics and properties of the iron(I1) complex with PAHB are described and a sensitive and very selective photometric method for determining trace amounts of iron is proposed.The determination of small amounts of iron in different .materials is described. Experimental Apparatus A Pye Unicam SP800 spectrophotometer was used for recording spectra in the ultraviolet and visible regions of the spectrum and a Coleman 55 (digital) instrument was used for measurements at fixed wavelengths, equipped with 1-cm glass or silica cells.Pye Unicam SPlOOO infrared and Perkin-Elmer 460 atomic-absorption spectrophotometers were also used. A Philips PW 9408 pH meter, with glass - calomel electrodes, was used for pH measure- ments. Reagents All solutions were prepared with analytical-reagent grade chemicals using distilled water. Pyridine-2-carbaldehyde 2-hydroxybenzoylhyd~azone reagent solution. A 0.075% m/V solu- tion was prepared by dissolving 0.075 g of recrystallised reagent in 3 ml of NN-dimethyl- formamide and diluting to 100ml with chloroform. This solution was stable for at least 1 week. The reagent was prepared in the pure form from 2-hydroxybenzoylhydrazide and pyridine-2-carbaldehyde as detailed in reference 10. This solution was prepared by dissolving ammonium iron( 11) sulphate hexahydrate in dilute sulphuric acid and standardising it gravimetrically. This solution was prepared fresh daily.Iro.n(I1) standard solution, 2.0436 g 1-1 of iron(1I). Ascorbic acid, 5% m/V.614 GALLEGO et al. : PYRIDINE-2-CARBALDEI-IYDE 2-HYDROXYBENZOYL Analyst, VOJ. 104 Bufer solution, pH 4.35. This solution was prepared by dissolving 105 g of sodium acetate trihydrate in distilled water, adding 100 ml of glacial acetic acid and diluting the mixture to 1 1. Procedure for the Determination of Iron( 11:) To 10-250 ml of sample solution in a separating funnel containing up to 120 pug of iron(II), add 1 ml of 5% m/V ascorbic acid solution and 5 ml of buffer solution and extract the mixture with one 10-ml volume of PAHB reagent solution.Shake the funnel vigorously for 2 min, allow the phases to separate and transfer the llower (organic) layer into a 10-ml flask con- taining anhydrous sodium sulphate. Measure the absorbance of the green chloroform extracts against water at 640 nm. The calibration graph is prepared by using standard solutions of iron(I1) treated in the same way. Results and Discussion Study of Iron - PAHB System Formation of iron complex in aqueous ethanolic solution green complex is formed immediately. Fig. 1. iron(I1) .12J3 When dilute iron(I1) solutions and a 0.1% m/V solution of PAHB in ethanol are mixed, a Absorption spectra of the complex are shown in PAHB differs from other related ligands in that a complex is formed only with 1.0 - 400 500 600 700 Wavelength/nm Fig. 1.Absorption spectra of green iron complex. Concentration of iron(I1) 10 pg ml-1. A, Green com- plex extracted into chloroform a t pH 4.35; B, green complex in aqueous ethanolic medium a t pH 7 ; and C, reagent blank. I 0.4 -0 -=f 'i 0.2 0 2 A 4 6 8 10 PH Fig. 2. Influence of pH on formation of iron complex. A, In aqueous ethanolic solution at 620 nm; and B, extracted into chloroform, at 640 nm.JuJy, 1979 AS A SELECTIVE SPECTROPHOTOMETRIC REAGENT FOR IRON(II) 61 5 Volumes (10ml) of a 0.1% m/V solution of PAHB in ethanol were allowed to react with 2.5ml of a 100.0pgml-l solution of iron(I1) at different pH values in a series of 25-ml calibrated flasks. The absorbances were measured at 620 nm after a 15-min reaction and are shown in Fig. 2. The green complex is formed immediately in aqueous media and is stable for 2 h in the optimum pH range, but high pH causes numerous interferences by metal ions that precipi- tate as hydroxides or basic salts.It is concluded that in aqueous media the complex of iron(I1) with PAHB is not of great analytical interest. The optimum pH range is 6.0-8.0. Stoicheiometry of the complex Job’s curves were plotted for the complex. The absorbance of the green complex obtained at pH 7.0 was measured at 620 nm immediately after preparation. Results showed [Fig. 3 (A)] a stoicheiometric ratio of metal to ligand of 1:2. The samples were then extracted with PAHB in chloroform at pH 4.35 and the absorbance was measured a t 640 nm; the same molar ratio (1 : 2) was found [Fig. 3 (B)]. 0 0.2 0.4 0.6 0.8 I F e ( l l ) l IFe(ll)l + IPAHBI Fig.3. Composition of iron(I1) - PAHB complex by the continuous variation method. A, In aqueous ethanolic solution (pH = 7, h = 620 nm, 2 x 10-3 M) ; and B, extracted into chloroform (pH = 4.35, h = 640 nm, 3 x 10-3 M). Oxidation state of iron and structure From experimental evidence it was concluded that the reagent reacts with iron in the bivalent state to give the green complex. In order to ensure that iron is present as iron(II), ascorbic acid was selected for use as a reducing agent. The green complex was not retained on either a cationic (Dowex 50-X8, sodium form) or an anionic (Dowex 1-X8, chloride form) ion-exchange resin, indicating that it was uncharged. In order to check the co-ordination in the complex, other related ligands have been tested.Benzaldehyde 2-hydroxybenzoylhydrazone does not form a green complex with iron( 11) , as the ligand does not contain the -N =C-C=N- chromophore group. Pyridine-2-carbaldehyde benzoylhydrazone forms a green 3 : l complex with iron(I1). The molecule contains the ferroin group, but not the hydroxide group. It is apparent that this reagent acts as a bidentate ligand. On the other hand, infrared spectra of the iron(I1) - PAHB complex in the solid state (using potassium bromide discs) show that the shift of the carbonyl stretching mode to a lower frequency is in accord with the co-ordination through oxygen; moreover, no imine stretching mode appears in the 3000 cm-1 region, owing to the de-protonation of61 6 GALLEGO et d. : PYRIDINE-2-CARBALDEHYDE 2-HYDROXYBENZOYL Analyst, VOl.104 the -CONH- group. The exact configuration of a complex of this type, bis(N-pyrid-2- ylidene-N‘-salicyloylhydrazinato)nickel( 11) , has been establi~hed.~ It is evident that PAHB acts as a tridentate ligand that forms an octahedral complex and four five-membered rings are produced. 1:2 iron- aroylhydrazone green complex Choice of extracting solvent, and absorption spectra When a solution of PAHB reagent in an organic solvent is shaken with a weakly acidic aqueous solution of iron(II), the green complex is formed immediately, in the organic phase. Solutions of the complex in chloroform are very stable, whereas those in aqueous ethanol are less stable. When benzene is used as the organic solvent the resulting complex is stable; however, the interferences of foreign ions are high.In oxygenated solvents, such as higher alcohols and 4-methylpentan-2-one, the colour system is less sensitive and less stable than in chloroform. The absorption spectrum of the iron(I1) - PAHB complex in chloroform is shown in Fig. 1 (A). It presentis two bands in the visible region, at 380 and 640nm. The latter wavelength was used in all subsequent measurements of absorbance because the reagent itself does not absorb at this wavelength. It is stable for at least 7 h. In$uence of p H The maximum constant absorbances were obtained in the pH range 4.04.7 [Fig. 2 (B)]. A decrease in absorbance below pH 4.0 can be attributed to incomplete formation of the iron - PAHB complex, owing to protonation of the pyridine nitrogen atom.1° A decrease in absorbance above pH 4.7 is probably caused by the precipitation of iron(I1).Influence of reagent concentration and shaking time An aliquot of 10-250ml of solution (acetate buffer) containing 40pg of iron(I1) was extracted with a solution of O.Ol-O.lyo m/V PAHB in chloroform (10 ml). The extraction was quantitative from 0.05% m/V of the reagent solution and remained constant with increasing concentration. Therefore, 10 ml of (3.075y0 m/V reagent solution was adopted as the concentration of solution containing the coinplexing ligand to be used. The shaking time was varied from 0.5 to 5 min, while the other variables were kept constant. These variations in shaking time did not produce any change in absorbance if the volume ratio Vors. : Vaq.was between 1 : 1 and 1 : 5. On the other hand, shaking for 2 min was neces- sary for the complete extraction of iron(I1) if tlhe volume ratio Vors. : Vaq. was 1 : 25. Spectrophotometric Determination of Iron( 11) with PAHB Based on the experimental work, a method. is proposed for the determination of trace amounts of iron involving the formation of the green complex with PAHB and its extraction into chloroform. Beer’s law is obeyed betweein 2 and 12 pg ml-l of iron(I1) in the organic phase at 640 nm. The optimum concentration range, evaluated by Ringborn’s method, is 3-10 pg ml-l of iron. The green complex gave a molar absorptivity of E = 3.67 x lo3 1 mol-l cm-l a t 640 nm [in the chloroform phase (10 ml)]. The sensitivity of the method, according to Sandell, is 0.015pgcm-2 of iron.The precision was estimated for 10-250-ml aliquots of 4Opg of iron(I1) solution, and the relative error of the method is 0.26%. For the determination of 40 pg of iron by this method, the foreign ions can be tolerated at the levels given in Tables I and 11. Alkali and alkaline earth metals, chloride, nitrate, sulphate, carbonate, perchlorate, thiocyanate , hydroxylamine and ascorbic acid can be tolerated at levels of 0.4 g. The Thioglycollic acid i(TGA) can be tolerated a t the l-g level.July, 19 79 AS A SELECTIVE SPECTROPHOTOMETRIC REAGENT FOR IRON(II) TABLE I TOLERANCE LIMITS OF EXTRACTIVE DETERMINATION OF IRON (11) 61 7 Results obtained using a 40-pg sample of iron(I1). Amount tolerated/ Ion added pg ml-l La(III), Mn(II), Cr(III), Hg(I), Hg(II), Se(IV), Ce(IV), Tl(I), Al(III), As(III), As(V), ammonium, alkali and alkaline earth metals, S20s2-, I-, Br-, C1-, PO,3-, NO,-, SO,%-, SOSB-, CO,a-, ClO,-, ClO,-, SCN-, Sa-, tartrate, dimethylglyoxime (DMG), thioglycollic acid (TGA), hydroxylamine and ascorbic acid .. .. .. 10 000 In(III),t 10,- . . .. .. .. .. .. .. .. .. .. .. 1000 Th(IV), Te(IV), Pt(IV), Bi(III),* citrate . . .. .. .. .. .. .. 5 000 F-, borate . . .. .. .. .. .. . . . . .. .. .. 2 500 * Centrifuged. t 0.1% m/V PAHB solution in chloroform. limiting value of the concentration of foreign ions was taken as that value which caused an error of not more than 2.5% in the absorbance. The method is remarkably free from interferences because most of the metallic chelates of PAHB are not extracted into chloroform and present their absorption peaks in the 350- 400-nm region.The good results obtained by masking were due to the fact that the precipitates remained in the aqueous phase. Further, the method has been compared, advantageously, with the other method previously reported from this laboratory with the use of related reagents (Table 111). Applications materials. The method has been applied satisfactorily to the determination of iron in different Determination of iron in minerals and non-ferro~s alloys Results of the analysis of iron in mineral and non-ferrous alloy samples from the Bureau TABLE I1 ELIMINATION OF INTERFERENCES BY ADDITION OF MASKING AGENTS Amount tolerated/pg ml-l Without With masking r L 3 Foreign ion masking agent Pd(I1) .. .. 1000 .. 5 000 V(V) . . .. 750 Bi(II1). . .. 2 500 In(II1). . .. 750 Cu(I1) . . .. 50 Cu(I1) . . .. 50 Sn(I1) . . .. 50 Mo(V1) .. 2 500 Ti(1V) . . .. 15 U(V1) . . .. 250 W(V1) .. .. 250 2[?ni * . . 1000 Zr (IV) .. 75 Co(I1) . . .. 20 Sb (111) .. 75 Zn(I1) . . .. 100 Ni(I1) . . .. 100 Cd(I1) . . .. 2 500 Pb(I1) . . .. 7 500 CN- .. .. 12 * Shaking for 5 min. t NH,OH.HCl; 0.3 g. Heating gently before extracting. agent 10 000 10 000 5 ooo* 6 000 10000 10000 2 500 10 000 5 OOOt 6 ooo* 5 000 10000 10000 2 500 5 000 2 500: 10000 10000 10000 10000: 10 ooot Masking agent NH,OH.HCl; 1 ml of 10% m/V solution NaCl; 0.2 g NH,OH.HCl; 1 ml of 10% m/V solution NH,OH.HCl; 1 ml of 10% m/V solution TGA; 1 ml of solution TGA; 1 ml of solution KSCN; 0.3 g TGA; 1 ml of solution TGA; 1 ml of solution NH,OH.HCl; 1 ml of 10% m/V solution F-; 2500 pg ml-l F-; 2500 pg ml-l F-; 2600 p g ml-l PO,s- + Ca2+; 10000 pg ml-' TGA; 1 ml of solution S,OS2-; 10000 pg ml-l TGA; 1 ml of solution DMG; 0.05 g TGA; 1 ml of solution TGA; 1 ml of solution Hg2+; 10000 pg ml-l61 8 GALLEGO et al.PYRIDINE-Z-CARBALDEHYDE 2-HYDROXYBENZOYL Analyst, VOE. 104 TABLE I11 COMPARISON WITH EXISTING METHODS Compound Di-2-pyridyl ketone azine . . Pyridine-2-carbaldehyde azine .. .. .. Di-2-pyridyl dihydrazone . . 2-Benzoylpyridine hydrazone Pyridine-2-carbaldehyde Z-hydroxybenzoyl- Diacetyldihydrazone .. hydrazone . . .. .. Optimum PH 4.2-5.2 4-5 4.7-7.8 4-7 4-7 4.0-4.7 Molar absorptivity/ X,,,./nm 1 mol-1 cm-1 Interferences* Reference 750 6.367 Cu(I1) 12 660 2.900 Au(III), Pd(II), Se(1V) 13 488 8.240 Mn(II), Co(II), Cu(I1) 14 490 11.500 Co(II), Cu(II), Cr(V1) 15 480 9.550 Co(II), CU(II), V(V) 16 640 3.670 - - * Cations that interfere a t an identical concentration. to iron.of Analysed Samples Ltd. and British Chemical Standards (Table IV) support the precision and reliability of this method. In order to prevent interferences due to foreign ions, 0.3 g of potassium thiocyanate was added to solutions of H.T. brass and 0.3g of potassium thiocyanate and 0.05 g of dimethylglyoxime (D:MG) were added to solutions of cupro-nickel. Each cupro-nickel solution was then heated gently. TABLE IV ANALYSIS OF MINERALS AND NON-FERROUS ALLOYS Sample Dolomite (BAS, No. 9h) . . . . Brass (BAS, No. 5g) .. .. .. H.T. brass (BAS, No. 10e) .. .. .. Cupro-nickel (BAS, No. 19e) . . .. Aluminium alloy (BAS, No. 20b) . . .. Portland cement (BCS, No. '372) .. Zinc concentrates (BAS, No. 4ldGj * . . * Average of five separate determinations. t Percentage as Fe,O,. Iron content, yo f A \ Reported value Found* 0.21t 0.207 f 0.002 2.491. 2.489 f 0.002 0.32 0.320 f 0.001 1.38 1.376 f 0.003 0.83 0.85 f 0.02 0.43 0.431 f 0.001 10.0 10.04 & 0.04 Determination of iron in milk, blood, tobacco, garkc and water Milk and blood were treated with equal voluines of 20% mlV trichloroacetic acid solution for 30 min at 30 "C in a water-bath and then the solid residues were removed. Fermented tobacco, in powder form, and garlic were ashed and then treated with a mixture of con- centrated perchloric, nitric and sulphuric acids (1 + 10 + 1) and boiled to ensure complete dissolution. The water from a sulphuric acid plant (pyrites process) was filtered through an TABLE V COMPARISON OF RESULTS FOR THE ANALYSIS OF DIFFERENT SAMPLES BY 1 ,lo-PHENANTHROLINE AND PAHB METHODS Iron content* r A 'L 11,lO-Phenanthroline Sample method PAHB method Milk ... . . . . . .. .. 1.38 p.p.m. 1.42 & 0.03 p.p.m. Bloodserum . . . . .. .. .. 0.93 p.p.m. 0.95 f 0.01 p.p.m. Tobacco . . . . . . . . . . .. 0.062% 0.061 f 0.001% Garlic . . . . . . .. .. .. 0.01 4 yo 0.014 f 0.001% Waste water (from sulphuric acid plant) . . 60.30 p.p.m. 60.25 f 0.02 p.p.m. * Average of six determinations.July, 1979 AS A SELECTIVE SPECTROPHOTOMETRIC REAGENT FOR IRON(II) 619 asbestos mat contained in a Gooch crucible.The results are compared with those obtained by the spectrophotometric method using 1 ,lo-phenanthroline in Table V. Determination of iron by the standard additions method in waters and analytical reagents The PAHB method can also be applied to the determination of iron a t parts per billion (loQ) levels, which decreases the lower limit of the iron determination by taking a larger volume of the aqueous phase in relation to the chloroform volume and applying the standard additions method. The method consists in adding several increasing known amounts of iron(I1) (0, 5 , 10, 15 and 20 pg) to several aliquots of sample solution: river water (200 ml of sample solution, 25 ml of acetate buffer) ; sea water (200 ml of sample solution, 25 ml of acetate buffer) ; water from a fertiliser plant (100 ml of sample solution, 15 ml of acetate buffer) ; concentrated nitric acid (5 ml of sample solution, 50 ml of acetate buffer) ; and sodium hydroxide solution (100 ml of 0.1545 N sample solution, 50 ml of acetate buffer).Then, all prepared solutions, after the extraction procedure described above, are measured at 640 nm. The procedure is repeated for blank solutions (distilled water) treated in the same way. The absorbances are plotted against the concentrations of the four iron-containing solutions of each sample. The straight lines (one for the sample solutions and the other for blank solutions) are extrapolated until they intersect the abscissa. The segment between the points of intersection give the concentration of the sample solution. The results are compared in some instances with those obtained by atomic-absorption spectroph~tometryl~ in Table VI.TABLE VI DETERMINATION OF IRON IN SAMPLES BY STANDARD ADDITIONS METHOD Iron content I A 1 Sample Reported value Found Concentrated nitric acid (Panreac, PRS) . . o . o o o l ~ o 0.000 15% Sodium hydroxide pellets (Merck, GR) . . 0.0005% 0.000 64% River water . . .. . . .. . . - 29.3 p.p.b. Sea water . . .. .. .. .. . . 15.7 p.p.b.* 13.5 p.p.b. Waste water (from fertiliser plant) . . . . 71.3 p.p.b.* 72.5 p.p.b. * Results from atomic-absorption spectrophotometry. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. References Katiyar, S. S., and Tandon, S. N., Talanta, 1964, 11, 892. Vasilikiotis, G. S., and Tossidis, J. A., Microchem. J., 1969, 14, 380. Uno, T., and Taniguchi, H., Bunseki Kagaku, 1971, 20, 997. Odashima, T., and Ishii, H., Nippon Kagaku Kaishi, 1973, 729. Vasilikiotis, G. S., Microchem. J., 1968, 13, 526. Capitan, F., Salinas, F., and Gimenez Plaza, J., A r s Pharm., 1975, 16, 293. Aggarwal, R. C., and Rao, T. R., Transition Met. Chem., 1977, 2, 21. Aggarwal, R. C., and Rao, T. R., Transition Met. Chem., 1977, 2, 59. Domiano, P., Musatti, A., Nardelli, M., and Pelizzi, C., J . Chem. SOC., Dalton Trans., 1975, 295. Gallego, M., Garcia-Vargas, M., Pino, F., and Valcarcel, M., Microchem. J., 1978, 23, 353. Rastogi, D. K., Sahni, S. K., Rana, V. B., and Dua, S. K., Transition Met. Chem., 1978, 3, 56. Valcarcel, M., Martinez, M. P., and Pino, F., Analyst, 1975, 100, 33. Luque de Castro, M. D., and Valcarcel, M., Analyt. Lett., 1978, 11, 1. Graciani Constante, E., An. Quim., 1971, 67, 607. Graciani Constante, E., and Olias Jimenez, J. M., An. Quim., 1971, 67, 615. Graciani Constante, E., An. Quim., 1974, 70, 695. Nix, J., and Goodwin, T., Atom. Absorption Newsl., 1970, 9, 119. Received December 29th, 1978 Accepted January 15th, 1979